#424

Reviewed-on: #429
Co-authored-by: miteruzo <miteruzo@naver.com>

backend/app/controllers/materials_controller.rb

@@ -70,7 +70,10 @@ class MaterialsController < ApplicationController
         .find_by(id: params[:id])
     return head :not_found unless material
 
-    wiki_page_body = material.tag&.tag_name('ja')&.wiki_page&.current_revision&.body
+    wiki_page_body =
+        TagDescription
+        .find_by(tag_id: material.tag_id, language_code: locale.language_code)
+        &.body
 
     render json: MaterialRepr.base(material, host: request.base_url,
                                    language_code: locale.language_code).merge(wiki_page_body:)

backend/app/controllers/nico_tags_controller.rb

@@ -26,7 +26,7 @@ class NicoTagsController < ApplicationController
         ExternalTag
         .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
                'ON post_tag_max.external_tag_id = external_tags.id')
-        .includes(linked_tags: { tag_names: :wiki_page })
+        .includes(linked_tags: [:tag_names, :tag_descriptions])
     if name
       q = q.where(('external_tags.name LIKE ? ' +
                    "OR CONCAT(external_tags.platform, ':', external_tags.name) LIKE ?"),

backend/app/controllers/posts_controller.rb

@@ -53,7 +53,7 @@ class PostsController < ApplicationController
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
         .preload(:external_tags, :uploaded_user, :parents, :children,
                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                { tag_names: :wiki_page }] }])
+                                                :tag_names, :tag_descriptions] }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -111,7 +111,7 @@ class PostsController < ApplicationController
         filtered_posts(locale)
         .preload(:uploaded_user, :parents, :children,
                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                { tag_names: :wiki_page }] }])
+                                                :tag_names, :tag_descriptions] }])
         .with_attached_thumbnail
         .order('RAND()')
         .first
@@ -198,7 +198,7 @@ class PostsController < ApplicationController
         Post
         .includes(:uploaded_user, :parents, :children,
                   post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                 { tag_names: :wiki_page }] }])
+                                                 :tag_names, :tag_descriptions] }])
         .with_attached_thumbnail
         .find_by(id: params[:id])
     return head :not_found unless post

backend/app/controllers/tag_groups_controller.rb

@@ -0,0 +1,39 @@
+class TagGroupsController < ApplicationController
+  def index
+    groups = TagGroup.includes(:created_by_user, :updated_by_user).order(:id)
+    render json: TagGroupRepr.many(groups.to_a)
+  end
+
+  def show
+    group = TagGroup.includes(:created_by_user, :updated_by_user).find_by(id: params[:id])
+    return head :not_found unless group
+
+    render json: TagGroupRepr.one(group)
+  end
+
+  def create
+    return head :unauthorized unless current_user
+    return head :forbidden unless current_user.gte_member?
+
+    group = TagGroupOperation.create!(**write_attributes, actor: current_user)
+    render json: TagGroupRepr.one(group), status: :created
+  end
+
+  def update
+    return head :unauthorized unless current_user
+    return head :forbidden unless current_user.gte_member?
+
+    group = TagGroup.find_by(id: params[:id])
+    return head :not_found unless group
+
+    TagGroupOperation.update!(group:, **write_attributes, actor: current_user)
+    render json: TagGroupRepr.one(group.reload)
+  end
+
+  private
+
+  def write_attributes
+    { name: params[:name], description: params[:description],
+      members: params[:members], placements: params[:placements] }
+  end
+end

backend/app/controllers/tag_versions_controller.rb

@@ -51,7 +51,7 @@ class TagVersionsController < ApplicationController
 
       tags_by_id =
           Tag
-          .includes(:tag_names, :materials, tag_names: :wiki_page)
+          .includes(:tag_names, :materials, :tag_descriptions, parents: :tag_descriptions)
           .where(id: all_parent_tag_ids)
           .index_by(&:id)
 

backend/app/controllers/tags_controller.rb

@@ -175,7 +175,7 @@ class TagsController < ApplicationController
     tags_by_id =
         Tag
         .joins(:tag_names)
-        .includes(:tag_names, :materials, tag_names: :wiki_page)
+        .includes(:tag_names, :materials, :tag_descriptions, parents: :tag_descriptions)
         .where(tag_names: { language_code: locale.language_code, primary_flg: true },
                id: tag_ids)
         .index_by(&:id)
@@ -238,7 +238,7 @@ class TagsController < ApplicationController
     base =
         Tag
         .joins(:tag_names)
-        .includes(:tag_names, :materials, tag_names: :wiki_page)
+        .includes(:tag_names, :materials, :tag_descriptions, parents: :tag_descriptions)
         .where(tag_names: { language_code: locale.language_code, primary_flg: true },
                deprecated_at: nil)
     base = base.where('tags.post_count > 0') if present_only
@@ -282,7 +282,7 @@ class TagsController < ApplicationController
     tag =
         Tag
         .joins(:tag_names)
-        .includes(:tag_names, :materials, tag_names: :wiki_page)
+        .includes(:tag_names, :materials, :tag_descriptions, parents: :tag_descriptions)
         .find_by(id: params[:id],
                  tag_names: { language_code: locale.language_code, primary_flg: true })
     return render json: TagRepr.base(tag, language_code: locale.language_code) if tag
@@ -302,7 +302,7 @@ class TagsController < ApplicationController
     tag =
         Tag
         .joins(:tag_names)
-        .includes(:tag_names, :materials, tag_names: :wiki_page)
+        .includes(:tag_names, :materials, :tag_descriptions, parents: :tag_descriptions)
         .find_by(tag_names: { name:, language_code: locale.language_code, primary_flg: true })
     return render json: TagRepr.base(tag, language_code: locale.language_code) if tag
 
@@ -320,7 +320,7 @@ class TagsController < ApplicationController
 
     tag = 
         Tag.joins(:tag_names)
-        .includes(:tag_names, tag_names: :wiki_page)
+        .includes(:tag_names, :tag_descriptions, parents: :tag_descriptions)
         .find_by(id: params[:id],
                  tag_names: { language_code: locale.language_code, primary_flg: true })
     return head :not_found unless tag
@@ -340,7 +340,7 @@ class TagsController < ApplicationController
     tag =
         Tag
         .joins(:tag_names)
-        .includes(:tag_names, tag_names: :wiki_page)
+        .includes(:tag_names, :tag_descriptions, parents: :tag_descriptions)
         .find_by(tag_names: { name:, language_code: locale.language_code, primary_flg: true })
     return head :not_found unless tag
 
@@ -359,7 +359,7 @@ class TagsController < ApplicationController
     tag =
         Tag
         .joins(:tag_names)
-        .includes(:tag_names, tag_names: :wiki_page)
+        .includes(:tag_names, :tag_descriptions, parents: :tag_descriptions)
         .find_by(id: params[:id],
                  tag_names: { language_code: locale.language_code, primary_flg: true })
     return head :not_found unless tag
@@ -424,7 +424,7 @@ class TagsController < ApplicationController
     tag =
         Tag
         .joins(:tag_names)
-        .includes(:tag_names, :materials, tag_names: :wiki_page)
+        .includes(:tag_names, :materials, :tag_descriptions, parents: :tag_descriptions)
         .find_by(tag_names: { name:, language_code: locale.language_code, primary_flg: true })
     return head :not_found unless tag
 

backend/app/controllers/wiki_pages_controller.rb

@@ -3,24 +3,29 @@ class WikiPagesController < ApplicationController
 
   def index
     # 互換性のための化石リソース
-    # TODO: ヘルプおよびタグ・グループ追加後に，
-    # TODO: タグ説明とヘルプ、タグ・グループから取得する作りに変更する．
-    #
-    title = params[:title].to_s.strip
-    descriptions = TagDescription.where(language_code: 'ja')
-        .includes(tag: :tag_names)
-    pages = unmigrated_wiki_pages.includes(tag_name: :tag)
 
+    title = params[:title].to_s.strip
+    descriptions =
+        TagDescription.where(language_code: 'ja')
+        .includes(tag: :tag_names)
+
+    descriptions = descriptions.order(updated_at: :desc, id: :asc)
+    tag_groups = TagGroup.order(updated_at: :desc, id: :asc)
     if title.present?
       pattern = "%#{ ApplicationRecord.sanitize_sql_like(title) }%"
-      descriptions = descriptions.joins(tag: :tag_names)
+      descriptions =
+          descriptions.joins(tag: :tag_names)
           .where(tag_names: { language_code: 'ja', primary_flg: true })
           .where('tag_names.name LIKE ?', pattern).limit(20)
-      pages = pages.where('tag_names.name LIKE ?', pattern).limit(20)
+      tag_groups = tag_groups.where(
+          'CONCAT(?, name) LIKE ?', TagGroupRepr::PREFIX, pattern).limit(20)
     end
 
     json = descriptions.map { |description| WikiPageRepr.tag_description(description) }
-    json.concat(WikiPageRepr.base(pages))
+    json.concat(tag_groups.map { WikiPageRepr.tag_group(_1) })
+    json.sort_by! do |row|
+      [-row['updated_at'].to_f, row.fetch('resource_type'), row.fetch('id')]
+    end
     json = json.first(20) if title.present?
     render json:
   end
@@ -30,39 +35,45 @@ class WikiPagesController < ApplicationController
 
     return render_wiki_page_by_tag_description tag_description if tag_description
 
-    page = WikiPage.joins(:tag_name)
-                   .includes(tag_name: :tag)
-                   .find_by(id: params[:id])
+    tag_group = TagGroup.find_by(id: params[:id])
+    return render_tag_group tag_group if tag_group
 
-    render_wiki_page_or_404 page
+    render_wiki_page_or_404 nil
   end
 
   def show_by_title
     title = params[:title].to_s.strip
 
+    if title.start_with?(TagGroupRepr::PREFIX)
+      name = title.delete_prefix(TagGroupRepr::PREFIX)
+      tag_group = TagGroup.find_by(name:)
+      return render_tag_group tag_group if tag_group
+      return head :not_found
+    end
+
     tag_description = tag_description_by_title(title)
 
     return render_wiki_page_by_tag_description tag_description if tag_description
 
-    page = WikiPage.joins(:tag_name)
-                   .includes(tag_name: :tag)
-                   .find_by(tag_name: { name: title })
-
-    render_wiki_page_or_404 page
+    render_wiki_page_or_404 nil
   end
 
   def exists
     return head :no_content if TagDescription.exists?(id: params[:id])
+    return head :no_content if TagGroup.exists?(id: params[:id])
 
-    head WikiPage.exists?(id: params[:id]) ? :no_content : :not_found
+    head :not_found
   end
 
   def exists_by_title
     title = params[:title].to_s.strip
+    if title.start_with?(TagGroupRepr::PREFIX)
+      exists = TagGroup.exists?(name: title.delete_prefix(TagGroupRepr::PREFIX))
+      return head exists ? :no_content : :not_found
+    end
     return head :no_content if tag_description_by_title(title)
 
-    exists = WikiPage.joins(:tag_name).exists?(tag_names: { name: title })
-    head exists ? :no_content : :not_found
+    head :not_found
   end
 
   def diff
@@ -79,29 +90,25 @@ class WikiPagesController < ApplicationController
       to_rev = to ? versions.find(to) : versions.find_by!(version_no: description.version_no)
       resource = WikiPageRepr.tag_description(description)
     else
-      page = WikiPage.joins(:tag_name).includes(tag_name: :tag).find(id)
-      from_rev = from && page.wiki_revisions.find(from)
-      to_rev = to ? page.wiki_revisions.find(to) : page.current_revision
-      if ((from_rev && !from_rev.content?) || !to_rev&.content?)
-        return render_unprocessable_entity('差分を表示できない版です．')
+      tag_group = TagGroup.find_by(id:)
+      if tag_group
+        from_rev = from && group_version(tag_group, from)
+        to_rev = to ? group_version(tag_group, to) : current_group_version(tag_group)
+        return head :not_found if (from && !from_rev) || !to_rev
+
+        from_body = from_rev && render_group_version(from_rev)
+        to_body = render_group_version(to_rev)
+        resource = { 'id' => tag_group.id,
+                     'title' => "#{ TagGroupRepr::PREFIX }#{ tag_group.name }",
+                     'deprecated_at' => nil }
+      else
+        return head :not_found
       end
-      resource = WikiPageRepr.base(page)
     end
 
-    diffs = Diff::LCS.sdiff(from_rev&.body&.lines || [], to_rev.body.lines)
-    diff_json = diffs.map { |change|
-      case change.action
-      when ?=
-        { type: 'context', content: change.old_element }
-      when ?!
-        [{ type: 'removed', content: change.old_element },
-         { type: 'added', content: change.new_element }]
-      when ?+
-        { type: 'added', content: change.new_element }
-      when ?-
-        { type: 'removed', content: change.old_element }
-      end
-    }.flatten.compact
+    from_body ||= from_rev&.body
+    to_body ||= to_rev.body
+    diff_json = wiki_diff(from_body, to_body)
 
     render json: { wiki_page_id:      resource.fetch('id'),
                    title:             resource.fetch('title'),
@@ -109,6 +116,8 @@ class WikiPagesController < ApplicationController
                    older_revision_id: from_rev&.id,
                    newer_revision_id: to_rev.id,
                    diff:              diff_json }
+  rescue TagGroupMarkdownRenderer::UnsupportedStructure => error
+    render_unprocessable_entity(error.message)
   end
 
   def create
@@ -122,17 +131,20 @@ class WikiPagesController < ApplicationController
     return render_unprocessable_entity('タイトルは必須です．', field: :title) if title.blank?
     return render_unprocessable_entity('本文は必須です．', field: :body) if body.blank?
 
-    page = if title.include?(':')
-      ApplicationRecord.transaction do
-        tag_name = TagName.find_or_create_by!(language_code: 'ja', name: title) do |name|
-          name.script_code = 'Jpan'
-          name.primary_flg = true
-          name.auto_generated = false
+    if title.start_with?(TagGroupRepr::PREFIX)
+      name = title.delete_prefix(TagGroupRepr::PREFIX)
+      parsed = TagGroupMarkdownParser.parse(body:, group: nil)
+      group = TagGroupOperation.create!(name:, **parsed, actor: current_user,
+                                         require_compatibility: true)
+      version = current_group_version(group)
+      return render_group_representation(TagGroupRepr.one(group), version,
+                                         body: parsed.fetch(:compatibility_body),
+                                         status: :created)
     end
 
-        Wiki::Commit.create_legacy_content!(
-            tag_name:, body:, created_by_user: current_user, message:)
-      end
+    page =
+        if title.include?(':')
+          return render_unprocessable_entity field: :title
         else
           tag_name = TagName.find_by(language_code: 'ja', name: title)
           return render_unprocessable_entity field: :title unless tag_name&.tag
@@ -141,11 +153,14 @@ class WikiPagesController < ApplicationController
               tag: tag_name.tag, body:, created_by_user: current_user, message:)
         end
 
-    render json: WikiPageRepr.base(page), status: :created
+    description = TagDescription.find_by!(tag_id: page.tag_name.tag_id, language_code: 'ja')
+    render json: WikiPageRepr.tag_description(description), status: :created
   rescue ActiveRecord::RecordInvalid => e
     render_validation_error e.record
   rescue ActiveRecord::RecordNotUnique
     render_record_not_unique
+  rescue TagGroupMarkdownParser::InvalidDocument => error
+    render_unprocessable_entity(error.message)
   end
 
   def update
@@ -158,36 +173,57 @@ class WikiPagesController < ApplicationController
     return render_unprocessable_entity('タイトルは必須です．', field: :title) if title.blank?
     return render_unprocessable_entity('本文は必須です．', field: :body) if body.blank?
 
-    page = WikiPage.find(params[:id])
+    group = TagGroup.find_by(id: params[:id])
+    if title.start_with?(TagGroupRepr::PREFIX)
+      return head :not_found unless group
+
+      name = title.delete_prefix(TagGroupRepr::PREFIX)
+      parsed = TagGroupMarkdownParser.parse(body:, group:)
+      TagGroupOperation.update!(group:, name:, **parsed, actor: current_user,
+                                require_compatibility: true)
+      return head :ok
+    end
+
+    return render_unprocessable_entity field: :title if title.include?(':')
+    description = TagDescription.find_by(id: params[:id])
+    unless description
+      return render_unprocessable_entity field: :title if group || WikiPage.exists?(id: params[:id])
+      return head :not_found
+    end
+    tag = description.tag
+    tag_name = TagDescriptionOperation.compatibility_tag_name(tag)
+    if tag_name.name.include?(':') || (group && title != tag_name.name)
+      return render_unprocessable_entity field: :title
+    end
+
     base_revision_id = params[:base_revision_id].presence
 
     ApplicationRecord.transaction do
-      page.lock!
-
-      old_title = page.title
-
-      tag = page.tag_name.tag
-
-      if tag && title != old_title
+      tag.lock!
+      old_title = tag_name.name
+      if title != old_title
         TagVersioning.ensure_snapshot!(tag, created_by_user: current_user)
       end
 
-      page.tag_name.update!(name: title) if title != old_title
+      tag_name.update!(name: title) if title != old_title
 
       message = params[:message].presence
-      Wiki::Commit.content!(page:,
-                            body:,
-                            created_user: current_user,
-                            message:,
-                            base_revision_id:)
+      TagDescriptionOperation.update!(tag:, body:, updated_by_user: current_user,
+                                      message:, base_revision_id:)
 
-      if tag && title != old_title
+      if title != old_title
         tag.reload
         TagVersionRecorder.record!(tag:, event_type: :update, created_by_user: current_user)
       end
     end
 
     head :ok
+  rescue ActiveRecord::RecordInvalid => e
+    render_validation_error e.record
+  rescue ActiveRecord::RecordNotUnique
+    render_record_not_unique
+  rescue TagGroupMarkdownParser::InvalidDocument => error
+    render_unprocessable_entity(error.message)
   end
 
   def changes
@@ -197,32 +233,130 @@ class WikiPagesController < ApplicationController
       return render json: description_changes(TagDescription.where(id: description.id))
     end
 
-    pages = id ? WikiPage.where(id:) : unmigrated_wiki_pages
-    revisions = WikiRevision.joins(wiki_page: :tag_name)
-        .where(wiki_page_id: pages.select(:id))
-        .includes(:created_user, wiki_page: { tag_name: :tag })
-        .order(id: :desc).limit(200)
-    json = revisions.map { |rev|
-      { revision_id: rev.id,
-        pred: rev.base_revision_id,
-        succ: nil,
-        wiki_page: { id: rev.wiki_page_id,
-                     title: rev.wiki_page.title,
-                     deprecated_at: rev.wiki_page.deprecated_at },
-        user: rev.created_user && { id: rev.created_user.id, name: rev.created_user.name },
-        kind: rev.kind,
-        message: rev.message,
-        timestamp: rev.created_at }
-    }
+    tag_group = id && TagGroup.find_by(id:)
+    if tag_group
+      return render json: tag_group_changes(TagGroupVersion.where(tag_group_id: tag_group.id))
+    end
+
+    json = []
     unless id
+      json.concat(tag_group_changes(TagGroupVersion.all))
       json.concat(description_changes(TagDescription.where(language_code: 'ja')))
-      json.sort_by! { |row| -row[:revision_id] }
+      json = json.each_with_index.sort_by do |row, index|
+        [-row.fetch(:timestamp).to_f, index]
+      end.map(&:first)
     end
     render json: json.first(200)
   end
 
   private
 
+  def legacy_group_page? page
+    return false unless page
+    return true if page.title.start_with?(TagGroupRepr::PREFIX)
+
+    prefix = ApplicationRecord.sanitize_sql_like(TagGroupRepr::PREFIX)
+    page.wiki_versions.where('title LIKE ?', "#{ prefix }%").exists?
+  end
+
+  def legacy_update_identified? page, title, group
+    return false unless page
+    return true if page.title == title
+    return false if title == group.name
+
+    namespace = page.title.split(':', 2)
+    return true if namespace.size == 2 && title.start_with?("#{ namespace.first }:")
+
+    base = params[:base_revision_id].presence
+    base && page.wiki_revisions.exists?(id: base)
+  end
+
+  def render_tag_group group
+    version = if params[:version].present?
+      group_version(group, params[:version])
+    else
+      current_group_version(group)
+    end
+    return head :not_found unless version
+
+    representation = if params[:version].present?
+      TagGroupRepr.historical(version)
+    else
+      TagGroupRepr.one(group)
+    end
+    render_group_representation(representation, version)
+  end
+
+  def render_group_version version
+    representation = TagGroupRepr.historical(version)
+    TagGroupMarkdownRenderer.render(representation)
+  end
+
+  def render_group_representation representation, version, body: nil, status: :ok
+    body ||= TagGroupMarkdownRenderer.render(representation)
+    versions = TagGroupVersion.where(tag_group_id: version.tag_group_id)
+    pred = versions.find_by(version_no: version.version_no - 1)&.id
+    succ = versions.find_by(version_no: version.version_no + 1)&.id
+    updated_at = version.created_at
+    render json: {
+        'id' => version.tag_group_id,
+        'resource_type' => 'tag_group',
+        'title' => "#{ TagGroupRepr::PREFIX }#{ representation.fetch('name') }",
+        'deprecated_at' => nil,
+        'body' => body,
+        'revision_id' => version.id,
+        'pred' => pred,
+        'succ' => succ,
+        'updated_at' => updated_at }, status:
+  rescue TagGroupMarkdownRenderer::UnsupportedStructure => error
+    render_unprocessable_entity(error.message)
+  end
+
+  def current_group_version group
+    TagGroupVersion.find_by(tag_group_id: group.id, version_no: group.version_no)
+  end
+
+  def group_version group, version_id
+    TagGroupVersion.find_by(tag_group_id: group.id, id: version_id)
+  end
+
+  def wiki_diff from_body, to_body
+    diffs = Diff::LCS.sdiff(from_body.to_s.lines, to_body.to_s.lines)
+    diffs.map { |change|
+      case change.action
+      when ?=
+        { type: 'context', content: change.old_element }
+      when ?!
+        [{ type: 'removed', content: change.old_element },
+         { type: 'added', content: change.new_element }]
+      when ?+
+        { type: 'added', content: change.new_element }
+      when ?-
+        { type: 'removed', content: change.old_element }
+      end
+    }.flatten.compact
+  end
+
+  def tag_group_changes versions
+    rows = versions.includes(:created_by_user).order(id: :desc).limit(200).to_a
+    groups = TagGroup.where(id: rows.map(&:tag_group_id).uniq).index_by(&:id)
+    predecessors = TagGroupVersion.where(tag_group_id: groups.keys,
+                                         version_no: rows.map { _1.version_no - 1 })
+        .pluck(:tag_group_id, :version_no, :id)
+        .to_h { |group_id, number, version_id| [[group_id, number], version_id] }
+    rows.map do |version|
+      group = groups.fetch(version.tag_group_id)
+      user = version.created_by_user
+      { revision_id: version.id,
+        pred: predecessors[[group.id, version.version_no - 1]],
+        succ: nil,
+        wiki_page: { id: group.id, title: "#{ TagGroupRepr::PREFIX }#{ version.name }",
+                     deprecated_at: nil },
+        user: user && { id: user.id, name: user.name },
+        kind: 'content', message: nil, timestamp: version.created_at }
+    end
+  end
+
   def render_wiki_page_or_404 page
     return head :not_found unless page
 
@@ -269,13 +403,6 @@ class WikiPagesController < ApplicationController
         .find_by(language_code: 'ja', tag_names: { language_code: 'ja', name: title })
   end
 
-  def unmigrated_wiki_pages
-    descriptions = TagDescription.where(language_code: 'ja')
-    WikiPage.joins(:tag_name).where.not(id: descriptions.select(:id))
-        .where('tag_names.tag_id IS NULL OR tag_names.tag_id NOT IN (?)',
-               descriptions.select(:tag_id))
-  end
-
   def description_changes descriptions
     versions = TagDescriptionVersion.where(language_code: 'ja',
                                            tag_id: descriptions.select(:tag_id))

backend/app/models/external_tag.rb

@@ -1,4 +1,6 @@
 class ExternalTag < ApplicationRecord
+  before_create { SharedResourceIdAllocator.assign!(self) }
+
   enum :platform, nico: 'nico'
   validates :platform, presence: true, inclusion: { in: ExternalTag.platforms.keys }
 

backend/app/models/tag.rb

@@ -2,6 +2,8 @@ require 'set'
 
 
 class Tag < ApplicationRecord
+  before_create { SharedResourceIdAllocator.assign!(self) }
+
   class NicoTagNormalisationError < ArgumentError
     ;
   end
@@ -51,6 +53,7 @@ class Tag < ApplicationRecord
   has_many :tag_versions
 
   has_many :tag_names
+  has_many :tag_descriptions
 
   attribute :version_no, :integer, default: 1
 
@@ -96,7 +99,13 @@ class Tag < ApplicationRecord
 
   def deprecated? = deprecated_at?
 
-  def has_wiki = wiki_page.present?
+  def has_description? language_code:
+    if tag_descriptions.loaded?
+      tag_descriptions.any? { _1.language_code == language_code }
+    else
+      tag_descriptions.exists?(language_code:)
+    end
+  end
 
   def material_id = materials.first&.id
 

backend/app/models/tag_description.rb

@@ -1,3 +1,5 @@
 class TagDescription < ApplicationRecord
+  before_create { SharedResourceIdAllocator.assign!(self) }
+
   belongs_to :tag
 end

backend/app/models/tag_group.rb

@@ -0,0 +1,18 @@
+class TagGroup < ApplicationRecord
+  before_create { SharedResourceIdAllocator.assign!(self) }
+
+  belongs_to :created_by_user, class_name: 'User', optional: true
+  belongs_to :updated_by_user, class_name: 'User', optional: true
+
+  has_many :tag_group_tags, dependent: :delete_all
+  has_many :tags, through: :tag_group_tags
+  has_many :tag_group_groups, dependent: :delete_all
+  has_many :child_tag_groups, through: :tag_group_groups
+  has_many :placements, -> { order(:position) },
+           class_name: 'TagGroupPlacement', dependent: :delete_all
+  has_many :versions, -> { order(:version_no) },
+           class_name: 'TagGroupVersion', dependent: :delete_all
+
+  validates :name, presence: true, uniqueness: true, length: { maximum: 255 }
+  validates :version_no, numericality: { only_integer: true, greater_than: 0 }
+end

backend/app/models/tag_group_group.rb

@@ -0,0 +1,7 @@
+class TagGroupGroup < ApplicationRecord
+  self.primary_key = [:tag_group_id, :child_tag_group_id]
+
+  belongs_to :tag_group
+  belongs_to :child_tag_group, class_name: 'TagGroup'
+  belongs_to :created_by_user, class_name: 'User', optional: true
+end

backend/app/models/tag_group_placement.rb

@@ -0,0 +1,9 @@
+class TagGroupPlacement < ApplicationRecord
+  self.primary_key = [:tag_group_id, :position]
+
+  belongs_to :tag_group
+  belongs_to :tag, optional: true
+  belongs_to :child_tag_group, class_name: 'TagGroup', optional: true
+  belongs_to :created_by_user, class_name: 'User', optional: true
+  belongs_to :updated_by_user, class_name: 'User', optional: true
+end

backend/app/models/tag_group_tag.rb

@@ -0,0 +1,7 @@
+class TagGroupTag < ApplicationRecord
+  self.primary_key = [:tag_group_id, :tag_id]
+
+  belongs_to :tag_group
+  belongs_to :tag
+  belongs_to :created_by_user, class_name: 'User', optional: true
+end

backend/app/models/tag_group_version.rb

@@ -0,0 +1,7 @@
+class TagGroupVersion < ApplicationRecord
+  include VersionRecord
+
+  belongs_to :tag_group
+
+  scope :ordered, -> { order(:version_no) }
+end

backend/app/representations/tag_group_repr.rb

@@ -0,0 +1,194 @@
+module TagGroupRepr
+  PREFIX = 'タググループ:'
+
+  module_function
+
+  def many groups
+    ids = groups.map(&:id)
+    return [] if ids.empty?
+
+    tag_members = TagGroupTag.where(tag_group_id: ids)
+        .group_by(&:tag_group_id)
+    group_members = TagGroupGroup.where(tag_group_id: ids)
+        .group_by(&:tag_group_id)
+    placements = TagGroupPlacement.where(tag_group_id: ids).order(:position)
+        .group_by(&:tag_group_id)
+
+    target_ids = tag_members.values.flatten.map(&:tag_id) +
+        group_members.values.flatten.map(&:child_tag_group_id) +
+        placements.values.flatten.filter_map(&:tag_id)
+    child_ids = group_members.values.flatten.map(&:child_tag_group_id) +
+        placements.values.flatten.filter_map(&:child_tag_group_id)
+    tags = Tag.select(:id, :category).where(id: target_ids.uniq).index_by(&:id)
+    names = TagName.where(tag_id: target_ids.uniq, language_code: 'ja', primary_flg: true)
+        .pluck(:tag_id, :name).to_h
+    child_groups = TagGroup.where(id: child_ids.uniq).index_by(&:id)
+
+    groups.map do |group|
+      group_json(group,
+                 tag_members.fetch(group.id, []),
+                 group_members.fetch(group.id, []),
+                 placements.fetch(group.id, []), tags, names, child_groups)
+    end
+  end
+
+  def one group
+    many([group]).first
+  end
+
+  def tag_group wiki_page
+    { 'id' => wiki_page.id,
+      'title' => "#{ PREFIX }#{ wiki_page.name }",
+      'deprecated_at' => nil,
+      'created_at' => wiki_page.created_at,
+      'updated_at' => wiki_page.updated_at }
+  end
+
+  def current_version group
+    TagGroupVersion.find_by(tag_group_id: group.id, version_no: group.version_no)
+  end
+
+  def historical version
+    members = Array(version.members)
+    placements = Array(version.placements)
+    tag_references = members.filter_map { |member| member['tag_id'] } +
+        collect_ids(placements, 'tag_id')
+    group_references = members.filter_map { |member| member['tag_group_id'] } +
+        collect_ids(placements, 'tag_group_id')
+    tag_versions = historical_tag_versions(tag_references)
+    group_versions = historical_group_versions(group_references)
+    tag_version_numbers = members.filter_map do |member|
+      next unless member['tag_id']
+
+      [member.fetch('tag_id'), member.fetch('version_no')]
+    end.to_h
+    group_version_numbers = members.filter_map do |member|
+      next unless member['tag_group_id']
+
+      [member.fetch('tag_group_id'), member.fetch('version_no')]
+    end.to_h
+    historical_members = members.map do |member|
+      historical_member(member, tag_versions, group_versions)
+    end
+    historical_placements = placements.map do |node|
+      historical_placement(node, tag_versions, group_versions,
+                           tag_version_numbers, group_version_numbers)
+    end
+    { 'id' => version.tag_group_id,
+      'name' => version.name,
+      'description' => version.description,
+      'version_no' => version.version_no,
+      'members' => historical_members,
+      'placements' => historical_placements,
+      'created_at' => version.created_at,
+      'updated_at' => version.created_at,
+      'created_by_user' => user_json(version.created_by_user),
+      'updated_by_user' => user_json(version.created_by_user) }
+  end
+
+  def group_json group, tag_members, group_members, rows, tags, names, child_groups
+    tree = placement_tree(rows, tags, names, child_groups)
+    members = tag_members.sort_by(&:tag_id).map do |member|
+      { 'tag' => tag_json(tags.fetch(member.tag_id), names.fetch(member.tag_id)) }
+    end
+    members.concat(group_members.sort_by(&:child_tag_group_id).map do |member|
+      { 'tag_group' => group_json_reference(child_groups.fetch(member.child_tag_group_id)) }
+    end)
+    { 'id' => group.id,
+      'name' => group.name,
+      'description' => group.description,
+      'version_no' => group.version_no,
+      'members' => members,
+      'placements' => tree,
+      'created_at' => group.created_at,
+      'updated_at' => group.updated_at,
+      'created_by_user' => user_json(group.created_by_user),
+      'updated_by_user' => user_json(group.updated_by_user) }
+  end
+
+  def user_json user
+    UserRepr.base(user) if user
+  end
+
+  def placement_tree rows, tags, names, child_groups
+    by_parent = rows.group_by(&:parent_position)
+    build = lambda do |parent|
+      by_parent.fetch(parent, []).sort_by(&:position).map do |row|
+        node = if row.tag_id
+          { 'tag' => tag_json(tags.fetch(row.tag_id), names.fetch(row.tag_id)) }
+        elsif row.child_tag_group_id
+          { 'tag_group' => group_json_reference(child_groups.fetch(row.child_tag_group_id)) }
+        else
+          { 'section_name' => row.section_name, 'description' => row.description }
+        end
+        node.merge('children' => build.call(row.position))
+      end
+    end
+    build.call(nil)
+  end
+
+  def tag_json tag, name
+    { 'id' => tag.id, 'name' => name, 'category' => tag.category }
+  end
+
+  def group_json_reference group
+    { 'id' => group.id, 'name' => group.name }
+  end
+
+  def collect_ids nodes, key
+    nodes.flat_map do |node|
+      [node[key], *collect_ids(Array(node['children']), key)].compact
+    end
+  end
+
+  def historical_tag_versions ids
+    TagVersion.where(tag_id: ids.uniq).to_a.index_by do |version|
+      [version.tag_id, version.version_no]
+    end
+  end
+
+  def historical_group_versions ids
+    TagGroupVersion.where(tag_group_id: ids.uniq).to_a.index_by do |version|
+      [version.tag_group_id, version.version_no]
+    end
+  end
+
+  def historical_member member, tag_versions, group_versions
+    if member['tag_id']
+      id = member.fetch('tag_id')
+      version = tag_versions.fetch([id, member.fetch('version_no')])
+      { 'tag' => { 'id' => id, 'name' => japanese_version_name(version),
+                   'category' => version.category } }
+    else
+      id = member.fetch('tag_group_id')
+      version = group_versions.fetch([id, member.fetch('version_no')])
+      { 'tag_group' => { 'id' => id, 'name' => version.name } }
+    end
+  end
+
+  def historical_placement(node, tag_versions, group_versions,
+                           tag_version_numbers, group_version_numbers)
+    result = if node['tag_id']
+      id = node.fetch('tag_id')
+      version_no = tag_version_numbers.fetch(id)
+      version = tag_versions.fetch([id, version_no])
+      { 'tag' => { 'id' => id, 'name' => japanese_version_name(version),
+                   'category' => version.category } }
+    elsif node['tag_group_id']
+      id = node.fetch('tag_group_id')
+      version_no = group_version_numbers.fetch(id)
+      version = group_versions.fetch([id, version_no])
+      { 'tag_group' => { 'id' => id, 'name' => version.name } }
+    else
+      { 'section_name' => node.fetch('section_name'), 'description' => node['description'] }
+    end
+    result.merge('children' => Array(node['children']).map do |child|
+      historical_placement(child, tag_versions, group_versions,
+                           tag_version_numbers, group_version_numbers)
+    end)
+  end
+
+  def japanese_version_name version
+    version.names.fetch('ja').fetch('name')
+  end
+end

backend/app/representations/tag_repr.rb

@@ -3,21 +3,28 @@
 
 module TagRepr
   BASE = { only: [:id, :category, :post_count, :created_at, :updated_at, :deprecated_at],
-           methods: [:has_wiki, :material_id, :has_deerjikists] }.freeze
+           methods: [:material_id, :has_deerjikists] }.freeze
 
   module_function
 
   def base tag, language_code: 'ja'
-    tag.as_json(BASE).merge(name: name_for(tag, language_code),
+    tag.as_json(BASE).merge('has_wiki' => tag.has_description?(language_code:),
+                            name: name_for(tag, language_code),
                             aliases: tag.snapshot_aliases(language_code),
                             parents: tag.parents.map { inline(_1, language_code:) })
   end
 
   def inline tag, language_code: 'ja'
-    tag.as_json(BASE).merge(name: name_for(tag, language_code), aliases: [], parents: [])
+    tag.as_json(BASE).merge('has_wiki' => tag.has_description?(language_code:),
+                            name: name_for(tag, language_code), aliases: [], parents: [])
   end
 
-  def many(tags, language_code: 'ja') = tags.map { |tag| base(tag, language_code:) }
+  def many tags, language_code: 'ja'
+    records = tags.to_a
+    ActiveRecord::Associations::Preloader.new(
+        records:, associations: [:tag_descriptions, { parents: :tag_descriptions }]).call
+    records.map { |tag| base(tag, language_code:) }
+  end
 
   def name_for tag, language_code
     tag.name(language_code) || (tag.system? ? tag.name('ja') : nil)

backend/app/representations/wiki_page_repr.rb

@@ -13,6 +13,7 @@ module WikiPageRepr
   def tag_description description
     tag = description.tag
     { 'id' => description.id,
+      'resource_type' => 'tag_description',
       'tag_name_id' => tag.tag_name('ja')&.id,
       'body' => description.body,
       'created_user_id' => description.created_by_user_id,
@@ -25,6 +26,10 @@ module WikiPageRepr
       'deprecated_at' => tag.deprecated_at }
   end
 
+  def tag_group tag_group
+    TagGroupRepr.tag_group(tag_group).merge('resource_type' => 'tag_group')
+  end
+
   def many wiki_pages
     wiki_pages.map { |p| base(p) }
   end

backend/app/services/shared_resource_id_allocator.rb

@@ -0,0 +1,56 @@
+require 'digest'
+
+
+class SharedResourceIdAllocator
+  SPACES = {
+      'tags' => ['tags', 'external_tags'],
+      'wiki' => ['tag_descriptions', 'tag_groups'] }.freeze
+  LOCK_TIMEOUT = 10
+
+  def self.assign! record
+    space, tables = SPACES.find { |_name, names| names.include?(record.class.table_name) }
+    raise ArgumentError, 'Unknown shared ID resource' unless tables
+
+    connection = record.class.connection
+    transaction = record.class.current_transaction
+    raise ActiveRecord::ActiveRecordError, 'Shared ID requires a transaction' unless transaction.open?
+
+    database = connection.pool.db_config.database
+    name = "btrc:ids:#{ Digest::SHA256.hexdigest(database)[0, 16] }:#{ space }"
+    quoted_name = connection.quote(name)
+    acquired = record.class.uncached do
+      connection.select_value("SELECT GET_LOCK(#{ quoted_name }, #{ LOCK_TIMEOUT })")
+    end
+    unless acquired.to_i == 1
+      raise ActiveRecord::LockWaitTimeout, "Shared ID lock unavailable: #{ space }"
+    end
+
+    release = lambda do
+      result = record.class.uncached do
+        connection.select_value("SELECT RELEASE_LOCK(#{ quoted_name })")
+      end
+      unless result.to_i == 1
+        raise ActiveRecord::ActiveRecordError, "Shared ID lock release failed: #{ space }"
+      end
+    end
+    begin
+      transaction.after_commit(&release)
+      transaction.after_rollback(&release)
+    rescue StandardError
+      release.call
+      raise
+    end
+
+    return if record.id.present?
+
+    # Locking reads see committed maxima even after earlier repeatable reads.
+    maxima = record.class.uncached do
+      tables.map do |table|
+        quoted_table = connection.quote_table_name(table)
+        connection.select_value(
+            "SELECT id FROM #{ quoted_table } ORDER BY id DESC LIMIT 1 FOR UPDATE").to_i
+      end
+    end
+    record.id = maxima.max + 1
+  end
+end

backend/app/services/tag_description_operation.rb

@@ -8,9 +8,9 @@ class TagDescriptionOperation
           tag_id: tag.id, language_code: LANGUAGE_CODE)
 
       page = Wiki::CompatibilityWriter.create!(
-          tag_name: tag.tag_name(LANGUAGE_CODE), body:, created_by_user:, message:)
+          tag_name: compatibility_tag_name(tag), body:, created_by_user:, message:)
       revision = page.wiki_revisions.order(id: :desc).first
-      record_version!(tag:, revision:, body:, user: created_by_user,
+      record_version!(tag:, revision:, body: revision.body, user: created_by_user,
                       description: nil)
       page
     end
@@ -22,7 +22,20 @@ class TagDescriptionOperation
       tag.lock!
       description = TagDescription.lock.find_by!(
           tag_id: tag.id, language_code: LANGUAGE_CODE)
-      page = WikiPage.lock.find(description.id)
+      tag_name = compatibility_tag_name(tag)
+      page = WikiPage.lock.find_by(tag_name_id: tag_name.id)
+      unless page
+        versions = TagDescriptionVersion.where(tag_id: tag.id, language_code: LANGUAGE_CODE)
+        if base_revision_id.present? && versions.maximum(:id) != base_revision_id.to_i
+          raise Wiki::Commit::Conflict, '競合が発生してゐます．'
+        end
+        page = Wiki::CompatibilityWriter.create!(
+            tag_name:, body:, created_by_user: updated_by_user, message:)
+        revision = page.current_revision
+        record_version!(tag:, revision:, body: revision.body,
+                        user: updated_by_user, description:)
+        next revision
+      end
       revision = Wiki::CompatibilityWriter.update!(
           page:, body:, updated_by_user:, message:,
           base_revision_id:)
@@ -34,6 +47,11 @@ class TagDescriptionOperation
     end
   end
 
+  def self.compatibility_tag_name tag
+    tag.tag_name(LANGUAGE_CODE) || TagName.find_by!(id: tag.tag_name_id,
+                                                  language_code: LANGUAGE_CODE)
+  end
+
   def self.record_version! tag:, revision:, body:, user:, description:
     version_no = description ? description.version_no + 1 : 1
     TagDescriptionVersion.create!(
@@ -44,7 +62,7 @@ class TagDescriptionOperation
       description.update!(body:, version_no:, updated_by_user_id: user.id)
     else
       TagDescription.create!(
-          id: revision.wiki_page_id, tag_id: tag.id,
+          tag_id: tag.id,
           language_code: LANGUAGE_CODE, version_no:, body:,
           created_by_user_id: user.id, updated_by_user_id: user.id)
     end

backend/app/services/tag_group_markdown_parser.rb

@@ -0,0 +1,177 @@
+require 'kramdown'
+
+class TagGroupMarkdownParser
+  PREFIX = TagGroupRepr::PREFIX
+
+  class InvalidDocument < StandardError
+    ;
+  end
+
+  def self.parse(body:, group: nil)
+    new(body:, group:).parse
+  end
+
+  def initialize(body:, group:)
+    @body = body.to_s.gsub(/\r\n?/, "\n")
+    @group = group
+  end
+
+  def parse
+    parse_body = @body.gsub(/([^\n])\n([\x23]{1,6}[ \t]+\S)/, "\\1\n\n\\2")
+    root = Kramdown::Document.new(parse_body, input: 'GFM', auto_ids: false).root
+    placements = []
+    description_parts = []
+    headings = []
+
+    root.children.each do |node|
+      case node.type
+      when :blank
+        next
+      when :header
+        level = node.options.fetch(:level)
+        name = inline_text(node).strip
+        invalid!('Section 名は必須です．') if name.blank?
+
+        headings.pop while headings.any? && headings.last.fetch(:level) >= level
+        parent = headings.last&.fetch(:node)
+        section = { 'section_name' => name, 'description' => nil,
+                    'children' => [] }
+        (parent ? parent.fetch('children') : placements) << section
+        headings << { level:, node: section }
+      when :p
+        text = inline_markdown(node).strip
+        next if text.empty?
+
+        if headings.any?
+          section = headings.last.fetch(:node)
+          section['description'] = [section['description'], text].compact.join("\n\n")
+        else
+          description_parts << text
+        end
+      when :ul
+        invalid!('未対応のリスト項目があります．') if node.attr.any?
+        target = headings.any? ? headings.last.fetch(:node).fetch('children') : placements
+        target.concat(node.children.map { parse_list_item(_1) })
+      else
+        invalid!("未対応の Markdown 構文 #{ node.type.inspect } です．")
+      end
+    end
+
+    description = description_parts.presence&.join("\n\n")
+    { description:, placements:, members: build_members(placements),
+      compatibility_body: @body.sub(/\n+\z/, '') }
+  rescue Kramdown::Error => error
+    invalid!("Markdown を解析できません：#{ error.message }")
+  end
+
+  private
+
+  def parse_list_item(item)
+    invalid!('リスト項目ではありません．') unless item.type == :li
+    invalid!('未対応のリスト項目があります．') if item.attr.any?
+
+    nested, own = item.children.partition { |child| child.type == :ul }
+    invalid!('順序付きリストは使用できません．') if own.any? { _1.type == :ol }
+    own.reject! { _1.type == :blank }
+    invalid!('未対応のリスト項目があります．') unless own.all? do |child|
+      [:p, :text].include?(child.type)
+    end
+    invalid!('リスト項目は一つの段落で指定してください．') if own.size != 1
+
+    label = own.map { inline_text(_1) }.join(' ').strip
+    invalid!('空のリスト項目は使用できません．') if label.blank?
+
+    node = if label.start_with?(PREFIX)
+      group_name = label.delete_prefix(PREFIX).strip
+      child_group = TagGroup.find_by(name: group_name)
+      invalid!("Tag Group #{ group_name.inspect } を解決できません．") unless child_group
+      invalid!('自分自身を含めることはできません．') if child_group.id == @group&.id
+      { 'tag_group_id' => child_group.id }
+    elsif label.end_with?(':', '：')
+      section_name = label.sub(/[:：]\z/, '').strip
+      invalid!('Section 名は必須です．') if section_name.blank?
+      { 'section_name' => section_name, 'description' => nil }
+    else
+      { 'tag_id' => resolve_tag(label) }
+    end
+
+    node['children'] = nested.flat_map do |list|
+      invalid!('未対応の入れ子リストです．') unless list.type == :ul
+      list.children.map { parse_list_item(_1) }
+    end
+    node
+  end
+
+  def resolve_tag(name)
+    names = TagName.where(language_code: 'ja', name:).to_a
+    tag_ids = names.filter_map(&:tag_id).uniq
+    invalid!("Tag #{ name.inspect } を解決できません．") if tag_ids.empty?
+    invalid!("Tag 名 #{ name.inspect } が曖昧です．") if tag_ids.many?
+
+    tag_ids.first
+  end
+
+  def build_members(nodes)
+    references = []
+    visit = lambda do |items|
+      items.each do |node|
+        references << node.slice('tag_id', 'tag_group_id') if
+            node.key?('tag_id') || node.key?('tag_group_id')
+        visit.call(node.fetch('children'))
+      end
+    end
+    visit.call(nodes)
+    references.uniq
+  end
+
+  def inline_text(node)
+    if node.attr.any?
+      invalid!("未対応のインライン構文 #{ node.type.inspect } です．")
+    end
+
+    case node.type
+    when :text
+      node.value.to_s
+    when :p, :header
+      node.children.map { inline_text(_1) }.join
+    else
+      invalid!("未対応のインライン構文 #{ node.type.inspect } です．")
+    end
+  end
+
+  def inline_markdown(node)
+    case node.type
+    when :text
+      node.value.to_s
+    when :codespan
+      text = node.value.to_s
+      longest = text.scan(/`+/).map(&:length).max || 0
+      delimiter = '`' * (longest + 1)
+      padded = text.start_with?('`', ' ') || text.end_with?('`', ' ')
+      content = padded ? " #{ text } " : text
+      "#{ delimiter }#{ content }#{ delimiter }"
+    when :softbreak
+      "\n"
+    when :br
+      "  \n"
+    when :p
+      node.children.map { inline_markdown(_1) }.join
+    when :a
+      href = node.attr.fetch('href')
+      label = node.children.map { inline_markdown(_1) }.join
+      title = node.attr['title']&.gsub('"', '\\"')
+      suffix = title ? " \"#{ title }\"" : ''
+      "[#{ label }](#{ href }#{ suffix })"
+    when :em
+      "*#{ node.children.map { inline_markdown(_1) }.join }*"
+    when :strong
+      "**#{ node.children.map { inline_markdown(_1) }.join }**"
+    else
+      invalid!("未対応の説明文構文 #{ node.type.inspect } です．")
+    end
+  end
+
+  def invalid! message
+    raise InvalidDocument, message
+  end
+end

backend/app/services/tag_group_markdown_renderer.rb

@@ -0,0 +1,118 @@
+class TagGroupMarkdownRenderer
+  class UnsupportedStructure < StandardError
+    ;
+  end
+
+  def self.render group
+    new(group).render
+  end
+
+  def initialize group
+    @group = group
+  end
+
+  def render
+    placements = @group.fetch('placements')
+    validate_membership!(placements)
+    blocks = []
+    if @group['description'].present?
+      blocks << clean_text(@group['description'])
+    end
+
+    sections = placements.select { _1.key?('section_name') }
+    if sections.any? && sections.size != placements.size
+      raise UnsupportedStructure,
+          '最上位の見出し Section と Placement の混在は Markdown にできません．'
+    end
+
+    if placements.all? { _1.key?('section_name') }
+      blocks.concat(placements.map { render_heading(_1, 2) })
+    else
+      blocks << render_list(placements, 0)
+    end
+
+    blocks.compact.reject(&:empty?).join("\n").sub(/\n+\z/, '')
+  end
+
+  private
+
+  def validate_membership! placements
+    placed = placement_members(placements).uniq.sort
+    members = @group.fetch('members').map { |member|
+      if member['tag']
+        ['tag', member.fetch('tag').fetch('id')]
+      else
+        ['tag_group', member.fetch('tag_group').fetch('id')]
+      end
+    }.uniq.sort
+    missing = members - placed
+    if missing.any?
+      raise UnsupportedStructure, '掲載位置のない membership は Markdown にできません．'
+    end
+  end
+
+  def placement_members nodes
+    nodes.flat_map do |node|
+      own = if node['tag']
+        [['tag', node.fetch('tag').fetch('id')]]
+      elsif node['tag_group']
+        [['tag_group', node.fetch('tag_group').fetch('id')]]
+      else
+        []
+      end
+      own + placement_members(node.fetch('children'))
+    end
+  end
+
+  def render_heading section, level
+    header = "#{ '#' * level } #{ clean_text(section.fetch('section_name')) }"
+    blocks = [header]
+    blocks << clean_text(section['description']) if section['description'].present?
+    children = section.fetch('children')
+    section_indexes = children.each_index.select { |index| children[index].key?('section_name') }
+    if section_indexes.any?
+      first_section = section_indexes.first
+      if children[first_section..].any? { !_1.key?('section_name') }
+        raise UnsupportedStructure,
+            '見出し Section と兄弟 Placement の交互配置は Markdown にできません．'
+      end
+      list_nodes = children[0...first_section]
+      blocks << render_list(list_nodes, 0) if list_nodes.any?
+      blocks.concat(children[first_section..].map { render_heading(_1, level + 1) })
+    elsif children.any?
+      blocks << render_list(children, 0)
+    end
+    blocks.join("\n")
+  end
+
+  def render_list nodes, depth
+    nodes.map do |node|
+      indent = '  ' * depth
+      if node.key?('section_name')
+        raise UnsupportedStructure,
+            'リスト内 Section の説明文は Markdown にできません．' if node['description'].present?
+
+        children = node.fetch('children')
+        head = "#{ indent }- #{ clean_text(node.fetch('section_name')) }:"
+        tail = render_list(children, depth + 1) if children.any?
+        [head, tail].compact.join("\n")
+      else
+        label = if node['tag']
+          node.fetch('tag').fetch('name')
+        else
+          "#{ TagGroupRepr::PREFIX }#{ node.fetch('tag_group').fetch('name') }"
+        end
+        raise UnsupportedStructure, '名称のない Placement は Markdown にできません．' if label.blank?
+
+        head = "#{ indent }- #{ clean_text(label) }"
+        children = node.fetch('children')
+        tail = render_list(children, depth + 1) if children.any?
+        [head, tail].compact.join("\n")
+      end
+    end.join("\n")
+  end
+
+  def clean_text value
+    value.to_s.gsub(/\r\n?/, "\n").lines.map(&:rstrip).join("\n").strip
+  end
+end

backend/app/services/tag_group_operation.rb

@@ -0,0 +1,410 @@
+class TagGroupOperation
+  MAX_PLACEMENT_DEPTH = 5
+  PREFIX = TagGroupRepr::PREFIX
+
+  def self.create!(name:, description:, members:, placements:, actor:,
+                   require_compatibility: false, compatibility_body: nil)
+    group, sync_issue = ApplicationRecord.transaction do
+      group = TagGroup.new(version_no: 1, created_by_user: actor,
+                           updated_by_user: actor)
+      attributes = prepare!(group:, name:, description:, members:, placements:)
+      group.assign_attributes(name: attributes.fetch(:name),
+                              description: attributes.fetch(:description))
+      group.save!
+      persist_structure!(group:, attributes:, actor:)
+      persist_version!(group:, attributes:, actor:, event_type: 'create')
+      issue = sync_compatibility!(group:, actor:, old_name: nil,
+                                  require_compatibility:, compatibility_body:)
+      [group, issue]
+    end
+    report_sync_issue(group, actor, sync_issue)
+    group
+  end
+
+  def self.update!(group:, name:, description:, members:, placements:, actor:,
+                   require_compatibility: false, compatibility_body: nil)
+    group, sync_issue = ApplicationRecord.transaction do
+      group.lock!
+      old_name = group.name
+      attributes = prepare!(group:, name:, description:, members:, placements:)
+      next [group, nil] if unchanged?(group, attributes)
+
+      group.update!(name: attributes.fetch(:name),
+                    description: attributes.fetch(:description),
+                    version_no: group.version_no + 1,
+                    updated_by_user: actor)
+      persist_structure!(group:, attributes:, actor:)
+      persist_version!(group:, attributes:, actor:, event_type: 'update')
+      issue = sync_compatibility!(group:, actor:, old_name:,
+                                  require_compatibility:, compatibility_body:)
+      [group, issue]
+    end
+    report_sync_issue(group, actor, sync_issue)
+    group
+  end
+
+  def self.prepare!(group:, name:, description:, members:, placements:)
+    record = group
+    normalized_name = name.to_s.strip
+    invalid!(record, :name, 'グループ名は必須です．') if normalized_name.blank?
+    if normalized_name.length > 255
+      invalid!(record, :name, 'グループ名は 255 文字以内で入力してください．')
+    end
+    invalid!(record, :description, '説明文は文字列で入力してください．') unless
+        description.nil? || description.is_a?(String)
+
+    record.name = normalized_name
+    record.description = description
+    unless record.valid?
+      raise ActiveRecord::RecordInvalid, record
+    end
+
+    member_rows = normalize_members(record, members)
+    placement_tree = normalize_placements(record, placements)
+    validate_references!(record, member_rows, placement_tree)
+    validate_containment!(record, member_rows)
+    { name: normalized_name, description:, members: member_rows,
+      placements: placement_tree }
+  end
+  private_class_method :prepare!
+
+  def self.unchanged?(group, attributes)
+    unless (group.name_in_database == attributes.fetch(:name) &&
+            group.description_in_database == attributes.fetch(:description))
+      return false
+    end
+
+    members = TagGroupTag.where(tag_group_id: group.id).pluck(:tag_id)
+        .map { { 'tag_id' => _1 } }
+    members.concat(TagGroupGroup.where(tag_group_id: group.id)
+        .pluck(:child_tag_group_id).map { { 'tag_group_id' => _1 } })
+    requested_members = attributes.fetch(:members).map do |member|
+      member.except('version_no')
+    end
+    return false unless members.to_set == requested_members.to_set
+
+    rows = TagGroupPlacement.where(tag_group_id: group.id).order(:position)
+        .group_by(&:parent_position)
+    build = lambda do |parent_position|
+      rows.fetch(parent_position, []).map do |row|
+        node = if row.tag_id
+          { 'tag_id' => row.tag_id }
+        elsif row.child_tag_group_id
+          { 'tag_group_id' => row.child_tag_group_id }
+        else
+          { 'section_name' => row.section_name, 'description' => row.description }
+        end
+        node.merge('children' => build.call(row.position))
+      end
+    end
+    build.call(nil) == comparable_placements(attributes.fetch(:placements))
+  end
+  private_class_method :unchanged?
+
+  def self.comparable_placements(nodes)
+    nodes.map do |node|
+      result = node.dup
+      result['description'] = node['description'] if node.key?('section_name')
+      result['children'] = comparable_placements(node.fetch('children'))
+      result
+    end
+  end
+  private_class_method :comparable_placements
+
+  def self.normalize_members(record, members)
+    invalid!(record, :members, 'members は配列で指定してください．') unless members.is_a?(Array)
+
+    result = members.map do |member|
+      member = stringify_keys(member)
+      unless member.is_a?(Hash) && member.keys.one? &&
+             (member.key?('tag_id') || member.key?('tag_group_id'))
+        invalid!(record, :members, '各 member は tag_id または tag_group_id を一つ指定してください．')
+      end
+      key = member.keys.first
+      id = integer_id(record, member.fetch(key), :members)
+      { key => id }
+    end
+    invalid!(record, :members, 'membership に重複があります．') unless result.uniq == result
+    result
+  end
+  private_class_method :normalize_members
+
+  def self.normalize_placements(record, placements)
+    invalid!(record, :placements, 'placements は配列で指定してください．') unless
+        placements.is_a?(Array)
+
+    normalize_placement_nodes(record, placements, 1)
+  end
+  private_class_method :normalize_placements
+
+  def self.normalize_placement_nodes(record, nodes, depth)
+    if depth > MAX_PLACEMENT_DEPTH && nodes.any?
+      invalid!(record, :placements, '配置の深さは 5 階層までです．')
+    end
+
+    nodes.map do |raw_node|
+      node = stringify_keys(raw_node)
+      unless node.is_a?(Hash) && node.key?('children')
+        invalid!(record, :placements, '各 Placement に children 配列が必要です．')
+      end
+      children = node.delete('children')
+      unless children.is_a?(Array)
+        invalid!(record, :placements, 'children は配列で指定してください．')
+      end
+
+      kinds = ['tag_id', 'tag_group_id', 'section_name'].select { node.key?(_1) }
+      invalid!(record, :placements, 'Placement の種類を一つ指定してください．') unless
+          kinds.one?
+      kind = kinds.first
+      allowed = [kind, 'description']
+      invalid!(record, :placements, '不明な Placement 項目があります．') unless
+          (node.keys - allowed).empty?
+
+      case kind
+      when 'tag_id', 'tag_group_id'
+        node[kind] = integer_id(record, node.fetch(kind), :placements)
+        if node['description'].present?
+          invalid!(record, :placements, 'Tag / Tag Group Placement に説明文は指定できません．')
+        end
+        node.delete('description')
+      when 'section_name'
+        section_name = node.fetch(kind)
+        unless section_name.is_a?(String) && section_name.strip.present? &&
+               section_name.length <= 255
+          invalid!(record, :placements, 'Section 名は 1〜255 文字で入力してください．')
+        end
+        node[kind] = section_name.strip
+        unless node['description'].nil? || node['description'].is_a?(String)
+          invalid!(record, :placements, 'Section の説明文は文字列で入力してください．')
+        end
+      end
+
+      node['children'] = normalize_placement_nodes(record, children, depth + 1)
+      node
+    end
+  end
+  private_class_method :normalize_placement_nodes
+
+  def self.validate_references!(record, members, placements)
+    membership = members.to_set
+    placed = placement_references(placements).uniq
+    missing = placed - membership.to_a
+    invalid!(record, :placements, 'Placement の対象は membership に含めてください．') if
+        missing.any?
+
+    tag_ids = members.filter_map { _1['tag_id'] }
+    group_ids = members.filter_map { _1['tag_group_id'] }
+    tags = Tag.where(id: tag_ids).index_by(&:id)
+    groups = TagGroup.where(id: group_ids).index_by(&:id)
+    invalid!(record, :members, '存在しない Tag が含まれています．') if
+        tags.size != tag_ids.uniq.size
+    invalid!(record, :members, '存在しない Tag Group が含まれています．') if
+        groups.size != group_ids.uniq.size
+
+    tag_versions = TagVersion.where(tag_id: tag_ids).pluck(:tag_id, :version_no).to_set
+    group_versions = TagGroupVersion.where(tag_group_id: group_ids)
+        .pluck(:tag_group_id, :version_no).to_set
+    tag_names = TagName.where(tag_id: tag_ids, language_code: 'ja', primary_flg: true)
+        .pluck(:tag_id).uniq
+
+    members.each do |member|
+      if member.key?('tag_id')
+        id = member.fetch('tag_id')
+        invalid!(record, :members, "Tag ##{ id } に日本語の正規名がありません．") unless
+            tag_names.include?(id)
+        version_no = tags.fetch(id).version_no
+        unless tag_versions.include?([id, version_no])
+          invalid!(record, :members, "Tag ##{ id } の Version がありません．")
+        end
+        member['version_no'] = version_no
+      else
+        id = member.fetch('tag_group_id')
+        version_no = groups.fetch(id).version_no
+        unless group_versions.include?([id, version_no])
+          invalid!(record, :members, "Tag Group ##{ id } の Version がありません．")
+        end
+        member['version_no'] = version_no
+      end
+    end
+  end
+  private_class_method :validate_references!
+
+  def self.validate_containment!(record, members)
+    target_id = record.id
+    return unless target_id
+
+    children = members.filter_map { _1['tag_group_id'] }
+    edges = TagGroupGroup.pluck(:tag_group_id, :child_tag_group_id)
+        .group_by(&:first).transform_values { _1.map(&:last) }
+    edges[target_id] = children
+    visiting = Set.new
+    visited = Set.new
+    visit = lambda do |group_id|
+      invalid!(record, :members, 'Tag Group の循環包含はできません．') if
+          group_id == target_id && visiting.include?(group_id)
+      return if visited.include?(group_id)
+
+      visiting.add(group_id)
+      edges.fetch(group_id, []).each do |child_id|
+        invalid!(record, :members, 'Tag Group の循環包含はできません．') if
+            child_id == target_id
+        visit.call(child_id)
+      end
+      visiting.delete(group_id)
+      visited.add(group_id)
+    end
+    children.each { visit.call(_1) }
+  end
+  private_class_method :validate_containment!
+
+  def self.persist_structure!(group:, attributes:, actor:)
+    tag_members = TagGroupTag.where(tag_group_id: group.id).index_by(&:tag_id)
+    group_members = TagGroupGroup.where(tag_group_id: group.id).index_by(&:child_tag_group_id)
+    attributes.fetch(:members).each do |member|
+      if member.key?('tag_id')
+        next if tag_members.delete(member.fetch('tag_id'))
+
+        TagGroupTag.create!(tag_group_id: group.id, tag_id: member.fetch('tag_id'),
+                            created_by_user: actor)
+      else
+        next if group_members.delete(member.fetch('tag_group_id'))
+
+        TagGroupGroup.create!(tag_group_id: group.id,
+                              child_tag_group_id: member.fetch('tag_group_id'),
+                              created_by_user: actor)
+      end
+    end
+    existing = TagGroupPlacement.where(tag_group_id: group.id).index_by(&:position)
+    persist_placements!(group:, nodes: attributes.fetch(:placements), actor:, existing:)
+    existing.values.sort_by(&:position).reverse_each(&:delete)
+    tag_members.each_value(&:delete)
+    group_members.each_value(&:delete)
+  end
+  private_class_method :persist_structure!
+
+  def self.persist_placements!(group:, nodes:, actor:, existing:, parent_position: nil,
+                               next_position: [0])
+    nodes.each do |node|
+      next_position[0] += 1
+      position = next_position[0]
+      row = existing.delete(position) || TagGroupPlacement.new(
+          tag_group_id: group.id, position:, created_by_user: actor)
+      row.assign_attributes(
+          parent_position:,
+          section_name: node['section_name'], description: node['description'],
+          tag_id: node['tag_id'], child_tag_group_id: node['tag_group_id'])
+      if row.new_record? || row.changed?
+        row.updated_by_user = actor
+        row.save!
+      end
+      persist_placements!(group:, nodes: node.fetch('children'), actor:, existing:,
+                          parent_position: position, next_position:)
+    end
+  end
+  private_class_method :persist_placements!
+
+  def self.persist_version!(group:, attributes:, actor:, event_type:)
+    TagGroupVersion.create!(tag_group: group, version_no: group.version_no,
+                            event_type:, name: group.name,
+                            description: group.description,
+                            members: attributes.fetch(:members),
+                            placements: attributes.fetch(:placements),
+                            created_by_user: actor)
+  end
+  private_class_method :persist_version!
+
+  def self.sync_compatibility!(group:, actor:, old_name:,
+                               require_compatibility:, compatibility_body:)
+    representation = TagGroupRepr.one(group)
+    body = begin
+      TagGroupMarkdownRenderer.render(representation)
+    rescue TagGroupMarkdownRenderer::UnsupportedStructure => error
+      if compatibility_body.present?
+        compatibility_body
+      elsif require_compatibility
+        invalid!(group, :base, error.message)
+      else
+        return error.message
+      end
+    end
+    invalid!(group, :base, '本文は必須です．') if body.blank?
+
+    title = "#{ PREFIX }#{ group.name }"
+    old_title = old_name && "#{ PREFIX }#{ old_name }"
+    page = if old_title
+      WikiPage.joins(:tag_name).find_by(tag_names: { name: old_title })
+    else
+      nil
+    end
+    page ||= WikiPage.joins(:tag_name).find_by(tag_names: { name: title })
+
+    if page
+      tag_name = page.tag_name
+      tag_name.update!(name: title) if tag_name.name != title
+      Wiki::Commit.legacy_content!(page:, body:, created_user: actor, message: nil,
+                                   base_revision_id: nil)
+    else
+      tag_name = TagName.find_or_initialize_by(language_code: 'ja', name: title)
+      tag_name.script_code = 'Jpan'
+      tag_name.primary_flg = true
+      tag_name.auto_generated = false
+      tag_name.save!
+      Wiki::CompatibilityWriter.create!(tag_name:, body:, created_by_user: actor)
+    end
+    nil
+  end
+  private_class_method :sync_compatibility!
+
+  def self.report_sync_issue(group, actor, reason)
+    return unless reason
+
+    payload = { status: 'unsupported_structure', tag_group_id: group.id,
+                version_no: group.version_no, user_id: actor&.id, reason: }
+    Rails.logger.warn("tag_group_wiki_sync_skipped #{ payload.to_json }")
+  end
+  private_class_method :report_sync_issue
+
+  def self.placement_references(nodes)
+    nodes.flat_map do |node|
+      own = if node.key?('tag_id')
+        [{ 'tag_id' => node.fetch('tag_id') }]
+      elsif node.key?('tag_group_id')
+        [{ 'tag_group_id' => node.fetch('tag_group_id') }]
+      else
+        []
+      end
+      own + placement_references(node.fetch('children'))
+    end
+  end
+  private_class_method :placement_references
+
+  def self.stringify_keys(value)
+    case value
+    when ActionController::Parameters
+      value.to_unsafe_h.deep_stringify_keys
+    when Hash
+      value.deep_stringify_keys
+    else
+      value
+    end
+  end
+  private_class_method :stringify_keys
+
+  def self.integer_id(record, value, attribute)
+    integer = if value.is_a?(Integer)
+      value
+    elsif value.is_a?(String) && value.match?(/\A[0-9]+\z/)
+      value.to_i
+    end
+    invalid!(record, attribute, 'ID は正の整数で指定してください．') unless integer
+    invalid!(record, attribute, 'ID は正の整数で指定してください．') unless integer.positive?
+    integer
+  end
+  private_class_method :integer_id
+
+  def self.invalid!(record, attribute, message)
+    record.errors.add(attribute, message)
+    raise ActiveRecord::RecordInvalid, record
+  end
+  private_class_method :invalid!
+end

backend/app/services/wiki/commit.rb

@@ -8,12 +8,13 @@ module Wiki
     end
 
     def self.create_content! tag_name:, body:, created_by_user:, message: nil
-      if tag_name.tag && tag_name.language_code == 'ja'
+      if tag_name.tag && tag_name.language_code == 'ja' && !tag_name.name.include?(':')
         return TagDescriptionOperation.create!(
             tag: tag_name.tag, body:, created_by_user:, message:)
       end
 
-      create_legacy_content!(tag_name:, body:, created_by_user:, message:)
+      tag_name.errors.add(:base, :invalid)
+      raise ActiveRecord::RecordInvalid, tag_name
     end
 
     def self.create_legacy_content! tag_name:, body:, created_by_user:, message: nil
@@ -42,14 +43,17 @@ module Wiki
     end
 
     def self.content! page:, body:, created_user:, message: nil, base_revision_id: nil
-      description = TagDescription.find_by(id: page.id)
-      if description
+      description = TagDescription.find_by(tag_id: page.tag_name.tag_id, language_code: 'ja')
+      description ||= TagDescription.joins(:tag).find_by(
+          tags: { tag_name_id: page.tag_name_id }, language_code: 'ja')
+      if description && !page.title.include?(':')
         return TagDescriptionOperation.update!(
             tag: description.tag, body:, updated_by_user: created_user,
             message:, base_revision_id:)
       end
 
-      legacy_content!(page:, body:, created_user:, message:, base_revision_id:)
+      page.errors.add(:base, :invalid)
+      raise ActiveRecord::RecordInvalid, page
     end
 
     def self.legacy_content! page:, body:, created_user:, message: nil,

backend/app/services/wiki/compatibility_writer.rb

@@ -1,8 +1,16 @@
 module Wiki
   class CompatibilityWriter
     def self.create! tag_name:, body:, created_by_user:, message: nil
-      Commit.create_legacy_content!(
-          tag_name:, body:, created_by_user:, message:)
+      ApplicationRecord.transaction do
+        tag_name.lock!
+        page = WikiPage.lock.find_by(tag_name_id: tag_name.id)
+        if page
+          update!(page:, body:, updated_by_user: created_by_user, message:)
+          page
+        else
+          Commit.create_legacy_content!(tag_name:, body:, created_by_user:, message:)
+        end
+      end
     end
 
     def self.update! page:, body:, updated_by_user:, message: nil,
@@ -23,7 +31,9 @@ module Wiki
         end
       end
 
-      return if Commit.normalise_body(page.body) == normalised
+      if Commit.normalise_body(page.body) == normalised && page.wiki_revisions.exists?
+        return
+      end
 
       Commit.legacy_content!(
           page:, body: normalised, created_user: updated_by_user, message:,

backend/config/routes.rb

@@ -1,4 +1,6 @@
 Rails.application.routes.draw do
+  resources :tag_groups, only: [:index, :show, :create, :update]
+
   resources :nico_tags, path: 'tags/nico', only: [:index, :show, :update]
 
   scope 'tags/:parent_id/children', controller: :tag_children do

backend/db/migrate/20261008000000_create_tag_groups.rb

@@ -0,0 +1,149 @@
+class CreateTagGroups < ActiveRecord::Migration[8.0]
+  def change
+    # 翻訳系は上流の Translation リソースが未完のため保留
+
+    create_table :tag_groups do |t|
+      t.integer :version_no, null: false
+      t.string :name, limit: 255, null: false
+      t.text :description
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.references :updated_by_user, foreign_key: { to_table: :users }
+      t.timestamps
+
+      t.check_constraint 'version_no > 0', name: 'chk_tag_groups_version_no_positive'
+      t.index :name, unique: true
+    end
+
+    create_table :tag_group_tags, primary_key: [:tag_group_id, :tag_id] do |t|
+      t.references :tag_group, null: false, foreign_key: true, index: false
+      t.references :tag, null: false, foreign_key: true
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.datetime :created_at, null: false
+    end
+
+    create_table :tag_group_groups, primary_key: [:tag_group_id, :child_tag_group_id] do |t|
+      t.references :tag_group, null: false, foreign_key: true, index: false
+      t.references :child_tag_group,
+                   null: false, foreign_key: { to_table: :tag_groups }
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.datetime :created_at, null: false
+
+      t.check_constraint 'tag_group_id <> child_tag_group_id',
+                         name: 'chk_tag_group_groups_tag_group_id_not_child_tag_group_id'
+    end
+
+    create_table :tag_group_placements, primary_key: [:tag_group_id, :position] do |t|
+      t.references :tag_group, null: false, foreign_key: true, index: false
+      t.integer :position, null: false
+      t.integer :parent_position
+      t.string :section_name, limit: 255
+      t.text :description
+      t.references :tag
+      t.references :child_tag_group
+      t.references :created_by_user, foreign_key: { to_table: :users }
+      t.references :updated_by_user, foreign_key: { to_table: :users }
+      t.timestamps
+
+      t.foreign_key :tag_group_placements,
+                    column: [:tag_group_id, :parent_position],
+                    primary_key: [:tag_group_id, :position]
+
+      t.foreign_key :tag_group_tags,
+                    column: [:tag_group_id, :tag_id],
+                    primary_key: [:tag_group_id, :tag_id]
+
+      t.foreign_key :tag_group_groups,
+                    column: [:tag_group_id, :child_tag_group_id],
+                    primary_key: [:tag_group_id, :child_tag_group_id]
+
+      t.check_constraint 'position > parent_position',
+                         name: 'chk_tag_group_placements_position_gt_parent_position'
+
+      t.check_constraint ('((section_name IS NOT NULL AND tag_id IS NULL AND child_tag_group_id IS NULL) OR ' +
+                          '(section_name IS NULL AND tag_id IS NOT NULL AND child_tag_group_id IS NULL) OR ' +
+                          '(section_name IS NULL AND tag_id IS NULL AND child_tag_group_id IS NOT NULL)) AND ' +
+                          '(section_name IS NOT NULL OR description IS NULL)'),
+                         name: 'chk_tag_group_placements_references_exclusive'
+    end
+
+    create_table :tag_group_versions do |t|
+      t.references :tag_group, null: false
+      t.integer :version_no, null: false
+      t.string :event_type, null: false
+      t.string :name, limit: 255, null: false
+      t.text :description
+      t.json :members, null: false
+      t.json :placements, null: false
+      t.references :created_by_user
+      t.datetime :created_at, null: false
+
+      t.check_constraint 'version_no > 0',
+                         name: 'chk_tag_group_versions_version_no_positive'
+      t.index [:tag_group_id, :version_no], unique: true
+
+      add_members_constraint!(t)
+      add_placements_constraint!(t)
+    end
+  end
+
+  private
+
+  def add_members_constraint! t
+    schema = { type: 'array',
+               uniqueItems: true,
+               items: member_schema }
+
+    quoted_schema = connection.quote(JSON.generate(schema))
+
+    t.check_constraint "JSON_SCHEMA_VALID(#{ quoted_schema }, members)", 
+                       name: 'chk_tag_group_versions_members_schema'
+  end
+
+  def member_schema
+    { oneOf: [tag_member_schema, group_member_schema] }
+  end
+
+  def tag_member_schema
+    { type: 'object',
+      required: ['tag_id', 'version_no'],
+      properties: { tag_id: { type: 'integer', minimum: 1 },
+                    version_no: { type: 'integer', minimum: 1 } },
+      additionalProperties: false }
+  end
+
+  def group_member_schema
+    { type: 'object',
+      required: ['tag_group_id', 'version_no'],
+      properties: { tag_group_id: { type: 'integer', minimum: 1 },
+                    version_no: { type: 'integer', minimum: 1 } },
+      additionalProperties: false }
+  end
+
+  def add_placements_constraint! t
+    schema = { type: 'array', items: placement_schema(5) }
+
+    quoted_schema = connection.quote(JSON.generate(schema))
+
+    t.check_constraint "JSON_SCHEMA_VALID(#{ quoted_schema }, placements)",
+                       name: 'chk_tag_group_versions_placements_schema'
+  end
+
+  def placement_schema rest
+    { type: 'object',
+      required: ['children'],
+      properties: { section_name: { type: 'string' },
+                    description: { type: ['string', 'null'] },
+                    tag_id: { type: 'integer', minimum: 1 },
+                    tag_group_id: { type: 'integer', minimum: 1 },
+                    children: if rest > 1
+                                { type: 'array', items: placement_schema(rest - 1) }
+                              else
+                                { type: 'array', maxItems: 0 }
+                              end },
+      oneOf: [{ required: ['section_name'] },
+              { required: ['tag_id'] },
+              { required: ['tag_group_id'] }],
+      dependencies: { description: ['section_name'] },
+      additionalProperties: false }
+  end
+end

backend/db/migrate/20261008010000_migrate_tag_groups_from_wiki_pages.rb

@@ -0,0 +1,518 @@
+class MigrateTagGroupsFromWikiPages < ActiveRecord::Migration[8.0]
+  PREFIX = 'タググループ:'
+
+  def up
+    require 'json'
+    require 'time'
+    require 'kramdown'
+    require 'kramdown-parser-gfm'
+
+    assert_empty_destination!
+
+    pages = connection.select_all(<<~SQL).to_a
+      SELECT
+        wp.id
+      , wp.created_user_id
+      , wp.updated_user_id
+      , wp.created_at
+      , wp.updated_at
+      , wp.discarded_at
+      , tn.name
+      FROM
+        wiki_pages wp
+        INNER JOIN
+          tag_names tn
+          ON
+            tn.id = wp.tag_name_id
+      WHERE
+        tn.language_code = 'ja'
+        AND tn.name LIKE #{ quote("#{ PREFIX }%") }
+      ORDER BY
+        wp.id
+    SQL
+
+    return if pages.empty?
+
+    pages.each do |page|
+      if page['discarded_at']
+        say "Wiki ##{ page.fetch('id') }: 破棄済みですが，Revision の移行を継続します．"
+      end
+    end
+
+    tag_ids = load_tag_ids
+    page_ids = pages.map { |page| page.fetch('id').to_i }
+
+    revisions = connection.select_all(<<~SQL).to_a
+      SELECT
+        wr.id
+      , wr.wiki_page_id
+      , wr.kind
+      , wr.redirect_page_id
+      , wr.created_user_id
+      , wr.created_at
+      FROM
+        wiki_revisions wr
+      WHERE
+        wr.wiki_page_id IN (#{ page_ids.join(', ') })
+      ORDER BY
+        wr.wiki_page_id
+      , wr.id
+    SQL
+
+    revisions_by_page = revisions.group_by { |row| row.fetch('wiki_page_id').to_i }
+
+    pages = pages.select do |page|
+      page_id = page.fetch('id').to_i
+      if revisions_by_page.fetch(page_id, []).empty?
+        # tag_groups.version_no > 0 のため，Revision 0 件の Wiki は作成しない．
+        say "Wiki ##{ page_id }: Revision が 0 件のため移行対象から除外します．"
+        false
+      else
+        true
+      end
+    end
+
+    return if pages.empty?
+
+    group_ids = pages.to_h { |page| [page.fetch('name'), page.fetch('id').to_i] }
+    revision_history = pages.to_h do |page|
+      page_id = page.fetch('id').to_i
+      history = revisions_by_page.fetch(page_id).each_with_index.map do |revision, index|
+        { 'id' => revision.fetch('id').to_i,
+          'version_no' => index + 1,
+          'created_at' => revision.fetch('created_at') }
+      end
+      [page_id, history]
+    end
+
+    tag_history = connection.select_all(<<~SQL).to_a.group_by { |row| row.fetch('tag_id').to_i }
+      SELECT
+        tag_id
+      , version_no
+      , created_at
+      FROM
+        tag_versions
+      ORDER BY
+        tag_id
+      , created_at
+      , version_no
+    SQL
+
+    prepared = { }
+    pages.each do |page|
+      page_id = page.fetch('id').to_i
+      history = revisions_by_page.fetch(page_id)
+
+      prepared[page_id] = history.each_with_index.map do |revision, index|
+        context = "Wiki ##{ page_id } / Revision ##{ revision.fetch('id') }"
+        body = revision_body(revision.fetch('id').to_i)
+
+        if revision.fetch('kind').to_i == 1
+          destination = revision['redirect_page_id']
+          description = "旧 Wiki のリダイレクト先: Wiki ##{ destination }"
+          placements = []
+          members = []
+          say "#{ context }: リダイレクトを説明文として移行します．"
+        else
+          begin
+            description, placements = parse_wiki(body, tag_ids, group_ids, context)
+            validate_tree!(placements, context)
+            members = build_members(placements, revision.fetch('created_at'),
+                                    tag_history, revision_history, context)
+          rescue StandardError => error
+            # 解析できない版も捨てず，当時の Markdown を説明文として保存する．
+            say "#{ context }: 構造化を保留し，本文を保存します (#{ error.message })．"
+            description = body
+            placements = []
+            members = []
+          end
+        end
+
+        { revision:, version_no: index + 1, description:, members:, placements:, body: }
+      end
+    end
+
+    # 自己包含は禁止されてゐるため，当該版は原文の保存を優先する．
+    prepared.each do |group_id, versions|
+      current = versions.last
+      next unless current.fetch(:members).any? { |member| member['tag_group_id'] == group_id }
+
+      say "Tag Group ##{ group_id }: 直接の自己参照を検出したため，現行構造化を保留します．"
+      current[:description] = current.fetch(:body)
+      current[:members] = []
+      current[:placements] = []
+    end
+
+    begin
+      validate_current_groups!(prepared)
+    rescue StandardError => error
+      say "Tag Group の包含関係に問題がありますが，移行を継続します (#{ error.message })．"
+    end
+
+    # 移行先の書込みはすべて検証の後．DDL は含めない．
+    connection.transaction do
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        current = prepared.fetch(page_id).last
+
+        insert_row('tag_groups', {
+            id: page_id,
+            # page.version_no ではなく，この Wiki に紐付く Revision の総数．
+            version_no: revisions_by_page.fetch(page_id).size,
+            name: page.fetch('name').delete_prefix(PREFIX),
+            description: current.fetch(:description),
+            created_by_user_id: page.fetch('created_user_id'),
+            updated_by_user_id: page.fetch('updated_user_id'),
+            created_at: page.fetch('created_at'),
+            updated_at: page.fetch('updated_at') })
+      end
+
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        name = page.fetch('name').delete_prefix(PREFIX)
+
+        prepared.fetch(page_id).each do |item|
+          revision = item.fetch(:revision)
+
+          insert_row('tag_group_versions', {
+              id: revision.fetch('id'),
+              tag_group_id: page_id,
+              version_no: item.fetch(:version_no),
+              event_type: item.fetch(:version_no) == 1 ? 'create' : 'update',
+              name:,
+              description: item.fetch(:description),
+              members: JSON.generate(item.fetch(:members)),
+              placements: JSON.generate(item.fetch(:placements)),
+              created_by_user_id: revision.fetch('created_user_id'),
+              created_at: revision.fetch('created_at') })
+        end
+      end
+
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        current = prepared.fetch(page_id).last
+        revision = current.fetch(:revision)
+
+        current.fetch(:members).each do |member|
+          if member.key?('tag_id')
+            insert_row('tag_group_tags', {
+                tag_group_id: page_id,
+                tag_id: member.fetch('tag_id'),
+                created_by_user_id: revision.fetch('created_user_id'),
+                created_at: revision.fetch('created_at') })
+          else
+            insert_row('tag_group_groups', {
+                tag_group_id: page_id,
+                child_tag_group_id: member.fetch('tag_group_id'),
+                created_by_user_id: revision.fetch('created_user_id'),
+                created_at: revision.fetch('created_at') })
+          end
+        end
+      end
+
+      pages.each do |page|
+        page_id = page.fetch('id').to_i
+        current = prepared.fetch(page_id).last
+        revision = current.fetch(:revision)
+        position = 0
+
+        append = -> nodes, parent_position do
+          nodes.each do |node|
+            position += 1
+            own_position = position
+
+            insert_row('tag_group_placements', {
+                tag_group_id: page_id,
+                position: own_position,
+                parent_position:,
+                section_name: node['section_name'],
+                description: node['description'],
+                tag_id: node['tag_id'],
+                child_tag_group_id: node['tag_group_id'],
+                created_by_user_id: revision.fetch('created_user_id'),
+                updated_by_user_id: revision.fetch('created_user_id'),
+                created_at: revision.fetch('created_at'),
+                updated_at: revision.fetch('created_at') })
+
+            append.(node.fetch('children'), own_position)
+          end
+        end
+
+        append.(current.fetch(:placements), nil)
+      end
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+
+  private
+
+  def assert_empty_destination!
+    ['tag_groups', 'tag_group_versions', 'tag_group_tags',
+     'tag_group_groups', 'tag_group_placements'].each do |table|
+      count = connection.select_value("SELECT COUNT(*) FROM #{ table }").to_i
+      raise "#{ table } が空ではありません．" if count.positive?
+    end
+  end
+
+  # 解決は tag_names.name -> tag_names.tag_id のみ．
+  # tags.tag_name_id は読まない．
+  def load_tag_ids
+    rows = connection.select_all(<<~SQL).to_a
+      SELECT
+        name
+      , tag_id
+      FROM
+        tag_names
+      WHERE
+        language_code = 'ja'
+        AND tag_id IS NOT NULL
+    SQL
+
+    rows.each_with_object({ }) do |row, by_name|
+      name = row.fetch('name')
+      id = row.fetch('tag_id').to_i
+      by_name[name] ||= []
+      by_name[name] << id unless by_name[name].include?(id)
+    end
+  end
+
+  def revision_body revision_id
+    connection.select_rows(<<~SQL).flatten.join("\n")
+      SELECT
+        wl.body
+      FROM
+        wiki_revision_lines wrl
+        INNER JOIN wiki_lines wl ON wl.id = wrl.wiki_line_id
+      WHERE
+        wrl.wiki_revision_id = #{ Integer(revision_id) }
+      ORDER BY
+        wrl.position
+    SQL
+  end
+
+  def parse_wiki markdown, tag_ids, group_ids, context
+    root = Kramdown::Document.new(markdown, input: 'GFM').root
+    placements = []
+    description_parts = []
+    headings = []
+
+    root.children.each_with_index do |node, index|
+      case node.type
+      when :blank
+        next
+      when :header
+        level = node.options.fetch(:level)
+        name = inline_text(node).strip
+        raise "#{ context }: 名前のない見出しがあります．" if name.empty?
+
+        headings.pop while headings.any? && headings.last.first >= level
+        parent = headings.last&.last
+        section = { 'section_name' => name, 'description' => nil, 'children' => [] }
+        (parent ? parent.fetch('children') : placements) << section
+        headings << [level, section]
+      when :p
+        text = inline_markdown(node).strip
+        next if text.empty?
+
+        if headings.any?
+          heading = headings.last.last
+          heading['description'] = [heading['description'], text].compact.join("\n\n")
+        else
+          description_parts << text
+        end
+      when :ul, :ol
+        if headings.any? && see_also_heading?(headings.last.last.fetch('section_name'))
+          heading = headings.last.last
+          raw = original_block(markdown, root.children, index)
+          heading['description'] = [heading['description'], raw].compact.join("\n\n")
+        else
+          target = headings.any? ? headings.last.last.fetch('children') : placements
+          target.concat(node.children.map { |item| parse_list_item(item, tag_ids, group_ids, context) })
+        end
+      else
+        raise "#{ context }: 未対応の Markdown 構文 #{ node.type.inspect }"
+      end
+    end
+
+    [description_parts.empty? ? nil : description_parts.join("\n\n"), placements]
+  end
+
+  def see_also_heading? name
+    ['see also', '関連項目'].include?(name.strip.downcase.sub(/[:：]\z/, ''))
+  end
+
+  def original_block markdown, siblings, index
+    current = siblings.fetch(index)
+    start_line = current.options[:location]
+    raise 'Markdown block の位置が不明です．' unless start_line
+
+    following = siblings[(index + 1)..].find do |node|
+      node.options[:location] && node.options[:location] > start_line
+    end
+    end_line = following ? following.options.fetch(:location) - 1 : markdown.lines.length
+    markdown.lines[(start_line - 1)...end_line].join.strip
+  end
+
+  def parse_list_item item, tag_ids, group_ids, context
+    raise "#{ context }: リスト項目ではありません．" if item.type != :li
+
+    nested, own = item.children.partition { |child| [:ul, :ol].include?(child.type) }
+    own.reject! { |child| child.type == :blank }
+    unless own.all? { |child| [:p, :text, :a, :codespan].include?(child.type) }
+      raise "#{ context }: 未対応のリスト項目があります．"
+    end
+
+    label = own.map { |child| inline_text(child) }.join(' ').strip
+    raise "#{ context }: 空のリスト項目があります．" if label.empty?
+
+    node =
+        if group_ids.key?(label)
+          { 'tag_group_id' => group_ids.fetch(label) }
+        elsif tag_ids.key?(label)
+          ids = tag_ids.fetch(label)
+          unless ids.one?
+            raise "#{ context }: タグ名 #{ label.inspect } が複数の ID を指します：#{ ids.inspect }"
+          end
+          { 'tag_id' => ids.first }
+        elsif label.end_with?(':', '：')
+          name = label.sub(/[:：]\z/, '')
+          raise "#{ context }: 空の区分名があります．" if name.empty?
+          if see_also_heading?(name) && nested.any?
+            raise "#{ context }: リスト内の See also は説明文への保管処理が未実装です．"
+          end
+          { 'section_name' => name, 'description' => nil }
+        else
+          raise "#{ context }: 解決不能なリスト項目 #{ label.inspect }"
+        end
+
+    node['children'] = nested.flat_map do |list|
+      list.children.map { |child| parse_list_item(child, tag_ids, group_ids, context) }
+    end
+
+    node
+  end
+
+  def inline_text node
+    case node.type
+    when :text, :codespan
+      node.value.to_s
+    when :softbreak, :br
+      ' '
+    when :a, :p, :header, :em, :strong, :span, :li
+      node.children.map { |child| inline_text(child) }.join
+    else
+      raise "未対応のインライン構文 #{ node.type.inspect }"
+    end
+  end
+
+  def inline_markdown node
+    case node.type
+    when :text
+      node.value.to_s
+    when :codespan
+      "`#{ node.value }`"
+    when :softbreak
+      "\n"
+    when :br
+      "  \n"
+    when :p
+      node.children.map { |child| inline_markdown(child) }.join
+    when :a
+      href = node.attr.fetch('href')
+      "[#{ node.children.map { |child| inline_markdown(child) }.join }](#{ href })"
+    when :em
+      "*#{ node.children.map { |child| inline_markdown(child) }.join }*"
+    when :strong
+      "**#{ node.children.map { |child| inline_markdown(child) }.join }**"
+    else
+      raise "未対応のインライン構文 #{ node.type.inspect }"
+    end
+  end
+
+  def validate_tree! nodes, context, depth = 1
+    raise "#{ context }: 配置の深さが 5 を超えてゐます．" if depth > 5 && nodes.any?
+
+    nodes.each do |node|
+      name = node['section_name']
+      if name && name.length > 255
+        raise "#{ context }: 区分名が 255 文字を超えてゐます．"
+      end
+      validate_tree!(node.fetch('children'), context, depth + 1)
+    end
+  end
+
+  def build_members nodes, date, tag_history, group_history, context
+    ids_of_tags = []
+    ids_of_groups = []
+
+    visit = -> items do
+      items.each do |node|
+        ids_of_tags << node['tag_id'] if node.key?('tag_id')
+        ids_of_groups << node['tag_group_id'] if node.key?('tag_group_id')
+        visit.(node.fetch('children'))
+      end
+    end
+    visit.(nodes)
+
+    tags = ids_of_tags.uniq.map do |id|
+      { 'tag_id' => id,
+        'version_no' => version_at(tag_history[id], date, context, "Tag ##{ id }") }
+    end
+
+    groups = ids_of_groups.uniq.map do |id|
+      { 'tag_group_id' => id,
+        'version_no' => version_at(group_history[id], date, context, "TagGroup ##{ id }") }
+    end
+
+    tags + groups
+  end
+
+  def version_at history, date, context, target
+    raise "#{ context }: #{ target } の履歴が存在しません．" unless history&.any?
+
+    timestamp = Time.parse(date.to_s)
+    candidates = history.select do |row|
+      Time.parse(row.fetch('created_at').to_s) <= timestamp
+    end
+    if candidates.empty?
+      raise "#{ context }: #{ target } は #{ date } 時点で存在しません．"
+    end
+
+    candidates.max_by { |row|
+      [Time.parse(row.fetch('created_at').to_s), row.fetch('version_no').to_i]
+    }.fetch('version_no').to_i
+  end
+
+  def validate_current_groups! prepared
+    edges = { }
+    prepared.each do |group_id, versions|
+      edges[group_id] = versions.last.fetch(:members)
+                              .filter_map { |member| member['tag_group_id'] }
+    end
+
+    visiting = { }
+    visited = { }
+    visit = -> group_id do
+      raise "Tag Group の循環参照を検出しました：#{ group_id }" if visiting[group_id]
+      return if visited[group_id]
+
+      visiting[group_id] = true
+      edges.fetch(group_id, []).each { |child_id| visit.(child_id) }
+      visiting.delete(group_id)
+      visited[group_id] = true
+    end
+
+    edges.keys.each { |group_id| visit.(group_id) }
+  end
+
+  def quote(value) = connection.quote(value)
+
+  def insert_row table, values
+    columns = values.keys.join(', ')
+    payload = values.values.map { |value| quote(value) }.join(', ')
+    execute "INSERT INTO #{ table } (#{ columns }) VALUES (#{ payload })"
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_10_05_000000) do
+ActiveRecord::Schema[8.0].define(version: 2026_10_08_010000) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -483,6 +483,80 @@ ActiveRecord::Schema[8.0].define(version: 2026_10_05_000000) do
     t.check_constraint "`version_no` > 0", name: "chk_tag_descriptions_version_no_positive"
   end
 
+  create_table "tag_group_groups", primary_key: ["tag_group_id", "child_tag_group_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_group_id", null: false
+    t.bigint "child_tag_group_id", null: false
+    t.bigint "created_by_user_id"
+    t.datetime "created_at", null: false
+    t.index ["child_tag_group_id"], name: "index_tag_group_groups_on_child_tag_group_id"
+    t.index ["created_by_user_id"], name: "index_tag_group_groups_on_created_by_user_id"
+    t.check_constraint "`tag_group_id` <> `child_tag_group_id`", name: "chk_tag_group_groups_tag_group_id_not_child_tag_group_id"
+  end
+
+  create_table "tag_group_placements", primary_key: ["tag_group_id", "position"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_group_id", null: false
+    t.integer "position", null: false
+    t.integer "parent_position"
+    t.string "section_name"
+    t.text "description"
+    t.bigint "tag_id"
+    t.bigint "child_tag_group_id"
+    t.bigint "created_by_user_id"
+    t.bigint "updated_by_user_id"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["child_tag_group_id"], name: "index_tag_group_placements_on_child_tag_group_id"
+    t.index ["created_by_user_id"], name: "index_tag_group_placements_on_created_by_user_id"
+    t.index ["tag_group_id", "child_tag_group_id"], name: "fk_rails_7342b54ced"
+    t.index ["tag_group_id", "parent_position"], name: "fk_rails_3af6ee71ca"
+    t.index ["tag_group_id", "tag_id"], name: "fk_rails_365dc60759"
+    t.index ["tag_id"], name: "index_tag_group_placements_on_tag_id"
+    t.index ["updated_by_user_id"], name: "index_tag_group_placements_on_updated_by_user_id"
+    t.check_constraint "(((`section_name` is not null) and (`tag_id` is null) and (`child_tag_group_id` is null)) or ((`section_name` is null) and (`tag_id` is not null) and (`child_tag_group_id` is null)) or ((`section_name` is null) and (`tag_id` is null) and (`child_tag_group_id` is not null))) and ((`section_name` is not null) or (`description` is null))", name: "chk_tag_group_placements_references_exclusive"
+    t.check_constraint "`position` > `parent_position`", name: "chk_tag_group_placements_position_gt_parent_position"
+  end
+
+  create_table "tag_group_tags", primary_key: ["tag_group_id", "tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_group_id", null: false
+    t.bigint "tag_id", null: false
+    t.bigint "created_by_user_id"
+    t.datetime "created_at", null: false
+    t.index ["created_by_user_id"], name: "index_tag_group_tags_on_created_by_user_id"
+    t.index ["tag_id"], name: "index_tag_group_tags_on_tag_id"
+  end
+
+  create_table "tag_group_versions", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_group_id", null: false
+    t.integer "version_no", null: false
+    t.string "event_type", null: false
+    t.string "name", null: false
+    t.text "description"
+    t.json "members", null: false
+    t.json "placements", null: false
+    t.bigint "created_by_user_id"
+    t.datetime "created_at", null: false
+    t.index ["created_by_user_id"], name: "index_tag_group_versions_on_created_by_user_id"
+    t.index ["tag_group_id", "version_no"], name: "index_tag_group_versions_on_tag_group_id_and_version_no", unique: true
+    t.index ["tag_group_id"], name: "index_tag_group_versions_on_tag_group_id"
+    t.check_constraint "`version_no` > 0", name: "chk_tag_group_versions_version_no_positive"
+    t.check_constraint "json_schema_valid(_utf8mb4'{\"type\":\"array\",\"items\":{\"type\":\"object\",\"required\":[\"children\"],\"properties\":{\"section_name\":{\"type\":\"string\"},\"description\":{\"type\":[\"string\",\"null\"]},\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"tag_group_id\":{\"type\":\"integer\",\"minimum\":1},\"children\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"required\":[\"children\"],\"properties\":{\"section_name\":{\"type\":\"string\"},\"description\":{\"type\":[\"string\",\"null\"]},\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"tag_group_id\":{\"type\":\"integer\",\"minimum\":1},\"children\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"required\":[\"children\"],\"properties\":{\"section_name\":{\"type\":\"string\"},\"description\":{\"type\":[\"string\",\"null\"]},\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"tag_group_id\":{\"type\":\"integer\",\"minimum\":1},\"children\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"required\":[\"children\"],\"properties\":{\"section_name\":{\"type\":\"string\"},\"description\":{\"type\":[\"string\",\"null\"]},\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"tag_group_id\":{\"type\":\"integer\",\"minimum\":1},\"children\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"required\":[\"children\"],\"properties\":{\"section_name\":{\"type\":\"string\"},\"description\":{\"type\":[\"string\",\"null\"]},\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"tag_group_id\":{\"type\":\"integer\",\"minimum\":1},\"children\":{\"type\":\"array\",\"maxItems\":0}},\"oneOf\":[{\"required\":[\"section_name\"]},{\"required\":[\"tag_id\"]},{\"required\":[\"tag_group_id\"]}],\"dependencies\":{\"description\":[\"section_name\"]},\"additionalProperties\":false}}},\"oneOf\":[{\"required\":[\"section_name\"]},{\"required\":[\"tag_id\"]},{\"required\":[\"tag_group_id\"]}],\"dependencies\":{\"description\":[\"section_name\"]},\"additionalProperties\":false}}},\"oneOf\":[{\"required\":[\"section_name\"]},{\"required\":[\"tag_id\"]},{\"required\":[\"tag_group_id\"]}],\"dependencies\":{\"description\":[\"section_name\"]},\"additionalProperties\":false}}},\"oneOf\":[{\"required\":[\"section_name\"]},{\"required\":[\"tag_id\"]},{\"required\":[\"tag_group_id\"]}],\"dependencies\":{\"description\":[\"section_name\"]},\"additionalProperties\":false}}},\"oneOf\":[{\"required\":[\"section_name\"]},{\"required\":[\"tag_id\"]},{\"required\":[\"tag_group_id\"]}],\"dependencies\":{\"description\":[\"section_name\"]},\"additionalProperties\":false}}',`placements`)", name: "chk_tag_group_versions_placements_schema"
+    t.check_constraint "json_schema_valid(_utf8mb4'{\"type\":\"array\",\"uniqueItems\":true,\"items\":{\"oneOf\":[{\"type\":\"object\",\"required\":[\"tag_id\",\"version_no\"],\"properties\":{\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"version_no\":{\"type\":\"integer\",\"minimum\":1}},\"additionalProperties\":false},{\"type\":\"object\",\"required\":[\"tag_group_id\",\"version_no\"],\"properties\":{\"tag_group_id\":{\"type\":\"integer\",\"minimum\":1},\"version_no\":{\"type\":\"integer\",\"minimum\":1}},\"additionalProperties\":false}]}}',`members`)", name: "chk_tag_group_versions_members_schema"
+  end
+
+  create_table "tag_groups", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.integer "version_no", null: false
+    t.string "name", null: false
+    t.text "description"
+    t.bigint "created_by_user_id"
+    t.bigint "updated_by_user_id"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["created_by_user_id"], name: "index_tag_groups_on_created_by_user_id"
+    t.index ["name"], name: "index_tag_groups_on_name", unique: true
+    t.index ["updated_by_user_id"], name: "index_tag_groups_on_updated_by_user_id"
+    t.check_constraint "`version_no` > 0", name: "chk_tag_groups_version_no_positive"
+  end
+
   create_table "tag_implications", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "tag_id", null: false
     t.bigint "parent_tag_id", null: false
@@ -823,6 +897,20 @@ ActiveRecord::Schema[8.0].define(version: 2026_10_05_000000) do
   add_foreign_key "tag_descriptions", "tags"
   add_foreign_key "tag_descriptions", "users", column: "created_by_user_id"
   add_foreign_key "tag_descriptions", "users", column: "updated_by_user_id"
+  add_foreign_key "tag_group_groups", "tag_groups"
+  add_foreign_key "tag_group_groups", "tag_groups", column: "child_tag_group_id"
+  add_foreign_key "tag_group_groups", "users", column: "created_by_user_id"
+  add_foreign_key "tag_group_placements", "tag_group_groups", column: ["tag_group_id", "child_tag_group_id"], primary_key: ["tag_group_id", "child_tag_group_id"]
+  add_foreign_key "tag_group_placements", "tag_group_placements", column: ["tag_group_id", "parent_position"], primary_key: ["tag_group_id", "position"]
+  add_foreign_key "tag_group_placements", "tag_group_tags", column: ["tag_group_id", "tag_id"], primary_key: ["tag_group_id", "tag_id"]
+  add_foreign_key "tag_group_placements", "tag_groups"
+  add_foreign_key "tag_group_placements", "users", column: "created_by_user_id"
+  add_foreign_key "tag_group_placements", "users", column: "updated_by_user_id"
+  add_foreign_key "tag_group_tags", "tag_groups"
+  add_foreign_key "tag_group_tags", "tags"
+  add_foreign_key "tag_group_tags", "users", column: "created_by_user_id"
+  add_foreign_key "tag_groups", "users", column: "created_by_user_id"
+  add_foreign_key "tag_groups", "users", column: "updated_by_user_id"
   add_foreign_key "tag_implications", "tags"
   add_foreign_key "tag_implications", "tags", column: "parent_tag_id"
   add_foreign_key "tag_names", "languages", column: "language_code", primary_key: "code"

backend/lib/tasks/migrate_help_pages.rake

@@ -0,0 +1,51 @@
+namespace :help do
+  desc 'ヘルプ移行'
+  task migrate: :environment do
+    help_path = ENV.fetch('BTRC_HUB_HELP_PATH')
+    abort 'ヘルプ置場指定してね．' if help_path.blank?
+
+    # ファイルがすでに存在する場合は通さなぃ．
+    abort 'ファイル消してね．' if Dir.glob("#{ help_path }/*").present?
+
+    # 現状物理削除はなぃはず
+    pages = 
+        WikiPage
+        .joins(:tag_name)
+        .where(tag_names: { tag_id: nil, language_code: 'ja' })
+        .where.not("tag_names.name LIKE 'タググループ:%'")
+
+    puts "移行対象: #{pages.count} 件"
+    abort '移行対象がありません．' if pages.empty?
+
+    pages.find_each do |page|
+      puts page.body
+      puts page.title
+
+      puts 'この identifier は何にする?'
+      identifier = $stdin.gets.chomp
+      Dir.mkdir "#{ help_path }/#{ identifier }"
+      Dir.mkdir "#{ help_path }/#{ identifier }/editions"
+      Dir.mkdir "#{ help_path }/#{ identifier }/editions/ja"
+
+      last_file_name = ''
+      WikiVersion.where(wiki_page_id: page.id).order(:version_no).each do |ver|
+        last_file_name, last_file_name_no = last_file_name.split('.')
+
+        file_name = ver.created_at.strftime('%Y-%m-%d')
+        if file_name == last_file_name
+          file_name += ".#{ (last_file_name_no || 1).to_i + 1 }"
+        end
+
+        body = "# #{ ver.title.sub(/^ヘルプ:/, '') }\n\n#{ ver.body }"
+        File.write "#{ help_path }/#{ identifier }/editions/ja/#{ file_name }.mdx", body
+
+        last_file_name = file_name
+      end
+
+      article_meta = { 'identifier' => identifier,
+                       'canonical_language' => 'ja',
+                       'languages' => { 'ja' => { 'current_edition' => last_file_name } } }
+      File.write "#{ help_path }/#{ identifier }/article.yml", article_meta.to_yaml
+    end
+  end
+end

backend/spec/db/tag_description_migration_spec.rb

@@ -5,11 +5,11 @@ RSpec.describe 'Tag description migration identity' do
   it 'preserves every revision ID and repeated body in an independent sequence' do
     user = create_member_user!
     tag_name = create(:tag_name, name: 'description_migration')
-    page = Wiki::Commit.create_content!(
+    page = Wiki::Commit.create_legacy_content!(
         tag_name:, body: 'A', created_by_user: user)
     revisions = [page.current_revision]
     ['A', 'B', 'B', 'A'].each do |body|
-      revisions << Wiki::Commit.content!(
+      revisions << Wiki::Commit.legacy_content!(
           page:, body:, created_user: user, base_revision_id: revisions.last.id)
     end
     tag = create(:tag, primary_tag_name: tag_name)

backend/spec/db/tag_group_constraints_spec.rb

@@ -0,0 +1,110 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag group persistence constraints' do
+  include TagGroupRecords
+
+  let!(:user) { create_member_user! }
+  let!(:tag) { group_tag('制約確認') }
+  let!(:group_id) do
+    seed_group(
+        name: '制約確認',
+        members: [{ 'tag_id' => tag.id, 'version_no' => tag.version_no }])
+  end
+
+  def placement attributes
+    insert_group_row('tag_group_placements', {
+        tag_group_id: group_id, position: 1,
+        created_at: Time.current, updated_at: Time.current }.merge(attributes))
+  end
+
+  def reject_statement
+    before_state = group_state
+    expect { yield }.to raise_error(ActiveRecord::StatementInvalid)
+    expect(group_state).to eq(before_state)
+  end
+
+  it 'allows membership without placement' do
+    expect(group_rows('tag_group_tags', tag_group_id: group_id).size).to eq(1)
+    expect(group_rows('tag_group_placements', tag_group_id: group_id)).to be_empty
+  end
+
+  it 'allows repeated Tag placements with unique membership' do
+    placement(tag_id: tag.id)
+    placement(tag_id: tag.id, position: 2)
+    expect(group_rows('tag_group_tags', tag_group_id: group_id).size).to eq(1)
+    expect(group_rows('tag_group_placements', tag_group_id: group_id).size).to eq(2)
+  end
+
+  it 'rejects duplicate Tag membership' do
+    reject_statement do
+      insert_group_row('tag_group_tags', {
+          tag_group_id: group_id, tag_id: tag.id, created_at: Time.current })
+    end
+  end
+
+  it 'rejects duplicate child membership' do
+    child_id = seed_group(name: '子')
+    attributes = { tag_group_id: group_id, child_tag_group_id: child_id,
+                   created_at: Time.current }
+    insert_group_row('tag_group_groups', attributes)
+    reject_statement { insert_group_row('tag_group_groups', attributes) }
+  end
+
+  it 'rejects direct self membership' do
+    reject_statement do
+      insert_group_row('tag_group_groups', {
+          tag_group_id: group_id, child_tag_group_id: group_id, created_at: Time.current })
+    end
+  end
+
+  it 'rejects a placement without a kind' do
+    reject_statement { placement({ }) }
+  end
+
+  it 'rejects simultaneous Section and Tag kinds' do
+    reject_statement { placement(section_name: '区分', tag_id: tag.id) }
+  end
+
+  it 'rejects a description attached to a Tag placement' do
+    reject_statement { placement(tag_id: tag.id, description: '不正') }
+  end
+
+  it 'allows an optional description on a Section' do
+    placement(section_name: '区分', description: '説明')
+    expect(group_rows('tag_group_placements', tag_group_id: group_id).sole)
+        .to include('section_name' => '区分', 'description' => '説明')
+  end
+
+  it 'rejects a Tag placement absent from membership' do
+    other = group_tag('非メンバー')
+    reject_statement { placement(tag_id: other.id) }
+  end
+
+  it 'rejects a missing parent placement' do
+    reject_statement { placement(section_name: '子', parent_position: 9) }
+  end
+
+  it 'rejects a child ordered before its parent' do
+    placement(section_name: '親', position: 2)
+    reject_statement { placement(section_name: '子', position: 1, parent_position: 2) }
+  end
+
+  it 'rejects duplicate group names' do
+    reject_statement { seed_group(name: '制約確認') }
+  end
+
+  it 'rejects a missing mandatory group name' do
+    reject_statement { seed_group(name: nil) }
+  end
+
+  it 'rejects a six-level snapshot' do
+    nodes = []
+    6.times { nodes = [{ 'tag_id' => tag.id, 'children' => nodes }] }
+    reject_statement do
+      insert_group_row('tag_group_versions', {
+          tag_group_id: group_id, version_no: 2, event_type: 'update', name: '制約確認',
+          members: [{ 'tag_id' => tag.id, 'version_no' => tag.version_no }].to_json,
+          placements: nodes.to_json, created_at: Time.current })
+    end
+  end
+end

backend/spec/db/tag_group_migration_spec.rb

@@ -0,0 +1,119 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag group migration identity and structure' do
+  include TagGroupRecords
+
+  let!(:user) { create_member_user! }
+
+  before { I18n.locale = :ja }
+
+  it 'preserves page and revision IDs, timestamps and repeated historical structures' do
+    tag = group_tag('洗澡鹿')
+    body = "説明文\n\n- 洗澡鹿\n- 洗澡鹿"
+    page = legacy_group('伊地知ニジカ亜種', body)
+    revisions = [page.current_revision]
+    [body, '# 空の区分', body].each do |text|
+      revisions << Wiki::Commit.legacy_content!(page:, body: text, created_user: user)
+    end
+    before_state = group_state.except(*TagGroupRecords::GROUP_TABLES)
+
+    migrate_groups
+
+    versions = group_rows('tag_group_versions', tag_group_id: page.id)
+        .sort_by { _1.fetch('version_no') }
+    expect(versions.map { _1.fetch('id') }).to eq(revisions.map(&:id))
+    expect(versions.map { _1.fetch('version_no') }).to eq([1, 2, 3, 4])
+    expect(versions.map { _1.fetch('created_at') }).to eq(revisions.map(&:created_at))
+    expect(versions.map { _1.fetch('created_by_user_id') }).to eq([user.id] * 4)
+    expect(versions.map { _1.fetch('event_type') }).to eq(['create', 'update', 'update', 'update'])
+    expect(group_version(page.id, 1).fetch('placements')).to eq(
+        group_version(page.id, 2).fetch('placements'))
+    expect(group_version(page.id, 3).fetch('placements')).to eq([
+        { 'section_name' => '空の区分', 'description' => nil, 'children' => [] }])
+    expect_group_snapshot(
+        page.id, number: 4, description: '説明文',
+        members: [{ 'tag_id' => tag.id, 'version_no' => tag.version_no }],
+        placements: [{ 'tag_id' => tag.id, 'children' => [] }] * 2)
+    expect(group_rows('tag_groups', id: page.id).sole).to include(
+        'created_at' => page.created_at, 'updated_at' => page.reload.updated_at)
+    expect(group_state.except(*TagGroupRecords::GROUP_TABLES)).to eq(before_state)
+  end
+
+  it 'migrates nested list Sections and child groups as different placement kinds' do
+    series = group_tag('ぼざろクリーチャーシリーズ')
+    deer = group_tag('洗澡鹿')
+    nijika = group_tag('伊地知ニジカ')
+    child = legacy_group('伊地知ニジカ亜種', <<~MARKDOWN)
+      - ぼざろクリーチャーシリーズ
+        - 伊地知ニジカ亜種:
+          - 洗澡鹿
+    MARKDOWN
+    parent = legacy_group('ぼざろクリーチャーシリーズのキャラクター', <<~MARKDOWN)
+      - ぼざろクリーチャーシリーズ
+        - 伊地知ニジカ
+          - タググループ:伊地知ニジカ亜種
+    MARKDOWN
+
+    migrate_groups
+
+    expect_group_snapshot(
+        child.id, number: 1,
+        members: [series, deer].map { { 'tag_id' => _1.id, 'version_no' => _1.version_no } },
+        placements: [{ 'tag_id' => series.id, 'children' => [
+            { 'section_name' => '伊地知ニジカ亜種', 'description' => nil,
+              'children' => [{ 'tag_id' => deer.id, 'children' => [] }] }] }])
+    expect_group_snapshot(
+        parent.id, number: 1,
+        members: [
+            { 'tag_id' => series.id, 'version_no' => series.version_no },
+            { 'tag_id' => nijika.id, 'version_no' => nijika.version_no },
+            { 'tag_group_id' => child.id, 'version_no' => 1 }],
+        placements: [{ 'tag_id' => series.id, 'children' => [
+            { 'tag_id' => nijika.id, 'children' => [
+                { 'tag_group_id' => child.id, 'children' => [] }] }] }])
+  end
+
+  it 'preserves heading order, group paragraphs and Section descriptions' do
+    names = ['タグ希望', 'キャラクター名希望', '楽曲情報希望', 'bot操作', 'ニジラー情報不詳']
+    tags = names.map { group_tag(_1) }
+    markdown = <<~MARKDOWN
+      全体の説明
+
+      # 要求
+      要求の説明
+
+      - タグ希望
+        - キャラクター名希望
+        - 楽曲情報希望
+
+      # 情報
+      - bot操作
+      - ニジラー情報不詳
+    MARKDOWN
+    page = legacy_group('メタタグ', markdown)
+    expect(page.current_revision.body).to eq(markdown.chomp)
+
+    migrate_groups
+
+    leaves = tags.map { { 'tag_id' => _1.id, 'children' => [] } }
+    leaves.first['children'] = leaves[1..2]
+    expect_group_snapshot(
+        page.id, number: 1, description: '全体の説明',
+        members: tags.map { { 'tag_id' => _1.id, 'version_no' => _1.version_no } },
+        placements: [
+            { 'section_name' => '要求', 'description' => '要求の説明',
+              'children' => [leaves.first] },
+            { 'section_name' => '情報', 'description' => nil,
+              'children' => leaves[3..4] }])
+  end
+
+  it 'leaves Help in the legacy domain' do
+    page = Wiki::Commit.create_legacy_content!(
+        tag_name: create(:tag_name, name: 'Help:移行対象外'),
+        body: 'Help 本文', created_by_user: user)
+    before_state = group_state
+    migrate_groups
+    expect(group_state).to eq(before_state)
+    expect(page.reload.body).to eq('Help 本文')
+  end
+end

backend/spec/models/shared_tag_ids_spec.rb

@@ -0,0 +1,84 @@
+require 'rails_helper'
+
+RSpec.describe 'Shared Tag and ExternalTag IDs', type: :model do
+  include TagGroupRecords
+  let(:user) { create_member_user! }
+
+  def create_resource resource
+    if resource == :tag
+      Tag.find_or_create_by_tag_name!(Locale.nipponese, 'shared_id_new', category: :general)
+    else
+      ExternalTag.create!(platform: :nico, name: 'shared_id_new')
+    end
+  end
+
+  [:tag, :external_tag].each do |resource|
+    it "starts #{ resource } at 1 when both tables are empty" do
+      expect(Tag.count).to eq(0)
+      expect(ExternalTag.count).to eq(0)
+      expect(create_resource(resource).id).to eq(1)
+    end
+
+    it "allocates #{ resource } above the other table's maximum" do
+      create(:tag, id: resource == :tag ? 1 : 20)
+      create(:external_tag, id: resource == :external_tag ? 1 : 20)
+
+      expect(create_resource(resource).id).to eq(21)
+    end
+
+    it "uses the current maximum after deleting earlier #{ resource } records" do
+      tag = create(:tag, id: 100)
+      TagName.where(tag_id: tag.id).update_all(tag_id: nil)
+      tag.destroy!
+      create(:external_tag, id: 200).destroy!
+      expect(Tag.unscoped.count).to eq(0)
+      expect(ExternalTag.unscoped.count).to eq(0)
+      expect(create_resource(resource).id).to eq(1)
+    end
+
+    it "preserves legacy overlaps and gaps when creating #{ resource }" do
+      [1, 2, 5].each { create(:tag, id: _1) }
+      [1, 3, 8].each { create(:external_tag, id: _1) }
+      tags = Tag.order(:id).map(&:attributes)
+      external_tags = ExternalTag.order(:id).map(&:attributes)
+
+      record = create_resource(resource)
+
+      aggregate_failures do
+        expect(record.id).to eq(9)
+        expect(Tag.where(id: [1, 2, 5]).order(:id).map(&:attributes)).to eq(tags)
+        expect(ExternalTag.where(id: [1, 3, 8]).order(:id).map(&:attributes)).to eq(external_tags)
+        expect(Tag.exists?(id: 3)).to be(false)
+        expect(ExternalTag.exists?(id: 2)).to be(false)
+      end
+    end
+
+    it "ignores the description and group ID space when creating #{ resource }" do
+      tag = create(:tag, id: 1)
+      create(:external_tag, id: 2)
+      TagDescription.create!(id: 100, tag:, language_code: 'ja', body: '', version_no: 1)
+      seed_group(name: 'separate_space', id: 200)
+
+      expect(create_resource(resource).id).to eq(3)
+    end
+  end
+
+  it 'preserves overlapping IDs during updates without rewriting the other resource' do
+    tag = create(:tag, id: 5)
+    external = create(:external_tag, id: 5)
+    tag.update!(category: :meta)
+    external.update!(name: 'updated_external')
+
+    expect(tag.reload.id).to eq(5)
+    expect(external.reload.id).to eq(5)
+    expect(tag.category).to eq('meta')
+    expect(external.name).to eq('updated_external')
+    expect(Tag.count).to eq(1)
+    expect(ExternalTag.count).to eq(1)
+  end
+
+  it 'allocates distinct IDs for successive creations across both tables' do
+    records = [:tag, :external_tag].map { create_resource(_1) }
+    expect(records.map(&:id)).to eq([1, 2])
+  end
+end

backend/spec/models/tag_repr_spec.rb

@@ -0,0 +1,50 @@
+require 'rails_helper'
+
+RSpec.describe TagRepr do
+  let(:tag) { create(:tag) }
+
+  def description_for record
+    TagDescription.create!(tag: record, language_code: 'ja', body: '', version_no: 1)
+  end
+
+  it 'keeps has_wiki boolean for the selected language in base and inline' do
+    description_for(tag)
+    [:base, :inline].each do |method|
+      expect(described_class.public_send(method, tag).as_json.fetch('has_wiki')).to be(true)
+      expect(described_class.public_send(method, tag, language_code: 'en')
+          .as_json.fetch('has_wiki')).to be(false)
+    end
+  end
+
+  it 'uses the same language for a tag and its parents' do
+    parent = create(:tag)
+    description_for(tag)
+    description_for(parent)
+    TagImplication.create!(tag:, parent_tag: parent)
+    ['ja', 'en'].each do |language_code|
+      row = described_class.base(tag, language_code:).as_json
+      expected = language_code == 'ja'
+      expect(row.fetch('has_wiki')).to be(expected)
+      expect(row.fetch('parents').sole.fetch('has_wiki')).to be(expected)
+    end
+  end
+
+  it 'preloads existence checks for many tags and their parents without Wiki reads' do
+    parent = create(:tag)
+    description_for(parent)
+    records = Array.new(3) do
+      create(:tag).tap { TagImplication.create!(tag: _1, parent_tag: parent) }
+    end
+    description_for(records.first)
+    queries = []
+    subscriber = ->(*args) { queries << args.last.fetch(:sql) }
+    rows = nil
+    ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') do
+      rows = described_class.many(records).as_json
+    end
+    expect(rows.map { _1.fetch('has_wiki') }).to eq([true, false, false])
+    expect(rows.map { _1.fetch('parents').sole.fetch('has_wiki') }).to eq([true] * 3)
+    expect(queries.grep(/\bSELECT\b.*\btag_descriptions\b/im).size).to be <= 2
+    expect(queries.grep(/\bwiki_(pages|revisions|versions)\b/i)).to be_empty
+  end
+end

backend/spec/models/tag_spec.rb

@@ -1,6 +1,47 @@
 require 'rails_helper'
 
 RSpec.describe Tag, type: :model do
+  describe '#has_description?' do
+    let(:tag) { create(:tag) }
+
+    it 'checks the requested language and treats an empty description as present' do
+      TagDescription.create!(tag:, language_code: 'ja', body: '', version_no: 1)
+      expect(tag.has_description?(language_code: 'ja')).to be(true)
+      expect(tag.has_description?(language_code: 'en')).to be(false)
+    end
+
+    it 'returns false without a description' do
+      expect(tag.has_description?(language_code: 'ja')).to be(false)
+    end
+
+    it 'does not use a Wiki page as evidence of a description' do
+      user = create_member_user!
+      Wiki::Commit.create_legacy_content!(tag_name: tag.tag_name('ja'), body: 'legacy',
+                                          created_by_user: user)
+      expect(tag.has_description?(language_code: 'ja')).to be(false)
+    end
+
+    it 'reuses preloaded descriptions without reading Wiki or querying again' do
+      TagDescription.create!(tag:, language_code: 'ja', body: '', version_no: 1)
+      loaded = Tag.includes(:tag_descriptions).find(tag.id)
+      queries = []
+      subscriber = ->(*args) { queries << args.last.fetch(:sql) }
+      ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') do
+        expect(loaded.has_description?(language_code: 'ja')).to be(true)
+        expect(loaded.has_description?(language_code: 'en')).to be(false)
+      end
+      expect(queries).to be_empty
+    end
+
+    it 'keeps language selection generic without enabling another Language record' do
+      tag.association(:tag_descriptions).target = [
+          TagDescription.new(tag:, language_code: 'en', body: '', version_no: 1)]
+      tag.association(:tag_descriptions).loaded!
+      expect(tag.has_description?(language_code: 'en')).to be(true)
+      expect(tag.has_description?(language_code: 'ja')).to be(false)
+    end
+  end
+
   describe 'external tag separation' do
     ['nico:reserved', 'NiCo:reserved'].each do |name|
       it "rejects the reserved prefix #{ name } for internal tags" do

backend/spec/requests/materials_spec.rb

@@ -178,6 +178,7 @@ RSpec.describe 'Materials API', type: :request do
   end
 
   describe 'GET /materials/:id' do
+    include WikiRecords
     let!(:tag) do
       create(:tag, primary_name: 'material_show', category: :material)
     end
@@ -205,6 +206,56 @@ RSpec.describe 'Materials API', type: :request do
       get '/materials/999999999'
       expect(response).to have_http_status(:not_found)
     end
+
+    def get_material_without_legacy_reads
+      queries = []
+      subscriber = lambda do |*args|
+        sql = args.last.fetch(:sql)
+        if sql.match?(/\bSELECT\b/i) && sql.match?(/\bwiki_(pages|revisions|versions)\b/i)
+          queries << sql
+        end
+      end
+      ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') do
+        get "/materials/#{ material.id }"
+      end
+      expect(response).to have_http_status(:ok)
+      expect(queries).to be_empty
+    end
+
+    it 'reads the description even when the legacy revision has different content' do
+      page = seed_wiki_page(tag_name: tag.tag_name('ja'), body: 'legacy body',
+                            created_by_user: member_user)
+      TagDescription.find_by!(tag_id: tag.id, language_code: 'ja')
+          .update!(body: 'description body')
+      get_material_without_legacy_reads
+      expect(json.fetch('wiki_page_body')).to eq('description body')
+      expect(page.current_revision.body).to eq('legacy body')
+    end
+
+    it 'uses tag_id rather than description ID without a Wiki page or TagName' do
+      description = TagDescription.create!(id: tag.id + 1000, tag:, language_code: 'ja',
+                                           body: 'independent body', version_no: 1)
+      tag.tag_names.update_all(tag_id: nil)
+      get_material_without_legacy_reads
+      expect(description.id).not_to eq(tag.id)
+      expect(json.fetch('wiki_page_body')).to eq('independent body')
+      expect(json.fetch('tag').fetch('has_wiki')).to be(true)
+    end
+
+    it 'returns nil when only legacy Wiki content exists' do
+      seed_wiki_page(tag_name: tag.tag_name('ja'), body: 'legacy only',
+                     created_by_user: member_user)
+      TagDescription.where(tag_id: tag.id).delete_all
+      get_material_without_legacy_reads
+      expect(json.fetch('wiki_page_body')).to be_nil
+      expect(json.fetch('tag').fetch('has_wiki')).to be(false)
+    end
+
+    it 'returns nil for an untagged material' do
+      material.update!(tag: nil)
+      get_material_without_legacy_reads
+      expect(json.fetch('wiki_page_body')).to be_nil
+    end
   end
 
   describe 'POST /materials' do

backend/spec/requests/tag_description_wiki_spec.rb

@@ -1,16 +1,17 @@
 require 'rails_helper'
 
 RSpec.describe 'Tag description Wiki compatibility', type: :request do
+  include WikiRecords
   let!(:user) { create_member_user! }
   let!(:tag_name) { create(:tag_name, name: 'description_contract') }
   let!(:page) do
-    Wiki::Commit.create_content!(
+    seed_wiki_page(
         tag_name:, body: 'old-revision', created_by_user: user)
   end
   let!(:revisions) do
     first = page.current_revision
-    second = Wiki::Commit.content!(page:, body: 'old-second', created_user: user)
-    third = Wiki::Commit.content!(page:, body: 'old-third', created_user: user)
+    second = Wiki::Commit.legacy_content!(page:, body: 'old-second', created_user: user)
+    third = Wiki::Commit.legacy_content!(page:, body: 'old-third', created_user: user)
     [first, second, third]
   end
   let!(:tag) { create(:tag, primary_tag_name: tag_name) }
@@ -242,8 +243,8 @@ RSpec.describe 'Tag description Wiki compatibility', type: :request do
   end
 
   describe 'mixed changes' do
-    it 'includes unmigrated history and ignores migrated legacy-only revisions' do
-      legacy_page = Wiki::Commit.create_content!(
+    it 'excludes unmigrated history and migrated legacy-only revisions' do
+      legacy_page = seed_wiki_page(
           tag_name: create(:tag_name, name: 'Help_mixed'),
           body: 'help', created_by_user: user)
       legacy_revision = legacy_page.current_revision
@@ -256,7 +257,7 @@ RSpec.describe 'Tag description Wiki compatibility', type: :request do
       migrated = json.select { |row| row.fetch('wiki_page').fetch('id') == description.id }
       expect(migrated.map { |row| row.fetch('revision_id') }).to eq(versions.reverse.map(&:id))
       expect(json.map { |row| row.fetch('revision_id') }).not_to include(extra.id)
-      expect(json).to include(include(
+      expect(json).not_to include(include(
           'revision_id' => legacy_revision.id,
           'wiki_page' => include('id' => legacy_page.id, 'title' => 'Help_mixed')))
     end

backend/spec/requests/tag_description_wiki_write_spec.rb

@@ -1,6 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe 'Tag description Wiki dual writes', type: :request do
+  include WikiRecords
   let!(:user) { create_member_user! }
   let!(:tag_name) { create(:tag_name, name: 'description_write') }
   let(:tag) { create(:tag, primary_tag_name: tag_name) }
@@ -31,7 +32,7 @@ RSpec.describe 'Tag description Wiki dual writes', type: :request do
            headers: headers
     end
 
-    it 'creates both current records and histories with a shared revision ID' do
+    it 'creates a description with matching Wiki content and histories' do
       expect { create_description }
           .to change(WikiPage, :count).by(1)
           .and change(WikiRevision, :count).by(1)
@@ -39,17 +40,28 @@ RSpec.describe 'Tag description Wiki dual writes', type: :request do
           .and change(TagDescription, :count).by(1)
           .and change(TagDescriptionVersion, :count).by(1)
       expect(response).to have_http_status(:created)
-
-      page = WikiPage.find(json.fetch('id'))
       description = TagDescription.find_by!(tag_id: tag.id, language_code: 'ja')
-      version = TagDescriptionVersion.find_by!(tag_id: tag.id, language_code: 'ja')
-      expect(description).to have_attributes(id: page.id, body: 'initial', version_no: 1)
-      expect(version).to have_attributes(
-          id: page.current_revision.id, body: 'initial', version_no: 1,
-          event_type: 'create', created_by_user_id: user.id)
+      expect(description).to have_attributes(body: 'initial', version_no: 1)
+      page = WikiPage.find_by!(tag_name:)
       expect(page.body).to eq(description.body)
-      expect(page.current_revision.body).to eq(version.body)
-      expect(page.wiki_versions.last.body).to eq(version.body)
+      expect(page.current_revision.body).to eq(description.body)
+      expect(page.wiki_versions.sole.body).to eq(description.body)
+      expect(json.fetch('id')).to eq(description.id)
+      expect(TagDescriptionVersion.find_by!(tag_id: tag.id, language_code: 'ja'))
+          .to have_attributes(body: 'initial', version_no: 1, created_by_user_id: user.id)
+    end
+
+    it 'reuses an existing compatibility page and synchronises its histories' do
+      page = Wiki::Commit.create_legacy_content!(
+          tag_name:, body: 'legacy', created_by_user: user)
+      expect { create_description }
+          .to change(WikiPage, :count).by(0)
+          .and change(WikiRevision, :count).by(1)
+          .and change(WikiVersion, :count).by(1)
+      expect(response).to have_http_status(:created)
+      description = TagDescription.find_by!(tag_id: tag.id)
+      expect(page.reload.body).to eq(description.body)
+      expect(TagDescriptionVersion.find(page.current_revision.id).body).to eq(page.body)
     end
 
     ['missing_description', 'orphan_description'].each do |title|
@@ -65,48 +77,19 @@ RSpec.describe 'Tag description Wiki dual writes', type: :request do
       end
     end
 
-    it 'creates a namespaced legacy Wiki resource without a Tag' do
-      expect do
-        post '/wiki', params: { title: 'Help:description_write', body: 'legacy' },
-             headers: headers
-      end.to change(WikiPage, :count).by(1)
-          .and change(WikiRevision, :count).by(1)
-          .and change(WikiVersion, :count).by(1)
-          .and change(TagDescription, :count).by(0)
-          .and change(TagDescriptionVersion, :count).by(0)
-
-      expect(response).to have_http_status(:created)
-      page = WikiPage.find(json.fetch('id'))
-      expect(page.tag_name.tag).to be_nil
-      expect(page.body).to eq('legacy')
-      expect(page.current_revision.body).to eq('legacy')
+    [false, true].each do |linked|
+      it "rejects namespaced creation with Tag present: #{ linked }" do
+        name = create(:tag_name, name: 'Help:frozen')
+        create(:tag, primary_tag_name: name) if linked
+        before_state = stored_state
+        post '/wiki', params: { title: name.name, body: 'legacy' }, headers: headers
+        expect(response.status).to be_between(400, 499)
+        expect(stored_state).to eq(before_state)
       end
-
-    it 'creates only legacy Wiki for a namespaced title with an existing Tag' do
-      namespaced_name = create(:tag_name, name: 'help:foo')
-      namespaced_tag = create(:tag, primary_tag_name: namespaced_name)
-
-      expect do
-        post '/wiki', params: { title: namespaced_name.name, body: 'legacy' },
-             headers: headers
-      end.to change(WikiPage, :count).by(1)
-          .and change(WikiRevision, :count).by(1)
-          .and change(WikiVersion, :count).by(1)
-          .and change(TagName, :count).by(0)
-          .and change(TagDescription, :count).by(0)
-          .and change(TagDescriptionVersion, :count).by(0)
-
-      expect(response).to have_http_status(:created)
-      page = WikiPage.find(json.fetch('id'))
-      expect(page).to have_attributes(tag_name_id: namespaced_name.id, body: 'legacy')
-      expect(page.tag_name.tag).to eq(namespaced_tag)
-      expect(page.current_revision.body).to eq('legacy')
-      expect(page.wiki_versions.last.body).to eq('legacy')
     end
 
     [WikiPage, WikiRevision, WikiVersion, TagDescription, TagDescriptionVersion].each do |model|
-      it "rolls back both systems when #{ model.name } persistence fails" do
-        tag_name
+      it "rolls back new description creation when #{ model.name } persistence fails" do
         before_state = stored_state
         aggregate_failures do
           fail_save(model) { create_description }
@@ -116,10 +99,32 @@ RSpec.describe 'Tag description Wiki dual writes', type: :request do
     end
   end
 
+  it 'creates missing compatibility data without writing an ID-colliding Help page' do
+    description = TagDescription.create!(
+        id: 500, tag:, language_code: 'ja', body: 'before', version_no: 1)
+    help = WikiPage.create!(id: description.id,
+                            tag_name: create(:tag_name, name: 'Help:unrelated'),
+                            body: 'unrelated', created_user: user, updated_user: user)
+    Wiki::Commit.legacy_content!(page: help, body: help.body, created_user: user)
+    before_help = help.attributes
+    expect do
+      put "/wiki/#{ description.id }", params: { title: tag_name.name, body: 'after' },
+          headers: headers
+    end.to change(WikiPage, :count).by(1)
+        .and change(WikiRevision, :count).by(1)
+        .and change(WikiVersion, :count).by(1)
+    expect(response).to have_http_status(:ok)
+    expect(description.reload).to have_attributes(id: 500, body: 'after')
+    expect(help.reload.attributes).to eq(before_help)
+    compatibility = WikiPage.find_by!(tag_name:)
+    expect(compatibility.body).to eq(description.body)
+    expect(compatibility.current_revision.body).to eq(description.body)
+  end
+
   describe 'PUT /wiki/:id' do
     let!(:page) do
       tag
-      Wiki::Commit.create_content!(
+      seed_wiki_page(
           tag_name:, body: 'before', created_by_user: user)
     end
     let!(:first_revision) { page.current_revision }
@@ -138,7 +143,8 @@ RSpec.describe 'Tag description Wiki dual writes', type: :request do
       expect(page.reload.version_no).to eq(1)
       expect(description.version_no).to eq(1)
       expect { update_description }
-          .to change(WikiRevision, :count).by(1)
+          .to change(WikiPage, :count).by(0)
+          .and change(WikiRevision, :count).by(1)
           .and change(WikiVersion, :count).by(1)
           .and change(TagDescriptionVersion, :count).by(1)
       expect(response).to have_http_status(:ok)
@@ -157,7 +163,8 @@ RSpec.describe 'Tag description Wiki dual writes', type: :request do
       tag_name.update_columns(tag_id: nil)
 
       expect { update_description }
-          .to change(WikiRevision, :count).by(1)
+          .to change(WikiPage, :count).by(0)
+          .and change(WikiRevision, :count).by(1)
           .and change(WikiVersion, :count).by(1)
           .and change(TagDescriptionVersion, :count).by(1)
 
@@ -177,10 +184,10 @@ RSpec.describe 'Tag description Wiki dual writes', type: :request do
       page_state = page.attributes
       description_state = description.attributes
 
-      update_description(body: 'before', title: 'Help:description_renamed')
+      update_description(body: 'before', title: 'description_renamed')
 
       expect(response).to have_http_status(:ok)
-      expect(tag_name.reload.name).to eq('Help:description_renamed')
+      expect(tag_name.reload.name).to eq('description_renamed')
       expect(WikiRevision.count).to eq(revision_count)
       expect(WikiVersion.count).to eq(wiki_version_count)
       expect(TagDescriptionVersion.count).to eq(description_version_count)

backend/spec/requests/tag_descriptions_spec.rb

@@ -206,7 +206,13 @@ RSpec.describe 'Tag description native API', type: :request do
     context 'without an existing description' do
       before { tag }
 
-      it 'creates both current resources and histories with matching compatibility IDs' do
+      it 'creates a description with synchronised Wiki content and histories' do
+        unrelated = Wiki::Commit.create_legacy_content!(
+            tag_name: create(:tag_name, name: 'Help:unrelated_native_description'),
+            body: 'unrelated body', created_by_user: author)
+        unrelated_state = [unrelated.attributes,
+                           unrelated.wiki_revisions.map(&:attributes),
+                           unrelated.wiki_versions.map(&:attributes)]
         expect { native_write(body: 'created natively') }
             .to change(TagDescription, :count).by(1)
             .and change(TagDescriptionVersion, :count).by(1)
@@ -215,7 +221,7 @@ RSpec.describe 'Tag description native API', type: :request do
             .and change(WikiVersion, :count).by(1)
         expect(response).to have_http_status(:created)
         description = TagDescription.find_by!(tag_id: tag.id, language_code: 'ja')
-        page = WikiPage.find(description.id)
+        page = WikiPage.find_by!(tag_name: tag.tag_name('ja'))
         revision = page.current_revision
         version = TagDescriptionVersion.find(revision.id)
         expect(description).to have_attributes(
@@ -227,6 +233,9 @@ RSpec.describe 'Tag description native API', type: :request do
         expect(page).to have_attributes(tag_name_id: tag_name.id, body: description.body)
         expect(revision.body).to eq(version.body)
         expect(page.wiki_versions.last.body).to eq(version.body)
+        unrelated.reload
+        expect([unrelated.attributes, unrelated.wiki_revisions.map(&:attributes),
+                unrelated.wiki_versions.map(&:attributes)]).to eq(unrelated_state)
         expect_current(description)
 
         get "/wiki/#{ description.id }"

backend/spec/requests/tag_group_markdown_spec.rb

@@ -0,0 +1,139 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag group Markdown round trips through the Wiki facade', type: :request do
+  include TagGroupRecords
+
+  let!(:user) { create_member_user! }
+  let!(:series) { group_tag('ぼざろクリーチャーシリーズ') }
+  let!(:deer) { group_tag('洗澡鹿') }
+  let!(:nizika) { group_tag('伊地知ニジカ') }
+  let!(:wanted) { group_tag('タグ希望') }
+  let!(:character) { group_tag('キャラクター名希望') }
+  let!(:music) { group_tag('楽曲情報希望') }
+  let!(:bot) { group_tag('bot操作') }
+  let!(:unknown) { group_tag('ニジラー情報不詳') }
+  let!(:child_id) { seed_group(name: '伊地知ニジカ亜種', description: '子の説明') }
+
+  before { I18n.locale = :ja }
+
+  def tag_node tag, children = []
+    { 'tag_id' => tag.id, 'children' => children }
+  end
+
+  def tag_member tag
+    { 'tag_id' => tag.id, 'version_no' => tag.version_no }
+  end
+
+  def check_round_trip placements:, members:, markdown:, description: nil
+    id = seed_group(name: '変換元', placements:, members:, description:)
+    without_group_legacy_reads { get group_title_path('変換元') }
+    expect(response).to have_http_status(:ok)
+    expect(json).to include('id' => id)
+    body = json.fetch('body')
+    expect(body).not_to include("\r")
+    expect(body.lines.map(&:chomp)).to all(satisfy { |line| line == line.rstrip })
+    # A trailing LF or a blank line between blocks is semantically irrelevant.
+    expect(body.lines.map(&:chomp).reject(&:empty?)).to eq(
+        markdown.lines.map(&:chomp).reject(&:empty?))
+
+    post '/wiki', params: { title: 'タググループ:再解析', body: },
+         headers: { 'X-Transfer-Code' => user.inheritance_code }
+    expect(response).to have_http_status(:created)
+    parsed = group_rows('tag_groups', name: '再解析').sole
+    expect_group_snapshot(parsed.fetch('id'), number: 1, placements:, members:, description:)
+  end
+
+  it 'renders a list Section under a Tag with two-space indentation' do
+    check_round_trip(
+        members: [series, deer].map { tag_member(_1) },
+        placements: [tag_node(series, [
+            { 'section_name' => '伊地知ニジカ亜種', 'description' => nil,
+              'children' => [tag_node(deer)] }])],
+        markdown: <<~MARKDOWN)
+          - ぼざろクリーチャーシリーズ
+            - 伊地知ニジカ亜種:
+              - 洗澡鹿
+        MARKDOWN
+  end
+
+  it 'renders a child Tag Group distinctly from its parent Tags' do
+    check_round_trip(
+        members: [tag_member(series), tag_member(nizika),
+                  { 'tag_group_id' => child_id, 'version_no' => 1 }],
+        placements: [tag_node(series, [tag_node(nizika, [
+            { 'tag_group_id' => child_id, 'children' => [] }])])],
+        markdown: <<~MARKDOWN)
+          - ぼざろクリーチャーシリーズ
+            - 伊地知ニジカ
+              - タググループ:伊地知ニジカ亜種
+        MARKDOWN
+  end
+
+  it 'renders top-level heading Sections, descriptions and ordered nested Tags' do
+    check_round_trip(
+        description: '全体の説明',
+        members: [wanted, character, music, bot, unknown].map { tag_member(_1) },
+        placements: [
+            { 'section_name' => '要求', 'description' => '要求の説明',
+              'children' => [tag_node(wanted, [tag_node(character), tag_node(music)])] },
+            { 'section_name' => '情報', 'description' => nil,
+              'children' => [tag_node(bot), tag_node(unknown)] }],
+        markdown: <<~MARKDOWN)
+          全体の説明
+
+          ## 要求
+          要求の説明
+
+          - タグ希望
+            - キャラクター名希望
+            - 楽曲情報希望
+
+          ## 情報
+          - bot操作
+          - ニジラー情報不詳
+        MARKDOWN
+  end
+
+  it 'renders nested heading Sections and all repeated Tag placements' do
+    check_round_trip(
+        members: [tag_member(deer)],
+        placements: [
+            { 'section_name' => '親', 'description' => '親の説明', 'children' => [
+                tag_node(deer),
+                { 'section_name' => '子', 'description' => '子の説明',
+                  'children' => [tag_node(deer)] }] }],
+        markdown: <<~MARKDOWN)
+          ## 親
+          親の説明
+
+          - 洗澡鹿
+
+          ### 子
+          子の説明
+
+          - 洗澡鹿
+        MARKDOWN
+  end
+
+  it 'rejects mixed top-level heading Sections and Tags without losing structure' do
+    id = seed_group(
+        name: '混在構造', members: [tag_member(series)],
+        placements: [
+            { 'section_name' => '区分', 'description' => nil, 'children' => [] },
+            tag_node(series)])
+
+    get group_title_path('混在構造')
+
+    expect(response).to have_http_status(:unprocessable_entity)
+    expect(group_rows('tag_groups', id:).sole.fetch('id')).to eq(id)
+  end
+
+  it 'rejects membership without a placement instead of omitting it' do
+    id = seed_group(name: '未配置構造', members: [tag_member(series)])
+
+    get group_title_path('未配置構造')
+
+    expect(response).to have_http_status(:unprocessable_entity)
+    expect(group_rows('tag_groups', id:).sole.fetch('id')).to eq(id)
+  end
+end

backend/spec/requests/tag_group_wiki_spec.rb

@@ -0,0 +1,309 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag group Wiki facade reads', type: :request do
+  include WikiRecords
+  include TagGroupRecords
+
+  let!(:user) { create_member_user! }
+  let!(:old_tag) { group_tag('旧タグ') }
+  let!(:new_tag) { group_tag('新タグ') }
+  let!(:page) { legacy_group('読取り契約', '- 旧タグ') }
+  let!(:first_revision) { page.current_revision }
+  let!(:last_revision) do
+    Wiki::Commit.legacy_content!(page:, body: '- 新タグ', created_user: user)
+  end
+
+  before do
+    I18n.locale = :ja
+    migrate_groups
+    page.update_columns(body: 'legacy stale current')
+    page.wiki_revisions.each do |revision|
+      revision.wiki_lines.update_all(body: 'legacy stale revision')
+    end
+    page.wiki_versions.update_all(body: 'legacy stale version')
+  end
+
+  [:id, :title].each do |lookup|
+    context "with #{ lookup } lookup" do
+      let(:path) { lookup == :id ? "/wiki/#{ page.id }" : group_title_path('読取り契約') }
+
+      it 'reads current structure without consulting migrated Wiki tables' do
+        without_group_legacy_reads { get path }
+        expect(response).to have_http_status(:ok)
+        expect(json).to include(
+            'id' => page.id, 'title' => 'タググループ:読取り契約',
+            'revision_id' => last_revision.id, 'pred' => first_revision.id, 'succ' => nil)
+        expect(json.fetch('body').strip).to eq('- 新タグ')
+      end
+
+      it 'reads the old snapshot rather than current membership or placement' do
+        without_group_legacy_reads { get path, params: { version: first_revision.id } }
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('body').strip).to eq('- 旧タグ')
+        expect(json).to include(
+            'revision_id' => first_revision.id, 'pred' => nil, 'succ' => last_revision.id,
+            'updated_at' => first_revision.created_at.iso8601(3))
+      end
+
+      it 'checks existence without consulting migrated Wiki tables' do
+        without_group_legacy_reads { get "#{ path }/exists" }
+        expect(response).to have_http_status(:no_content)
+      end
+
+      it 'does not fall back to legacy history when a group version is missing' do
+        group_connection.execute(
+            "DELETE FROM tag_group_versions WHERE id = #{ first_revision.id }")
+        without_group_legacy_reads { get path, params: { version: first_revision.id } }
+        expect(response).to have_http_status(:not_found)
+      end
+    end
+  end
+
+  ['/wiki', '/wiki/search'].each do |path|
+    it "lists migrated groups once with current titles at #{ path }" do
+      get path, params: { title: 'タググループ:読取り' }
+      expect(response).to have_http_status(:ok)
+      rows = json.select { _1.fetch('title') == 'タググループ:読取り契約' }
+      expect(rows.size).to eq(1)
+      expect(rows.sole).to include('id' => page.id)
+      expect(response.body).not_to include('legacy stale')
+    end
+  end
+
+  it 'combines Tag Groups and Tag Descriptions without Help in the unfiltered list' do
+    help = seed_wiki_page(
+        tag_name: create(:tag_name, name: 'Help:混在'), body: 'help', created_by_user: user)
+    TagDescriptionOperation.create!(tag: new_tag, body: 'tag description', created_by_user: user)
+    get '/wiki'
+    expect(response).to have_http_status(:ok)
+    expect(json.map { _1.fetch('title') }).to include(
+        'タググループ:読取り契約', '新タグ')
+    expect(json.map { _1.fetch('title') }).not_to include(help.title)
+  end
+
+  it 'excludes Help when its ID collides with a new group' do
+    help = seed_wiki_page(
+        tag_name: create(:tag_name, name: 'Help:ID重複'), body: 'help body',
+        created_by_user: user)
+    seed_group(name: '別 ID重複', id: help.id, description: 'group body')
+
+    get '/wiki'
+    expect(response).to have_http_status(:ok)
+    expect(json.map { _1.fetch('title') }).to include('タググループ:別 ID重複')
+    expect(json.map { _1.fetch('title') }).not_to include(help.title)
+
+    get "/wiki/title/#{ CGI.escapeURIComponent(help.title) }"
+    expect(response).to have_http_status(:not_found)
+
+    get '/wiki/changes'
+    expect(response).to have_http_status(:ok)
+    expect(json.map { _1.dig('wiki_page', 'title') }).to include(
+        'タググループ:別 ID重複')
+    expect(json.map { _1.dig('wiki_page', 'title') }).not_to include(help.title)
+  end
+
+  it 'finds groups by a partial prefix of their displayed title' do
+    get '/wiki', params: { title: 'タググル' }
+
+    expect(response).to have_http_status(:ok)
+    expect(json.map { _1.fetch('title') }).to include('タググループ:読取り契約')
+    expect(json.find { _1.fetch('title') == 'タググループ:読取り契約' })
+        .to include('updated_at' => be_present)
+  end
+
+  it 'does not restore a stale namespaced Wiki page after a group rename' do
+    old_page = legacy_group('旧名称', '- 旧本文')
+    seed_group(name: '新名称', id: old_page.id, description: '正本')
+
+    get group_title_path('旧名称')
+    expect(response).to have_http_status(:not_found)
+    get "#{ group_title_path('旧名称') }/exists"
+    expect(response).to have_http_status(:not_found)
+    get group_title_path('新名称')
+    expect(response).to have_http_status(:ok)
+    expect(json.fetch('body').strip).to eq('正本')
+  end
+
+  it 'orders mixed resource history by timestamp rather than independent IDs' do
+    help = seed_wiki_page(
+        tag_name: create(:tag_name, name: 'Help:時系列'), body: 'help',
+        created_by_user: user)
+    TagDescriptionOperation.create!(tag: new_tag, body: 'description', created_by_user: user)
+    TagGroupVersion.update_all(created_at: 3.days.ago)
+    WikiRevision.where(wiki_page_id: help.id).update_all(created_at: 2.days.ago)
+    TagDescriptionVersion.update_all(created_at: 1.day.ago)
+
+    get '/wiki/changes'
+
+    expect(response).to have_http_status(:ok)
+    timestamps = json.map { Time.iso8601(_1.fetch('timestamp')) }
+    expect(timestamps).to eq(timestamps.sort.reverse)
+    titles = json.map { _1.dig('wiki_page', 'title') }
+    expect(titles).not_to include(help.title)
+    expect(titles.index('新タグ')).to be < titles.index('タググループ:読取り契約')
+  end
+
+  it 'renders nullable group editors without raising' do
+    group_connection.execute(
+        "UPDATE tag_groups SET created_by_user_id = NULL, updated_by_user_id = NULL " +
+        "WHERE id = #{ page.id }")
+    group_connection.execute(
+        "UPDATE tag_group_versions SET created_by_user_id = NULL " +
+        "WHERE tag_group_id = #{ page.id }")
+
+    get "/tag_groups/#{ page.id }"
+    expect(response).to have_http_status(:ok)
+    expect(json).to include('created_by_user' => nil, 'updated_by_user' => nil)
+    get "/wiki/title/#{ CGI.escapeURIComponent('タググループ:読取り契約') }"
+    expect(response).to have_http_status(:ok)
+  end
+
+  it 'reads group version IDs, editor and timestamps in scoped changes' do
+    without_group_legacy_reads { get '/wiki/changes', params: { id: page.id } }
+    expect(response).to have_http_status(:ok)
+    expect(json.map { _1.fetch('revision_id') }).to eq([last_revision.id, first_revision.id])
+    expect(json.first).to include(
+        'pred' => first_revision.id, 'succ' => nil, 'kind' => 'content',
+        'timestamp' => last_revision.created_at.iso8601(3),
+        'user' => { 'id' => user.id, 'name' => user.name },
+        'wiki_page' => include('id' => page.id, 'title' => 'タググループ:読取り契約'))
+  end
+
+  it 'includes group history but excludes all legacy-only revisions' do
+    extra = Wiki::Commit.legacy_content!(page:, body: 'legacy only', created_user: user)
+    help = seed_wiki_page(
+        tag_name: create(:tag_name, name: 'Help:変更一覧'), body: 'help', created_by_user: user)
+    get '/wiki/changes'
+    expect(response).to have_http_status(:ok)
+    rows = json.select { _1.fetch('wiki_page').fetch('title') == 'タググループ:読取り契約' }
+    expect(rows.map { _1.fetch('revision_id') }).to eq([last_revision.id, first_revision.id])
+    expect(rows.map { _1.fetch('revision_id') }).not_to include(extra.id)
+    expect(json).not_to include(include(
+        'revision_id' => help.current_revision.id,
+        'wiki_page' => include('title' => help.title)))
+  end
+
+  [false, true].each do |explicit|
+    it "diffs structured snapshots with #{ explicit ? 'explicit' : 'current' } target" do
+      params = { from: first_revision.id }
+      params[:to] = last_revision.id if explicit
+      without_group_legacy_reads { get "/wiki/#{ page.id }/diff", params: }
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'wiki_page_id' => page.id, 'older_revision_id' => first_revision.id,
+          'newer_revision_id' => last_revision.id)
+      differences = json.fetch('diff').map { [_1.fetch('type'), _1.fetch('content').strip] }
+      expect(differences).to include(['removed', '- 旧タグ'], ['added', '- 新タグ'])
+    end
+  end
+
+  it 'does not use a version belonging to another group' do
+    other_id = seed_group(name: '別グループ', description: '別本文')
+    other_version = group_version(other_id, 1)
+    get group_title_path('読取り契約'), params: { version: other_version.fetch('id') }
+    expect(response).to have_http_status(:not_found)
+    get "/wiki/#{ page.id }/diff",
+        params: { from: other_version.fetch('id'), to: last_revision.id }
+    expect(response).to have_http_status(:not_found)
+  end
+
+  it 'resolves a group without any compatibility Wiki page' do
+    id = seed_group(name: '独立グループ', description: '独立した説明')
+    without_group_legacy_reads { get group_title_path('独立グループ') }
+    expect(response).to have_http_status(:ok)
+    expect(json).to include('id' => id, 'title' => 'タググループ:独立グループ')
+    expect(json.fetch('body').strip).to eq('独立した説明')
+  end
+
+  context 'when Help, description and group share the same numeric ID' do
+    let(:colliding_help) do
+      seed_wiki_page(
+          tag_name: create(:tag_name, name: 'Help:三者ID衝突'),
+          body: 'help', created_by_user: user)
+    end
+    let(:colliding_group_id) do
+      seed_group(name: '三者ID衝突', id: colliding_help.id, description: 'group')
+    end
+    let(:colliding_description) do
+      TagDescription.create!(
+          id: colliding_help.id, tag: new_tag, language_code: 'ja',
+          body: 'description', version_no: 1)
+    end
+    let(:colliding_description_version) do
+      TagDescriptionVersion.create!(
+          tag_id: new_tag.id, language_code: 'ja', version_no: 1,
+          event_type: 'create', body: 'description', created_by_user_id: user.id,
+          created_at: Time.current)
+    end
+
+    before do
+      colliding_group_id
+      colliding_description
+      colliding_description_version
+    end
+
+    it 'lists both new resources sharing a numeric ID without legacy Help' do
+      get '/wiki'
+
+      expect(response).to have_http_status(:ok)
+      rows = json.select { _1.fetch('id') == colliding_help.id }
+      expect(rows.map { [_1.fetch('id'), _1.fetch('title')] }).to contain_exactly(
+          [colliding_description.id, '新タグ'],
+          [colliding_group_id, 'タググループ:三者ID衝突'])
+    end
+
+    it 'lists each resource history without deduplicating the shared resource ID' do
+      get '/wiki/changes'
+
+      expect(response).to have_http_status(:ok)
+      rows = json.select { _1.fetch('wiki_page').fetch('id') == colliding_help.id }
+      histories = rows.map do |row|
+        [row.fetch('revision_id'), row.fetch('wiki_page').fetch('title')]
+      end
+      expect(histories).to contain_exactly(
+          [colliding_description_version.id, '新タグ'],
+          [group_version(colliding_group_id, 1).fetch('id'), 'タググループ:三者ID衝突'])
+    end
+
+    it 'still excludes legacy copies of an actually migrated group and its history' do
+      extra = Wiki::Commit.legacy_content!(page:, body: 'legacy only', created_user: user)
+
+      get '/wiki'
+
+      expect(response).to have_http_status(:ok)
+      rows = json.select { _1.fetch('id') == page.id }
+      expect(rows.map { _1.fetch('title') }).to eq(['タググループ:読取り契約'])
+
+      get '/wiki/changes'
+
+      expect(response).to have_http_status(:ok)
+      rows = json.select { _1.fetch('wiki_page').fetch('id') == page.id }
+      expect(rows.map { _1.fetch('revision_id') })
+          .to contain_exactly(first_revision.id, last_revision.id)
+      expect(rows.map { _1.fetch('revision_id') }).not_to include(extra.id)
+      expect(rows.map { _1.fetch('wiki_page').fetch('title') })
+          .to all(eq('タググループ:読取り契約'))
+    end
+  end
+
+  it 'resolves new resources by title while excluding colliding legacy Help' do
+    help = seed_wiki_page(
+        tag_name: create(:tag_name, name: 'Help:ID衝突'), body: 'help', created_by_user: user)
+    seed_group(name: 'ID衝突', id: help.id, description: 'group')
+    TagDescription.create!(
+        id: help.id, tag: new_tag, language_code: 'ja', body: 'description', version_no: 1)
+    TagDescriptionVersion.create!(
+        tag_id: new_tag.id, language_code: 'ja', version_no: 1,
+        event_type: 'create', body: 'description', created_at: Time.current)
+    { 'タググループ:ID衝突' => 'group', '新タグ' => 'description' }
+        .each do |title, body|
+      get "/wiki/title/#{ CGI.escapeURIComponent(title) }"
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('body').strip).to eq(body)
+    end
+    without_group_legacy_reads do
+      get "/wiki/title/#{ CGI.escapeURIComponent(help.title) }"
+    end
+    expect(response).to have_http_status(:not_found)
+  end
+end

backend/spec/requests/tag_group_wiki_write_spec.rb

@@ -0,0 +1,307 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag group Wiki facade writes', type: :request do
+  include TagGroupRecords
+  include ActiveSupport::Testing::TimeHelpers
+
+  let!(:user) { create_member_user! }
+  let!(:tag) { group_tag('洗澡鹿') }
+  let!(:other_tag) { group_tag('伊地知ニジカ') }
+  let(:headers) { { 'X-Transfer-Code' => user.inheritance_code } }
+  let(:members) { [{ 'tag_id' => tag.id, 'version_no' => tag.version_no }] }
+  let(:placements) { [{ 'tag_id' => tag.id, 'children' => [] }] }
+
+  before { I18n.locale = :ja }
+
+  def create_group title: 'タググループ:新規', body: '- 洗澡鹿', request_headers: headers
+    post '/wiki', params: { title:, body: }, headers: request_headers
+  end
+
+  def expect_rejected_without_writes
+    before_state = group_state
+    yield
+    aggregate_failures do
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(group_state).to eq(before_state)
+      expect(response.media_type).to eq('application/json')
+      expect(response.body).not_to be_empty
+      next if response.body.empty?
+
+      expect(json).to include(
+          'type' => 'validation_error', 'errors' => a_kind_of(Hash),
+          'base_errors' => a_kind_of(Array))
+    end
+  end
+
+  it 'creates the group, snapshot and matching compatibility histories' do
+    expect { create_group }
+        .to change(WikiPage, :count).by(1)
+        .and change(WikiRevision, :count).by(1)
+        .and change(WikiVersion, :count).by(1)
+    expect(response).to have_http_status(:created)
+    group = group_rows('tag_groups', name: '新規').sole
+    expect(json).to include('id' => group.fetch('id'), 'title' => 'タググループ:新規')
+    expect_group_snapshot(group.fetch('id'), number: 1, members:, placements:)
+    version = group_version(group.fetch('id'), 1)
+    expect(version).to include('event_type' => 'create', 'created_by_user_id' => user.id)
+    expect(group).to include('created_by_user_id' => user.id, 'updated_by_user_id' => user.id)
+    page = WikiPage.joins(:tag_name).find_by!(tag_names: { name: 'タググループ:新規' })
+    expect(page.body).to eq('- 洗澡鹿')
+    expect(page.current_revision.body).to eq(page.body)
+    expect(page.wiki_versions.sole.body).to eq(page.body)
+  end
+
+  it 'keeps repeated placements but only one membership and one snapshot member' do
+    create_group(body: "- 洗澡鹿\n# 再掲\n- 洗澡鹿")
+    expect(response).to have_http_status(:created)
+    id = group_rows('tag_groups', name: '新規').sole.fetch('id')
+    expect_group_snapshot(
+        id, number: 1, members:,
+        placements: placements + [
+            { 'section_name' => '再掲', 'description' => nil, 'children' => placements }])
+  end
+
+  it 'resolves a Japanese alias through tag_names.tag_id' do
+    create(:tag_name, :alias, tag:, name: '鹿の別名')
+    create_group(body: '- 鹿の別名')
+    expect(response).to have_http_status(:created)
+    id = group_rows('tag_groups', name: '新規').sole.fetch('id')
+    expect_group_snapshot(id, number: 1, members:, placements:)
+    get group_title_path('新規')
+    expect(response).to have_http_status(:ok)
+    expect(json.fetch('body').strip).to eq('- 洗澡鹿')
+  end
+
+  {
+      'unknown Tag' => '- 解決不能',
+      'unknown Tag Group' => '- タググループ:存在しない',
+      'empty Section' => '- :',
+      'unsupported placement syntax' => "```\n洗澡鹿\n```",
+      'six levels' => "- 洗澡鹿\n  - 洗澡鹿\n    - 洗澡鹿\n      - 洗澡鹿\n" +
+          "        - 洗澡鹿\n          - 洗澡鹿",
+      'blank body' => '',
+      'normalised blank body' => "\r\n\r\n" }.each do |label, body|
+    it "rejects #{ label } without fallback or partial create" do
+      expect_rejected_without_writes { create_group(body:) }
+    end
+  end
+
+  ['', 'タググループ:', "タググループ:#{ '長' * 256 }"].each do |title|
+    it "rejects an invalid required title of length #{ title.length }" do
+      expect_rejected_without_writes { create_group(title:) }
+    end
+  end
+
+  it 'rejects a duplicate group name even without a matching legacy Wiki' do
+    seed_group(name: '新規', description: '既存')
+    expect_rejected_without_writes { create_group }
+  end
+
+  it 'accepts the existing five-level placement depth boundary' do
+    body = (0...5).map { "#{ '  ' * _1 }- 洗澡鹿" }.join("\n")
+    create_group(body:)
+    expect(response).to have_http_status(:created)
+    id = group_rows('tag_groups', name: '新規').sole.fetch('id')
+    tree = []
+    5.times { tree = [{ 'tag_id' => tag.id, 'children' => tree }] }
+    expect_group_snapshot(id, number: 1, members:, placements: tree)
+  end
+
+  context 'updating a migrated group' do
+    let!(:page) { legacy_group('更新', '- 洗澡鹿') }
+    let!(:old_revision) { page.current_revision }
+
+    before { migrate_groups }
+
+    def update_group body: '- 伊地知ニジカ', title: 'タググループ:更新', request_headers: headers
+      put "/wiki/#{ page.id }", params: { title:, body: }, headers: request_headers
+    end
+
+    [1, 3].each do |repetitions|
+      it "keeps identical Markdown unchanged after #{ repetitions } Wiki PUTs" do
+        body = page.body
+        travel_to(1.hour.from_now) do
+          expect_noop_group_update(group_id: page.id, page:) do
+            repetitions.times do
+              update_group(body:)
+              expect(response).to have_http_status(:ok)
+            end
+          end
+        end
+      end
+    end
+
+    it 'keeps identical Markdown unchanged with a stale base_revision_id' do
+      update_group
+      expect(response).to have_http_status(:ok)
+      body = page.reload.body
+      expect(page.current_revision.id).not_to eq(old_revision.id)
+
+      travel_to(1.hour.from_now) do
+        expect_noop_group_update(group_id: page.id, page:) do
+          put "/wiki/#{ page.id }",
+              params: { title: page.title, body:, base_revision_id: old_revision.id },
+              headers: headers
+        end
+      end
+    end
+
+    it 'keeps the original editor on identical Markdown from another member' do
+      editor = create(:user, :member)
+      body = page.body
+      travel_to(1.hour.from_now) do
+        expect_noop_group_update(group_id: page.id, page:) do
+          update_group(body:, request_headers: { 'X-Transfer-Code' => editor.inheritance_code })
+        end
+      end
+    end
+
+    it 'does not add history for Wiki content already changed through Native' do
+      changed_members = [{ 'tag_id' => other_tag.id }]
+      changed_placements = [{ 'tag_id' => other_tag.id, 'children' => [] }]
+      put "/tag_groups/#{ page.id }",
+          params: { name: '更新', description: nil,
+                    members: changed_members, placements: changed_placements },
+          headers:, as: :json
+      expect(response).to have_http_status(:ok)
+      expect_group_snapshot(
+          page.id, number: 2,
+          members: [{ 'tag_id' => other_tag.id, 'version_no' => other_tag.version_no }],
+          placements: changed_placements)
+      body = page.reload.body
+      expect(body).to eq('- 伊地知ニジカ')
+
+      travel_to(1.hour.from_now) do
+        expect_noop_group_update(group_id: page.id, page:) { update_group(body:) }
+      end
+    end
+
+    it 'replaces current membership and placement, retaining the old snapshot' do
+      old = group_version(page.id, 1)
+      update_group
+      expect(response).to have_http_status(:ok)
+      expect_group_snapshot(
+          page.id, number: 2,
+          members: [{ 'tag_id' => other_tag.id, 'version_no' => other_tag.version_no }],
+          placements: [{ 'tag_id' => other_tag.id, 'children' => [] }])
+      expect(group_version(page.id, 1)).to eq(old)
+      expect(group_version(page.id, 2)).to include(
+          'event_type' => 'update', 'created_by_user_id' => user.id)
+      expect(page.reload.body.strip).to eq('- 伊地知ニジカ')
+      expect(page.wiki_revisions.count).to eq(2)
+      expect(page.wiki_versions.count).to eq(2)
+      expect(page.current_revision.body).to eq(page.body)
+      expect(page.wiki_versions.order(:version_no).last.body).to eq(page.body)
+    end
+
+    it 'accepts the current base_revision_id used by the existing frontend' do
+      put "/wiki/#{ page.id }",
+          params: { title: page.title, body: '- 伊地知ニジカ',
+                    base_revision_id: old_revision.id }, headers: headers
+      expect(response).to have_http_status(:ok)
+      expect(group_rows('tag_groups', id: page.id).sole.fetch('version_no')).to eq(2)
+    end
+
+    it 'ignores a stale base_revision_id without rejecting the update' do
+      update_group
+      expect(response).to have_http_status(:ok)
+
+      put "/wiki/#{ page.id }",
+          params: { title: page.title, body: '- 洗澡鹿',
+                    base_revision_id: old_revision.id }, headers: headers
+
+      expect(response).to have_http_status(:ok)
+      expect(group_rows('tag_groups', id: page.id).sole.fetch('version_no')).to eq(3)
+      expect(page.reload.body.strip).to eq('- 洗澡鹿')
+    end
+
+    it 'records the editor and update time' do
+      editor = create(:user, :member, name: '編輯者')
+      travel_to(Time.current + 1.hour) do
+        update_group(request_headers: { 'X-Transfer-Code' => editor.inheritance_code })
+        expect(response).to have_http_status(:ok)
+        current = group_rows('tag_groups', id: page.id).sole
+        version = group_version(page.id, 2)
+        expect(current).to include('updated_by_user_id' => editor.id)
+        expect(version).to include('created_by_user_id' => editor.id)
+        expect(current.fetch('updated_at')).to be_within(1.second).of(Time.current)
+        expect(version.fetch('created_at')).to be_within(1.second).of(Time.current)
+      end
+    end
+
+    ['- 不明なタグ', '- タググループ:不明', '', '- :'].each do |body|
+      it "rejects invalid update #{ body.inspect } atomically" do
+        expect_rejected_without_writes { update_group(body:) }
+      end
+    end
+
+    it 'rejects direct self inclusion' do
+      expect_rejected_without_writes { update_group(body: '- タググループ:更新') }
+    end
+
+    it 'rejects indirect cyclic membership' do
+      seed_group(
+          name: '子', members: [{ 'tag_group_id' => page.id, 'version_no' => 1 }],
+          placements: [{ 'tag_group_id' => page.id, 'children' => [] }])
+      expect_rejected_without_writes { update_group(body: '- タググループ:子') }
+    end
+
+    it 'rejects duplicate rename without retaining a partial title or version' do
+      seed_group(name: '既存', description: '既存')
+      expect_rejected_without_writes { update_group(title: 'タググループ:既存') }
+    end
+  end
+
+  [:create, :update].each do |operation|
+    context "#{ operation } access and atomicity" do
+      let!(:page) do
+        next unless operation == :update
+
+        legacy = legacy_group('更新', '- 洗澡鹿')
+        seed_group(name: '更新', id: legacy.id, members:, placements:)
+        legacy
+      end
+      let!(:child_id) { seed_group(name: '子グループ', description: '子') }
+
+      def write_group operation, request_headers
+        body = "- 伊地知ニジカ\n- タググループ:子グループ"
+        if operation == :create
+          create_group(body:, request_headers:)
+        else
+          put "/wiki/#{ page.id }",
+              params: { title: 'タググループ:更新', body: }, headers: request_headers
+        end
+      end
+
+      [nil, :guest, :banned].each do |role|
+        it "rejects #{ role || 'anonymous' } without writes" do
+          actor = case role
+          when :guest then create(:user, :guest)
+          when :banned then create(:user, :member, :banned)
+          end
+          request_headers = actor ? { 'X-Transfer-Code' => actor.inheritance_code } : { }
+          before_state = group_state
+          write_group(operation, request_headers)
+          expect(response).to have_http_status(role ? :forbidden : :unauthorized)
+          expect(group_state).to eq(before_state)
+        end
+      end
+
+      it 'rejects a banned IP even for a member' do
+        IpAddress.create!(ip_address: IPAddr.new('127.0.0.1').hton, banned_at: Time.current)
+        before_state = group_state
+        write_group(operation, headers)
+        expect(response).to have_http_status(:forbidden)
+        expect(group_state).to eq(before_state)
+      end
+
+      tables = TagGroupRecords::GROUP_TABLES
+      tables += ['wiki_pages', 'wiki_revisions', 'wiki_versions']
+      tables.each do |table|
+        it "rolls back all writes after a real #{ table } write fails" do
+          expect_atomic_group_write(table) { write_group(operation, headers) }
+        end
+      end
+    end
+  end
+end

backend/spec/requests/tag_group_write_integrity_spec.rb

@@ -0,0 +1,331 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag group write integrity', type: :request do
+  include WikiRecords
+  include TagGroupRecords
+  include ActiveSupport::Testing::TimeHelpers
+
+  let!(:user) { create_member_user! }
+  let!(:tag) { group_tag('洗澡鹿') }
+  let!(:other_tag) { group_tag('伊地知ニジカ') }
+  let(:headers) { { 'X-Transfer-Code' => user.inheritance_code } }
+  let(:members) { [{ 'tag_id' => tag.id, 'version_no' => tag.version_no }] }
+  let(:placements) { [{ 'tag_id' => tag.id, 'children' => [] }] }
+
+  before { I18n.locale = :ja }
+
+  def native_payload name: '更新', nodes: placements, member_rows: members
+    { name:, description: nil, members: member_rows.map do |member|
+      member.except('version_no')
+    end, placements: nodes }
+  end
+
+  def put_wiki id, title:, body: '- 伊地知ニジカ', base: nil
+    put "/wiki/#{ id }", params: { title:, body:, base_revision_id: base }, headers: headers
+  end
+
+  def create_help body
+    seed_wiki_page(
+        tag_name: create(:tag_name, name: 'Help:既存'), body:, created_by_user: user)
+  end
+
+  def expect_no_writes
+    before_state = group_state
+    yield
+    expect(response).to have_http_status(:unprocessable_entity)
+    expect(group_state).to eq(before_state)
+  end
+
+  context 'resource identity' do
+    it 'rejects removing the prefix from a migrated group atomically' do
+      page = legacy_group('更新', '- 洗澡鹿')
+      seed_group(name: '更新', id: page.id, members:, placements:)
+
+      expect_no_writes do
+        put_wiki(page.id, title: '更新', base: page.current_revision.id)
+      end
+    end
+
+    it 'rejects an unprefixed update to an independent group' do
+      id = seed_group(name: '独立', members:, placements:)
+      expect_no_writes { put_wiki(id, title: '独立') }
+    end
+
+    it 'does not revive legacy writes when the compatibility title is stale' do
+      page = legacy_group('更新', '- 洗澡鹿')
+      seed_group(name: '更新', id: page.id, members:, placements:)
+      page.tag_name.update!(name: '古い別名')
+
+      expect_no_writes { put_wiki(page.id, title: '古い別名') }
+    end
+
+    it 'renames the group and its compatibility page through the operation' do
+      page = legacy_group('更新', '- 洗澡鹿')
+      seed_group(name: '更新', id: page.id, members:, placements:)
+
+      put_wiki(page.id, title: 'タググループ:改名')
+
+      expect(response).to have_http_status(:ok)
+      expect(TagGroup.find(page.id)).to have_attributes(name: '改名', version_no: 2)
+      expect(page.reload.title).to eq('タググループ:改名')
+      expect(page.body).to eq('- 伊地知ニジカ')
+    end
+
+    it 'rejects a standalone Help update despite a colliding group ID' do
+      page = create_help('Help body')
+      id = seed_group(name: '別グループ', id: page.id, members:, placements:)
+      group_before = TagGroup.find(id).attributes
+      version_before = group_version(id, 1)
+
+      expect_no_writes { put_wiki(id, title: 'Help:改名', body: 'Help after') }
+
+      expect(page.reload.body).to eq('Help body')
+      expect(page.title).to eq('Help:既存')
+      expect(TagGroup.find(id).attributes).to eq(group_before)
+      expect(group_version(id, 1)).to eq(version_before)
+    end
+
+    it 'rejects an ambiguous unprefixed rename when Help and group IDs collide' do
+      page = create_help('Help body')
+      seed_group(name: '更新', id: page.id, members:, placements:)
+
+      expect_no_writes { put_wiki(page.id, title: '更新') }
+      expect_no_writes do
+        put_wiki(page.id, title: '更新', base: page.current_revision.id)
+      end
+    end
+
+    it 'keeps a description update separate from a colliding group' do
+      page = Wiki::Commit.create_content!(
+          tag_name: tag.tag_name('ja'), body: 'description before', created_by_user: user)
+      description = TagDescription.find_by!(tag_id: tag.id)
+      id = seed_group(name: '更新', id: description.id, members:, placements:)
+      group_before = TagGroup.find(id).attributes
+
+      put_wiki(id, title: '洗澡鹿', body: 'description after')
+
+      expect(response).to have_http_status(:ok)
+      expect(TagDescription.find(id).body).to eq('description after')
+      expect(page.reload.body).to eq('description after')
+      expect(TagGroup.find(id).attributes).to eq(group_before)
+    end
+
+    it 'does not route a colliding Help write into an unrelated description' do
+      page = create_help('Help before')
+      id = seed_group(name: '更新', id: page.id, members:, placements:)
+      description = TagDescription.create!(
+          id:, tag:, language_code: 'ja', version_no: 1, body: 'description')
+
+      expect_no_writes { put_wiki(id, title: 'Help:既存', body: 'Help after') }
+
+      expect(page.reload.body).to eq('Help before')
+      expect(description.reload.body).to eq('description')
+      expect(TagGroup.find(id).version_no).to eq(1)
+      put_wiki(id, title: '洗澡鹿', body: 'description after')
+      expect(response).to have_http_status(:ok)
+      expect(description.reload.body).to eq('description after')
+      put_wiki(id, title: 'タググループ:更新')
+      expect(response).to have_http_status(:ok)
+      expect(TagGroup.find(id).version_no).to eq(2)
+      expect(page.reload.body).to eq('Help before')
+      expect(description.reload.body).to eq('description after')
+    end
+  end
+
+  context 'compatibility synchronisation' do
+    it 'synchronises a native rename without modifying an ID-colliding Help page' do
+      help = create_help('Help before')
+      id = seed_group(name: '更新', id: help.id, members:, placements:)
+      help_before = help.attributes
+
+      expect do
+        put "/tag_groups/#{ id }", params: native_payload(name: '改名'), headers:, as: :json
+      end.to change(WikiPage, :count).by(1)
+          .and change(WikiRevision, :count).by(1)
+          .and change(WikiVersion, :count).by(1)
+
+      expect(response).to have_http_status(:ok)
+      expect(TagGroup.find(id).name).to eq('改名')
+      expect(help.reload.attributes).to eq(help_before)
+    end
+
+    ['wiki_lines', 'wiki_revision_lines'].each do |table|
+      it "rolls back native and compatibility state on #{ table } failure" do
+        expect_atomic_group_write(table) do
+          post '/tag_groups', params: native_payload, headers:, as: :json
+        end
+      end
+    end
+
+    it 'rolls back a renderer failure instead of treating it as unsupported structure' do
+      allow(TagGroupMarkdownRenderer).to receive(:render).and_raise('renderer failure')
+      before_state = group_state
+
+      expect do
+        post '/tag_groups', params: native_payload, headers:, as: :json
+      end.to raise_error(RuntimeError, 'renderer failure')
+
+      expect(group_state).to eq(before_state)
+    end
+
+    it 'preserves unplaced membership and reports the skipped sync with its version' do
+      page = legacy_group('更新', '- 洗澡鹿')
+      id = seed_group(name: '更新', id: page.id, members:, placements:)
+      wiki_before = group_state.slice(*TagGroupRecords::WRITE_TABLES.drop(5))
+      warnings = []
+      allow(Rails.logger).to receive(:warn) { warnings << _1 }
+
+      put "/tag_groups/#{ id }",
+          params: native_payload(member_rows: members + [{ 'tag_id' => other_tag.id }]),
+          headers:, as: :json
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('members').map { _1.fetch('tag').fetch('id') })
+          .to contain_exactly(tag.id, other_tag.id)
+      expect(group_version(id, 2).fetch('members')).to include(
+          { 'tag_id' => other_tag.id, 'version_no' => other_tag.version_no })
+      expect(group_state.slice(*TagGroupRecords::WRITE_TABLES.drop(5))).to eq(wiki_before)
+      payload = JSON.parse(warnings.grep(/tag_group_wiki_sync_skipped/).sole.split(' ', 2).last)
+      expect(payload).to include(
+          'status' => 'unsupported_structure', 'tag_group_id' => id,
+          'version_no' => 2, 'user_id' => user.id,
+          'reason' => '掲載位置のない membership は Markdown にできません．')
+    end
+
+    it 'preserves a list Section description and records why no Wiki was created' do
+      nodes = [{ 'tag_id' => tag.id, 'children' => [
+          { 'section_name' => '区分', 'description' => '構造化の説明', 'children' => [] }] }]
+      warnings = []
+      allow(Rails.logger).to receive(:warn) { warnings << _1 }
+      before_count = WikiPage.count
+
+      post '/tag_groups', params: native_payload(nodes:), headers:, as: :json
+
+      expect(response).to have_http_status(:created)
+      id = json.fetch('id')
+      expect(group_version(id, 1).fetch('placements')).to eq(nodes)
+      expect(json.fetch('placements').sole.fetch('children').sole)
+          .to include('description' => '構造化の説明')
+      expect(WikiPage.count).to eq(before_count)
+      expect(warnings.grep(/tag_group_wiki_sync_skipped/).sole)
+          .to include('リスト内 Section の説明文は Markdown にできません．')
+    end
+  end
+
+  context 'identifier Markdown' do
+    ['[洗澡鹿](https://example.com)', '*洗澡鹿*', '**洗澡鹿**', '`洗澡鹿`',
+     '![洗澡鹿](https://example.com/image.png)',
+     '[タググループ:子](https://example.com)', '*区分*:',
+     '[区分](https://example.com):'].each do |label|
+      [:create, :update].each do |operation|
+        it "rejects decorated #{ label } on #{ operation } without writes" do
+          seed_group(name: '子', description: '子')
+          id = seed_group(name: '更新', members:, placements:)
+          expect_no_writes do
+            if operation == :create
+              post '/wiki', params: { title: 'タググループ:新規', body: "- #{ label }" },
+                   headers: headers
+            else
+              put_wiki(id, title: 'タググループ:更新', body: "- #{ label }")
+            end
+          end
+        end
+      end
+    end
+
+    ['# [区分](https://example.com)', '# *区分*', '# `区分`', '# 区分 {#custom}',
+     '# ![区分](https://example.com/image.png)'].each do |heading|
+      it "rejects a decorated heading #{ heading } without writes" do
+        expect_no_writes do
+          post '/wiki', params: { title: 'タググループ:新規', body: "#{ heading }\n- 洗澡鹿" },
+               headers: headers
+        end
+      end
+    end
+
+    it 'preserves allowed markup in group and heading descriptions' do
+      description = '[参照](https://example.com "リンクの説明") と *強調* と ``a`b``'
+      body = "#{ description }\n\n# 区分\n#{ description }\n\n- 洗澡鹿"
+      post '/wiki', params: { title: 'タググループ:説明構文', body: }, headers: headers
+
+      expect(response).to have_http_status(:created)
+      id = json.fetch('id')
+      version = group_version(id, 1)
+      expect(version.fetch('description')).to eq(description)
+      expect(version.fetch('placements').sole.fetch('description')).to eq(description)
+    end
+  end
+
+  context 'structure audit information' do
+    let!(:child_id) { seed_group(name: '子', description: '子') }
+    let(:members) do
+      [{ 'tag_id' => tag.id, 'version_no' => tag.version_no },
+       { 'tag_group_id' => child_id, 'version_no' => 1 }]
+    end
+    let(:placements) do
+      [{ 'tag_id' => tag.id, 'children' => [
+          { 'tag_group_id' => child_id, 'children' => [] }] }]
+    end
+    let!(:id) { seed_group(name: '更新', members:, placements:) }
+    let!(:editor) { create(:user, :member) }
+    let(:headers) { { 'X-Transfer-Code' => editor.inheritance_code } }
+
+    it 'keeps all membership and placement audit fields on a name-only edit' do
+      before_rows = group_state.slice('tag_group_tags', 'tag_group_groups', 'tag_group_placements')
+      travel_to(1.hour.from_now) do
+        put "/tag_groups/#{ id }", params: native_payload(name: '改名'), headers:, as: :json
+      end
+
+      expect(response).to have_http_status(:ok)
+      expect(group_state.slice(*before_rows.keys)).to eq(before_rows)
+      expect(group_version(id, 2)).to include('name' => '改名', 'members' => members)
+    end
+
+    it 'keeps membership and placement audit fields on a description-only edit' do
+      before_rows = group_state.slice('tag_group_tags', 'tag_group_groups', 'tag_group_placements')
+      travel_to(1.hour.from_now) do
+        put "/tag_groups/#{ id }",
+            params: native_payload.merge(description: '説明を追加'), headers:, as: :json
+      end
+
+      expect(response).to have_http_status(:ok)
+      expect(group_state.slice(*before_rows.keys)).to eq(before_rows)
+      expect(group_version(id, 2)).to include('description' => '説明を追加', 'members' => members)
+    end
+
+    it 'rolls back membership removal and placement edits on a late sync failure' do
+      new_members = [{ 'tag_id' => tag.id }]
+      new_nodes = [{ 'tag_id' => tag.id, 'children' => [] }]
+      expect_atomic_group_write('wiki_versions') do
+        put "/tag_groups/#{ id }",
+            params: native_payload(nodes: new_nodes, member_rows: new_members), headers:, as: :json
+      end
+    end
+
+    it 'changes only added or removed membership and keeps placement creation audit' do
+      tag_row = TagGroupTag.find([id, tag.id]).attributes
+      placement_row = TagGroupPlacement.find([id, 1]).attributes
+      new_members = [{ 'tag_id' => tag.id }, { 'tag_id' => other_tag.id }]
+      new_nodes = [{ 'tag_id' => tag.id, 'children' => [
+          { 'tag_id' => other_tag.id, 'children' => [] }] }]
+      child_row = TagGroupPlacement.find([id, 2]).attributes
+      travel_to(1.hour.from_now) do
+        put "/tag_groups/#{ id }",
+            params: native_payload(nodes: new_nodes, member_rows: new_members), headers:, as: :json
+      end
+
+      expect(response).to have_http_status(:ok)
+      expect(TagGroupTag.find([id, tag.id]).attributes).to eq(tag_row)
+      expect(TagGroupGroup.exists?(tag_group_id: id, child_tag_group_id: child_id)).to be(false)
+      expect(TagGroupTag.find([id, other_tag.id]).created_by_user_id).to eq(editor.id)
+      expect(TagGroupPlacement.find([id, 1]).attributes).to eq(placement_row)
+      changed = TagGroupPlacement.find([id, 2])
+      expect(changed).to have_attributes(
+          tag_id: other_tag.id, child_tag_group_id: nil, updated_by_user_id: editor.id,
+          created_at: child_row.fetch('created_at'),
+          created_by_user_id: child_row.fetch('created_by_user_id'))
+      expect(changed.updated_at).to be > child_row.fetch('updated_at')
+      expect(group_version(id, 1).fetch('members')).to eq(members)
+    end
+  end
+end

backend/spec/requests/tag_groups_spec.rb

@@ -0,0 +1,467 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag group native API', type: :request do
+  include TagGroupRecords
+  include ActiveSupport::Testing::TimeHelpers
+
+  let!(:user) { create_member_user! }
+  let!(:group_id) { seed_group(name: '専用API', description: '説明') }
+
+  before { I18n.locale = :ja }
+
+  def expect_native_group value
+    expect(value).to include(
+        'id' => group_id, 'name' => '専用API', 'description' => '説明',
+        'version_no' => 1, 'members' => [], 'placements' => [],
+        'created_by_user' => UserRepr.base(user), 'updated_by_user' => UserRepr.base(user))
+    current = group_rows('tag_groups', id: group_id).sole
+    expect(value).to include(
+        'created_at' => current.fetch('created_at').iso8601(3),
+        'updated_at' => current.fetch('updated_at').iso8601(3))
+  end
+
+  it 'serves the collection as JSON without reading Wiki tables' do
+    without_group_legacy_reads { get '/tag_groups' }
+    expect(response).to have_http_status(:ok)
+    expect(response.media_type).to eq('application/json')
+    expect(json).to be_a(Array)
+    expect_native_group(json.find { _1.fetch('id') == group_id })
+  end
+
+  it 'serves an independent group as JSON without reading Wiki tables' do
+    without_group_legacy_reads { get "/tag_groups/#{ group_id }" }
+    expect(response).to have_http_status(:ok)
+    expect(response.media_type).to eq('application/json')
+    expect_native_group(json)
+  end
+
+  it 'provides the confirmed PUT update route' do
+    route = Rails.application.routes.recognize_path("/tag_groups/#{ group_id }", method: :put)
+    expect(route).to include(action: 'update', id: group_id.to_s)
+  end
+
+  [:create, :update].each do |operation|
+    [nil, :guest, :banned].each do |role|
+      it "rejects #{ role || 'anonymous' } native #{ operation } without writes" do
+        actor = case role
+        when :guest then create(:user, :guest)
+        when :banned then create(:user, :member, :banned)
+        end
+        headers = actor ? { 'X-Transfer-Code' => actor.inheritance_code } : { }
+        before_state = group_state
+        if operation == :create
+          post '/tag_groups', params: { }, headers:, as: :json
+        else
+          put "/tag_groups/#{ group_id }", params: { }, headers:, as: :json
+        end
+        expect(response).to have_http_status(role ? :forbidden : :unauthorized)
+        expect(group_state).to eq(before_state)
+      end
+    end
+
+    it "rejects a banned IP for native #{ operation }" do
+      IpAddress.create!(ip_address: IPAddr.new('127.0.0.1').hton, banned_at: Time.current)
+      headers = { 'X-Transfer-Code' => user.inheritance_code }
+      before_state = group_state
+      if operation == :create
+        post '/tag_groups', params: { }, headers:, as: :json
+      else
+        put "/tag_groups/#{ group_id }", params: { }, headers:, as: :json
+      end
+      expect(response).to have_http_status(:forbidden)
+      expect(group_state).to eq(before_state)
+    end
+  end
+
+  context 'structured membership and placements' do
+    let!(:tag) { group_tag('掲載タグ') }
+    let!(:unplaced) { group_tag('未掲載タグ') }
+    let!(:child_id) { seed_group(name: '子', description: '子の説明') }
+    let(:headers) { { 'X-Transfer-Code' => user.inheritance_code } }
+    let(:members) do
+      [{ 'tag_id' => tag.id }, { 'tag_group_id' => child_id }, { 'tag_id' => unplaced.id }]
+    end
+    let(:snapshot_members) do
+      [{ 'tag_id' => tag.id, 'version_no' => tag.version_no },
+       { 'tag_group_id' => child_id, 'version_no' => 1 },
+       { 'tag_id' => unplaced.id, 'version_no' => unplaced.version_no }]
+    end
+    let(:placements) do
+      [{ 'section_name' => '区分', 'description' => '区分の説明', 'children' => [
+          { 'tag_id' => tag.id, 'children' => [
+              { 'tag_group_id' => child_id, 'children' => [] }] },
+          { 'tag_id' => tag.id, 'children' => [] }] }]
+    end
+    let(:payload) { { name: '構造化', description: '全体の説明', members:, placements: } }
+
+    def native_write operation, attributes = payload
+      if operation == :create
+        post '/tag_groups', params: attributes, headers:, as: :json
+      else
+        put "/tag_groups/#{ group_id }", params: attributes, headers:, as: :json
+      end
+    end
+
+    def expect_structured_response
+      tag_repr = { 'id' => tag.id, 'name' => '掲載タグ', 'category' => tag.category }
+      unplaced_repr = { 'id' => unplaced.id, 'name' => '未掲載タグ',
+                        'category' => unplaced.category }
+      expect(json.fetch('members')).to contain_exactly(
+          include('tag' => include(tag_repr)),
+          include('tag' => include(unplaced_repr)),
+          include('tag_group' => include('id' => child_id, 'name' => '子')))
+      expect(json.fetch('placements')).to match([
+          include('section_name' => '区分', 'description' => '区分の説明', 'children' => [
+              include('tag' => include(tag_repr), 'children' => [
+                  include('tag_group' => include('id' => child_id, 'name' => '子'),
+                          'children' => [])]),
+              include('tag' => include(tag_repr), 'children' => [])])])
+    end
+
+    [:create, :update].each do |operation|
+      it "#{ operation }s structured data and keeps unplaced membership" do
+        previous = group_version(group_id, 1)
+        native_write(operation)
+        expect(response).to have_http_status(operation == :create ? :created : :ok)
+        id = operation == :create ? json.fetch('id') : group_id
+        number = operation == :create ? 1 : 2
+        expect_group_snapshot(
+            id, number:, members: snapshot_members, placements:, description: '全体の説明')
+        expect(group_version(group_id, 1)).to eq(previous)
+        get "/tag_groups/#{ id }"
+        expect(response).to have_http_status(:ok)
+        expect(json).to include(
+            'id' => id, 'name' => '構造化', 'description' => '全体の説明', 'version_no' => number,
+            'created_by_user' => UserRepr.base(user), 'updated_by_user' => UserRepr.base(user))
+        expect_structured_response
+      end
+
+      {
+          'blank name' => { name: '' },
+          'missing name' => { name: nil },
+          'oversized name' => { name: '長' * 256 },
+          'duplicate name' => { name: '子' },
+          'placement without a kind' => { placements: [{ children: [] }] },
+          'empty Section name' => { placements: [{ section_name: '', children: [] }] } }
+          .each do |label, invalid|
+        it "rejects #{ label } on #{ operation } without partial writes" do
+          before_state = group_state
+          native_write(operation, payload.merge(invalid))
+          aggregate_failures do
+            expect(response).to have_http_status(:unprocessable_entity)
+            expect(group_state).to eq(before_state)
+          end
+        end
+      end
+
+      it "rejects multiple placement kinds on #{ operation }" do
+        before_state = group_state
+        invalid = [{ tag_id: tag.id, section_name: '不正', children: [] }]
+        native_write(operation, payload.merge(placements: invalid))
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(group_state).to eq(before_state)
+      end
+
+      it "rejects a missing referenced Tag on #{ operation }" do
+        missing = Tag.maximum(:id).to_i + 1
+        before_state = group_state
+        native_write(operation, payload.merge(
+            members: [{ tag_id: missing }], placements: [{ tag_id: missing, children: [] }]))
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(group_state).to eq(before_state)
+      end
+
+      it "rejects a missing referenced group on #{ operation }" do
+        missing = group_rows('tag_groups').map { _1.fetch('id') }.max + 1
+        before_state = group_state
+        native_write(operation, payload.merge(
+            members: [{ tag_group_id: missing }],
+            placements: [{ tag_group_id: missing, children: [] }]))
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(group_state).to eq(before_state)
+      end
+
+      it "rejects six descendant placement levels on #{ operation }" do
+        nodes = []
+        6.times { nodes = [{ tag_id: tag.id, children: nodes }] }
+        before_state = group_state
+        native_write(operation, payload.merge(placements: nodes))
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(group_state).to eq(before_state)
+      end
+    end
+
+    it 'preserves membership-only data in structured reads without inventing placements' do
+      id = seed_group(name: '未掲載のみ', members: [snapshot_members.last])
+      without_group_legacy_reads { get "/tag_groups/#{ id }" }
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('placements')).to eq([])
+      expect(json.fetch('members')).to match([
+          include('tag' => include('id' => unplaced.id, 'name' => '未掲載タグ'))])
+    end
+
+    it 'rejects indirect containment cycles' do
+      parent = seed_group(
+          name: '親', members: [{ 'tag_group_id' => group_id, 'version_no' => 1 }])
+      before_state = group_state
+      native_write(:update, payload.merge(
+          members: [{ tag_group_id: parent }], placements: []))
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(group_state).to eq(before_state)
+    end
+
+    it 'rejects direct containment even without a corresponding placement' do
+      before_state = group_state
+      native_write(:update, payload.merge(
+          members: [{ tag_group_id: group_id }], placements: []))
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(group_state).to eq(before_state)
+    end
+
+    it 'preserves list Section descriptions in native structured data' do
+      nodes = [{ tag_id: tag.id, children: [
+          { section_name: 'リスト内区分', description: '消してはいけない説明', children: [] }] }]
+      native_write(:update, payload.merge(placements: nodes))
+      expect(response).to have_http_status(:ok)
+      expect(group_version(group_id, 2).fetch('placements')).to eq(
+          JSON.parse(nodes.to_json))
+      get "/tag_groups/#{ group_id }"
+      expect(response).to have_http_status(:ok)
+      section = json.fetch('placements').sole.fetch('children').sole
+      expect(section).to include(
+          'section_name' => 'リスト内区分', 'description' => '消してはいけない説明')
+    end
+
+    context 'compatibility writes for representable structures' do
+      let(:group_id) do
+        page = legacy_group('専用API', '説明')
+        seed_group(name: '専用API', id: page.id, description: '説明')
+      end
+      let(:members) { [{ 'tag_id' => tag.id }, { 'tag_group_id' => child_id }] }
+      let(:snapshot_members) do
+        [{ 'tag_id' => tag.id, 'version_no' => tag.version_no },
+         { 'tag_group_id' => child_id, 'version_no' => 1 }]
+      end
+
+      it 'treats identical native content with unplaced membership as a no-op' do
+        unplaced_payload = payload.merge(
+            name: '専用API', members: members + [{ 'tag_id' => unplaced.id }])
+        native_write(:update, unplaced_payload)
+        expect(response).to have_http_status(:ok)
+        expect_group_snapshot(
+            group_id, number: 2, description: '全体の説明', placements:,
+            members: snapshot_members + [
+                { 'tag_id' => unplaced.id, 'version_no' => unplaced.version_no }])
+        page = WikiPage.joins(:tag_name).find_by!(tag_names: { name: 'タググループ:専用API' })
+
+        travel_to(1.hour.from_now) do
+          expect_noop_group_update(group_id:, page:) do
+            native_write(:update, unplaced_payload)
+          end
+        end
+      end
+
+      context 'no-op updates against synchronised state' do
+        let(:synchronised_payload) do
+          { name: '構造化', description: '全体の説明',
+            members: [{ 'tag_id' => tag.id }, { 'tag_group_id' => child_id }], placements: }
+        end
+        let(:additional_tag) { group_tag('追加対象タグ') }
+        let(:compatibility_page) do
+          WikiPage.joins(:tag_name).find_by!(tag_names: { name: 'タググループ:構造化' })
+        end
+
+        before do
+          native_write(:update, synchronised_payload)
+          expect(response).to have_http_status(:ok)
+          expect(compatibility_page.body).to include('- 掲載タグ', '- タググループ:子')
+        end
+
+        [1, 3].each do |repetitions|
+          it "keeps identical native content unchanged after #{ repetitions } PUTs" do
+            travel_to(1.hour.from_now) do
+              expect_noop_group_update(group_id:, page: compatibility_page) do
+                repetitions.times do
+                  native_write(:update, synchronised_payload)
+                  expect(response).to have_http_status(:ok)
+                end
+              end
+            end
+          end
+        end
+
+        it 'treats membership order alone as a no-op' do
+          travel_to(1.hour.from_now) do
+            expect_noop_group_update(group_id:, page: compatibility_page) do
+              native_write(:update, synchronised_payload.merge(
+                  members: synchronised_payload.fetch(:members).reverse))
+            end
+          end
+        end
+
+        it 'keeps the original editor when another member submits identical content' do
+          editor = create(:user, :member)
+          travel_to(1.hour.from_now) do
+            expect_noop_group_update(group_id:, page: compatibility_page) do
+              put "/tag_groups/#{ group_id }", params: synchronised_payload,
+                  headers: { 'X-Transfer-Code' => editor.inheritance_code }, as: :json
+            end
+          end
+        end
+
+        it 'does not add history for Native content already changed through Wiki' do
+          put "/wiki/#{ group_id }",
+              params: { title: 'タググループ:構造化', body: '- 未掲載タグ' }, headers: headers
+          expect(response).to have_http_status(:ok)
+          expect(compatibility_page.reload.body).to eq('- 未掲載タグ')
+          same_content = {
+              name: '構造化', description: nil, members: [{ 'tag_id' => unplaced.id }],
+              placements: [{ 'tag_id' => unplaced.id, 'children' => [] }] }
+          expect_group_snapshot(
+              group_id, number: 3, description: nil,
+              members: [{ 'tag_id' => unplaced.id, 'version_no' => unplaced.version_no }],
+              placements: same_content.fetch(:placements))
+
+          travel_to(1.hour.from_now) do
+            expect_noop_group_update(group_id:, page: compatibility_page) do
+              native_write(:update, same_content)
+            end
+          end
+        end
+
+        ['name', 'description', 'membership addition', 'membership removal',
+         'Section name', 'Section description', 'placement order', 'placement hierarchy']
+            .each do |change|
+          it "records a real #{ change } change in both histories" do
+            changed = synchronised_payload.deep_dup
+            section = changed.fetch(:placements).first
+            children = section.fetch('children')
+            case change
+            when 'name' then changed[:name] = '実際の改名'
+            when 'description' then changed[:description] = '実際の説明変更'
+            when 'membership addition'
+              expect(changed.fetch(:members)).not_to include('tag_id' => additional_tag.id)
+              changed[:members] << { 'tag_id' => additional_tag.id }
+              children << { 'tag_id' => additional_tag.id, 'children' => [] }
+            when 'membership removal'
+              changed[:members].reject! { _1.key?('tag_group_id') }
+              children.first['children'] = []
+            when 'Section name' then section['section_name'] = '別区分'
+            when 'Section description' then section['description'] = '別の区分説明'
+            when 'placement order' then children.reverse!
+            when 'placement hierarchy'
+              children << children.first.fetch('children').pop
+            end
+
+            old_versions = group_rows('tag_group_versions', tag_group_id: group_id)
+            group = TagGroup.find(group_id)
+            number = group.version_no
+            page = compatibility_page
+            old_revision_id = page.current_revision.id
+            old_wiki_version_id = page.wiki_versions.order(version_no: :desc).first.id
+            revision_count = page.wiki_revisions.count
+            version_count = page.wiki_versions.count
+            travel_to(1.hour.from_now) { native_write(:update, changed) }
+
+            expect(response).to have_http_status(:ok)
+            expected_members = changed.fetch(:members).map do |member|
+              target = if member.key?('tag_id')
+                Tag.find(member.fetch('tag_id'))
+              else
+                TagGroup.find(member.fetch('tag_group_id'))
+              end
+              member.merge('version_no' => target.version_no)
+            end
+            expect_group_snapshot(
+                group_id, number: number + 1, members: expected_members,
+                placements: changed.fetch(:placements), description: changed.fetch(:description))
+            expect(group.reload.name).to eq(changed.fetch(:name))
+            versions = group_rows('tag_group_versions', tag_group_id: group_id)
+            expect(versions.size).to eq(old_versions.size + 1)
+            expect(versions.select { _1.fetch('version_no') <= number }).to eq(old_versions)
+            expect(page.wiki_revisions.count).to eq(revision_count + 1)
+            expect(page.wiki_versions.count).to eq(version_count + 1)
+            expect(page.reload.current_revision.id).not_to eq(old_revision_id)
+            expect(page.wiki_versions.order(version_no: :desc).first.id)
+                .not_to eq(old_wiki_version_id)
+            get group_title_path(changed.fetch(:name))
+            expect(response).to have_http_status(:ok)
+            expect(json.fetch('body')).to eq(page.body)
+          end
+        end
+      end
+
+      it 'creates native state and its compatibility histories' do
+        expect { native_write(:create) }
+            .to change(WikiPage, :count).by(1)
+            .and change(WikiRevision, :count).by(1)
+            .and change(WikiVersion, :count).by(1)
+        expect(response).to have_http_status(:created)
+        expect_group_snapshot(
+            json.fetch('id'), number: 1, members: snapshot_members,
+            placements:, description: '全体の説明')
+      end
+
+      [:create, :update].each do |operation|
+        it "synchronises Wiki current state and histories on native #{ operation }" do
+          native_write(operation)
+          expect(response).to have_http_status(operation == :create ? :created : :ok)
+          id = operation == :create ? json.fetch('id') : group_id
+          number = operation == :create ? 1 : 2
+          expect_group_snapshot(
+              id, number:, members: snapshot_members, placements:, description: '全体の説明')
+          page = WikiPage.joins(:tag_name).find_by!(tag_names: { name: 'タググループ:構造化' })
+          expect(page.body).to include('全体の説明', '# 区分', '- 掲載タグ', '- タググループ:子')
+          expect(page.current_revision.body).to eq(page.body)
+          expect(page.wiki_versions.order(:version_no).last.body).to eq(page.body)
+          expect(page.wiki_revisions.count).to eq(number)
+          expect(page.wiki_versions.count).to eq(number)
+          get group_title_path('構造化')
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('body')).to eq(page.body)
+        end
+
+        (TagGroupRecords::GROUP_TABLES + ['wiki_pages', 'wiki_revisions', 'wiki_versions'])
+            .each do |table|
+          it "rolls back native #{ operation } after #{ table } writes" do
+            expect_atomic_group_write(table) { native_write(operation) }
+          end
+        end
+      end
+
+      it 'assigns the next group ID from the existing maximum' do
+        child_id
+        expected_id = [TagGroup.maximum(:id).to_i, TagDescription.maximum(:id).to_i].max + 1
+        native_write(:create)
+        expect(response).to have_http_status(:created)
+        expect(json.fetch('id')).to eq(expected_id)
+      end
+
+      TagGroupRecords::GROUP_TABLES.each do |table|
+        it "rolls back native creation after #{ table } writes" do
+          expect_atomic_group_write(table) { native_write(:create) }
+        end
+      end
+
+      it 'updates a native group through Wiki and then reads structured current data' do
+        native_write(:create)
+        expect(response).to have_http_status(:created)
+        id = json.fetch('id')
+        previous = group_version(id, 1)
+        put "/wiki/#{ id }",
+            params: { title: 'タググループ:構造化', body: '- 未掲載タグ' }, headers: headers
+        expect(response).to have_http_status(:ok)
+        expect_group_snapshot(
+            id, number: 2,
+            members: [{ 'tag_id' => unplaced.id, 'version_no' => unplaced.version_no }],
+            placements: [{ 'tag_id' => unplaced.id, 'children' => [] }])
+        expect(group_version(id, 1)).to eq(previous)
+        get "/tag_groups/#{ id }"
+        expect(response).to have_http_status(:ok)
+        expect(json).to include('version_no' => 2)
+        expect(json.fetch('members')).to match([
+            include('tag' => include('id' => unplaced.id, 'name' => '未掲載タグ'))])
+      end
+    end
+  end
+end

backend/spec/requests/tag_resource_ids_spec.rb

@@ -0,0 +1,101 @@
+require 'rails_helper'
+
+RSpec.describe 'Tag resource ID allocation', type: :request do
+  include TagGroupRecords
+  include WikiRecords
+
+  let!(:user) { create_member_user! }
+  let(:headers) { { 'X-Transfer-Code' => user.inheritance_code } }
+
+  def seed_description id
+    tag = create(:tag, primary_name: "description_id_#{ id }")
+    TagDescription.create!(id:, tag:, language_code: 'ja', body: 'existing', version_no: 1)
+  end
+
+  def create_resource resource
+    if resource == :description
+      tag = create(:tag, primary_name: 'new_description')
+      post '/wiki', params: { title: tag.name('ja'), body: 'new description' }, headers: headers
+    else
+      post '/tag_groups', params: {
+          name: 'new_group', description: 'new group', members: [], placements: [] },
+          headers:, as: :json
+    end
+    expect(response).to have_http_status(:created)
+  end
+
+  [:description, :group].each do |resource|
+    it "keeps #{ resource } independent of the Tag and ExternalTag ID space" do
+      create(:tag, id: 100)
+      create(:external_tag, id: 200)
+      seed_description(1)
+      seed_group(name: 'shared_space', id: 2)
+
+      create_resource(resource)
+
+      expect(json.fetch('id')).to eq(3)
+    end
+
+    it "uses the other resource's larger maximum when allocating #{ resource }" do
+      seed_description(resource == :description ? 1 : 20)
+      seed_group(name: 'other_space', id: resource == :group ? 1 : 20)
+      create_resource(resource)
+      expect(json.fetch('id')).to eq(21)
+    end
+
+    it "starts #{ resource } at 1 even after previous allocated rows were removed" do
+      expect(TagDescription.count).to eq(0)
+      expect(TagGroup.count).to eq(0)
+      if resource == :description
+        seed_description(100).destroy!
+      else
+        id = seed_group(name: 'removed', id: 100)
+        TagGroup.find(id).destroy!
+      end
+      create_resource(resource)
+      expect(json.fetch('id')).to eq(1)
+    end
+
+    it "allocates #{ resource } above both maxima, preserving legacy overlaps and gaps" do
+      [1, 2, 5].each { seed_description(_1) }
+      [1, 3, 8].each { seed_group(name: "group_#{ _1 }", id: _1) }
+      descriptions = TagDescription.order(:id).map(&:attributes)
+      groups = TagGroup.order(:id).map(&:attributes)
+
+      create_resource(resource)
+
+      expect(json.fetch('id')).to eq(9)
+      expect(TagDescription.where(id: [1, 2, 5]).order(:id).map(&:attributes)).to eq(descriptions)
+      expect(TagGroup.where(id: [1, 3, 8]).order(:id).map(&:attributes)).to eq(groups)
+      expect(TagDescription.exists?(id: 3)).to be(false)
+      expect(TagGroup.exists?(id: 2)).to be(false)
+    end
+  end
+
+  it 'preserves group ID and unrelated rows on update' do
+    id = seed_group(name: 'existing', id: 5, description: 'before')
+    other_id = seed_group(name: 'other', id: 8, description: 'other')
+    other = TagGroup.find(other_id).attributes
+    put "/tag_groups/#{ id }", params: {
+        name: 'renamed', description: 'after', members: [], placements: [] }, headers:, as: :json
+    expect(response).to have_http_status(:ok)
+    expect(json.fetch('id')).to eq(id)
+    expect(TagGroup.find(id).description).to eq('after')
+    expect(TagGroup.find(other_id).attributes).to eq(other)
+    expect(TagGroup.count).to eq(2)
+  end
+
+  it 'preserves description identity on a compatibility update' do
+    tag = create(:tag, primary_name: 'existing_description')
+    page = seed_wiki_page(tag_name: tag.tag_name('ja'), body: 'before', created_by_user: user)
+    description = TagDescription.find_by!(tag_id: tag.id, language_code: 'ja')
+    id = description.id
+    put "/wiki/#{ id }", params: {
+        title: tag.name('ja'), body: 'after', base_revision_id: page.current_revision.id },
+        headers: headers
+    expect(response).to have_http_status(:ok)
+    expect(description.reload.id).to eq(id)
+    expect(description.body).to eq('after')
+    expect(TagDescription.count).to eq(1)
+  end
+end

backend/spec/requests/tag_wiki_history_integrity_spec.rb

@@ -1,6 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe 'Tag and wiki history integrity', type: :request do
+  include WikiRecords
   let(:member_user) { create(:user, role: 'member') }
 
   def stub_current_user user
@@ -13,7 +14,7 @@ RSpec.describe 'Tag and wiki history integrity', type: :request do
   end
 
   def create_wiki_for_tag! tag:, body: 'wiki body', user: member_user
-    Wiki::Commit.create_content!(
+    seed_wiki_page(
         tag_name: primary_tag_name_for(tag, 'ja'),
         body:,
         created_by_user: user,

backend/spec/requests/tags_spec.rb

@@ -38,6 +38,22 @@ RSpec.describe 'Tags API', type: :request do
   end
 
   describe 'GET /tags' do
+    it 'serialises descriptions and parents without loading legacy Wiki pages' do
+      TagDescription.create!(tag:, language_code: 'ja', body: '', version_no: 1)
+      TagDescription.create!(tag: tag2, language_code: 'ja', body: '', version_no: 1)
+      TagImplication.create!(tag:, parent_tag: tag2)
+      queries = []
+      subscriber = ->(*args) { queries << args.last.fetch(:sql) }
+      ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') do
+        get '/tags', params: { name: 'spec_tag' }
+      end
+      expect(response).to have_http_status(:ok)
+      row = response_tags.sole
+      expect(row.fetch('has_wiki')).to be(true)
+      expect(row.fetch('parents').sole.fetch('has_wiki')).to be(true)
+      expect(queries.grep(/\bSELECT\b.*\bwiki_(pages|revisions|versions)\b/im)).to be_empty
+    end
+
     it 'includes legacy external JSON alongside an internal tag with the same id' do
       external = create(:external_tag, id: tag.id, name: 'spec_external', post_count: 3)
 

backend/spec/requests/wiki_conflict_spec.rb

@@ -1,6 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe 'Wiki conflict handling', type: :request do
+  include WikiRecords
   let!(:user) { create_member_user! }
 
   def auth_headers user
@@ -9,8 +10,8 @@ RSpec.describe 'Wiki conflict handling', type: :request do
 
   it 'returns 409 when base_revision_id is stale' do
     page =
-        Wiki::Commit.create_content!(
-            tag_name: create(:tag_name, name: 'wiki_conflict_request'),
+        seed_wiki_page(
+            tag_name: create(:tag, primary_name: 'wiki_conflict_request').tag_name('ja'),
             body: 'first',
             created_by_user: user,
             message: 'init')
@@ -25,12 +26,10 @@ RSpec.describe 'Wiki conflict handling', type: :request do
         base_revision_id: stale_id)
 
     put "/wiki/#{ page.id }",
-        params: {
-          title: 'wiki_conflict_request',
+        params: { title: 'wiki_conflict_request',
                   body: 'third',
                   message: 'stale',
-          base_revision_id: stale_id,
-        },
+                  base_revision_id: stale_id },
         headers: auth_headers(user)
 
     expect(response).to have_http_status(:conflict)

backend/spec/requests/wiki_creation_freeze_spec.rb

@@ -0,0 +1,63 @@
+require 'rails_helper'
+
+RSpec.describe 'Wiki creation freeze', type: :request do
+  include TagGroupRecords
+  include WikiRecords
+
+  let!(:user) { create_member_user! }
+  let(:headers) { { 'X-Transfer-Code' => user.inheritance_code } }
+
+  it 'creates missing compatibility history when updating an independent group' do
+    id = seed_group(name: 'independent', description: 'before')
+    expect do
+      put "/tag_groups/#{ id }", params: {
+          name: 'independent', description: 'after', members: [], placements: [] },
+          headers:, as: :json
+    end.to change(WikiPage, :count).by(1)
+        .and change(WikiRevision, :count).by(1)
+        .and change(WikiVersion, :count).by(1)
+    expect(response).to have_http_status(:ok)
+    expect(TagGroup.find(id).description).to eq('after')
+    page = WikiPage.joins(:tag_name).find_by!(tag_names: { name: 'タググループ:independent' })
+    expect(page.body).to include('after')
+    expect(page.current_revision.body).to eq(page.body)
+    expect(page.wiki_versions.sole.body).to eq(page.body)
+  end
+
+  context 'existing description write protection' do
+    let!(:tag) { create(:tag, primary_name: 'protected_description') }
+    let!(:page) do
+      seed_wiki_page(tag_name: tag.tag_name('ja'), body: 'before', created_by_user: user)
+    end
+
+    [nil, :guest, :banned].each do |role|
+      it "rejects #{ role || 'anonymous' } without changing either history" do
+        actor = role && create(:user, role == :banned ? :member : role)
+        actor.update!(banned_at: Time.current) if role == :banned
+        request_headers = actor ? { 'X-Transfer-Code' => actor.inheritance_code } : { }
+        before_state = group_state
+        put "/wiki/#{ page.id }", params: { title: tag.name('ja'), body: 'after' },
+            headers: request_headers
+        expect(response).to have_http_status(role ? :forbidden : :unauthorized)
+        expect(group_state).to eq(before_state)
+      end
+    end
+
+    it 'rejects banned IPs without writes' do
+      IpAddress.create!(ip_address: IPAddr.new('127.0.0.1').hton, banned_at: Time.current)
+      before_state = group_state
+      put "/wiki/#{ page.id }", params: { title: tag.name('ja'), body: 'after' }, headers: headers
+      expect(response).to have_http_status(:forbidden)
+      expect(group_state).to eq(before_state)
+    end
+
+    ['', "\r\n\r\n"].each do |body|
+      it "rejects invalid description body #{ body.inspect } atomically" do
+        before_state = group_state
+        put "/wiki/#{ page.id }", params: { title: tag.name('ja'), body: }, headers: headers
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(group_state).to eq(before_state)
+      end
+    end
+  end
+end

backend/spec/requests/wiki_history_integrity_spec.rb

@@ -2,6 +2,7 @@ require 'cgi'
 require 'rails_helper'
 
 RSpec.describe 'Wiki history integrity', type: :request do
+  include WikiRecords
   let!(:user) { create_member_user! }
 
   def auth_headers user
@@ -9,49 +10,22 @@ RSpec.describe 'Wiki history integrity', type: :request do
   end
 
   def create_wiki_page title:, body: 'body', message: 'init', user: self.user
-    Wiki::Commit.create_content!(
-        tag_name: create(:tag_name, name: title),
+    seed_wiki_page(
+        tag_name: create(:tag, primary_name: title).tag_name('ja'),
         body:,
         created_by_user: user,
         message:)
   end
 
   describe 'POST /wiki' do
-    it 'creates wiki_page, wiki_revision, and wiki_version atomically' do
-      expect {
-        post '/wiki',
-             params: {
-               title: 'Help:wiki_history_create_atomic',
-               body: "a\nb\nc",
-               message: 'initial commit',
-             },
+    it 'rejects frozen creation without partial page or history' do
+      expect do
+        post '/wiki', params: { title: 'Help:wiki_history_create_atomic', body: 'initial' },
              headers: auth_headers(user)
-      }
-          .to change(WikiPage, :count).by(1)
-          .and change(WikiRevision, :count).by(1)
-          .and change(WikiVersion, :count).by(1)
-
-      expect(response).to have_http_status(:created)
-
-      page = WikiPage.find(json.fetch('id'))
-      revision = page.current_revision
-      version = page.wiki_versions.order(:version_no).last
-
-      expect(page.title).to eq('Help:wiki_history_create_atomic')
-      expect(page.body).to eq("a\nb\nc")
-
-      expect(revision).to be_content
-      expect(revision.message).to eq('initial commit')
-      expect(revision.lines_count).to eq(3)
-
-      expect(version).to have_attributes(
-        version_no: 1,
-        event_type: 'create',
-        title: 'Help:wiki_history_create_atomic',
-        body: "a\nb\nc",
-        reason: 'initial commit',
-        created_by_user_id: user.id
-      )
+      end.to change(WikiPage, :count).by(0)
+          .and change(WikiRevision, :count).by(0)
+          .and change(WikiVersion, :count).by(0)
+      expect(response.status).to be_between(400, 499)
     end
 
     it 'returns 422 and creates nothing when normalised body is blank' do
@@ -71,21 +45,19 @@ RSpec.describe 'Wiki history integrity', type: :request do
           .where(tag_names: { name: 'Help:wiki_history_blank_body' })).not_to exist
     end
 
-    it 'returns 422 and creates no partial page when title already exists' do
+    it 'rejects frozen creation without modifying an existing page or history' do
       create_wiki_page(title: 'Help:wiki_history_duplicate_title', body: 'first')
 
-      expect {
+      expect do
         post '/wiki',
-             params: {
-               title: 'Help:wiki_history_duplicate_title',
-               body: 'second',
-               message: 'duplicate',
-             },
+             params: { title: 'Help:wiki_history_duplicate_title',
+                       body: 'second', message: 'duplicate' },
              headers: auth_headers(user)
-      }
-          .not_to change(WikiPage, :count)
+      end.to change(WikiPage, :count).by(0)
+          .and change(WikiRevision, :count).by(0)
+          .and change(WikiVersion, :count).by(0)
 
-      expect(response).to have_http_status(:unprocessable_entity)
+      expect(response.status).to be_between(400, 499)
       expect(WikiPage.joins(:tag_name)
           .where(tag_names: { name: 'Help:wiki_history_duplicate_title' }).count).to eq(1)
     end

backend/spec/requests/wiki_index_order_spec.rb

@@ -0,0 +1,42 @@
+require 'rails_helper'
+
+RSpec.describe 'Wiki resource ordering', type: :request do
+  include TagGroupRecords
+  let(:user) { create_member_user! }
+
+  it 'selects the latest twenty across both resources before applying the global limit' do
+    start = Time.utc(2026, 1, 1)
+    descriptions = 24.times.map do |index|
+      tag = create(:tag, primary_name: "rank_#{ index }")
+      TagDescription.create!(tag:, language_code: 'ja', body: 'current', version_no: 1,
+                             updated_at: start + index.hours)
+    end
+    groups = [24, 25].map do |hour|
+      group = TagGroup.find(seed_group(name: "rank_group_#{ hour }"))
+      group.update_columns(updated_at: start + hour.hours)
+      group
+    end
+
+    get '/wiki', params: { title: 'rank' }
+
+    expect(response).to have_http_status(:ok)
+    expected = groups.reverse.map { ['tag_group', _1.id] } +
+        descriptions.last(18).reverse.map { ['tag_description', _1.id] }
+    expect(json.map { [_1.fetch('resource_type'), _1.fetch('id')] }).to eq(expected)
+  end
+
+  it 'breaks timestamp ties by resource kind and ID while retaining legacy overlaps' do
+    time = Time.utc(2026, 1, 1)
+    tag = create(:tag, primary_name: 'tied')
+    TagDescription.create!(id: 51, tag:, language_code: 'ja', body: 'current',
+                           version_no: 1, updated_at: time)
+    group = TagGroup.find(seed_group(name: 'tied', id: 51))
+    group.update_columns(updated_at: time)
+
+    2.times do
+      get '/wiki', params: { title: 'tied' }
+      expect(json.map { [_1.fetch('resource_type'), _1.fetch('id')] })
+          .to eq([['tag_description', 51], ['tag_group', 51]])
+    end
+  end
+end

backend/spec/requests/wiki_spec.rb

@@ -4,6 +4,7 @@ require 'securerandom'
 
 
 RSpec.describe 'Wiki API', type: :request do
+  include WikiRecords
   def auth_headers(user)
     { 'X-Transfer-Code' => user.inheritance_code }
   end
@@ -18,7 +19,7 @@ RSpec.describe 'Wiki API', type: :request do
         deprecated_at: Time.zone.local(2026, 6, 1))
   end
   let!(:page) do
-    Wiki::Commit.create_content!(
+    seed_wiki_page(
         tag_name: tn,
         body: 'init',
         created_by_user: user,
@@ -108,94 +109,14 @@ RSpec.describe 'Wiki API', type: :request do
       end
     end
 
-    context 'when success' do
-      it 'creates wiki_page and first content revision' do
+    ['Help:TestPage', 'ヘルプ:ホーム'].each do |title|
+      it "rejects frozen creation of #{ title } without history writes" do
         expect do
-          post endpoint, params: { title: 'Help:TestPage', body: "a\nb\nc", message: 'init' },
-                         headers: auth_headers(member)
-        end
-            .to change(WikiPage, :count).by(1)
-            .and change(WikiRevision, :count).by(1)
-            .and change(WikiVersion, :count).by(1)
-
-        expect(response).to have_http_status(:created)
-
-        page_id = json.fetch('id')
-        expect(json.fetch('title')).to eq('Help:TestPage')
-
-        created_page = WikiPage.find(page_id)
-        version = created_page.wiki_versions.order(:version_no).last
-
-        expect(version).to have_attributes(
-          version_no: 1,
-          event_type: 'create',
-          title: 'Help:TestPage',
-          body: "a\nb\nc",
-          created_by_user_id: member.id
-        )
-
-        rev = created_page.current_revision
-        expect(rev).to be_present
-        expect(rev).to be_content
-        expect(rev.message).to eq('init')
-
-        expect(created_page.body).to eq("a\nb\nc")
-
-        expect(rev.lines_count).to eq(3)
-        expect(rev.wiki_revision_lines.order(:position).pluck(:position)).to eq([0, 1, 2])
-        expect(rev.wiki_lines.pluck(:body)).to match_array(['a', 'b', 'c'])
-      end
-
-      it 'reuses existing WikiLine rows by sha256' do
-        # 先に同じ行を作っておく
-        WikiLine.create!(sha256: Digest::SHA256.hexdigest('a'), body: 'a', created_at: Time.current, updated_at: Time.current)
-
-        post endpoint,
-             params: { title: 'Help:Reuse', body: "a\na" },
-             headers: auth_headers(member)
-
-        page = WikiPage.find(JSON.parse(response.body).fetch('id'))
-        rev = page.current_revision
-        expect(rev.lines_count).to eq(2)
-
-        # "a" の WikiLine が増殖しない（1行のはず）
-        expect(WikiLine.where(body: 'a').count).to eq(1)
-      end
-
-      it 'deduplicates duplicated new lines before upsert' do
-        duplicated = 'duplicated_line_for_wiki_line_upsert_spec'
-
-        post endpoint,
-             params: { title: 'Help:DuplicateNewLine', body: "#{ duplicated }\n#{ duplicated }" },
-             headers: auth_headers(member)
-
-        expect(response).to have_http_status(:created)
-
-        page = WikiPage.find(json.fetch('id'))
-        rev = page.current_revision
-
-        expect(rev.lines_count).to eq(2)
-        expect(WikiLine.where(body: duplicated).count).to eq(1)
-        expect(rev.wiki_revision_lines.count).to eq(2)
-        expect(rev.wiki_revision_lines.pluck(:wiki_line_id).uniq.size).to eq(1)
-      end
-
-      it 'normalises CRLF and strips trailing newlines' do
-        post endpoint,
-             params: { title: 'Help:NormalisedBody',
-                       body: "a\r\nb\r\n\r\n", message: 'normalise' },
-             headers: auth_headers(member)
-
-        expect(response).to have_http_status(:created)
-
-        page = WikiPage.find(json.fetch('id'))
-        rev = page.current_revision
-        version = page.wiki_versions.order(:version_no).last
-
-        expect(page.body).to eq("a\nb")
-        expect(version.body).to eq("a\nb")
-        expect(rev.lines_count).to eq(2)
-        expect(rev.wiki_lines.order('wiki_revision_lines.position').map(&:body)).to eq(['a', 'b'])
+          post endpoint, params: { title:, body: 'initial' }, headers: auth_headers(member)
+        end.to change(WikiPage, :count).by(0)
+            .and change(WikiRevision, :count).by(0)
+            .and change(WikiVersion, :count).by(0)
+        expect(response.status).to be_between(400, 499)
       end
     end
   end
@@ -208,10 +129,10 @@ RSpec.describe 'Wiki API', type: :request do
       { 'X-Transfer-Code' => user.inheritance_code }
     end
 
-    let!(:test_tag_name) { create(:tag_name, name: 'TestPage') }
+    let!(:test_tag_name) { create(:tag, primary_name: 'TestPage').tag_name('ja') }
 
     let!(:page) do
-      Wiki::Commit.create_content!(
+      seed_wiki_page(
           tag_name: test_tag_name,
           body: "a\nb",
           created_by_user: member,
@@ -249,7 +170,8 @@ RSpec.describe 'Wiki API', type: :request do
 
         expect do
           put "/wiki/#{page.id}",
-              params: { title: 'TestPage', body: "x\ny", message: 'edit', base_revision_id: current_id },
+              params: { title: 'TestPage', body: "x\ny", message: 'edit',
+                        base_revision_id: current_id },
               headers: auth_headers(member)
         end
             .to change(WikiRevision, :count).by(1)
@@ -284,7 +206,7 @@ RSpec.describe 'Wiki API', type: :request do
             created_by_user: member)
 
         linked_page =
-            Wiki::Commit.create_content!(
+            seed_wiki_page(
                 tag_name: linked_tag_name,
                 body: 'before',
                 created_by_user: member,
@@ -358,13 +280,14 @@ RSpec.describe 'Wiki API', type: :request do
 
   describe 'GET /wiki/search' do
     before do
-      Wiki::Commit.create_content!(
-          tag_name: create(:tag_name, name: 'spec_wiki_title_2'),
+      search_tag = create(:tag, primary_name: 'spec_wiki_title_2')
+      seed_wiki_page(
+          tag_name: search_tag.tag_name('ja'),
           body: 'search body 2',
           created_by_user: user,
           message: 'init')
 
-      Wiki::Commit.create_content!(
+      seed_wiki_page(
           tag_name: create(:tag_name, name: 'unrelated_title'),
           body: 'unrelated body',
           created_by_user: user,
@@ -516,7 +439,7 @@ RSpec.describe 'Wiki API', type: :request do
     it 'raises because redirect revisions are deprecated' do
       target_tag_name = create(:tag_name, name: 'redirect_deprecated_target')
       target =
-          Wiki::Commit.create_content!(
+          seed_wiki_page(
               tag_name: target_tag_name,
               body: 'target',
               created_by_user: user,
@@ -539,7 +462,7 @@ RSpec.describe 'Wiki API', type: :request do
     linked_tag = create(:tag, primary_tag_name: linked_tag_name, category: :general)
 
     linked_page =
-        Wiki::Commit.create_content!(
+        seed_wiki_page(
             tag_name: linked_tag_name,
             body: 'before',
             created_by_user: user,

backend/spec/requests/wiki_title_collision_spec.rb

@@ -1,6 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe 'Wiki title collision', type: :request do
+  include WikiRecords
   let!(:user) { create_member_user! }
 
   def auth_headers user
@@ -8,7 +9,7 @@ RSpec.describe 'Wiki title collision', type: :request do
   end
 
   def create_wiki_page title:, body:
-    Wiki::Commit.create_content!(
+    seed_wiki_page(
         tag_name: create(:tag_name, name: title),
         body:,
         created_by_user: user,
@@ -43,20 +44,18 @@ RSpec.describe 'Wiki title collision', type: :request do
     expect(source.wiki_versions.count).to eq(source_version_count)
   end
 
-  it 'returns 422 when creating wiki with existing title' do
+  it 'rejects frozen creation with an existing title without adding history' do
     create_wiki_page(title: 'Help:wiki_collision_create', body: 'already exists')
 
-    expect {
+    expect do
       post '/wiki',
-           params: {
-             title: 'Help:wiki_collision_create',
-             body: 'new body',
-             message: 'duplicate create',
-           },
+           params: { title: 'Help:wiki_collision_create', body: 'new body',
+                     message: 'duplicate create' },
            headers: auth_headers(user)
-    }
-        .not_to change(WikiPage, :count)
+    end.to change(WikiPage, :count).by(0)
+        .and change(WikiRevision, :count).by(0)
+        .and change(WikiVersion, :count).by(0)
 
-    expect(response).to have_http_status(:unprocessable_entity)
+    expect(response.status).to be_between(400, 499)
   end
 end

backend/spec/requests/wiki_unmigrated_contract_spec.rb

@@ -1,115 +1,78 @@
 require 'rails_helper'
 
 RSpec.describe 'Unmigrated Wiki compatibility', type: :request do
+  include WikiRecords
   let!(:user) { create_member_user! }
   let!(:tag_name) { create(:tag_name, name: 'Help_contract') }
   let!(:page) do
-    Wiki::Commit.create_content!(tag_name:, body: 'before', created_by_user: user)
+    seed_wiki_page(tag_name:, body: 'before', created_by_user: user)
   end
   let!(:first_revision) { page.current_revision }
   let!(:last_revision) do
-    Wiki::Commit.content!(page:, body: 'after', created_user: user,
+    Wiki::Commit.legacy_content!(page:, body: 'after', created_user: user,
                                  message: 'legacy edit', base_revision_id: first_revision.id)
   end
 
   before { expect(TagDescription.exists?(id: page.id)).to be(false) }
 
-  it 'keeps a Tag-linked legacy page in the legacy domain when its title changes' do
+  it 'rejects standalone updates even after a Tag is linked to the legacy page' do
     create(:tag, primary_tag_name: tag_name)
+    original = page.reload.attributes
 
     expect do
       put "/wiki/#{ page.id }",
           params: { title: 'legacy_tag_renamed', body: 'changed legacy body',
                     message: 'legacy rename', base_revision_id: last_revision.id },
           headers: { 'X-Transfer-Code' => user.inheritance_code }
-    end.to change(WikiRevision, :count).by(1)
-        .and change(WikiVersion, :count).by(1)
+    end.to change(WikiRevision, :count).by(0)
+        .and change(WikiVersion, :count).by(0)
         .and change(TagDescription, :count).by(0)
         .and change(TagDescriptionVersion, :count).by(0)
 
-    expect(response).to have_http_status(:ok)
-    expect(page.reload).to have_attributes(body: 'changed legacy body')
-    expect(page.title).to eq('legacy_tag_renamed')
-    expect(page.current_revision).to have_attributes(message: 'legacy rename')
+    expect(response).to have_http_status(:unprocessable_entity)
+    expect(page.reload.attributes).to eq(original)
+    expect(page.current_revision).to have_attributes(message: 'legacy edit')
   end
 
-  it 'shows legacy current and historical content by ID' do
-    get "/wiki/#{ page.id }"
-    expect(response).to have_http_status(:ok)
-    expect(json).to include('body' => 'after', 'revision_id' => last_revision.id,
-                           'pred' => first_revision.id, 'succ' => nil)
-    get "/wiki/#{ page.id }", params: { version: first_revision.id }
-    expect(response).to have_http_status(:ok)
-    expect(json).to include('body' => 'before', 'revision_id' => first_revision.id,
-                           'pred' => nil, 'succ' => last_revision.id)
-  end
+  include TagGroupRecords
 
-  it 'shows legacy current and historical content by title' do
-    get "/wiki/title/#{ tag_name.name }"
-    expect(response).to have_http_status(:ok)
-    expect(json).to include('body' => 'after', 'revision_id' => last_revision.id)
-    get "/wiki/title/#{ tag_name.name }", params: { version: first_revision.id }
-    expect(response).to have_http_status(:ok)
-    expect(json).to include('body' => 'before', 'revision_id' => first_revision.id)
-  end
-
-  it 'recognises legacy IDs' do
-    get "/wiki/#{ page.id }/exists"
-    expect(response).to have_http_status(:no_content)
-  end
-
-  it 'recognises legacy titles' do
-    get "/wiki/title/#{ tag_name.name }/exists"
-    expect(response).to have_http_status(:no_content)
-  end
-
-  it 'diffs legacy revisions with explicit and current target' do
-    [last_revision.id, nil].each do |to|
-      get "/wiki/#{ page.id }/diff", params: { from: first_revision.id, to: }
-      expect(response).to have_http_status(:ok)
-      expect(json).to include(
-          'wiki_page_id' => page.id, 'title' => tag_name.name,
-          'older_revision_id' => first_revision.id, 'newer_revision_id' => last_revision.id,
-          'diff' => [{ 'type' => 'removed', 'content' => 'before' },
-                     { 'type' => 'added', 'content' => 'after' }])
-    end
-  end
-
-  describe 'revision resource scope' do
-    let!(:other_page) do
-      Wiki::Commit.create_content!(
-          tag_name: create(:tag_name, name: 'Help_other_contract'),
-          body: 'other page history', created_by_user: user)
-    end
-    let(:other_revision) { other_page.current_revision }
-
-    it 'returns 404 when show specifies another page revision' do
-      get "/wiki/#{ page.id }", params: { version: other_revision.id }
-      expect(response).to have_http_status(:not_found)
-    end
-
-    it 'returns 404 when diff from belongs to another page' do
-      get "/wiki/#{ page.id }/diff",
-          params: { from: other_revision.id, to: last_revision.id }
-      expect(response).to have_http_status(:not_found)
-    end
-
-    it 'returns 404 when diff to belongs to another page' do
-      get "/wiki/#{ page.id }/diff",
-          params: { from: first_revision.id, to: other_revision.id }
+  [:id, :title].each do |lookup|
+    [nil, :historical].each do |edition|
+      it "rejects legacy #{ lookup } #{ edition || 'current' } without legacy SELECTs" do
+        path = lookup == :id ? "/wiki/#{ page.id }" : "/wiki/title/#{ tag_name.name }"
+        params = edition ? { version: first_revision.id } : { }
+        without_group_legacy_reads { get path, params: }
         expect(response).to have_http_status(:not_found)
       end
     end
 
-  it 'retains legacy history metadata and ordering' do
-    get '/wiki/changes', params: { id: page.id }
+    it "does not recognise legacy #{ lookup } existence" do
+      path = lookup == :id ? "/wiki/#{ page.id }" : "/wiki/title/#{ tag_name.name }"
+      without_group_legacy_reads { get "#{ path }/exists" }
+      expect(response).to have_http_status(:not_found)
+    end
+  end
+
+  ['/wiki', '/wiki/search', '/wiki/changes'].each do |path|
+    it "excludes unmigrated content from #{ path } without legacy SELECTs" do
+      without_group_legacy_reads { get path }
       expect(response).to have_http_status(:ok)
-    expect(json.map { |row| row.fetch('revision_id') }).to eq(
-        [last_revision.id, first_revision.id])
-    expect(json.first).to include(
-        'pred' => first_revision.id, 'succ' => nil, 'kind' => 'content',
-        'message' => 'legacy edit', 'timestamp' => last_revision.created_at.iso8601(3),
-        'wiki_page' => { 'id' => page.id, 'title' => tag_name.name, 'deprecated_at' => nil },
-        'user' => { 'id' => user.id, 'name' => user.name })
+      expect(json).to eq([])
+    end
+  end
+
+  it 'returns empty scoped history without legacy SELECTs' do
+    without_group_legacy_reads { get '/wiki/changes', params: { id: page.id } }
+    expect(response).to have_http_status(:ok)
+    expect(json).to eq([])
+  end
+
+  [false, true].each do |explicit|
+    it "rejects legacy diff with #{ explicit ? 'explicit' : 'current' } target" do
+      params = { from: first_revision.id }
+      params[:to] = last_revision.id if explicit
+      without_group_legacy_reads { get "/wiki/#{ page.id }/diff", params: }
+      expect(response).to have_http_status(:not_found)
+    end
   end
 end

backend/spec/services/shared_resource_id_allocator_mysql_spec.rb

@@ -0,0 +1,143 @@
+require 'rails_helper'
+require 'timeout'
+
+RSpec.describe 'Shared resource IDs across MySQL connections', type: :model do
+  self.use_transactional_tests = false
+
+  before do
+    expect(Rails.env.test?).to be(true)
+    expect(ApplicationRecord.connection.adapter_name).to match(/mysql/i)
+    expect(ApplicationRecord.connection_pool.size).to be >= 3
+    @prefix = "shared_ids_#{ SecureRandom.hex(8) }"
+    @names = [0, 1].map { create(:tag_name, name: "#{ @prefix }_#{ _1 }") }
+    @tag = Tag.create!(tag_name_id: @names.first.id, category: :general)
+    @names.first.update!(tag_id: @tag.id)
+    @threads = []
+    @release = Queue.new
+  end
+
+  after do
+    @release << true if @release
+    begin
+      Array(@threads).each do |thread|
+        raise 'Shared ID worker did not finish' unless thread.join(15)
+      end
+    ensure
+      if @prefix
+        ExternalTag.where(name: "#{ @prefix }_external").delete_all
+        TagGroup.where(name: "#{ @prefix }_group").delete_all
+      end
+      TagDescription.where(tag_id: @tag.id).delete_all if @tag
+      if @names
+        ids = @names.map(&:id)
+        TagName.where(id: ids).update_all(tag_id: nil)
+        Tag.where(tag_name_id: ids).delete_all
+        TagName.where(id: ids).delete_all
+      end
+    end
+  end
+
+  def await_queue queue
+    Timeout.timeout(15) { queue.pop }
+  end
+
+  def worker &block
+    Thread.new do
+      Thread.current.report_on_exception = false
+      ApplicationRecord.connection_pool.with_connection(&block)
+    end.tap { @threads << _1 }
+  end
+
+  def create_resource kind
+    case kind
+    when :tag
+      Tag.create!(tag_name_id: @names.last.id, category: :general)
+    when :external
+      ExternalTag.create!(platform: :nico, name: "#{ @prefix }_external")
+    when :description
+      TagDescription.create!(tag: @tag, language_code: 'ja', body: '', version_no: 1)
+    when :group
+      TagGroup.create!(name: "#{ @prefix }_group", version_no: 1)
+    end
+  end
+
+  def hold_creation kind, rollback: false
+    ready = Queue.new
+    thread = worker do |connection|
+      id = nil
+      connection.transaction do
+        lock_name = nil
+        subscriber = lambda do |*args|
+          match = args.last.fetch(:sql).match(/GET_LOCK\('([^']+)'/)
+          lock_name = match[1] if match
+        end
+        ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') do
+          id = create_resource(kind).id
+        end
+        ready << [id, connection.select_value('SELECT CONNECTION_ID()').to_i, lock_name]
+        await_queue(@release)
+        raise ActiveRecord::Rollback if rollback
+      end
+      id
+    end
+    [thread, await_queue(ready)]
+  end
+
+  def result thread
+    raise 'Shared ID worker did not finish' unless thread.join(15)
+    thread.value
+  end
+
+  { tags: [:external, :tag], wiki: [:description, :group] }.each do |space, kinds|
+    [false, true].each do |rollback|
+      it "serialises #{ space } inserts through #{ rollback ? 'rollback' : 'commit' }" do
+        tables = SharedResourceIdAllocator::SPACES.fetch(space.to_s)
+        maxima = tables.map do |table|
+          ApplicationRecord.connection.select_value("SELECT MAX(id) FROM #{ table }").to_i
+        end
+        first, (first_id, connection_id, lock_name) = hold_creation(kinds.first, rollback:)
+        expect(first_id).to eq(maxima.max + 1)
+        connection = ApplicationRecord.connection
+        quoted_lock = connection.quote(lock_name)
+        expect(connection.select_value("SELECT IS_USED_LOCK(#{ quoted_lock })").to_i)
+            .to eq(connection_id)
+
+        started = Queue.new
+        second = worker do |other|
+          started << other.select_value('SELECT CONNECTION_ID()').to_i
+          create_resource(kinds.last).id
+        end
+        expect(await_queue(started)).not_to eq(connection_id)
+        @release << true
+        expect(result(first)).to eq(first_id)
+        expect(result(second)).to eq(rollback ? first_id : first_id + 1)
+        expect(connection.select_value("SELECT IS_FREE_LOCK(#{ quoted_lock })").to_i).to eq(1)
+      end
+    end
+  end
+
+  it 'allows the other ID space to commit while the first space remains locked' do
+    first, (_id, connection_id, lock_name) = hold_creation(:external)
+    second = worker { create_resource(:group).id }
+    expect(result(second)).to be_positive
+    connection = ApplicationRecord.connection
+    quoted_lock = connection.quote(lock_name)
+    expect(connection.select_value("SELECT IS_USED_LOCK(#{ quoted_lock })").to_i)
+        .to eq(connection_id)
+    @release << true
+    result(first)
+  end
+
+  it 'raises on a real lock acquisition failure without inserting a record' do
+    first, (_id, _connection_id, lock_name) = hold_creation(:external, rollback: true)
+    stub_const('SharedResourceIdAllocator::LOCK_TIMEOUT', 0)
+    expect { create_resource(:external) }.to raise_error(ActiveRecord::LockWaitTimeout)
+    expect(ExternalTag.where(name: "#{ @prefix }_external")).not_to exist
+    @release << true
+    result(first)
+    connection = ApplicationRecord.connection
+    quoted_lock = connection.quote(lock_name)
+    expect(connection.select_value("SELECT IS_FREE_LOCK(#{ quoted_lock })").to_i)
+        .to eq(1)
+  end
+end

backend/spec/services/shared_resource_id_allocator_spec.rb

@@ -0,0 +1,88 @@
+require 'rails_helper'
+
+RSpec.describe SharedResourceIdAllocator do
+  let(:record) { build(:external_tag) }
+  let(:connection) { double('connection') }
+  let(:transaction) { double('transaction', open?: true) }
+  let(:callbacks) { { } }
+  let(:sql) { [] }
+
+  before do
+    config = double('config', database: 'test_shared_ids')
+    allow(connection).to receive_message_chain(:pool, :db_config).and_return(config)
+    allow(connection).to receive(:quote) { "'#{ _1 }'" }
+    allow(connection).to receive(:quote_table_name) { "`#{ _1 }`" }
+    allow(connection).to receive(:select_value) do |statement|
+      sql << statement
+      if statement.include?('GET_LOCK') || statement.include?('RELEASE_LOCK')
+        1
+      elsif statement.include?('`tags`')
+        8
+      else
+        3
+      end
+    end
+    allow(ExternalTag).to receive(:connection).and_return(connection)
+    allow(ExternalTag).to receive(:current_transaction).and_return(transaction)
+    allow(ExternalTag).to receive(:uncached).and_yield
+    [:after_commit, :after_rollback].each do |event|
+      allow(transaction).to receive(event) { |&callback| callbacks[event] = callback }
+    end
+  end
+
+  [:after_commit, :after_rollback].each do |event|
+    it "holds the shared lock until #{ event }" do
+      described_class.assign!(record)
+      expect(record.id).to eq(9)
+      expect(sql.grep(/RELEASE_LOCK/)).to be_empty
+      expect(sql.grep(/FOR UPDATE/).size).to eq(2)
+      callbacks.fetch(event).call
+      expect(sql.grep(/RELEASE_LOCK/).size).to eq(1)
+    end
+  end
+
+  it 'does not silently proceed after an acquisition timeout' do
+    allow(connection).to receive(:select_value).and_return(0)
+    expect { described_class.assign!(record) }.to raise_error(ActiveRecord::LockWaitTimeout)
+    expect(record.id).to be_nil
+    expect(callbacks).to be_empty
+  end
+
+  it 'releases a successfully acquired lock when allocation fails and rolls back' do
+    allow(connection).to receive(:select_value).with(/FOR UPDATE/).and_raise('query failed')
+    expect { described_class.assign!(record) }.to raise_error(RuntimeError, 'query failed')
+    callbacks.fetch(:after_rollback).call
+    expect(sql.grep(/RELEASE_LOCK/).size).to eq(1)
+  end
+
+  it 'preserves explicitly supplied historical IDs' do
+    record.id = 5
+    described_class.assign!(record)
+    expect(record.id).to eq(5)
+    expect(sql.grep(/FOR UPDATE/)).to be_empty
+    callbacks.fetch(:after_commit).call
+  end
+
+  it 'uses the same named lock within each space and different locks between spaces' do
+    locks = [Tag, ExternalTag, TagDescription, TagGroup].map do |model|
+      allow(model).to receive(:connection).and_return(connection)
+      allow(model).to receive(:current_transaction).and_return(transaction)
+      allow(model).to receive(:uncached).and_yield
+      record = model.new(id: 50)
+      described_class.assign!(record)
+      statement = sql.grep(/GET_LOCK/).last
+      callbacks.fetch(:after_commit).call
+      statement
+    end
+    expect(locks[0]).to eq(locks[1])
+    expect(locks[2]).to eq(locks[3])
+    expect(locks[0]).not_to eq(locks[2])
+  end
+
+  it 'reports lock release failures' do
+    described_class.assign!(record)
+    allow(connection).to receive(:select_value).with(/RELEASE_LOCK/).and_return(nil)
+    expect { callbacks.fetch(:after_commit).call }
+        .to raise_error(ActiveRecord::ActiveRecordError, /lock release failed/)
+  end
+end

backend/spec/services/wiki/commit_integrity_spec.rb

@@ -2,41 +2,42 @@ require 'digest'
 require 'rails_helper'
 
 RSpec.describe Wiki::Commit do
+  include WikiRecords
   let(:user) { create_member_user! }
 
   def create_page title:, body: 'initial body'
-    described_class.create_content!(
-        tag_name: create(:tag_name, name: title),
+    seed_wiki_page(
+        tag_name: create(:tag, primary_name: title).tag_name('ja'),
         body:,
         created_by_user: user,
         message: 'init')
   end
 
   describe '.create_content!' do
-    it 'creates page, revision, and version with normalised body' do
-      expect {
-        described_class.create_content!(
-            tag_name: create(:tag_name, name: 'commit_integrity_create'),
-            body: "a\r\nb\r\n\r\n",
-            created_by_user: user,
-            message: 'init')
-      }
-          .to change(WikiPage, :count).by(1)
+    it 'rejects standalone Help creation without adding page or history' do
+      name = create(:tag_name, name: 'Help:frozen_service')
+      counts = [WikiPage.count, WikiRevision.count, WikiVersion.count]
+      expect do
+        described_class.create_content!(tag_name: name, body: 'initial', created_by_user: user)
+      end.to raise_error(ActiveRecord::RecordInvalid)
+      expect([WikiPage.count, WikiRevision.count, WikiVersion.count]).to eq(counts)
+    end
+
+    it 'keeps the internal compatibility writer available' do
+      name = create(:tag_name, name: 'タググループ:compatibility_service')
+      page = nil
+      expect do
+        page = described_class.create_legacy_content!(
+            tag_name: name, body: 'initial', created_by_user: user)
+      end.to change(WikiPage, :count).by(1)
           .and change(WikiRevision, :count).by(1)
           .and change(WikiVersion, :count).by(1)
-
-      page = WikiPage.joins(:tag_name).find_by!(tag_names: { name: 'commit_integrity_create' })
-      revision = page.current_revision
-      version = page.wiki_versions.order(:version_no).last
-
-      expect(page.body).to eq("a\nb")
-      expect(revision.lines_count).to eq(2)
-      expect(version.body).to eq("a\nb")
-      expect(version.reason).to eq('init')
+      expect(page.current_revision.body).to eq('initial')
+      expect(page.wiki_versions.sole.body).to eq('initial')
     end
 
     it 'rejects body that becomes blank after normalisation' do
-      tag_name = create(:tag_name, name: 'commit_integrity_blank')
+      tag_name = create(:tag, primary_name: 'commit_integrity_blank').tag_name('ja')
 
       expect {
         described_class.create_content!(
@@ -52,6 +53,17 @@ RSpec.describe Wiki::Commit do
   end
 
   describe '.content!' do
+    it 'rejects standalone Help updates without modifying histories' do
+      page = seed_wiki_page(tag_name: create(:tag_name, name: 'Help:standalone'),
+                            body: 'before', created_by_user: user)
+      counts = [WikiRevision.count, WikiVersion.count]
+      expect do
+        described_class.content!(page:, body: 'after', created_user: user)
+      end.to raise_error(ActiveRecord::RecordInvalid)
+      expect(page.reload.body).to eq('before')
+      expect([WikiRevision.count, WikiVersion.count]).to eq(counts)
+    end
+
     it 'updates page body, revision, and version' do
       page = create_page(title: 'commit_integrity_update', body: 'before')
       current_id = page.current_revision.id
@@ -80,7 +92,7 @@ RSpec.describe Wiki::Commit do
       tag = create(:tag, primary_tag_name: tag_name, category: :general)
 
       page =
-          described_class.create_content!(
+          seed_wiki_page(
               tag_name:,
               body: 'before',
               created_by_user: user,

backend/spec/services/wiki/commit_spec.rb

@@ -1,12 +1,13 @@
 require 'rails_helper'
 
 RSpec.describe Wiki::Commit do
+  include WikiRecords
   let(:user) { create_member_user! }
 
   def create_page(title: 'commit_spec_page', body: 'initial body')
-    tag_name = create(:tag_name, name: title)
+    tag_name = create(:tag, primary_name: title).tag_name('ja')
 
-    Wiki::Commit.create_content!(
+    seed_wiki_page(
         tag_name:,
         body:,
         created_by_user: user,
@@ -84,7 +85,7 @@ RSpec.describe Wiki::Commit do
       tag = create(:tag, primary_tag_name: tag_name, category: :general)
 
       page =
-          described_class.create_content!(
+          seed_wiki_page(
               tag_name:,
               body: 'before',
               created_by_user: user,
@@ -111,7 +112,7 @@ RSpec.describe Wiki::Commit do
       tag = create(:tag, primary_tag_name: tag_name, category: :general)
 
       page =
-          described_class.create_content!(
+          seed_wiki_page(
               tag_name:,
               body: 'before',
               created_by_user: user,

backend/spec/services/wiki_version_recorder_spec.rb

@@ -1,10 +1,11 @@
 require 'rails_helper'
 
 RSpec.describe WikiVersionRecorder do
+  include WikiRecords
   let(:user) { create_member_user! }
 
   def create_page title:, body: 'body'
-    Wiki::Commit.create_content!(
+    seed_wiki_page(
         tag_name: create(:tag_name, name: title),
         body:,
         created_by_user: user,

backend/spec/support/tag_group_records.rb

@@ -0,0 +1,220 @@
+require 'cgi'
+
+# SQL fixtures use the migration contract without inventing application models.
+# Include explicitly; these helpers must not affect unrelated examples.
+module TagGroupRecords
+  GROUP_TABLES = [
+      'tag_groups', 'tag_group_tags', 'tag_group_groups',
+      'tag_group_placements', 'tag_group_versions'].freeze
+  WRITE_TABLES = GROUP_TABLES + [
+      'wiki_pages', 'wiki_revisions', 'wiki_versions', 'wiki_lines',
+      'wiki_revision_lines', 'tag_names', 'tag_description_versions',
+      'tag_descriptions']
+
+  def group_connection = ApplicationRecord.connection
+
+  def group_rows table, conditions = { }
+    clauses = conditions.map do |column, value|
+      "#{ group_connection.quote_column_name(column) } = #{ group_connection.quote(value) }"
+    end
+    sql = "SELECT * FROM #{ group_connection.quote_table_name(table) }"
+    sql += " WHERE #{ clauses.join(' AND ') }" if clauses.any?
+    group_connection.select_all(sql).to_a.sort_by { |row| row.values.map(&:to_s) }
+  end
+
+  def insert_group_row table, attributes
+    columns = attributes.keys.map { group_connection.quote_column_name(_1) }.join(', ')
+    values = attributes.values.map { group_connection.quote(_1) }.join(', ')
+    group_connection.insert(
+        "INSERT INTO #{ group_connection.quote_table_name(table) } (#{ columns }) " +
+        "VALUES (#{ values })")
+  end
+
+  def group_state
+    WRITE_TABLES.to_h { |table| [table, group_rows(table)] }
+  end
+
+  def expect_noop_group_update group_id:, page:
+    before_state = group_state
+    group = TagGroup.find(group_id)
+    version_id = TagGroupVersion.find_by!(
+        tag_group_id: group_id, version_no: group.version_no).id
+    revision_id = page.reload.current_revision.id
+    wiki_version_id = page.wiki_versions.order(version_no: :desc).first.id
+    writes = []
+    tables = Regexp.union(WRITE_TABLES)
+    subscriber = lambda do |*args|
+      sql = args.last.fetch(:sql)
+      if sql.match?(/\A\s*(?:INSERT INTO|UPDATE|DELETE FROM)\s+`?(?:#{ tables })`?\s/i)
+        writes << sql
+      end
+    end
+
+    aggregate_failures 'no-op current state, history and SQL writes' do
+      ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') { yield }
+      expect(response).to have_http_status(:ok)
+      after_state = group_state
+      before_state.each do |table, rows|
+        expect(after_state.fetch(table)).to eq(rows), "No-op changed #{ table }"
+      end
+      expect(TagGroupVersion.find_by!(
+          tag_group_id: group_id, version_no: group.reload.version_no).id).to eq(version_id)
+      expect(page.reload.current_revision.id).to eq(revision_id)
+      expect(page.wiki_versions.order(version_no: :desc).first.id).to eq(wiki_version_id)
+      expect(writes).to be_empty
+    end
+  end
+
+  def expect_atomic_group_write table
+    before_state = group_state
+    failure = Class.new(StandardError)
+    injected = false
+    subscriber = lambda do |*args|
+      sql = args.last.fetch(:sql)
+      next unless sql.match?(/\A\s*(?:INSERT INTO|UPDATE)\s+`?#{ table }`?\s/i)
+
+      injected = true
+      raise failure, 'injected after SQL write'
+    end
+    begin
+      ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') { yield }
+    rescue failure
+      # Inspect the database after the operation's transaction unwinds.
+    end
+    aggregate_failures do
+      expect(injected).to be(true), "No INSERT/UPDATE reached #{ table }"
+      expect(group_state).to eq(before_state)
+    end
+  end
+
+  def group_version group_id, number
+    row = group_rows('tag_group_versions', tag_group_id: group_id, version_no: number).sole
+    row.merge('members' => JSON.parse(row.fetch('members')),
+              'placements' => JSON.parse(row.fetch('placements')))
+  end
+
+  def group_tag name
+    tag = create(:tag, primary_name: name)
+    TagVersioning.ensure_snapshot!(tag, created_by_user: user)
+    tag.reload
+  end
+
+  def legacy_group name, body
+    # Historical fixtures preserve blank lines independently of today's writer.
+    body = body.sub(/\n+\z/, '')
+    page = WikiPage.create!(
+        tag_name: create(:tag_name, name: "タググループ:#{ name }"),
+        body:, created_user: user, updated_user: user)
+    WikiVersionRecorder.record!(page:, event_type: :create, created_by_user: user)
+    lines = body.split("\n")
+    shas = lines.map { Digest::SHA256.hexdigest(_1) }
+    revision = WikiRevision.create!(
+        wiki_page: page, kind: :content, created_user: user, lines_count: lines.size,
+        tree_sha256: Digest::SHA256.hexdigest(shas.join(',')))
+    lines.each_with_index do |line, position|
+      sha = shas.fetch(position)
+      line_id = WikiLine.find_by(sha256: sha)&.id || insert_group_row('wiki_lines', {
+          body: line, sha256: sha, created_at: Time.current, updated_at: Time.current })
+      WikiRevisionLine.create!(wiki_revision: revision, wiki_line_id: line_id, position:)
+    end
+    page
+  end
+
+  def seed_group name:, placements: [], members: [], description: nil, id: nil
+    now = Time.current.change(usec: 0)
+    attributes = {
+        name:, description:, version_no: 1,
+        created_by_user_id: user.id, updated_by_user_id: user.id,
+        created_at: now, updated_at: now }
+    attributes[:id] = id if id
+    group_id = insert_group_row('tag_groups', attributes)
+    insert_group_row('tag_group_versions', {
+        tag_group_id: group_id, version_no: 1, event_type: 'create',
+        name:, description:, members: members.to_json, placements: placements.to_json,
+        created_by_user_id: user.id, created_at: now })
+    members.each do |member|
+      tag_member = member.key?('tag_id')
+      table = tag_member ? 'tag_group_tags' : 'tag_group_groups'
+      key = tag_member ? :tag_id : :child_tag_group_id
+      value = member.fetch(tag_member ? 'tag_id' : 'tag_group_id')
+      insert_group_row(table, {
+          tag_group_id: group_id, key => value,
+          created_by_user_id: user.id, created_at: now })
+    end
+    position = 0
+    append = lambda do |nodes, parent|
+      nodes.each do |node|
+        position += 1
+        own_position = position
+        insert_group_row('tag_group_placements', {
+            tag_group_id: group_id, position:, parent_position: parent,
+            section_name: node['section_name'], description: node['description'],
+            tag_id: node['tag_id'], child_tag_group_id: node['tag_group_id'],
+            created_by_user_id: user.id, updated_by_user_id: user.id,
+            created_at: now, updated_at: now })
+        append.call(node.fetch('children'), own_position)
+      end
+    end
+    append.call(placements, nil)
+    group_id
+  end
+
+  def without_group_legacy_reads
+    queries = []
+    subscriber = lambda do |*args|
+      sql = args.last.fetch(:sql)
+      if (sql.match?(/\bSELECT\b/i) &&
+          sql.match?(/\bwiki_(pages|revisions|versions|lines|revision_lines)\b/i))
+        queries << sql
+      end
+    end
+    ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') { yield }
+    expect(queries).to be_empty
+  end
+
+  def group_title_path name
+    "/wiki/title/#{ CGI.escapeURIComponent("タググループ:#{ name }") }"
+  end
+
+  def migrate_groups
+    require_relative '../../db/migrate/20261008010000_migrate_tag_groups_from_wiki_pages'
+    migration = MigrateTagGroupsFromWikiPages.new
+    @group_migration_messages = []
+    allow(migration).to receive(:say) do |message, *_args|
+      @group_migration_messages << message if message.start_with?('Wiki #', 'Tag Group')
+    end
+    migration.suppress_messages { migration.up }
+  end
+
+  def expect_group_snapshot group_id, number:, placements:, members:, description: nil
+    current = group_rows('tag_groups', id: group_id).sole
+    version = group_version(group_id, number)
+    expect(current).to include('version_no' => number, 'description' => description),
+                       @group_migration_messages&.join("\n")
+    expect(version).to include(
+        'name' => current.fetch('name'), 'description' => description,
+        'version_no' => number, 'placements' => placements)
+    expect(version.fetch('members')).to match_array(members)
+    expect(group_rows('tag_group_tags', tag_group_id: group_id).map { _1.fetch('tag_id') })
+        .to match_array(members.filter_map { _1['tag_id'] })
+    children = group_rows('tag_group_groups', tag_group_id: group_id)
+    expect(children.map { _1.fetch('child_tag_group_id') })
+        .to match_array(members.filter_map { _1['tag_group_id'] })
+
+    rows = group_rows('tag_group_placements', tag_group_id: group_id)
+    tree = lambda do |parent|
+      rows.select { _1['parent_position'] == parent }.sort_by { _1.fetch('position') }
+          .map do |row|
+        node = if row['tag_id']
+          { 'tag_id' => row.fetch('tag_id') }
+        elsif row['child_tag_group_id']
+          { 'tag_group_id' => row.fetch('child_tag_group_id') }
+        else
+          { 'section_name' => row.fetch('section_name'), 'description' => row['description'] }
+        end
+        node.merge('children' => tree.call(row.fetch('position')))
+      end
+    end
+    expect(tree.call(nil)).to eq(placements)
+  end
+end

backend/spec/support/wiki_records.rb

@@ -0,0 +1,20 @@
+# Historical records must remain seedable after application creation is frozen.
+module WikiRecords
+  def seed_wiki_page(tag_name:, body:, created_by_user:, message: nil)
+    page = WikiPage.create!(tag_name:, body:,
+                            created_user: created_by_user, updated_user: created_by_user)
+    revision = Wiki::Commit.legacy_content!(
+        page:, body:, created_user: created_by_user, message:)
+    if tag_name.tag && tag_name.language_code == 'ja'
+      TagDescription.create!(
+          id: page.id, tag: tag_name.tag, language_code: 'ja', body: page.reload.body,
+          version_no: 1, created_by_user_id: created_by_user.id,
+          updated_by_user_id: created_by_user.id)
+      TagDescriptionVersion.create!(
+          id: revision.id, tag_id: tag_name.tag_id, language_code: 'ja',
+          version_no: 1, event_type: 'create', body: page.body,
+          created_by_user_id: created_by_user.id, created_at: revision.created_at)
+    end
+    page
+  end
+end

backend/spec/tasks/nico_export_spec.rb

@@ -18,8 +18,8 @@ RSpec.describe 'nico:export' do
   before do
     task.reenable
 
-    allow(ENV).to receive(:fetch).with('MYSQL_USER').and_return('mysql-user')
-    allow(ENV).to receive(:fetch).with('MYSQL_PASS').and_return('mysql-pass')
+    allow(ENV).to receive(:fetch).with('NIZIKA_NICO_MYSQL_USER').and_return('mysql-user')
+    allow(ENV).to receive(:fetch).with('NIZIKA_NICO_MYSQL_PASS').and_return('mysql-pass')
     allow(ENV).to receive(:fetch).with('NIZIKA_NICO_PATH').and_return('/srv/nizika-nico')
   end
 
@@ -33,8 +33,8 @@ RSpec.describe 'nico:export' do
       expect(Open3).to receive(:capture3) do |env, *args, **kwargs|
         expect(env).to eq(
           {
-            'MYSQL_USER' => 'mysql-user',
-            'MYSQL_PASS' => 'mysql-pass',
+            'NIZIKA_NICO_MYSQL_USER' => 'mysql-user',
+            'NIZIKA_NICO_MYSQL_PASS' => 'mysql-pass',
           },
         )
 

frontend/.gitignore

@@ -22,3 +22,5 @@ dist-ssr
 *.njsproj
 *.sln
 *.sw?
+
+/help

frontend/package-lock.json

@@ -38,6 +38,7 @@
         "remark-gfm": "^4.0.1",
         "tailwind-merge": "^3.3.0",
         "unist-util-visit-parents": "^6.0.1",
+        "yaml": "^2.9.1",
         "zustand": "^5.0.8"
       },
       "devDependencies": {
@@ -2060,9 +2061,6 @@
         "arm"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2077,9 +2075,6 @@
         "arm"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2094,9 +2089,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2111,9 +2103,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2128,9 +2117,6 @@
         "loong64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2145,9 +2131,6 @@
         "loong64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2162,9 +2145,6 @@
         "ppc64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2179,9 +2159,6 @@
         "ppc64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2196,9 +2173,6 @@
         "riscv64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2213,9 +2187,6 @@
         "riscv64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2230,9 +2201,6 @@
         "s390x"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2247,9 +2215,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -2264,9 +2229,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -9114,10 +9076,9 @@
       "license": "ISC"
     },
     "node_modules/yaml": {
-      "version": "2.8.3",
-      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.8.3.tgz",
-      "integrity": "sha512-AvbaCLOO2Otw/lW5bmh9d/WEdcDFdQp2Z2ZUH3pX9U2ihyUY0nvLv7J6TrWowklRGPYbB/IuIMfYgxaCPg5Bpg==",
-      "dev": true,
+      "version": "2.9.1",
+      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.9.1.tgz",
+      "integrity": "sha512-3NxN8+78OdzbT7C/WjGsyfPAtJaN3FNDsWxv7Y7mcDsT/oOmgW8BpyQQFFBnvZE3j9Y2Sdz1ULFLezL7Eb2yFw==",
       "license": "ISC",
       "bin": {
         "yaml": "bin.mjs"

frontend/package.json

@@ -42,6 +42,7 @@
     "remark-gfm": "^4.0.1",
     "tailwind-merge": "^3.3.0",
     "unist-util-visit-parents": "^6.0.1",
+    "yaml": "^2.9.1",
     "zustand": "^5.0.8"
   },
   "devDependencies": {

frontend/scripts/help-assets.test.ts

@@ -0,0 +1,112 @@
+import { mkdir, mkdtemp, rm, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import path from 'node:path'
+import { afterAll, afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { generateHelpAssets } from './help-assets'
+
+import type { ArticleMeta, HelpChange, HelpSearchEntry, HelpTitleIndex } from '@/lib/help'
+
+vi.hoisted (() => {
+  vi.stubGlobal ('Uint8Array', Object.getPrototypeOf (new TextEncoder ().encode ('')).constructor)
+})
+afterAll (() => vi.unstubAllGlobals ())
+
+
+const readAsset = <T,> (modules: Map<string, string>, name: string): T => {
+  const source = modules.get (`virtual:help/${ name }`)!
+  const argument = source.slice ('export default JSON.parse('.length, -1)
+  return JSON.parse (JSON.parse (argument))
+}
+
+
+describe ('Build-time Help assets', () => {
+  let root: string
+
+  beforeEach (async () => {
+    root = await mkdtemp (path.join (tmpdir (), 'btrc-help-assets-'))
+  })
+
+  afterEach (async () => {
+    await rm (root, { recursive: true, force: true })
+  })
+
+  const writeArticle = async (
+      identifier: string,
+      editions: Record<string, string>,
+      current = '2026-10-02.10',
+  ) => {
+    const articleRoot = path.join (root, identifier)
+    const editionRoot = path.join (articleRoot, 'editions', 'ja')
+    await mkdir (editionRoot, { recursive: true })
+    await writeFile (path.join (articleRoot, 'article.yml'), [
+	`identifier: ${ identifier }`, 'canonical_language: ja', 'title: stale YAML title',
+	'languages:', '  ja:', `    current_edition: ${ current }`].join ('\n'))
+    await Promise.all (Object.entries (editions).map (([edition, source]) =>
+	writeFile (path.join (editionRoot, `${ edition }.mdx`), source)))
+  }
+
+  it ('generates current search and ordered published history from each edition H1', async () => {
+    await writeArticle ('guide', {
+	'2026-10-01': '# 旧ホーム\n\n過去本文',
+	'2026-10-02': '# ホーム\n\n最初の同日版',
+	'2026-10-02.2': '# ホーム\n\n第二版',
+	'2026-10-02.3': '# ホーム\n\n第三版',
+	'2026-10-02.10': '\uFEFF# 最新ホーム\r\n\r\n現在本文\r\n# 別見出し',
+	'2026-10-02.11': '# 未公開\n\n検索に含めない' })
+    const { modules } = await generateHelpAssets (root)
+    const articles = readAsset<Record<string, ArticleMeta>> (modules, 'articles')
+    expect (articles.guide.languages.ja).toEqual ({
+	currentEdition: '2026-10-02.10', publishedEditions: [
+	    '2026-10-01', '2026-10-02', '2026-10-02.2', '2026-10-02.3', '2026-10-02.10'] })
+    expect (readAsset<Record<string, HelpSearchEntry[]>> (modules, 'search').ja).toEqual ([{
+	identifier: 'guide', edition: '2026-10-02.10', title: '最新ホーム',
+	body: '\r\n現在本文\r\n# 別見出し' }])
+    const changes = readAsset<Record<string, HelpChange[]>> (modules, 'changes').ja
+    expect (changes.map (row => [row.edition, row.title, row.pred, row.change, row.date]))
+	.toEqual ([
+	    ['2026-10-02.10', '最新ホーム', '2026-10-02.3', 'update', '2026-10-02'],
+	    ['2026-10-02.3', 'ホーム', '2026-10-02.2', 'update', '2026-10-02'],
+	    ['2026-10-02.2', 'ホーム', '2026-10-02', 'update', '2026-10-02'],
+	    ['2026-10-02', 'ホーム', '2026-10-01', 'update', '2026-10-02'],
+	    ['2026-10-01', '旧ホーム', null, 'new', '2026-10-01']])
+    const titles = readAsset<Record<string, HelpTitleIndex>> (modules, 'titles').ja
+    expect (titles).toEqual ({ '旧ホーム': ['guide'], 'ホーム': ['guide'],
+	'最新ホーム': ['guide'] })
+    expect (modules.get ('virtual:help/loaders')).not.toContain ('2026-10-02.11')
+    expect (modules.get ('virtual:help/loaders')).toContain ('2026-10-02.10.mdx?raw')
+    expect (modules.get ('virtual:help/search')).not.toContain ('import(')
+  })
+
+  it ('retains ambiguous title identifiers and normalises width and case', async () => {
+    await writeArticle ('alpha', { '2026-10-02': '# ＨＯＭＥ\n本文' }, '2026-10-02')
+    await writeArticle ('beta', { '2026-10-02': '# Home\n本文' }, '2026-10-02')
+    const { modules } = await generateHelpAssets (root)
+    expect (readAsset<Record<string, HelpTitleIndex>> (modules, 'titles').ja.home)
+	.toEqual (['alpha', 'beta'])
+    expect (readAsset<Record<string, HelpChange[]>> (modules, 'changes').ja
+	.map (row => row.identifier)).toEqual (['alpha', 'beta'])
+  })
+
+  it ('rejects missing current files during generation', async () => {
+    await writeArticle ('guide', { '2026-10-02': '# ホーム' })
+    await expect (generateHelpAssets (root)).rejects.toThrow ('Missing current Help edition')
+  })
+
+  it.each (['本文\n# 遅い見出し', '#   \n本文']) (
+      'rejects missing leading H1 at build time: %s', async source => {
+	await writeArticle ('guide', { '2026-10-02.10': source })
+	await expect (generateHelpAssets (root)).rejects.toThrow ('Missing leading Help H1')
+      })
+
+  it ('rejects absent canonical metadata instead of generating incomplete assets', async () => {
+    await writeArticle ('guide', { '2026-10-02.10': '# ホーム' })
+    await writeFile (path.join (root, 'guide', 'article.yml'),
+	'identifier: guide\nlanguages:\n  ja:\n    current_edition: 2026-10-02.10')
+    await expect (generateHelpAssets (root)).rejects.toThrow ('Invalid Help metadata')
+  })
+
+  it ('rejects an empty Help checkout', async () => {
+    await expect (generateHelpAssets (root)).rejects.toThrow ('No Help articles found')
+  })
+})

frontend/scripts/help-assets.ts

@@ -0,0 +1,192 @@
+import { readFile, readdir } from 'node:fs/promises'
+import path from 'node:path'
+import { normalizePath as normalisePath } from 'vite'
+import { parse as parseYAML } from 'yaml'
+
+import type { Plugin } from 'vite'
+
+type Language = { currentEdition: string; publishedEditions: string[] }
+type Article = {
+    identifier: string
+    canonicalLanguage: string
+    languages: Record<string, Language> }
+type Text = { title: string; body: string }
+type SearchEntry = Text & { identifier: string; edition: string }
+type Change = {
+    identifier: string
+    edition: string
+    title: string
+    pred: string | null
+    change: 'new' | 'update'
+    date: string }
+type RawArticle = {
+    identifier: string
+    canonical_language: string
+    languages: Record<string, { current_edition: string }> }
+
+const prefix = 'virtual:help/'
+
+
+const compareEditions = (a: string, b: string): number => {
+  const [dateA, revisionA = '1'] = a.split ('.')
+  const [dateB, revisionB = '1'] = b.split ('.')
+  return dateA.localeCompare (dateB) || Number (revisionA) - Number (revisionB)
+}
+
+
+const validateEdition = (edition: string): void => {
+  const [date, revision = '1'] = edition.split ('.')
+  if (!(/^\d{4}-\d{2}-\d{2}(?:\.[1-9]\d*)?$/.test (edition))
+      || new Date (date).toISOString ().slice (0, 10) !== date
+      || !(Number.isSafeInteger (Number (revision))))
+    throw new Error (`Invalid Help edition: ${ edition }`)
+}
+
+
+export const generateHelpAssets = async (
+    root: string,
+): Promise<{ modules: Map<string, string>; files: string[] }> => {
+  const modules = new Map<string, string> ()
+  const files: string[] = [root]
+  const articles: Record<string, Article> = Object.create (null)
+  const search: Record<string, SearchEntry[]> = Object.create (null)
+  const changes: Record<string, Change[]> = Object.create (null)
+  const titles: Record<string, Record<string, string[]>> = Object.create (null)
+  const components: string[] = []
+  const sources: string[] = []
+  const texts: string[] = []
+  const directories = (await readdir (root, { withFileTypes: true }))
+      .filter (entry => entry.isDirectory () && !(entry.name.startsWith ('.')))
+      .sort ((a, b) => a.name.localeCompare (b.name))
+
+  for (const directory of directories)
+  {
+    const articlePath = path.join (root, directory.name, 'article.yml')
+    files.push (articlePath)
+    const raw = parseYAML (await readFile (articlePath, 'utf8')) as RawArticle | null
+    if (raw == null || raw.identifier !== directory.name
+	|| typeof raw.canonical_language !== 'string'
+	|| raw.languages == null || typeof raw.languages !== 'object'
+	|| !(Object.hasOwn (raw.languages, raw.canonical_language)))
+      throw new Error (`Invalid Help metadata: ${ articlePath }`)
+
+    const article: Article = {
+	identifier: raw.identifier,
+	canonicalLanguage: raw.canonical_language,
+	languages: Object.create (null) }
+    articles[raw.identifier] = article
+
+    for (const [language, metadata] of Object.entries (raw.languages))
+    {
+      if (!(/^[a-zA-Z0-9-]+$/.test (language))
+	  || typeof metadata?.current_edition !== 'string')
+	throw new Error (`Invalid Help language: ${ articlePath }/${ language }`)
+
+      const currentEdition = metadata.current_edition
+      validateEdition (currentEdition)
+      const editionRoot = path.join (root, directory.name, 'editions', language)
+      files.push (editionRoot)
+      const editions = (await readdir (editionRoot)).filter (name => name.endsWith ('.mdx'))
+	  .map (name => name.slice (0, -4))
+      editions.forEach (validateEdition)
+      if (!(editions.includes (currentEdition)))
+	throw new Error (`Missing current Help edition: ${ editionRoot }/${ currentEdition }`)
+
+      const publishedEditions = editions
+	  .filter (edition => compareEditions (edition, currentEdition) <= 0)
+	  .sort (compareEditions)
+      article.languages[language] = { currentEdition, publishedEditions }
+      search[language] ??= []
+      changes[language] ??= []
+      titles[language] ??= Object.create (null)
+
+      for (const [index, edition] of publishedEditions.entries ())
+      {
+	const sourcePath = path.join (editionRoot, `${ edition }.mdx`)
+	files.push (sourcePath)
+	const mdx = (await readFile (sourcePath, 'utf8')).replace (/^\uFEFF/, '')
+	const heading = mdx.match (/^# +([^\r\n]+)(?:\r?\n|$)/)
+	if (heading == null || !(heading[1].trim ()))
+	  throw new Error (`Missing leading Help H1: ${ sourcePath }`)
+
+	const text = { title: heading[1].trim (), body: mdx.slice (heading[0].length) }
+	const key = JSON.stringify (`${ raw.identifier }/${ language }/${ edition }`)
+	const textId = `${ prefix }text/${ raw.identifier }/${ language }/${ edition }`
+	const componentId = JSON.stringify (normalisePath (sourcePath))
+	const sourceId = JSON.stringify (normalisePath (sourcePath) + '?raw')
+	modules.set (textId, `export default ${ JSON.stringify (text) }`)
+	texts.push (`${ key }: () => import(${ JSON.stringify (textId) })`)
+	components.push (`${ key }: () => import(${ componentId })`)
+	sources.push (`${ key }: () => import(${ sourceId })`)
+
+	if (edition === currentEdition)
+	  search[language].push ({ identifier: raw.identifier, edition, ...text })
+	changes[language].push ({ identifier: raw.identifier, edition, title: text.title,
+	    pred: publishedEditions[index - 1] ?? null,
+	    change: index === 0 ? 'new' : 'update', date: edition.split ('.')[0] })
+	const title = text.title.trim ().normalize ('NFKC').toLowerCase ()
+	const identifiers = titles[language][title] ??= []
+	if (!(identifiers.includes (raw.identifier)))
+	  identifiers.push (raw.identifier)
+      }
+    }
+  }
+
+  if (Object.keys (articles).length === 0)
+    throw new Error (`No Help articles found: ${ root }`)
+
+  Object.values (changes).forEach (rows => rows.sort ((a, b) =>
+      compareEditions (b.edition, a.edition) || a.identifier.localeCompare (b.identifier)))
+  for (const [name, data] of Object.entries ({ articles, search, changes, titles }))
+  {
+    modules.set (`${ prefix }${ name }`,
+	`export default JSON.parse(${ JSON.stringify (JSON.stringify (data)) })`)
+  }
+  modules.set (`${ prefix }loaders`, [
+      `export const components = { ${ components.join (',\n') } };`,
+      `export const sources = { ${ sources.join (',\n') } };`,
+      `export const texts = { ${ texts.join (',\n') } };`].join ('\n'))
+  return { modules, files }
+}
+
+
+export const helpAssets = (): Plugin => {
+  const state: { root: string; data?: ReturnType<typeof generateHelpAssets> } = { root: '' }
+  const data = () => state.data ??= generateHelpAssets (state.root)
+
+  return {
+      name: 'help-assets',
+      configResolved (config) {
+	state.root = path.resolve (config.root, 'help')
+      },
+      async buildStart () {
+	state.data = generateHelpAssets (state.root)
+	const generated = await data ()
+	generated.files.forEach (file => this.addWatchFile (file))
+      },
+      resolveId (id) {
+	return id.startsWith (prefix) ? `\0${ id }` : null
+      },
+      async load (id) {
+	if (!(id.startsWith (`\0${ prefix }`)))
+	  return null
+	const source = (await data ()).modules.get (id.slice (1))
+	if (source == null)
+	  throw new Error (`Unknown Help asset: ${ id.slice (1) }`)
+	return source
+      },
+      configureServer (server) {
+	server.watcher.add (state.root)
+	server.watcher.on ('all', (_event, file) => {
+	  if (!(file.startsWith (state.root + path.sep)) || !(/\.(mdx|yml)$/.test (file)))
+	    return
+	  state.data = undefined
+	  for (const module of server.moduleGraph.idToModuleMap.values ())
+	  {
+	    if (module.id?.startsWith (`\0${ prefix }`))
+	      server.moduleGraph.invalidateModule (module)
+	  }
+	  server.ws.send ({ type: 'full-reload' })
+	})
+      } }
+}

frontend/src/App.tsx

@@ -15,7 +15,6 @@ import { Toaster } from '@/components/ui/toaster'
 import { apiPost, isApiError } from '@/lib/api'
 import { applyClientAnimationMode,
 	 applyClientAppearance,
-	 type ClientAnimationMode,
 	 fetchUserThemeSlots,
 	 fetchUserSettings,
 	 getClientThemeMode,
@@ -25,7 +24,6 @@ import { applyClientAnimationMode,
 	 seedClientThemeMode } from '@/lib/settings'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
 import { UnsavedChangesGuardProvider } from '@/lib/useUnsavedChangesGuard'
-
 import { KeyboardShortcutsProvider } from '@/lib/useKeyboardShortcuts'
 import DeerjikistDetailPage from '@/pages/deerjikists/DeerjikistDetailPage'
 import MaterialBasePage from '@/pages/materials/MaterialBasePage'
@@ -34,12 +32,15 @@ import MaterialHistoryPage from '@/pages/materials/MaterialHistoryPage'
 import MaterialListPage from '@/pages/materials/MaterialListPage'
 import MaterialNewPage from '@/pages/materials/MaterialNewPage'
 import MaterialSyncSuppressionsPage from '@/pages/materials/MaterialSyncSuppressionsPage'
-// import MaterialSearchPage from '@/pages/materials/MaterialSearchPage'
 import MorePage from '@/pages/MorePage'
 import GekanatorPage from '@/pages/GekanatorPage'
 import NicoTagListPage from '@/pages/tags/NicoTagListPage'
 import NotFound from '@/pages/NotFound'
 import TOSPage from '@/pages/TOSPage.mdx'
+import HelpDetailPage from '@/pages/help/HelpDetailPage'
+import HelpChangesPage from '@/pages/help/HelpChangesPage'
+import HelpDiffPage from '@/pages/help/HelpDiffPage'
+import HelpListPage from '@/pages/help/HelpListPage'
 import PostDetailPage from '@/pages/posts/PostDetailPage'
 import PostHistoryPage from '@/pages/posts/PostHistoryPage'
 import PostNewPage from '@/pages/posts/PostNewPage'
@@ -60,6 +61,7 @@ import WikiSearchPage from '@/pages/wiki/WikiSearchPage'
 
 import type { Dispatch, FC, SetStateAction } from 'react'
 
+import type { ClientAnimationMode } from '@/lib/settings'
 import type { User } from '@/types'
 
 
@@ -69,9 +71,7 @@ const RouteTransitionWrapper = ({ animationMode, user, setUser }: {
     setUser:        Dispatch<SetStateAction<User | null>> }) => {
   const location = useLocation ()
 
-  if (animationMode === 'off')
-    {
-      return (
+  const routes = (
       <Routes location={location}>
 	<Route path="/" element={<Navigate to="/posts" replace/>}/>
 	<Route path="/posts" element={<PostListPage/>}/>
@@ -99,6 +99,10 @@ const RouteTransitionWrapper = ({ animationMode, user, setUser }: {
 	<Route path="/wiki/:id/edit" element={<WikiEditPage user={user}/>}/>
 	<Route path="/wiki/:id/diff" element={<WikiDiffPage/>}/>
 	<Route path="/wiki/changes" element={<WikiHistoryPage/>}/>
+	<Route path="/help" element={<HelpListPage/>}/>
+	<Route path="/help/changes" element={<HelpChangesPage/>}/>
+	<Route path="/help/:identifier/diff" element={<HelpDiffPage/>}/>
+	<Route path="/help/:identifier" element={<HelpDetailPage/>}/>
 	<Route path="/users/settings" element={<SettingPage user={user} setUser={setUser}/>}/>
 	<Route path="/settings" element={<Navigate to="/users/settings" replace/>}/>
 	<Route path="/tos" element={<TOSPage/>}/>
@@ -106,44 +110,13 @@ const RouteTransitionWrapper = ({ animationMode, user, setUser }: {
 	<Route path="/more" element={<MorePage/>}/>
 	<Route path="*" element={<NotFound/>}/>
       </Routes>)
-    }
+
+  if (animationMode === 'off')
+    return routes
 
   return (
       <AnimatePresence mode="wait">
-	<Routes location={location}>
-	  <Route path="/" element={<Navigate to="/posts" replace/>}/>
-	  <Route path="/posts" element={<PostListPage/>}/>
-	  <Route path="/posts/new" element={<PostNewPage user={user}/>}/>
-	  <Route path="/posts/search" element={<PostSearchPage/>}/>
-	  <Route path="/posts/:id" element={<PostDetailRoute user={user}/>}/>
-	  <Route path="/posts/changes" element={<PostHistoryPage/>}/>
-	  <Route path="/tags" element={<TagListPage/>}/>
-	  <Route path="/tags/:id" element={<TagDetailPage/>}/>
-	  <Route path="/tags/:id/deerjikists" element={<DeerjikistDetailPage/>}/>
-	  <Route path="/tags/nico" element={<NicoTagListPage user={user}/>}/>
-	  <Route path="/nico/tags" element={<NicoTagListPage user={user}/>}/>
-	  <Route path="/tags/changes" element={<TagHistoryPage/>}/>
-	  <Route path="/theatres/:id" element={<TheatreDetailPage user={user}/>}/>
-	  <Route path="/materials" element={<MaterialBasePage/>}>
-	    <Route index element={<MaterialListPage/>}/>
-	    <Route path="changes" element={<MaterialHistoryPage/>}/>
-	    <Route path="new" element={<MaterialNewPage/>}/>
-	    <Route path="suppressions" element={<MaterialSyncSuppressionsPage/>}/>
-	    <Route path=":id" element ={<MaterialDetailPage/>}/>
-	  </Route>
-	  <Route path="/wiki" element={<WikiSearchPage/>}/>
-	  <Route path="/wiki/:title" element={<WikiDetailPage/>}/>
-	  <Route path="/wiki/new" element={<WikiNewPage user={user}/>}/>
-	  <Route path="/wiki/:id/edit" element={<WikiEditPage user={user}/>}/>
-	  <Route path="/wiki/:id/diff" element={<WikiDiffPage/>}/>
-	  <Route path="/wiki/changes" element={<WikiHistoryPage/>}/>
-	  <Route path="/users/settings" element={<SettingPage user={user} setUser={setUser}/>}/>
-	  <Route path="/settings" element={<Navigate to="/users/settings" replace/>}/>
-	  <Route path="/tos" element={<TOSPage/>}/>
-	  <Route path="/gekanator" element={<GekanatorPage user={user}/>}/>
-	  <Route path="/more" element={<MorePage/>}/>
-	  <Route path="*" element={<NotFound/>}/>
-	</Routes>
+	{routes}
       </AnimatePresence>)
 }
 
@@ -166,9 +139,9 @@ const RoutedApp: FC = () => {
       () => (
 	  animationMode === 'off'
 	  ? { duration: 0 }
-	  : animationMode === 'reduced'
+	  : (animationMode === 'reduced'
 	     ? { duration: .08, ease: 'linear' as const }
-	    : { duration: .2, ease: 'easeOut' as const }
+	     : { duration: .2, ease: 'easeOut' as const })
       ),
       [animationMode],
   )
@@ -285,10 +258,12 @@ const RoutedApp: FC = () => {
       </DialogueProvider>)
 }
 
+
 const router = createBrowserRouter ([{
     path: '*',
     element: <RoutedApp/> }])
 
+
 const App: FC = () => (
     <>
       <RouteBlockerOverlay/>
@@ -296,4 +271,5 @@ const App: FC = () => (
       <RouterProvider router={router}/>
     </>)
 
+
 export default App

frontend/src/components/TopNav.tsx

@@ -20,6 +20,7 @@ import type { FC, MouseEvent } from 'react'
 import type { Material, Menu, MenuVisibleItem, Tag, User } from '@/types'
 
 type Props = { user: User | null }
+type NavDensity = 'auto' | 'comfortable' | 'compact'
 
 
 export const menuOutline = (
@@ -40,6 +41,9 @@ export const menuOutline = (
 
   const materialFlg = /^\/materials\/\d+/.test (pathName)
 
+  const helpVsbl = /\/help\/((?!changes)[^\/?]+)/.test (pathName)
+  const helpIdentifier = pathName.split ('/')[2] ?? ''
+
   return [
       { name: '広場', to: '/posts', subMenu: [
 	  { name: '一覧', to: '/posts' },
@@ -71,7 +75,7 @@ export const menuOutline = (
 	    visible: materialFlg && Boolean (material?.tag) },
 	  { name: '履歴', to: `/materials/changes?material_id=${ material?.id }`,
 	    visible: materialFlg }] },
-      { name: 'Wiki', to: '/wiki/ヘルプ:ホーム', base: '/wiki', subMenu: [
+      { name: 'Wiki', to: '/wiki', subMenu: [
 	  { name: '検索', to: '/wiki' },
 	  { name: '新規', to: '/wiki/new', visible: editable },
 	  { name: '全体履歴', to: '/wiki/changes' },
@@ -82,6 +86,13 @@ export const menuOutline = (
 	  { name: '履歴', to: `/wiki/changes?id=${ wikiId }`, visible: wikiPageFlg },
 	  { name: '編輯', to: `/wiki/${ wikiId || wikiTitle }/edit`,
 	    visible: wikiPageFlg && editable }] },
+      { name: 'ヘルプ', to: '/help/home', base: '/help', subMenu: [
+	  { name: '検索', to: '/help' },
+	  { name: '全体履歴', to: '/help/changes' },
+	  { component: <Separator/>, visible: helpVsbl },
+	  { name: '履歴',
+	    to: `/help/changes?identifier=${ helpIdentifier }`,
+	    visible: helpVsbl }] },
       { name: 'おたのしみ', visible: false, subMenu: [
 	  { name: '上映会 (β)', to: '/theatres/1' },
 	  { name: 'グカネータ (β)', to: '/gekanator' }] },
@@ -104,6 +115,8 @@ const TopNav: FC<Props> = ({ user }) => {
   const dirRef = useRef<(-1) | 1> (1)
   const itemsRef = useRef<(HTMLAnchorElement | null)[]> ([])
   const navRef = useRef<HTMLDivElement | null> (null)
+  const layoutRef = useRef<HTMLDivElement | null> (null)
+  const barRef = useRef<HTMLElement | null> (null)
 
   const measure = (idx: number) => {
     const nav = navRef.current
@@ -128,6 +141,8 @@ const TopNav: FC<Props> = ({ user }) => {
       width:    0,
       visible:  false })
   const [menuOpen, setMenuOpen] = useState (false)
+  const [desktop, setDesktop] = useState (false)
+  const [density, setDensity] = useState<NavDensity> ('auto')
   const [moreVsbl, setMoreVsbl] = useState (false)
   const [openItemIdx, setOpenItemIdx] = useState (-1)
   const [wikiId, setWikiId] = useState<number | null> (WikiIdBus.get ())
@@ -194,15 +209,81 @@ const TopNav: FC<Props> = ({ user }) => {
   const dir = dirRef.current
 
   useLayoutEffect (() => {
-    const raf = requestAnimationFrame (() => measure (moreVsbl ? -1 : activeIdx))
-    const onResize = () => requestAnimationFrame (() => measure (moreVsbl ? -1 : activeIdx))
+    const layout = layoutRef.current
+    const bar = barRef.current
+    const primary = bar?.querySelector<HTMLElement> ('.top-nav-primary')
+    if (layout == null || bar == null || primary == null)
+      return
 
-    addEventListener ('resize', onResize)
-    return () => {
-      cancelAnimationFrame (raf)
-      removeEventListener ('resize', onResize)
+    const userLink = bar.querySelector<HTMLElement> ('.top-nav-user-desktop')
+    let raf = 0
+    let active = true
+
+    const update = () => {
+      if (!(window.matchMedia ('(min-width: 768px)').matches))
+      {
+	setDesktop (false)
+	setDensity ('auto')
+	return
       }
-  })
+
+      // Measure the same desktop elements while the mobile layout is active.
+      layout.setAttribute ('data-measuring', '')
+      const previousDensity = layout.getAttribute ('data-density') ?? 'auto'
+      const fittingDensity = (() => {
+	try
+	{
+	  for (const candidate of ['auto', 'comfortable', 'compact'] as const)
+	  {
+	    layout.setAttribute ('data-density', candidate)
+	    const style = getComputedStyle (bar)
+	    const required = primary.getBoundingClientRect ().width
+		+ (userLink?.getBoundingClientRect ().width ?? 0)
+		+ parseFloat (style.paddingLeft) + parseFloat (style.paddingRight)
+		+ (userLink == null ? 0 : parseFloat (style.columnGap))
+	    if (required <= bar.clientWidth)
+	      return candidate
+	  }
+	  return null
+	}
+	finally
+	{
+	  layout.setAttribute ('data-density', previousDensity)
+	  layout.removeAttribute ('data-measuring')
+	}
+      }) ()
+      setDensity (fittingDensity ?? 'compact')
+      setDesktop (fittingDensity != null)
+      if (fittingDensity != null)
+	measure (moreVsbl ? -1 : activeIdx)
+    }
+
+    const schedule = () => {
+      if (!(active))
+	return
+      cancelAnimationFrame (raf)
+      raf = requestAnimationFrame (update)
+    }
+
+    const observer =
+	typeof ResizeObserver === 'undefined' ? null : new ResizeObserver (schedule)
+    observer?.observe (bar)
+    observer?.observe (primary)
+    if (userLink != null)
+      observer?.observe (userLink)
+    update ()
+    addEventListener ('resize', schedule)
+    document.fonts?.addEventListener ('loadingdone', schedule)
+    void document.fonts?.ready.then (schedule)
+
+    return () => {
+      active = false
+      cancelAnimationFrame (raf)
+      observer?.disconnect ()
+      removeEventListener ('resize', schedule)
+      document.fonts?.removeEventListener ('loadingdone', schedule)
+    }
+  }, [activeIdx, moreVsbl, desktop, density, user?.name])
 
   useEffect (() => {
     const unsubscribe = WikiIdBus.subscribe (setWikiId)
@@ -215,19 +296,22 @@ const TopNav: FC<Props> = ({ user }) => {
   }, [activeIdx, location])
 
   return (
-      <>
-	<nav className="top-nav-root px-3 flex justify-between items-center w-full">
-	  <div className="flex items-center gap-2 h-12">
+      <div ref={layoutRef} className="top-nav-layout w-full min-w-0 shrink-0"
+	  data-desktop={desktop} data-density={density}>
+	<nav ref={barRef}
+	    className="top-nav-root top-nav-bar flex justify-between items-center w-full">
+	  <div className="top-nav-primary flex shrink-0 items-center h-12">
 	    <PrefetchLink
 		to="/posts"
-		className="top-nav-brand-link mx-4 text-xl font-bold"
+		className="top-nav-brand-link shrink-0 whitespace-nowrap font-bold"
 		onClick={() => {
 		  scroll (0, 0)
 		}}>
 	      ぼざクリ タグ広場
 	    </PrefetchLink>
 
-	    <div ref={navRef} className="relative hidden md:flex h-12 items-center">
+	    <div ref={navRef}
+		className="top-nav-desktop-menu relative shrink-0 h-12 items-center">
 	      <div aria-hidden
 		   className={cn ('absolute inset-y-0 h-12',
 				  'top-nav-highlight',
@@ -239,6 +323,7 @@ const TopNav: FC<Props> = ({ user }) => {
 	      {visibleMenu.map ((item, i) => (
 		  <motion.div
 		      key={item.to}
+		      className="shrink-0 whitespace-nowrap"
 		      {...(animationsOff
 			   ? { }
 			   : { layoutId: `menu-${ item.name }` })}
@@ -252,7 +337,7 @@ const TopNav: FC<Props> = ({ user }) => {
 			ref={(el: (HTMLAnchorElement | null)) => {
 			  itemsRef.current[i] = el
 			}}
-			className={cn ('top-nav-menu-link relative z-10 flex h-full items-center px-5',
+			className={cn ('top-nav-menu-link relative z-10 flex h-full items-center',
 				       (i === openItemIdx) && 'font-bold')}>
 		      {item.name}
 		    </PrefetchLink>
@@ -267,7 +352,8 @@ const TopNav: FC<Props> = ({ user }) => {
 		    setMoreVsbl (true)
 		    measure (-1)
 		  }}
-		  className={cn ('top-nav-menu-link relative z-10 flex h-full items-center px-5',
+		  className={cn ('top-nav-menu-link relative z-10 flex h-full items-center',
+				 'shrink-0 whitespace-nowrap',
 				 (openItemIdx < 0 || moreVsbl) && 'font-bold')}>
 		その他 &raquo;
 	      </PrefetchLink>
@@ -278,7 +364,8 @@ const TopNav: FC<Props> = ({ user }) => {
 
 	  <button
 	     type="button"
-	     className="top-nav-mobile-toggle md:hidden ml-auto border-0 bg-transparent pr-4"
+	     className="top-nav-mobile-toggle shrink-0 whitespace-nowrap
+			ml-auto border-0 bg-transparent pr-4"
 	     onClick={() => {
 	       setMenuOpen (!(menuOpen))
 	     }}>
@@ -291,7 +378,8 @@ const TopNav: FC<Props> = ({ user }) => {
 	      key="submenu-shell"
 	      {...(animationsOff ? { }
 		   : { layout: true })}
-	      className="relative z-20 hidden md:block overflow-hidden top-nav-submenu"
+	      className="top-nav-desktop-submenu relative z-20 shrink-0
+			 overflow-hidden top-nav-submenu"
 	      animate={{ height: submenuHeight }}
 	      onMouseLeave={() => {
 		if (moreVsbl)
@@ -305,7 +393,8 @@ const TopNav: FC<Props> = ({ user }) => {
 	   ? (
 	      moreMenu.map ((item, i) => (
 		  <div key={i} className="relative h-[40px]">
-		    <div className="absolute inset-0 flex items-center px-3">
+		    <div className="absolute inset-0 flex items-center px-3
+				    overflow-x-auto whitespace-nowrap">
 		      <motion.div
 			  transition={topNavTransition}
 			  {...(animationsOff
@@ -315,7 +404,7 @@ const TopNav: FC<Props> = ({ user }) => {
 			       : { initial: { x: 40, y: -40, opacity: 0 },
 				   animate: { x: 0, y: 0, opacity: 1 },
 				   exit: { x: 40, y: -40, opacity: 0 } }))}
-			  className="z-10 h-full flex items-center px-3 font-bold w-28">
+			  className="z-10 h-full flex shrink-0 items-center px-3 font-bold w-28">
 			<h2>{item.name}</h2>
 		      </motion.div>
 		      {item.subMenu
@@ -363,7 +452,8 @@ const TopNav: FC<Props> = ({ user }) => {
 		  <div className="relative h-[40px]">
 		    {animationsOff
 		     ? (
-			 <div className="absolute inset-0 flex items-center px-3">
+			 <div className="absolute inset-0 flex items-center px-3
+					 overflow-x-auto whitespace-nowrap">
 			   {(visibleMenu[activeIdx]?.subMenu ?? [])
 			    .filter (item => item.visible ?? true)
 			    .map ((item, i) => (
@@ -392,7 +482,8 @@ const TopNav: FC<Props> = ({ user }) => {
 			       variants={{ enter: (d: -1 | 1) => ({ y: d * 24, opacity: 0 }),
 					   centre: { y: 0, opacity: 1 },
 					   exit: (d: -1 | 1) => ({ y: (-d) * 24, opacity: 0 }) }}
-			       className="absolute inset-0 flex items-center px-3"
+			       className="absolute inset-0 flex items-center px-3
+					  overflow-x-auto whitespace-nowrap"
 			       initial="enter"
 			       animate="centre"
 			       exit="exit"
@@ -432,7 +523,7 @@ const TopNav: FC<Props> = ({ user }) => {
 	 ? (
 	     menuOpen && (
 		 <div
-		     className={cn ('top-nav-root top-nav-mobile-menu flex flex-col md:hidden',
+		     className={cn ('top-nav-root top-nav-mobile-menu flex flex-col',
 				    'items-start')}>
 		   <Separator/>
 		   {visibleMenu.map ((item, i) => (
@@ -476,9 +567,6 @@ const TopNav: FC<Props> = ({ user }) => {
 		       </Fragment>))}
 		   <PrefetchLink
 		       to="/more"
-		       ref={(el: (HTMLAnchorElement | null)) => {
-			 itemsRef.current[visibleMenu.length] = el
-		       }}
 		       className={cn ('top-nav-mobile-row w-full min-h-[40px] flex items-center pl-8',
 				      ((openItemIdx < 0)
 				       && 'top-nav-mobile-active font-bold'))}>
@@ -492,7 +580,7 @@ const TopNav: FC<Props> = ({ user }) => {
 	       {menuOpen && (
 		   <motion.div
 		       key="spmenu"
-		       className={cn ('top-nav-root top-nav-mobile-menu flex flex-col md:hidden',
+		       className={cn ('top-nav-root top-nav-mobile-menu flex flex-col',
 				      'items-start')}
 		       variants={{ closed: { clipPath: 'inset(0 0 100% 0)',
 					     height:   0 },
@@ -558,9 +646,6 @@ const TopNav: FC<Props> = ({ user }) => {
 			 </Fragment>))}
 		     <PrefetchLink
 			 to="/more"
-			 ref={(el: (HTMLAnchorElement | null)) => {
-			   itemsRef.current[visibleMenu.length] = el
-			 }}
 			 className={cn ('top-nav-mobile-row w-full min-h-[40px] flex items-center pl-8',
 					((openItemIdx < 0)
 					 && 'top-nav-mobile-active font-bold'))}>
@@ -570,7 +655,7 @@ const TopNav: FC<Props> = ({ user }) => {
 		     <Separator/>
 		   </motion.div>)}
 	     </AnimatePresence>)}
-      </>)
+      </div>)
 }
 
 export default TopNav

frontend/src/components/TopNavUser.tsx

@@ -15,8 +15,8 @@ const TopNavUser: FC<Props> = ({ user, sp }) => {
     return
 
   const className = cn ((sp
-			 ? 'md:hidden w-full min-h-[40px] flex items-center pl-8'
-			 : 'hidden md:block ml-auto pr-4'),
+			 ? 'top-nav-user-mobile w-full min-h-[40px] flex items-center pl-8'
+			 : 'top-nav-user-desktop shrink-0 whitespace-nowrap ml-auto'),
 		        'font-bold text-red-600 hover:text-red-400',
 		        'dark:text-yellow-400 dark:hover:text-yellow-200')
 

frontend/src/index.css

@@ -197,12 +197,244 @@ body
   background: var(--top-nav-root-bg-mobile);
 }
 
+.top-nav-bar
+{
+  padding-inline: 8px;
+  column-gap: 4px;
+}
+
+.top-nav-primary
+{
+  column-gap: 4px;
+}
+
+.top-nav-layout .top-nav-brand-link
+{
+  margin-inline: 4px;
+  font-size: 1rem;
+}
+
+.top-nav-desktop-menu,
+.top-nav-desktop-submenu,
+.top-nav-user-desktop
+{
+  display: none;
+}
+
+.top-nav-desktop-menu
+{
+  font-size: .875rem;
+}
+
+.top-nav-desktop-menu .top-nav-menu-link
+{
+  padding-inline: 8px;
+}
+
+.top-nav-user-desktop
+{
+  padding-right: 4px;
+  font-size: .875rem;
+}
+
+.top-nav-user-mobile
+{
+  overflow-wrap: anywhere;
+}
+
 @media (min-width: 768px)
 {
-  .top-nav-root
+  .top-nav-layout .top-nav-brand-link
+  {
+    font-size: 1.125rem;
+  }
+
+  .top-nav-layout[data-desktop='true'] .top-nav-root
   {
     background: var(--top-nav-root-bg-desktop);
   }
+
+  .top-nav-layout[data-desktop='true'] .top-nav-desktop-menu,
+  .top-nav-layout[data-measuring] .top-nav-desktop-menu
+  {
+    display: flex;
+  }
+
+  .top-nav-layout[data-desktop='true'] .top-nav-user-desktop,
+  .top-nav-layout[data-measuring] .top-nav-user-desktop,
+  .top-nav-layout[data-desktop='true'] .top-nav-desktop-submenu
+  {
+    display: block;
+  }
+
+  .top-nav-layout[data-desktop='true'] .top-nav-mobile-toggle,
+  .top-nav-layout[data-measuring] .top-nav-mobile-toggle,
+  .top-nav-layout[data-desktop='true'] .top-nav-mobile-menu,
+  .top-nav-layout[data-desktop='true'] .top-nav-user-mobile
+  {
+    display: none;
+  }
+
+  .top-nav-layout[data-measuring]
+  {
+    visibility: hidden;
+  }
+}
+
+@media (min-width: 1024px)
+{
+  .top-nav-bar
+  {
+    padding-inline: 12px;
+    column-gap: 8px;
+  }
+
+  .top-nav-primary
+  {
+    column-gap: 6px;
+  }
+
+  .top-nav-layout .top-nav-brand-link
+  {
+    margin-inline: 8px;
+    font-size: 1.25rem;
+  }
+
+  .top-nav-desktop-menu
+  {
+    column-gap: 2px;
+  }
+
+  .top-nav-desktop-menu,
+  .top-nav-user-desktop
+  {
+    font-size: .9375rem;
+  }
+
+  .top-nav-desktop-menu .top-nav-menu-link
+  {
+    padding-inline: 12px;
+  }
+
+  .top-nav-user-desktop
+  {
+    padding-right: 8px;
+  }
+}
+
+@media (min-width: 1280px)
+{
+  .top-nav-bar
+  {
+    column-gap: 12px;
+  }
+
+  .top-nav-primary
+  {
+    column-gap: 8px;
+  }
+
+  .top-nav-layout .top-nav-brand-link
+  {
+    margin-inline: 16px;
+  }
+
+  .top-nav-desktop-menu
+  {
+    column-gap: 4px;
+  }
+
+  .top-nav-desktop-menu,
+  .top-nav-user-desktop
+  {
+    font-size: 1rem;
+  }
+
+  .top-nav-desktop-menu .top-nav-menu-link
+  {
+    padding-inline: 20px;
+  }
+
+  .top-nav-user-desktop
+  {
+    padding-right: 16px;
+  }
+}
+
+.top-nav-layout[data-density='comfortable'] .top-nav-bar
+{
+  padding-inline: 12px;
+  column-gap: 8px;
+}
+
+.top-nav-layout[data-density='comfortable'] .top-nav-primary
+{
+  column-gap: 6px;
+}
+
+.top-nav-layout[data-density='comfortable'] .top-nav-brand-link
+{
+  margin-inline: 8px;
+  font-size: 1.25rem;
+}
+
+.top-nav-layout[data-density='comfortable'] .top-nav-desktop-menu
+{
+  column-gap: 2px;
+}
+
+.top-nav-layout[data-density='comfortable'] .top-nav-desktop-menu,
+.top-nav-layout[data-density='comfortable'] .top-nav-user-desktop
+{
+  font-size: .9375rem;
+}
+
+.top-nav-layout[data-density='comfortable'] .top-nav-desktop-menu .top-nav-menu-link
+{
+  padding-inline: 12px;
+}
+
+.top-nav-layout[data-density='comfortable'] .top-nav-user-desktop
+{
+  padding-right: 8px;
+}
+
+.top-nav-layout[data-density='compact'] .top-nav-bar
+{
+  padding-inline: 8px;
+  column-gap: 4px;
+}
+
+.top-nav-layout[data-density='compact'] .top-nav-primary
+{
+  column-gap: 4px;
+}
+
+.top-nav-layout[data-density='compact'] .top-nav-brand-link
+{
+  margin-inline: 4px;
+  font-size: 1.125rem;
+}
+
+.top-nav-layout[data-density='compact'] .top-nav-desktop-menu
+{
+  column-gap: 0;
+}
+
+.top-nav-layout[data-density='compact'] .top-nav-desktop-menu,
+.top-nav-layout[data-density='compact'] .top-nav-user-desktop
+{
+  font-size: .875rem;
+}
+
+.top-nav-layout[data-density='compact'] .top-nav-desktop-menu .top-nav-menu-link
+{
+  padding-inline: 8px;
+}
+
+.top-nav-layout[data-density='compact'] .top-nav-user-desktop
+{
+  padding-right: 4px;
 }
 
 .top-nav-brand-link,

frontend/src/lib/help.test.ts

@@ -0,0 +1,130 @@
+import { describe, expect, it } from 'vitest'
+
+import { createHelpFixture } from '@/test/helpFixtures'
+
+
+describe ('Help file catalogue', () => {
+  it.each ([['Éclair', 'eclair'], ['home', 'HOME'], ['e\u0301clair', 'Éclair']]) (
+      'compares title %s and %s equally at primary strength', (a, b) => {
+	const { api } = createHelpFixture ()
+	expect (api.compareHelpTitles (a, b)).toBe (0)
+      })
+
+  it ('orders distinct Latin titles in both directions', () => {
+    const { api } = createHelpFixture ()
+    expect (api.compareHelpTitles ('Apple', 'Zebra')).toBeLessThan (0)
+    expect (api.compareHelpTitles ('Zebra', 'Apple')).toBeGreaterThan (0)
+  })
+
+  it.each ([['ほーむ', 'ホーム'], ['ホーム', 'ほーむ'], ['ﾎｰﾑ', 'ホーム'],
+      ['ＡＢＣ', 'abc'], ['ｶﾞｲﾄﾞ', 'がいど']]) (
+      'normalises search text %s and %s identically', (query, text) => {
+	const { api } = createHelpFixture ()
+	expect (api.normaliseHelpSearchText (query)).toBe (api.normaliseHelpSearchText (text))
+      })
+
+  it.each ([['2026-10-02', null], ['2026-10-02.2', 2],
+      ['2026-10-02.3', 3], ['2026-10-02.10', 10]] as const) (
+      'separates the date and revision of %s', (edition, revision) => {
+	const { api } = createHelpFixture ()
+	expect (api.parseHelpEdition (edition)).toEqual ({ date: '2026-10-02', revision })
+	const date = api.helpEditionDate (edition)!
+	expect ([date.getFullYear (), date.getMonth (), date.getDate ()]).toEqual ([2026, 9, 2])
+      })
+
+  it.each (['missing', '2026-02-30', '2026-10-02.0', '2026-10-02.2.extra',
+      '2026-10-02.9007199254740992']) ('rejects invalid edition %s', edition => {
+    const { api } = createHelpFixture ()
+    expect (api.parseHelpEdition (edition)).toBeNull ()
+    expect (api.helpEditionDate (edition)).toBeNull ()
+  })
+
+  it ('compares numeric revision suffixes without losing the edition identifiers', () => {
+    const { api } = createHelpFixture ()
+    expect (['2026-10-02.10', '2026-10-02.3', '2026-10-02', '2026-10-02.2']
+	.sort (api.compareHelpEditions)).toEqual ([
+	'2026-10-02', '2026-10-02.2', '2026-10-02.3', '2026-10-02.10'])
+  })
+
+  it ('uses current_edition rather than the newest file, without YAML titles', async () => {
+    const { api } = createHelpFixture ()
+    expect ((await api.getHelpArticle ('guide'))?.languages.ja.currentEdition)
+	.toBe ('2026-10-02.10')
+    expect (await api.getHelpSearchEntries ('ja')).toEqual ([
+	{ identifier: 'guide', edition: '2026-10-02.10',
+	  title: 'ホーム第十版', body: '\nタグを検索する 第十版本文' },
+	{ identifier: '設定 手順', edition: '2026-10-03',
+	  title: '設定', body: '\nタグを設定する' }])
+  })
+
+  it ('orders published editions numerically and separates languages', async () => {
+    const { api } = createHelpFixture ()
+    expect (await api.getHelpEditions ('guide', 'ja')).toEqual ([
+	'2026-10-01', '2026-10-02', '2026-10-02.2', '2026-10-02.3', '2026-10-02.10'])
+    expect (await api.getHelpEditions ('guide', 'en')).toEqual (['2026-10-02'])
+    expect (await api.getHelpPublishedEditions ('guide', 'ja'))
+	.not.toContain ('2026-10-02.11')
+  })
+
+  it ('uses generated edition titles without reparsing raw sources', async () => {
+    const { api, raw } = createHelpFixture ()
+    expect (await api.getHelpEditionText ('guide', 'ja', '2026-10-01'))
+	.toEqual ({ title: '旧ホーム', body: '\n過去限定本文' })
+    raw['../../help/guide/editions/ja/2026-10-02.mdx']
+	.mockResolvedValue ('\uFEFF# 改題\r\n\r\n本文\r\n# 別見出し')
+    expect (await api.getHelpEditionText ('guide', 'ja', '2026-10-02'))
+	.toEqual ({ title: 'ホーム', body: '\nタグを検索する' })
+    expect (raw['../../help/guide/editions/ja/2026-10-02.mdx']).not.toHaveBeenCalled ()
+    expect ((await api.getHelpEditionSource ('guide', 'ja', '2026-10-02'))?.source)
+	.toBe ('\uFEFF# 改題\r\n\r\n本文\r\n# 別見出し')
+  })
+
+  it ('returns missing resources without substituting another edition', async () => {
+    const { api } = createHelpFixture ()
+    expect (await api.getHelpArticle ('absent')).toBeNull ()
+    expect (await api.getHelpEditions ('absent', 'ja')).toEqual ([])
+    expect (await api.getHelpEdition ('guide', 'ja', 'missing')).toBeNull ()
+    expect (await api.getHelpEditionText ('guide', 'fr', '2026-10-02')).toBeNull ()
+  })
+
+  it ('reports generated search asset failures instead of hiding articles', async () => {
+    const { api, loads } = createHelpFixture ()
+    loads.search.mockImplementationOnce (() => { throw new Error ('asset failure') })
+    await expect (api.getHelpSearchEntries ('ja')).rejects.toThrow ('asset failure')
+  })
+
+  it ('reports raw loading failures and excludes unpublished editions', async () => {
+    const { api, raw } = createHelpFixture ()
+    const loader = raw['../../help/guide/editions/ja/2026-10-02.mdx']
+    loader.mockRejectedValueOnce (new Error ('fixture failure'))
+    await expect (api.getHelpEditionSource ('guide', 'ja', '2026-10-02'))
+	.rejects.toThrow ('fixture failure')
+    expect (await api.getHelpEdition ('guide', 'ja', '2026-10-02.11')).toBeNull ()
+    expect (await api.getHelpEditionSource ('guide', 'ja', '2026-10-02.11')).toBeNull ()
+  })
+
+  it ('compares raw H1 and body lines in the requested direction', async () => {
+    const { api } = createHelpFixture ()
+    const forward = await api.getHelpDiff ('guide', 'ja', '2026-10-01', '2026-10-02.10')
+    expect (forward?.from.title).toBe ('旧ホーム')
+    expect (forward?.to.title).toBe ('ホーム第十版')
+    expect (forward?.lines).toEqual (expect.arrayContaining ([
+	{ type: 'removed', content: '# 旧ホーム\n' },
+	{ type: 'added', content: '# ホーム第十版\n' },
+	{ type: 'unchanged', content: '\n' }]))
+    const reverse = await api.getHelpDiff ('guide', 'ja', '2026-10-02.10', '2026-10-01')
+    expect (reverse?.lines).toEqual (expect.arrayContaining ([
+	{ type: 'added', content: '# 旧ホーム\n' },
+	{ type: 'removed', content: '# ホーム第十版\n' }]))
+  })
+
+  it.each ([['missing', 'ja', '2026-10-01', '2026-10-02.10'],
+      ['guide', 'ja', '2026-10-01', '2026-10-02.11'],
+      ['guide', 'en', '2026-10-01', '2026-10-02']]) (
+      'rejects invalid diff editions for %s/%s without reading sources',
+      async (id, language, from, to) => {
+	const { api, raw } = createHelpFixture ()
+	expect (await api.getHelpDiff (id, language, from, to)).toBeNull ()
+	Object.values (raw).forEach (load => expect (load).not.toHaveBeenCalled ())
+      })
+})

frontend/src/lib/help.ts

@@ -0,0 +1,237 @@
+import type { ComponentType } from 'react'
+
+type ArticleLanguage = { currentEdition: string; publishedEditions: string[] }
+
+export type ArticleMeta = {
+    identifier: string
+    canonicalLanguage: string
+    languages:  Record<string, ArticleLanguage> }
+
+export type HelpEditionText = {
+    title: string
+    body:  string }
+
+export type HelpSearchEntry = HelpEditionText & {
+    identifier: string
+    edition:    string }
+
+export type HelpEditionContent = HelpEditionText & { content: ComponentType }
+
+export type HelpEditionSource = HelpEditionText & { source: string }
+export type HelpChange = Pick<HelpSearchEntry, 'identifier' | 'edition' | 'title'> & {
+    pred: string | null
+    change: 'new' | 'update'
+    date: string }
+export type HelpTitleIndex = Record<string, string[]>
+export type HelpDiffLine = {
+    type: 'added' | 'removed' | 'unchanged'
+    content: string }
+export type HelpDiff = {
+    from: HelpEditionSource
+    to: HelpEditionSource
+    lines: HelpDiffLine[] }
+
+export type HelpEditionIdentifier = { date: string; revision: number | null }
+
+// Fix the locale and primary strength; ICU versions still differ from MySQL UCA 9.0.
+const helpTitleCollator = new Intl.Collator ('en', {
+    usage: 'sort', sensitivity: 'base', ignorePunctuation: false, numeric: false })
+
+
+export const compareHelpTitles = (a: string, b: string): number =>
+    helpTitleCollator.compare (a, b)
+
+
+export const normaliseHelpSearchText = (value: string): string =>
+    value.normalize ('NFKC').toLowerCase ().replace (/[ァ-ヶヽヾ]/g,
+	character => String.fromCodePoint (character.codePointAt (0)! - 0x60))
+
+
+export const parseHelpEdition = (edition: string): HelpEditionIdentifier | null => {
+  const matched = edition.match (/^(\d{4}-\d{2}-\d{2})(?:\.([1-9]\d*))?$/)
+  if (matched == null)
+    return null
+  const date = new Date (matched[1])
+  const revision = matched[2] == null ? null : Number (matched[2])
+  if (Number.isNaN (date.getTime ()) || date.toISOString ().slice (0, 10) !== matched[1]
+      || (revision != null && !(Number.isSafeInteger (revision))))
+    return null
+  return { date: matched[1], revision }
+}
+
+
+export const compareHelpEditions = (a: string, b: string): number => {
+  const before = parseHelpEdition (a)
+  const after = parseHelpEdition (b)
+  if (before == null || after == null)
+    return a.localeCompare (b)
+  return before.date.localeCompare (after.date)
+      || (before.revision ?? 1) - (after.revision ?? 1)
+}
+
+
+export const helpEditionDate = (edition: string): Date | null => {
+  const parsed = parseHelpEdition (edition)
+  return parsed == null ? null : new Date (`${ parsed.date }T00:00:00`)
+}
+
+
+export const getHelpArticle = async (identifier: string): Promise<ArticleMeta | null> => {
+  const { default: articles } = await import ('virtual:help/articles')
+  return Object.hasOwn (articles, identifier) ? articles[identifier] : null
+}
+
+
+export const getHelpEditions = async (
+    identifier: string,
+    language: string,
+): Promise<string[]> => {
+  const article = await getHelpArticle (identifier)
+  return article?.languages[language]?.publishedEditions ?? []
+}
+
+
+export const getHelpPublishedEditions = async (
+    identifier: string,
+    language: string,
+): Promise<string[]> => await getHelpEditions (identifier, language)
+
+
+export const getHelpEdition = async (
+    identifier: string,
+    language: string,
+    edition: string,
+): Promise<ComponentType | null> => {
+  const { components } = await import ('virtual:help/loaders')
+  const loader = components[`${ identifier }/${ language }/${ edition }`]
+
+  if (!(loader))
+    return null
+
+  return (await loader ()).default
+}
+
+
+export const getHelpEditionSource = async (
+    identifier: string,
+    language: string,
+    edition: string,
+): Promise<HelpEditionSource | null> => {
+  const { sources } = await import ('virtual:help/loaders')
+  const loader = sources[`${ identifier }/${ language }/${ edition }`]
+
+  if (!(loader))
+    return null
+
+  const [source, text] = await Promise.all ([
+      loader (), getHelpEditionText (identifier, language, edition)])
+  return text == null ? null : { source: source.default, ...text }
+}
+
+
+export const getHelpEditionText = async (
+    identifier: string,
+    language: string,
+    edition: string,
+): Promise<HelpEditionText | null> => {
+  const { texts } = await import ('virtual:help/loaders')
+  const loader = texts[`${ identifier }/${ language }/${ edition }`]
+  return loader == null ? null : (await loader ()).default
+}
+
+
+export const getHelpSearchEntries = async (
+    language: string,
+): Promise<HelpSearchEntry[]> => {
+  const { default: search } = await import ('virtual:help/search')
+  return search[language] ?? []
+}
+
+
+export const getHelpEditionContent = async (
+    identifier: string,
+    language: string,
+    edition: string,
+): Promise<HelpEditionContent | null> => {
+  const [content, text] = await Promise.all ([
+      getHelpEdition (identifier, language, edition),
+      getHelpEditionText (identifier, language, edition)])
+
+  return content != null && text != null ? { content, ...text } : null
+}
+
+
+export const getHelpChanges = async (language: string): Promise<HelpChange[]> => {
+  const { default: changes } = await import ('virtual:help/changes')
+  return changes[language] ?? []
+}
+
+
+export const getHelpTitleIndex = async (language: string): Promise<HelpTitleIndex> => {
+  const { default: titles } = await import ('virtual:help/titles')
+  return titles[language] ?? { }
+}
+
+
+const diffHelpLines = (from: string, to: string): HelpDiffLine[] => {
+  const before = from.match (/[^\n]*\n|[^\n]+$/g) ?? []
+  const after = to.match (/[^\n]*\n|[^\n]+$/g) ?? []
+  const lengths = Array.from ({ length: before.length + 1 },
+      () => new Uint32Array (after.length + 1))
+
+  for (let i = before.length - 1; i >= 0; --i)
+  {
+    for (let j = after.length - 1; j >= 0; --j)
+    {
+      lengths[i][j] =
+	  before[i] === after[j]
+	  ? lengths[i + 1][j + 1] + 1
+	  : Math.max (lengths[i + 1][j], lengths[i][j + 1])
+    }
+  }
+
+  const lines: HelpDiffLine[] = []
+  let i = 0
+  let j = 0
+  while (i < before.length || j < after.length)
+  {
+    if (i < before.length && j < after.length && before[i] === after[j])
+    {
+      lines.push ({ type: 'unchanged', content: before[i] })
+      ++i
+      ++j
+    }
+    else if (i < before.length
+	     && (j === after.length || lengths[i + 1][j] >= lengths[i][j + 1]))
+    {
+      lines.push ({ type: 'removed', content: before[i] })
+      ++i
+    }
+    else
+    {
+      lines.push ({ type: 'added', content: after[j] })
+      ++j
+    }
+  }
+  return lines
+}
+
+
+export const getHelpDiff = async (
+    identifier: string,
+    language: string,
+    from: string,
+    to: string,
+): Promise<HelpDiff | null> => {
+  const editions = await getHelpPublishedEditions (identifier, language)
+  if (!(editions.includes (from)) || !(editions.includes (to)))
+    return null
+
+  const [before, after] = await Promise.all ([
+      getHelpEditionSource (identifier, language, from),
+      getHelpEditionSource (identifier, language, to)])
+  if (before == null || after == null)
+    return null
+
+  return { from: before, to: after, lines: diffHelpLines (before.source, after.source) }
+}

frontend/src/lib/helpPrefetch.test.tsx

@@ -0,0 +1,126 @@
+import { QueryClient } from '@tanstack/react-query'
+import { fireEvent, screen, waitFor } from '@testing-library/react'
+import { Route, Routes, useLocation } from 'react-router-dom'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import PrefetchLink from '@/components/PrefetchLink'
+import * as prefetchers from '@/lib/prefetchers'
+import HelpDetailPage from '@/pages/help/HelpDetailPage'
+import HelpDiffPage from '@/pages/help/HelpDiffPage'
+import { helpFixture } from '@/test/helpFixtures'
+import { renderWithProviders } from '@/test/render'
+
+const settings = vi.hoisted (() => ({ linkPreload: 'intent' }))
+vi.mock ('@/lib/useClientBehaviourSettings', () => ({
+    useClientBehaviourSettings: () => settings }))
+vi.mock ('@/lib/help', async () => (await import ('@/test/helpFixtures')).helpFixture.api)
+
+
+const Location = () => {
+  const location = useLocation ()
+  return <output data-testid="location">{location.pathname}</output>
+}
+
+
+describe ('Help prefetch integration', () => {
+  beforeEach (() => {
+    helpFixture.reset ()
+    settings.linkPreload = 'intent'
+    vi.clearAllMocks ()
+  })
+
+  it.each (['/help', '/help?title=ホーム&text=検索&page=2', '/help/guide', '/help/changes',
+      '/help/guide/diff?from=2026-10-01&to=2026-10-02.10',
+      '/help/%E8%A8%AD%E5%AE%9A%20%E6%89%8B%E9%A0%86?edition=2026-10-03']) (
+      'registers a unique prefetcher for %s', path => {
+	const url = new URL (path, window.location.origin)
+	expect (prefetchers.routePrefetchers.filter (entry => entry.test (url))).toHaveLength (1)
+      })
+
+  it ('loads and reuses the same MDX module for repeated prefetch and display', async () => {
+    const { queryClient } = renderWithProviders (
+	<>
+	  <PrefetchLink to="/help/guide">Help</PrefetchLink>
+	  <Routes>
+	    <Route path="/" element={null}/>
+	    <Route path="/help/:identifier" element={<HelpDetailPage/>}/>
+	  </Routes>
+	</>)
+    const loader = helpFixture.moduleLoads['../../help/guide/editions/ja/2026-10-02.10.mdx']
+    await Promise.all ([
+	prefetchers.prefetchForURL (queryClient, '/help/guide'),
+	prefetchers.prefetchForURL (queryClient, '/help/guide')])
+    expect (loader).toHaveBeenCalledTimes (1)
+    fireEvent.click (screen.getByRole ('link', { name: 'Help' }))
+    await screen.findByRole ('heading', { name: 'ホーム第十版' })
+    expect (loader).toHaveBeenCalledTimes (1)
+  })
+
+  it ('prefetches the requested identifier and edition independently', async () => {
+    const client = new QueryClient ()
+    await prefetchers.prefetchForURL (client, '/help/guide?edition=2026-10-02.3')
+    await prefetchers.prefetchForURL (client, `/help/${encodeURIComponent ('設定 手順')}`)
+    expect (helpFixture.modules['../../help/guide/editions/ja/2026-10-02.3.mdx'])
+	.toHaveBeenCalledTimes (1)
+    expect (helpFixture.modules['../../help/設定 手順/editions/ja/2026-10-03.mdx'])
+	.toHaveBeenCalledTimes (1)
+    expect (helpFixture.modules['../../help/guide/editions/en/2026-10-02.mdx'])
+	.not.toHaveBeenCalled ()
+  })
+
+  it ('loads history editions without treating changes as an article', async () => {
+    const article = vi.spyOn (helpFixture.api, 'getHelpArticle')
+    await prefetchers.prefetchForURL (new QueryClient (), '/help/changes')
+    expect (article).not.toHaveBeenCalledWith ('changes')
+    expect (helpFixture.loads.changes).toHaveBeenCalledTimes (1)
+    Object.values (helpFixture.raw).forEach (load => expect (load).not.toHaveBeenCalled ())
+    Object.values (helpFixture.modules).forEach (load => expect (load).not.toHaveBeenCalled ())
+  })
+
+  it ('reuses two prefetched raw editions when displaying their diff', async () => {
+    const path = '/help/guide/diff?from=2026-10-01&to=2026-10-02.10'
+    const { queryClient } = renderWithProviders (
+	<>
+	  <PrefetchLink to={path}>diff</PrefetchLink>
+	  <Routes>
+	    <Route path="/" element={null}/>
+	    <Route path="/help/:identifier/diff" element={<HelpDiffPage/>}/>
+	  </Routes>
+	</>)
+    await Promise.all ([prefetchers.prefetchForURL (queryClient, path),
+	prefetchers.prefetchForURL (queryClient, path)])
+    fireEvent.click (screen.getByRole ('link', { name: 'diff' }))
+    expect (await screen.findByRole ('link', { name: '旧ホーム (2026-10-01)' }))
+	.toHaveAttribute ('href', '/help/guide?edition=2026-10-01')
+    expect (screen.getByText ('+ # ホーム第十版')).toBeInTheDocument ()
+    Object.entries (helpFixture.raw).forEach (([name, load]) => {
+      const expected = /\/ja\/(2026-10-01|2026-10-02\.10)\.mdx$/.test (name) ? 1 : 0
+      expect (load).toHaveBeenCalledTimes (expected)
+    })
+    Object.values (helpFixture.modules).forEach (load => expect (load).not.toHaveBeenCalled ())
+  })
+
+  it.each (['intent', 'off']) ('respects %s intent settings and still prefetches clicks',
+      async mode => {
+	settings.linkPreload = mode
+	const run = vi.spyOn (prefetchers, 'prefetchForURL').mockResolvedValue ()
+	renderWithProviders (<><Location/><PrefetchLink to="/help/guide">Help</PrefetchLink></>)
+	const link = screen.getByRole ('link', { name: 'Help' })
+	fireEvent.mouseEnter (link)
+	fireEvent.touchStart (link)
+	expect (run).toHaveBeenCalledTimes (mode === 'intent' ? 2 : 0)
+	fireEvent.click (link)
+	await waitFor (() => expect (screen.getByTestId ('location'))
+	    .toHaveTextContent ('/help/guide'))
+	expect (run).toHaveBeenCalledTimes (mode === 'intent' ? 3 : 1)
+      })
+
+  it ('continues navigation after prefetch failure', async () => {
+    vi.spyOn (console, 'error').mockImplementation (() => {})
+    vi.spyOn (prefetchers, 'prefetchForURL').mockRejectedValue (new Error ('offline'))
+    renderWithProviders (<><Location/><PrefetchLink to="/help/guide">Help</PrefetchLink></>)
+    fireEvent.click (screen.getByRole ('link', { name: 'Help' }))
+    await waitFor (() => expect (screen.getByTestId ('location'))
+	.toHaveTextContent ('/help/guide'))
+  })
+})

frontend/src/lib/prefetchers.ts

@@ -1,8 +1,13 @@
 import { QueryClient } from '@tanstack/react-query'
 import { match } from 'path-to-regexp'
 
+import { getHelpArticle,
+	 getHelpChanges,
+	 getHelpDiff,
+	 getHelpEditionContent,
+	 getHelpSearchEntries } from '@/lib/help'
 import { fetchPost, fetchPosts, fetchPostChanges } from '@/lib/posts'
-import { postsKeys, tagsKeys, wikiKeys } from '@/lib/queryKeys'
+import { helpKeys, postsKeys, tagsKeys, wikiKeys } from '@/lib/queryKeys'
 import { fetchNicoTags, fetchTagByName, fetchTag, fetchTagChanges, fetchTags } from '@/lib/tags'
 import { fetchWikiPage,
 	 fetchWikiPageByTitle,
@@ -15,12 +20,69 @@ type Prefetcher = (qc: QueryClient, url: URL) => Promise<void>
 const mPost = match<{ id: string }> ('/posts/:id')
 const mWiki = match<{ title: string }> ('/wiki/:title')
 const mTag = match<{ id: string }> ('/tags/:id')
+const mHelp = match<{ identifier: string }> ('/help/:identifier')
+const mHelpDiff = match<{ identifier: string }> ('/help/:identifier/diff')
 
 
 const boolFromQuery = (value: string | null): boolean =>
     ['1', 'true', 'on', 'yes', ''].includes ((value ?? '').toLowerCase ())
 
 
+const prefetchHelpIndex: Prefetcher = async qc => {
+  await qc.prefetchQuery ({
+      queryKey: helpKeys.index ('ja'),
+      queryFn: () => getHelpSearchEntries ('ja'),
+      staleTime: Infinity })
+}
+
+
+const prefetchHelpShow: Prefetcher = async (qc, url) => {
+  const matched = mHelp (url.pathname)
+  if (!(matched))
+    return
+
+  const { identifier } = matched.params
+  const language = 'ja'
+  const article = await qc.fetchQuery ({
+      queryKey: helpKeys.article (identifier),
+      queryFn: () => getHelpArticle (identifier),
+      staleTime: Infinity })
+  const edition = url.searchParams.get ('edition') ?? article?.languages[language]?.currentEdition
+  if (edition == null)
+    return
+
+  await qc.prefetchQuery ({
+      queryKey: helpKeys.edition (identifier, language, edition),
+      queryFn: () => getHelpEditionContent (identifier, language, edition),
+      staleTime: Infinity })
+}
+
+
+const prefetchHelpChanges: Prefetcher = async qc => {
+  await qc.prefetchQuery ({
+      queryKey: helpKeys.changes ('ja'),
+      queryFn: () => getHelpChanges ('ja'),
+      staleTime: Infinity })
+}
+
+
+const prefetchHelpDiff: Prefetcher = async (qc, url) => {
+  const matched = mHelpDiff (url.pathname)
+  const fromValues = url.searchParams.getAll ('from')
+  const toValues = url.searchParams.getAll ('to')
+  const [from] = fromValues
+  const [to] = toValues
+  if (!(matched) || fromValues.length !== 1 || toValues.length !== 1 || !(from) || !(to))
+    return
+
+  const { identifier } = matched.params
+  await qc.prefetchQuery ({
+      queryKey: helpKeys.diff (identifier, 'ja', from, to),
+      queryFn: () => getHelpDiff (identifier, 'ja', from, to),
+      staleTime: Infinity })
+}
+
+
 const prefetchWikiPagesIndex: Prefetcher = async (qc, url) => {
   const title = url.searchParams.get ('title') ?? ''
 
@@ -223,6 +285,13 @@ const prefetchTagChanges: Prefetcher = async (qc, url) => {
 
 
 export const routePrefetchers: { test: (u: URL) => boolean; run: Prefetcher }[] = [
+    { test: u => /^\/help\/?$/.test (u.pathname), run: prefetchHelpIndex },
+    { test: u => /^\/help\/changes\/?$/.test (u.pathname), run: prefetchHelpChanges },
+    { test: u => Boolean (mHelpDiff (u.pathname)), run: prefetchHelpDiff },
+    { test: u => {
+	const matched = mHelp (u.pathname)
+	return Boolean (matched && matched.params.identifier !== 'changes')
+      }, run: prefetchHelpShow },
     { test: u => ['/', '/posts', '/posts/search'].includes (u.pathname),
       run: prefetchPostsIndex },
     { test: u => (!(['/posts/new', '/posts/changes', '/posts/search'].includes (u.pathname))

frontend/src/lib/queryKeys.ts

@@ -61,3 +61,14 @@ export const wikiKeys = {
     root:   ['wiki'] as const,
     index:  (p: { title?: string }) => ['wiki', 'index', p] as const,
     show:   (title: string, p: { version?: string }) => ['wiki', title, p] as const }
+
+export const helpKeys = {
+    root:    ['help'] as const,
+    index:   (language: string) => ['help', 'index', language] as const,
+    titles:  (language: string) => ['help', 'titles', language] as const,
+    article: (identifier: string) => ['help', 'article', identifier] as const,
+    edition: (identifier: string, language: string, edition: string) =>
+	     ['help', 'edition', identifier, language, edition] as const,
+    diff: (identifier: string, language: string, from: string, to: string) =>
+	  ['help', 'diff', identifier, language, from, to] as const,
+    changes: (language: string) => ['help', 'changes', language] as const }

frontend/src/lib/tags.test.ts

@@ -72,11 +72,25 @@ describe ('tags API functions', () => {
     )
   })
 
-  it ('returns null when tag fetches fail', async () => {
-    api.apiGet.mockRejectedValueOnce (new Error ('missing'))
+  it ('returns null for a missing named tag while retaining the ID helper contract', async () => {
     api.apiGet.mockRejectedValueOnce (new Error ('missing'))
+    api.apiGet.mockRejectedValueOnce ({ isAxiosError: true, response: { status: 404 } })
 
     await expect (fetchTag ('1')).resolves.toBeNull ()
     await expect (fetchTagByName ('unknown')).resolves.toBeNull ()
   })
+
+  it.each ([403, 500]) ('propagates named tag HTTP %s', async status => {
+    const error = { isAxiosError: true, response: { status } }
+    api.apiGet.mockRejectedValueOnce (error)
+    await expect (fetchTagByName ('ホーム')).rejects.toBe (error)
+  })
+
+  it ('propagates named tag network failures and encodes the name', async () => {
+    const error = new Error ('offline')
+    api.apiGet.mockRejectedValueOnce (error)
+    await expect (fetchTagByName ('名前/#%')).rejects.toBe (error)
+    expect (api.apiGet).toHaveBeenLastCalledWith (
+	`/tags/name/${ encodeURIComponent ('名前/#%') }`)
+  })
 })

frontend/src/lib/tags.ts

@@ -1,3 +1,5 @@
+import { isAxiosError } from 'axios'
+
 import { apiGet } from '@/lib/api'
 
 import type { Deerjikist,
@@ -68,11 +70,13 @@ export const fetchExternalTag = async (id: string): Promise<Tag | null> => {
 export const fetchTagByName = async (name: string): Promise<Tag | null> => {
   try
   {
-    return await apiGet (`/tags/name/${ name }`)
+    return await apiGet (`/tags/name/${ encodeURIComponent (name) }`)
   }
-  catch
+  catch (error)
   {
+    if (isAxiosError (error) && error.response?.status === 404)
       return null
+    throw error
   }
 }
 

frontend/src/lib/wiki.test.ts

@@ -2,9 +2,7 @@ import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import { fetchWikiPage, fetchWikiPageByTitle, fetchWikiPages } from '@/lib/wiki'
 
-const api = vi.hoisted (() => ({
-  apiGet: vi.fn (),
-}))
+const api = vi.hoisted (() => ({ apiGet: vi.fn () }))
 
 vi.mock ('@/lib/api', () => api)
 
@@ -23,18 +21,16 @@ describe ('wiki API functions', () => {
     expect (api.apiGet).toHaveBeenNthCalledWith (
 	1,
 	'/wiki',
-        { params: { title: '虹' } },
-    )
+	{ params: { title: '虹' } })
     expect (api.apiGet).toHaveBeenNthCalledWith (
 	2,
 	'/wiki/1',
-        { params: { version: '3' } },
-    )
+	{ params: { version: '3' } })
   })
 
   it ('encodes title path segments and returns null on misses', async () => {
     api.apiGet.mockResolvedValueOnce ({ id: 2 })
-    api.apiGet.mockRejectedValueOnce (new Error ('missing'))
+    api.apiGet.mockRejectedValueOnce ({ isAxiosError: true, response: { status: 404 } })
 
     await fetchWikiPageByTitle ('a/b c', { version: undefined })
     await expect (fetchWikiPageByTitle ('missing', { })).resolves.toBeNull ()
@@ -42,7 +38,18 @@ describe ('wiki API functions', () => {
     expect (api.apiGet).toHaveBeenNthCalledWith (
 	1,
 	'/wiki/title/a%2Fb%20c',
-        { params: { version: undefined } },
-    )
+	{ params: { version: undefined } })
+  })
+
+  it.each ([403, 500]) ('preserves HTTP %s errors', async status => {
+    const error = { isAxiosError: true, response: { status } }
+    api.apiGet.mockRejectedValueOnce (error)
+    await expect (fetchWikiPageByTitle ('ホーム', { })).rejects.toBe (error)
+  })
+
+  it ('preserves network failures for callers', async () => {
+    const error = new Error ('network failure')
+    api.apiGet.mockRejectedValueOnce (error)
+    await expect (fetchWikiPageByTitle ('ホーム', { })).rejects.toBe (error)
   })
 })

frontend/src/lib/wiki.ts

@@ -1,3 +1,5 @@
+import { isAxiosError } from 'axios'
+
 import { apiGet } from '@/lib/api'
 
 import type { WikiPage } from '@/types'
@@ -21,8 +23,10 @@ export const fetchWikiPageByTitle = async (
   {
     return await apiGet (`/wiki/title/${ encodeURIComponent (title) }`, { params: { version } })
   }
-  catch
+  catch (error)
   {
+    if (isAxiosError (error) && error.response?.status === 404)
       return null
+    throw error
   }
 }

frontend/src/mdx-components.test.tsx

@@ -0,0 +1,77 @@
+import { fireEvent, screen, within } from '@testing-library/react'
+import { afterAll, beforeEach, describe, expect, it, vi } from 'vitest'
+import { Route, Routes } from 'react-router-dom'
+
+import HelpDetailPage from '@/pages/help/HelpDetailPage'
+import { compileMdx, transformMdx } from '@/test/compileMdx'
+import { helpFixture } from '@/test/helpFixtures'
+import { renderWithProviders } from '@/test/render'
+
+const prefetch = vi.hoisted (() => vi.fn (async () => {}))
+vi.hoisted (() => {
+  vi.stubGlobal ('Uint8Array', Object.getPrototypeOf (new TextEncoder ().encode ('')).constructor)
+})
+afterAll (() => vi.unstubAllGlobals ())
+vi.mock ('@/lib/prefetchers', () => ({ prefetchForURL: prefetch }))
+vi.mock ('@/lib/help', async () => (await import ('@/test/helpFixtures')).helpFixture.api)
+
+const tableSource = [
+    '# 表のある記事', '', '| 項目 | 説明 |', '| --- | --- |',
+    '| [ヘルプ](/help/guide) | **強調** |', '| 通常 | 本文 |'].join ('\n')
+
+
+describe ('Help MDX with the application compiler', () => {
+  beforeEach (() => helpFixture.reset ())
+
+  it ('compiles GFM table structure, links and emphasis', async () => {
+    const Content = await compileMdx (tableSource)
+    renderWithProviders (<Content/>)
+    const table = screen.getByRole ('table')
+    expect (table.querySelector ('thead')).not.toBeNull ()
+    expect (table.querySelector ('tbody')).not.toBeNull ()
+    expect (within (table).getAllByRole ('columnheader')).toHaveLength (2)
+    expect (within (table).getAllByRole ('cell')).toHaveLength (4)
+    expect (within (table).getByRole ('link', { name: 'ヘルプ' }))
+	.toHaveAttribute ('href', '/help/guide')
+    expect (table.querySelector ('strong')).toHaveTextContent ('強調')
+  })
+
+  it ('retains headings, paragraphs and lists', async () => {
+    const Content = await compileMdx ('# 見出し\n\n本文\n\n- 第一\n- 第二')
+    renderWithProviders (<Content/>)
+    expect (screen.getByRole ('heading', { level: 1 })).toHaveTextContent ('見出し')
+    expect (screen.getByText ('本文').tagName).toBe ('P')
+    expect (screen.getAllByRole ('listitem')).toHaveLength (2)
+  })
+
+  it ('keeps tables inside the Help dark-mode typography container', async () => {
+    const Content = await compileMdx (tableSource)
+    helpFixture.modules['../../help/guide/editions/ja/2026-10-02.10.mdx']
+	.mockResolvedValue ({ default: Content })
+    renderWithProviders (
+	<Routes><Route path="/help/:identifier" element={<HelpDetailPage/>}/></Routes>,
+	{ route: '/help/guide' })
+    const table = await screen.findByRole ('table')
+    expect (table.closest ('article')).toHaveClass ('prose', 'dark:prose-invert')
+  })
+
+  it ('preserves long cell contents when rendering a wide table', async () => {
+    const cell = 'wide'.repeat (100)
+    const Content = await compileMdx (`| 項目 |\n| --- |\n| ${ cell } |`)
+    renderWithProviders (<Content/>)
+    expect (screen.getByRole ('cell')).toHaveTextContent (cell)
+  })
+
+  it.each (['.mdx?raw', '.mdx?url']) ('preserves the %s transform bypass', async suffix => {
+    expect (await transformMdx (tableSource, suffix)).toBeNull ()
+  })
+
+  it ('uses the common PrefetchLink interaction for internal MDX links', async () => {
+    prefetch.mockClear ()
+    const Content = await compileMdx ('[内部リンク](/help/guide)')
+    renderWithProviders (<Content/>)
+    fireEvent.mouseEnter (screen.getByRole ('link', { name: '内部リンク' }))
+    expect (prefetch).toHaveBeenCalledWith (expect.anything (),
+	new URL ('/help/guide', window.location.origin).href)
+  })
+})

frontend/src/mdx-components.tsx

@@ -1,4 +1,52 @@
+import { useMemo } from 'react'
+import { useLocation } from 'react-router-dom'
+
+import PrefetchLink from '@/components/PrefetchLink'
+
 import type { MDXComponents } from 'mdx/types'
+import type { ComponentProps } from 'react'
 
 
-export const useMDXComponents = (): MDXComponents => ({ })
+const MDXLink = (
+    { href, children, ...props }: ComponentProps<'a'>,
+) => {
+  const location = useLocation ()
+  const url = useMemo (() => {
+    try
+    {
+      const base = new URL (location.pathname + location.search, window.location.origin)
+      return href ? new URL (href, base) : null
+    }
+    catch
+    {
+      return null
+    }
+  }, [href, location.pathname, location.search])
+  const internal = url != null
+      && url.origin === window.location.origin
+      && (url.protocol === 'http:' || url.protocol === 'https:')
+  const anchor = href?.startsWith ('#')
+      || (url?.pathname === location.pathname
+	  && url.search === location.search && Boolean (url.hash))
+  const external = url != null && !(internal)
+      && (url.protocol === 'http:' || url.protocol === 'https:')
+
+  if (internal && !(anchor) && props.download == null)
+  {
+    return (
+	<PrefetchLink to={url.pathname + url.search + url.hash} {...props}>
+	  {children}
+	</PrefetchLink>)
+  }
+
+  return (
+      <a href={href}
+	 {...props}
+	 target={external ? props.target ?? '_blank' : props.target}
+	 rel={props.rel ?? 'noopener noreferrer'}>
+	{children}
+      </a>)
+}
+
+
+export const useMDXComponents = (): MDXComponents => ({ a: MDXLink })

frontend/src/pages/help/HelpChangesPage.test.tsx

@@ -0,0 +1,55 @@
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { render, screen } from '@testing-library/react'
+import { HelmetProvider } from 'react-helmet-async'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { apiGet } from '@/lib/api'
+import App from '@/App'
+import { helpFixture } from '@/test/helpFixtures'
+
+vi.hoisted (() => {
+  window.history.replaceState (null, '', '/help/changes')
+})
+
+vi.mock ('@/lib/help', async () => (await import ('@/test/helpFixtures')).helpFixture.api)
+vi.mock ('@/lib/api', () => ({ apiGet: vi.fn (), apiPost: async () => ({}),
+			    isApiError: () => false }))
+vi.mock ('@/components/TopNav', () => ({ default: () => null }))
+vi.mock ('@/components/DevModeWatermark', () => ({ default: () => null }))
+vi.mock ('@/lib/useClientBehaviourSettings', () => ({
+    useClientBehaviourSettings: () => ({ animation: 'off', linkPreload: 'off' }) }))
+
+
+describe ('Help global edition history', () => {
+  beforeEach (() => helpFixture.reset ())
+
+  it ('routes /help/changes to published editions in date and numeric order', async () => {
+    const client = new QueryClient ({ defaultOptions: { queries: { retry: false } } })
+    const { container } = render (
+	<QueryClientProvider client={client}>
+	  <HelmetProvider><App/></HelmetProvider>
+	</QueryClientProvider>)
+    const links = await screen.findAllByRole ('link')
+    const editions = links.map (link => link.getAttribute ('href'))
+	.filter (href => href?.startsWith ('/help/') && href.includes ('edition='))
+    expect (editions).toEqual ([
+	`/help/${encodeURIComponent ('設定 手順')}?edition=2026-10-03`,
+	'/help/guide?edition=2026-10-02.10', '/help/guide?edition=2026-10-02.3',
+	'/help/guide?edition=2026-10-02.2', '/help/guide?edition=2026-10-02',
+	'/help/guide?edition=2026-10-01'])
+    expect (container).toHaveTextContent ('旧ホーム')
+    expect (screen.getAllByRole ('columnheader').map (cell => cell.textContent))
+	.toEqual (['差分', 'タイトル', '有効開始年月日'])
+    expect (container).not.toHaveTextContent ('未公開ホーム')
+    expect (container).not.toHaveTextContent ('Invalid Date')
+    expect (container.textContent).not.toMatch (/\d{1,2}:\d{2}/)
+    expect (screen.getAllByRole ('link', { name: '差分' }).map (link =>
+	link.getAttribute ('href'))).toEqual ([
+	'/help/guide/diff?from=2026-10-02.3&to=2026-10-02.10',
+	'/help/guide/diff?from=2026-10-02.2&to=2026-10-02.3',
+	'/help/guide/diff?from=2026-10-02&to=2026-10-02.2',
+	'/help/guide/diff?from=2026-10-01&to=2026-10-02'])
+    expect (apiGet).not.toHaveBeenCalled ()
+    expect (window.location.pathname).toBe ('/help/changes')
+  })
+})

frontend/src/pages/help/HelpChangesPage.tsx

@@ -0,0 +1,119 @@
+import { useQuery } from '@tanstack/react-query'
+import { useMemo } from 'react'
+import { Helmet } from 'react-helmet-async'
+import { useSearchParams } from 'react-router-dom'
+
+import PrefetchLink from '@/components/PrefetchLink'
+import PageTitle from '@/components/common/PageTitle'
+import Pagination from '@/components/common/Pagination'
+import MainArea from '@/components/layout/MainArea'
+import { SITE_TITLE } from '@/config'
+import { getHelpChanges, helpEditionDate } from '@/lib/help'
+import { helpKeys } from '@/lib/queryKeys'
+import { LIST_LIMIT_OPTIONS } from '@/lib/settings'
+import { dateString } from '@/lib/utils'
+
+import type { FC } from 'react'
+
+
+const HelpChangesPage: FC = () => {
+  const [searchParams, setSearchParams] = useSearchParams ()
+  const identifier = searchParams.get ('identifier') ?? ''
+  const pageValue = searchParams.get ('page') ?? '1'
+  const pageNumber = Number (pageValue)
+  const requestedPage =
+      (/^[1-9]\d*$/.test (pageValue) && Number.isSafeInteger (pageNumber))
+      ? pageNumber : 1
+  const limit = LIST_LIMIT_OPTIONS.find (
+      value => String (value) === searchParams.get ('limit')) ?? 20
+  const { data: changes = [], isLoading, error } = useQuery ({
+      queryKey: helpKeys.changes ('ja'),
+      queryFn: () => getHelpChanges ('ja'),
+      staleTime: Infinity })
+
+  const results = useMemo (() => identifier
+      ? changes.filter (entry => entry.identifier === identifier)
+      : changes, [changes, identifier])
+  const totalPages = Math.ceil (results.length / limit)
+  const page = requestedPage <= Math.max (1, totalPages) ? requestedPage : 1
+  const pageResults = results.slice ((page - 1) * limit, page * limit)
+
+  const handleLimit = (value: string) => {
+    const next = new URLSearchParams (searchParams)
+    next.set ('limit', value)
+    next.set ('page', '1')
+    setSearchParams (next)
+  }
+
+  return (
+      <MainArea className="min-w-0 max-w-full">
+	<Helmet>
+	  <title>{`ヘルプ改定履歴 | ${ SITE_TITLE }`}</title>
+	</Helmet>
+	<div className="flex flex-wrap items-center justify-between gap-3">
+	  <PageTitle>ヘルプ改定履歴</PageTitle>
+	  <label className="flex items-center gap-2 text-xs text-muted-foreground">
+	    <span>件数</span>
+	    <select value={limit} onChange={ev => handleLimit (ev.target.value)}
+		className="rounded border border-border bg-background px-2 py-1 text-sm
+			   text-foreground">
+	      {LIST_LIMIT_OPTIONS.map (value => (
+		  <option key={value} value={value}>{value}</option>))}
+	    </select>
+	  </label>
+	</div>
+
+	{isLoading
+	 ? <p className="mt-4">読み込み中...</p>
+	 : (error
+	    ? <p className="mt-4 text-red-600" role="alert">{error.message}</p>
+	    : (
+		<div className="mt-4">
+		  <div className="min-w-0 max-w-full overflow-x-auto">
+		    <table className="table-auto w-full min-w-[900px] whitespace-nowrap
+				      border-collapse">
+		      <thead className="border-b-2 border-black dark:border-white">
+			<tr>
+			  <th className="p-2 text-left">差分</th>
+			  <th className="p-2 text-left">タイトル</th>
+			  <th className="p-2 text-left">有効開始年月日</th>
+			</tr>
+		      </thead>
+		      <tbody>
+			{pageResults.map (entry => {
+			  const path = `/help/${ encodeURIComponent (entry.identifier) }`
+			  const diffQuery = new URLSearchParams ({
+			      from: entry.pred ?? '', to: entry.edition })
+			  const diffTo = `${ path }/diff?${ diffQuery.toString () }`
+			  const updatedAt = helpEditionDate (entry.edition)
+
+			  return (
+			      <tr key={`${ entry.identifier }/${ entry.edition }`}
+				  className="even:bg-gray-100 text-gray-900
+					     dark:even:bg-gray-700 dark:text-gray-100">
+				<td className="p-2">
+				  {entry.pred != null && (
+				      <PrefetchLink to={diffTo}>
+					差分
+				      </PrefetchLink>)}
+				</td>
+				<td className="p-2">
+				  <PrefetchLink to={`${ path }?edition=${ entry.edition }`}>
+				    {entry.title}
+				  </PrefetchLink>
+				</td>
+				<td className="p-2">
+				  {updatedAt != null && dateString (updatedAt, 'hour')}
+				</td>
+			      </tr>)
+			})}
+		      </tbody>
+		    </table>
+		  </div>
+		  {totalPages > 0 && <Pagination page={page} totalPages={totalPages}/>}
+		</div>))}
+      </MainArea>)
+}
+
+
+export default HelpChangesPage

frontend/src/pages/help/HelpDetailPage.test.tsx

@@ -0,0 +1,128 @@
+import { act, fireEvent, screen, waitFor } from '@testing-library/react'
+import { Route, Routes, useLocation, useNavigate } from 'react-router-dom'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import HelpDetailPage from '@/pages/help/HelpDetailPage'
+import { helpFixture } from '@/test/helpFixtures'
+import { renderWithProviders } from '@/test/render'
+
+import type { ReactElement } from 'react'
+
+vi.mock ('@/lib/help', async () => (await import ('@/test/helpFixtures')).helpFixture.api)
+
+
+const Navigation = () => {
+  const navigate = useNavigate ()
+  const location = useLocation ()
+  return (
+      <>
+	<output data-testid="location">{location.pathname}{location.search}</output>
+	<button onClick={() => navigate ('/help/設定%20手順')}>switch article</button>
+	<button onClick={() => navigate ('/help/guide?edition=2026-10-02.3')}>
+	  switch edition
+	</button>
+      </>)
+}
+
+
+const renderDetail = (route = '/help/guide') => renderWithProviders (
+    <>
+      <Navigation/>
+      <Routes>
+	<Route path="/help/:identifier" element={<HelpDetailPage/>}/>
+      </Routes>
+    </>, { route })
+
+
+describe ('Help details', () => {
+  beforeEach (() => helpFixture.reset ())
+
+  it.each (['2026-10-02', '2026-10-02.2', '2026-10-02.3', '2026-10-02.10']) (
+      'displays a date without inventing a time for edition %s', async edition => {
+	const { container } = renderDetail (`/help/guide?edition=${ edition }`)
+	await screen.findByRole ('heading')
+	expect (container).toHaveTextContent ('10月2日')
+	expect (container).not.toHaveTextContent ('Invalid Date')
+	expect (container.textContent).not.toMatch (/\d{1,2}:\d{2}/)
+	const revision = edition.split ('.')[1]
+	if (revision != null)
+	  expect (container).toHaveTextContent (` .${ revision }`)
+	expect (helpFixture.modules[`../../help/guide/editions/ja/${ edition }.mdx`])
+	    .toHaveBeenCalled ()
+      })
+
+  it ('displays current_edition H1 and body even when newer files exist', async () => {
+    renderDetail ()
+    expect (await screen.findByRole ('heading', { name: 'ホーム第十版' }))
+	.toBeInTheDocument ()
+    expect (screen.getByText ('タグを検索する 第十版本文')).toBeInTheDocument ()
+    expect (screen.queryByText ('未公開限定本文')).not.toBeInTheDocument ()
+    await waitFor (() => expect (document.title).toContain ('ホーム第十版 ヘルプ'))
+  })
+
+  it ('uses the historical H1 in both page and document titles', async () => {
+    renderDetail ('/help/guide?edition=2026-10-01')
+    expect (await screen.findByRole ('heading', { name: '旧ホーム' })).toBeInTheDocument ()
+    expect (screen.getByText ('過去限定本文')).toBeInTheDocument ()
+    await waitFor (() => expect (document.title).toContain ('旧ホーム ヘルプ'))
+    expect (screen.getByText ('(最古)')).toBeInTheDocument ()
+    expect (screen.getByRole ('link', { name: '新 >' }))
+	.toHaveAttribute ('href', '/help/guide?edition=2026-10-02')
+  })
+
+  it ('navigates .2, .3 and .10 in numeric edition order', async () => {
+    renderDetail ('/help/guide?edition=2026-10-02.3')
+    await screen.findByRole ('heading', { name: 'ホーム第三版' })
+    expect (screen.getByRole ('link', { name: '< 古' }))
+	.toHaveAttribute ('href', '/help/guide?edition=2026-10-02.2')
+    const next = screen.getByRole ('link', { name: '新 >' })
+    expect (next).toHaveAttribute ('href', '/help/guide?edition=2026-10-02.10')
+    fireEvent.click (next)
+    await screen.findByRole ('heading', { name: 'ホーム第十版' })
+    expect (screen.getByText ('(最新)')).toBeInTheDocument ()
+    expect (screen.queryByText ('第三版本文')).not.toBeInTheDocument ()
+  })
+
+  it.each (['/help/missing', '/help/guide?edition=missing',
+      '/help/guide?edition=2026-02-30', '/help/guide?edition=2026-10-02.2.extra',
+      '/help/guide?edition=2026-10-02.11']) (
+      'shows NotFound without redirecting %s', async route => {
+	renderDetail (route)
+	expect (await screen.findByText ('404')).toBeInTheDocument ()
+	expect (screen.getByTestId ('location')).toHaveTextContent (route)
+      })
+
+  it ('hides the old body and title immediately while another article loads', async () => {
+    let finish!: (value: { default: () => ReactElement }) => void
+    const pending = new Promise<{ default: () => ReactElement }> (resolve => {
+      finish = resolve
+    })
+    helpFixture.modules['../../help/設定 手順/editions/ja/2026-10-03.mdx']
+	.mockReturnValueOnce (pending)
+    renderDetail ()
+    await screen.findByRole ('heading', { name: 'ホーム第十版' })
+    fireEvent.click (screen.getByRole ('button', { name: 'switch article' }))
+    expect (screen.queryByText ('タグを検索する 第十版本文')).not.toBeInTheDocument ()
+    await waitFor (() => expect (document.title).not.toContain ('ホーム'))
+    await act (async () => finish ({ default: () => <h1>設定</h1> }))
+    await screen.findByRole ('heading', { name: '設定' })
+    await waitFor (() => expect (document.title).toContain ('設定 ヘルプ'))
+  })
+
+  it ('ignores a late response for the previous edition', async () => {
+    let finish!: (value: { default: () => ReactElement }) => void
+    const pending = new Promise<{ default: () => ReactElement }> (resolve => {
+      finish = resolve
+    })
+    const loader = helpFixture.modules['../../help/guide/editions/ja/2026-10-02.10.mdx']
+    loader.mockReturnValueOnce (pending)
+    renderDetail ()
+    await waitFor (() => expect (loader).toHaveBeenCalled ())
+    fireEvent.click (screen.getByRole ('button', { name: 'switch edition' }))
+    await screen.findByRole ('heading', { name: 'ホーム第三版' })
+    await act (async () => finish ({ default: () => <h1>遅延した旧本文</h1> }))
+    expect (screen.queryByText ('遅延した旧本文')).not.toBeInTheDocument ()
+    expect (screen.getByRole ('heading', { name: 'ホーム第三版' })).toBeInTheDocument ()
+    await waitFor (() => expect (document.title).toContain ('ホーム第三版 ヘルプ'))
+  })
+})

frontend/src/pages/help/HelpDetailPage.tsx

@@ -0,0 +1,106 @@
+import { useQuery } from '@tanstack/react-query'
+import { Helmet } from 'react-helmet-async'
+import { useLocation, useParams } from 'react-router-dom'
+
+import PrefetchLink from '@/components/PrefetchLink'
+import MainArea from '@/components/layout/MainArea'
+import { SITE_TITLE } from '@/config'
+import { getHelpArticle,
+	 getHelpEditionContent,
+	 helpEditionDate,
+	 parseHelpEdition } from '@/lib/help'
+import { dateString } from '@/lib/utils'
+import { helpKeys } from '@/lib/queryKeys'
+import NotFound from '@/pages/NotFound'
+
+import type { FC } from 'react'
+
+
+const HelpDetailPage: FC = () => {
+  const { identifier = '' } = useParams ()
+  const location = useLocation ()
+
+  // Locale 対応が入るまでは ja 固定
+  const language = 'ja'
+
+  const { data: article, isLoading: articleLoading } = useQuery ({
+      queryKey: helpKeys.article (identifier),
+      queryFn: () => getHelpArticle (identifier),
+      staleTime: Infinity })
+
+  const query = new URLSearchParams (location.search)
+  const requestedEdition = query.get ('edition')
+
+  const languageMeta = article?.languages[language]
+
+  const edition = requestedEdition ?? languageMeta?.currentEdition
+  const editionInfo = parseHelpEdition (edition ?? '')
+  const editionDate = helpEditionDate (edition ?? '')
+
+  const editions = languageMeta?.publishedEditions ?? []
+
+  const index = edition ? editions.indexOf (edition) : -1
+  const pred = index > 0 ? editions[index - 1] : undefined
+  const succ =
+      index >= 0 && index < editions.length - 1
+      ? editions[index + 1]
+      : undefined
+
+  const { data: current, isSuccess, isError } = useQuery ({
+      enabled: edition != null && editionInfo != null && editions.includes (edition),
+      queryKey: helpKeys.edition (identifier, language, edition ?? ''),
+      queryFn: () => getHelpEditionContent (identifier, language, edition ?? ''),
+      staleTime: Infinity })
+
+  if (!articleLoading
+      && (article == null
+	  || languageMeta == null
+	  || edition == null
+	  || editionInfo == null
+	  || !(editions.includes (edition))))
+    return <NotFound/>
+
+  if (isError || (isSuccess && current == null))
+    return <NotFound/>
+
+  const Content = current?.content
+
+  return (
+      <MainArea>
+	<Helmet>
+	  <title>{current?.title || identifier} ヘルプ | {SITE_TITLE}</title>
+	</Helmet>
+
+	{requestedEdition && (
+	    <div className="text-sm flex gap-3 items-center justify-center
+			    border border-gray-700 rounded px-2 py-1 mb-4">
+	      {pred
+	       ? (
+		  <PrefetchLink
+		      to={`/help/${ encodeURIComponent (identifier) }?edition=${ pred }`}>
+		    &lt; 古
+		  </PrefetchLink>)
+	       : '(最古)'}
+
+	      <span>
+		{editionDate != null && dateString (editionDate, 'hour')}
+		{editionInfo?.revision != null && ` .${ editionInfo.revision }`}
+	      </span>
+
+	      {succ
+	       ? (
+		  <PrefetchLink
+		      to={`/help/${ encodeURIComponent (identifier) }?edition=${ succ }`}>
+		    新 &gt;
+		  </PrefetchLink>)
+	       : '(最新)'}
+	    </div>)}
+
+	<article className="prose dark:prose-invert mx-auto p-4">
+	  {Content ? <Content/> : <div>Loading...</div>}
+	</article>
+      </MainArea>)
+}
+
+
+export default HelpDetailPage

frontend/src/pages/help/HelpDiffPage.tsx

@@ -0,0 +1,81 @@
+import { useQuery } from '@tanstack/react-query'
+import { Helmet } from 'react-helmet-async'
+import { useParams, useSearchParams } from 'react-router-dom'
+
+import PrefetchLink from '@/components/PrefetchLink'
+import PageTitle from '@/components/common/PageTitle'
+import MainArea from '@/components/layout/MainArea'
+import { SITE_TITLE } from '@/config'
+import { getHelpDiff } from '@/lib/help'
+import { helpKeys } from '@/lib/queryKeys'
+import { cn } from '@/lib/utils'
+import NotFound from '@/pages/NotFound'
+
+import type { FC } from 'react'
+
+const markers = { added: '+ ', removed: '- ', unchanged: '  ' }
+
+
+const HelpDiffPage: FC = () => {
+  const { identifier = '' } = useParams ()
+  const [searchParams] = useSearchParams ()
+  const from = searchParams.get ('from') ?? ''
+  const to = searchParams.get ('to') ?? ''
+  const validParams = Boolean (identifier && from && to)
+      && searchParams.getAll ('from').length === 1
+      && searchParams.getAll ('to').length === 1
+  const { data: diff, isLoading, isError } = useQuery ({
+      enabled: validParams,
+      queryKey: helpKeys.diff (identifier, 'ja', from, to),
+      queryFn: () => getHelpDiff (identifier, 'ja', from, to),
+      staleTime: Infinity })
+
+  if (!(validParams) || isError || (!(isLoading) && diff == null))
+    return <NotFound/>
+
+  const title = `${ diff?.to.title ?? identifier } (${ from } → ${ to })`
+  const path = `/help/${ encodeURIComponent (identifier) }`
+
+  return (
+      <MainArea className="min-w-0 max-w-full">
+	<Helmet>
+	  <title>{`ヘルプ 差分: ${ title } | ${ SITE_TITLE }`}</title>
+	</Helmet>
+	<PageTitle>{title}</PageTitle>
+
+	{diff
+	 ? (
+	    <>
+	      <div className="flex flex-wrap items-center gap-3">
+		<PrefetchLink to={`${ path }?edition=${ encodeURIComponent (from) }`}>
+		  {diff.from.title} ({from})
+		</PrefetchLink>
+		<span>→</span>
+		<PrefetchLink to={`${ path }?edition=${ encodeURIComponent (to) }`}>
+		  {diff.to.title} ({to})
+		</PrefetchLink>
+	      </div>
+	      <pre className="mt-4 overflow-x-auto whitespace-pre">
+		<code>
+		  {diff.lines.map ((line, index) => {
+		    const colour = cn (
+			'block text-gray-900 dark:text-gray-100',
+			line.type === 'added' && 'bg-green-200 dark:bg-green-800',
+			line.type === 'added' && 'text-green-950 dark:text-green-50',
+			line.type === 'removed' && 'bg-red-200 dark:bg-red-800',
+			line.type === 'removed' && 'text-red-950 dark:text-red-50')
+
+		    return (
+			<span key={index} className={colour}>
+			  {markers[line.type]}{line.content.replace (/\r?\n$/, '')}
+			</span>)
+		  })}
+		</code>
+	      </pre>
+	    </>)
+	 : <div>Loading...</div>}
+      </MainArea>)
+}
+
+
+export default HelpDiffPage

frontend/src/pages/help/HelpListPage.test.tsx

@@ -0,0 +1,130 @@
+import { fireEvent, screen } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import HelpListPage from '@/pages/help/HelpListPage'
+import { helpFixture } from '@/test/helpFixtures'
+import { renderWithProviders } from '@/test/render'
+
+vi.mock ('@/lib/help', async () => (await import ('@/test/helpFixtures')).helpFixture.api)
+
+
+describe ('Help search', () => {
+  beforeEach (() => helpFixture.reset ())
+
+  it ('initially lists each current article once with encoded detail links', async () => {
+    renderWithProviders (<HelpListPage/>, { route: '/help' })
+    expect (await screen.findByRole ('link', { name: 'ホーム第十版' }))
+	.toHaveAttribute ('href', '/help/guide')
+    expect (screen.getByRole ('link', { name: '設定' }))
+	.toHaveAttribute ('href', `/help/${encodeURIComponent ('設定 手順')}`)
+    expect (screen.getByText ('2 件')).toBeInTheDocument ()
+    expect (screen.queryByText ('旧ホーム')).not.toBeInTheDocument ()
+    expect (screen.queryByText ('未公開ホーム')).not.toBeInTheDocument ()
+  })
+
+  it.each ([
+      ['ホーム', '', ['ホーム第十版']],
+      ['', '検索', ['ホーム第十版']],
+      ['', 'タグ', ['設定', 'ホーム第十版']],
+      ['ホーム', '設定', []],
+      ['ホーム', '検索', ['ホーム第十版']],
+      ['旧ホーム', '', []],
+      ['', '過去限定本文', []]]) (
+      'searches title %s and body %s together', async (title, body, expected) => {
+	const { container } = renderWithProviders (<HelpListPage/>)
+	await screen.findByText ('2 件')
+	fireEvent.change (screen.getByRole ('textbox', { name: 'タイトル' }), {
+	    target: { value: title } })
+	fireEvent.change (screen.getByRole ('textbox', { name: '内容' }), {
+	    target: { value: body } })
+	fireEvent.click (screen.getByRole ('button', { name: '検索' }))
+	expect (Array.from (container.querySelectorAll ('tbody a'), link => link.textContent))
+	    .toEqual (expected)
+	if (expected.length === 0)
+	  expect (screen.getByText ('該当する記事がありません．')).toBeInTheDocument ()
+      })
+
+  it ('displays loading failures without stale results', async () => {
+    helpFixture.loads.search.mockImplementationOnce (() => {
+      throw new Error ('fixture loading failed')
+    })
+    renderWithProviders (<HelpListPage/>)
+    expect (await screen.findByRole ('alert')).toHaveTextContent ('fixture loading failed')
+    expect (screen.queryByRole ('link', { name: 'ホーム第十版' })).not.toBeInTheDocument ()
+  })
+
+  it ('sorts edition suffixes numerically and displays their date without a time', async () => {
+    const editions = ['2026-10-02', '2026-10-02.2', '2026-10-02.3', '2026-10-02.10']
+    helpFixture.assets.search.default.ja = editions.map ((edition, index) => ({
+	identifier: `article-${ index }`, edition, title: `article-${ index }`, body: '' }))
+    const { container } = renderWithProviders (<HelpListPage/>, { route: '/help' })
+    await screen.findByText ('4 件')
+    expect (Array.from (container.querySelectorAll ('tbody a'), link => link.textContent))
+	.toEqual (['article-3', 'article-2', 'article-1', 'article-0'])
+    const dates = Array.from (container.querySelectorAll ('tbody tr'), row =>
+	row.children[1].textContent)
+    expect (dates).toHaveLength (4)
+    dates.forEach (date => {
+      expect (date).toContain ('10月2日')
+      expect (date).not.toContain ('Invalid Date')
+      expect (date).not.toMatch (/\d{1,2}:\d{2}/)
+    })
+  })
+
+  it.each ([['ホーム', 'ほーむ', '', true], ['ほーむ', 'ホーム', '', true],
+      ['ホーム', 'ﾎｰﾑ', '', true], ['ホーム', '', 'ホーム', true],
+      ['ホーム', 'ほーむ', 'ﾎｰﾑ', true], ['ホーム', 'ほーむ', '不一致', false]] as const) (
+      'searches title %s with %s and body with %s', async (title, query, text, found) => {
+	const entry = { identifier: 'guide', edition: '2026-10-02.10',
+	    title, body: 'ほーむの案内' }
+	helpFixture.assets.search.default.ja = [entry]
+	const params = new URLSearchParams ({ title: query, text })
+	renderWithProviders (<HelpListPage/>, { route: `/help?${ params }` })
+	await screen.findByText (`${ found ? 1 : 0 } 件`)
+	if (found)
+	  expect (screen.getByRole ('link', { name: title })).toHaveAttribute ('href', '/help/guide')
+	else
+	  expect (screen.queryByRole ('link', { name: title })).not.toBeInTheDocument ()
+	expect (entry).toEqual ({ identifier: 'guide', edition: '2026-10-02.10',
+	    title, body: 'ほーむの案内' })
+      })
+
+  // ai_ci equality is documented by MySQL; these are not captured DB sort results.
+  it.each (['asc', 'desc']) ('orders different titles before identifiers in order %s',
+      async direction => {
+	helpFixture.assets.search.default.ja = [
+	    { identifier: 'alpha', edition: '2026-10-02', title: 'Zebra', body: '' },
+	    { identifier: 'beta', edition: '2026-10-02', title: 'Apple', body: '' }]
+	const { container } = renderWithProviders (<HelpListPage/>, {
+	    route: `/help?order=title:${ direction }` })
+	await screen.findByText ('2 件')
+	const expected = direction === 'asc'
+	    ? ['/help/beta', '/help/alpha'] : ['/help/alpha', '/help/beta']
+	expect (Array.from (container.querySelectorAll ('tbody a'), link =>
+	    link.getAttribute ('href'))).toEqual (expected)
+      })
+
+  it.each (['asc', 'desc']) ('uses identifier ties for MySQL ai_ci title order %s',
+      async direction => {
+	helpFixture.assets.search.default.ja = [
+	    { identifier: 'alpha', edition: '2026-10-02', title: 'Éclair', body: '' },
+	    { identifier: 'beta', edition: '2026-10-02', title: 'eclair', body: '' }]
+	const { container } = renderWithProviders (<HelpListPage/>, {
+	    route: `/help?order=title:${ direction }` })
+	await screen.findByText ('2 件')
+	expect (Array.from (container.querySelectorAll ('tbody a'), link =>
+	    link.getAttribute ('href'))).toEqual (['/help/alpha', '/help/beta'])
+      })
+
+  it.each (['asc', 'desc']) ('ignores title case before identifier ties in order %s',
+      async direction => {
+	helpFixture.assets.search.default.ja = [
+	    { identifier: 'alpha', edition: '2026-10-02', title: 'home', body: '' },
+	    { identifier: 'beta', edition: '2026-10-02', title: 'HOME', body: '' }]
+	const { container } = renderWithProviders (<HelpListPage/>, {
+	    route: `/help?order=title:${ direction }` })
+	await screen.findByText ('2 件')
+	expect (Array.from (container.querySelectorAll ('tbody a'), link =>
+	    link.getAttribute ('href'))).toEqual (['/help/alpha', '/help/beta'])
+      })
+})

frontend/src/pages/help/HelpListPage.tsx

@@ -0,0 +1,212 @@
+import { useQuery } from '@tanstack/react-query'
+import { useEffect, useMemo, useState } from 'react'
+import { Helmet } from 'react-helmet-async'
+import { useSearchParams } from 'react-router-dom'
+
+import PrefetchLink from '@/components/PrefetchLink'
+import SortHeader from '@/components/SortHeader'
+import FormField from '@/components/common/FormField'
+import PageTitle from '@/components/common/PageTitle'
+import Pagination from '@/components/common/Pagination'
+import MainArea from '@/components/layout/MainArea'
+import { SITE_TITLE } from '@/config'
+import { compareHelpEditions,
+	 compareHelpTitles,
+	 getHelpSearchEntries,
+	 helpEditionDate,
+	 normaliseHelpSearchText } from '@/lib/help'
+import { helpKeys } from '@/lib/queryKeys'
+import { LIST_LIMIT_OPTIONS } from '@/lib/settings'
+import { dateString, inputClass } from '@/lib/utils'
+
+import type { FormEvent, FC } from 'react'
+
+type SearchConditions = { title: string; text: string }
+type HelpOrderField = 'title' | 'updated_at'
+type HelpOrder = `${ HelpOrderField }:${ 'asc' | 'desc' }`
+
+const defaultDirection: Record<HelpOrderField, 'asc' | 'desc'> = {
+    title: 'asc', updated_at: 'desc' }
+
+
+const HelpSearchPage: FC = () => {
+  const [searchParams, setSearchParams] = useSearchParams ()
+  const pageValue = searchParams.get ('page') ?? '1'
+  const pageNumber = Number (pageValue)
+  const requestedPage =
+      (/^[1-9]\d*$/.test (pageValue) && Number.isSafeInteger (pageNumber))
+      ? pageNumber : 1
+  const limit = LIST_LIMIT_OPTIONS.find (
+      value => String (value) === searchParams.get ('limit')) ?? 20
+  const requestedOrder = searchParams.get ('order') ?? 'updated_at:desc'
+  const order: HelpOrder =
+      ['title:asc', 'title:desc', 'updated_at:asc', 'updated_at:desc'].includes (requestedOrder)
+      ? requestedOrder as HelpOrder
+      : 'updated_at:desc'
+  const conditions = useMemo<SearchConditions> (() => ({
+      title: searchParams.get ('title') ?? '',
+      text: searchParams.get ('text') ?? '' }), [searchParams])
+  const [title, setTitle] = useState (conditions.title)
+  const [text, setText] = useState (conditions.text)
+  const { data: entries = [], isLoading: loading, error } = useQuery ({
+      queryKey: helpKeys.index ('ja'),
+      queryFn: () => getHelpSearchEntries ('ja'),
+      staleTime: Infinity })
+
+  useEffect (() => {
+    setTitle (conditions.title)
+    setText (conditions.text)
+  }, [conditions.title, conditions.text])
+
+  const results = useMemo (() => {
+    const titleQuery = normaliseHelpSearchText (conditions.title.trim ())
+    const textQuery = normaliseHelpSearchText (conditions.text.trim ())
+    const [field, direction] = order.split (':')
+
+    return (
+	entries.filter (entry => (
+	    (!(titleQuery)
+	     || normaliseHelpSearchText (entry.title).includes (titleQuery))
+	    && (!(textQuery)
+		|| normaliseHelpSearchText (entry.body).includes (textQuery))))
+	.sort ((a, b) => {
+	  const comparison = field === 'title'
+	      ? compareHelpTitles (a.title, b.title)
+	      : compareHelpEditions (a.edition, b.edition)
+
+	  return (direction === 'asc' ? comparison : -comparison)
+	      || a.identifier.localeCompare (b.identifier)
+	}))
+  }, [entries, conditions, order])
+
+  const totalPages = Math.ceil (results.length / limit)
+  const page = requestedPage <= Math.max (1, totalPages) ? requestedPage : 1
+  const pageResults = results.slice ((page - 1) * limit, page * limit)
+
+  const handleLimit = (value: string) => {
+    const next = new URLSearchParams (searchParams)
+    next.set ('limit', value)
+    next.set ('page', '1')
+    setSearchParams (next)
+  }
+
+  const handleSearch = (ev: FormEvent<HTMLFormElement>) => {
+    ev.preventDefault ()
+    const next = new URLSearchParams (searchParams)
+    if (title)
+      next.set ('title', title)
+    else
+      next.delete ('title')
+    if (text)
+      next.set ('text', text)
+    else
+      next.delete ('text')
+    next.set ('page', '1')
+    setSearchParams (next)
+  }
+
+  return (
+      <MainArea className="min-w-0 max-w-full">
+	<Helmet>
+	  <title>{`ヘルプ | ${ SITE_TITLE }`}</title>
+	</Helmet>
+
+	<div className="max-w-xl">
+	  <div className="flex flex-wrap items-center justify-between gap-3">
+	    <PageTitle>ヘルプ検索</PageTitle>
+	    <label className="flex items-center gap-2 text-xs text-muted-foreground">
+	      <span>件数</span>
+	      <select value={limit} onChange={ev => handleLimit (ev.target.value)}
+		  className="rounded border border-border bg-background px-2 py-1 text-sm
+			     text-foreground">
+		{LIST_LIMIT_OPTIONS.map (value => (
+		    <option key={value} value={value}>{value}</option>))}
+	      </select>
+	    </label>
+	  </div>
+	  <form onSubmit={handleSearch} className="space-y-2">
+	    <FormField label="タイトル">
+	      {({ invalid }) => (
+		  <input type="text"
+		      aria-label="タイトル"
+		      value={title}
+		      onChange={e => setTitle (e.target.value)}
+		      className={inputClass (invalid)}/>)}
+	    </FormField>
+
+	    <FormField label="内容">
+	      {({ invalid }) => (
+		  <input type="text"
+		      aria-label="内容"
+		      value={text}
+		      onChange={e => setText (e.target.value)}
+		      className={inputClass (invalid)}/>)}
+	    </FormField>
+
+	    <div className="py-3">
+	      <button type="submit"
+		  className="bg-blue-500 text-white px-4 py-2 rounded">
+		検索
+	      </button>
+	    </div>
+	  </form>
+	</div>
+
+	{loading
+	 ? <p className="mt-4">読み込み中...</p>
+	 : (error
+	    ? <p className="mt-4 text-red-600" role="alert">{error.message}</p>
+	    : (
+		<div className="mt-4">
+		  <p className="mb-2 text-sm" aria-live="polite">{results.length} 件</p>
+		  <div className="min-w-0 max-w-full overflow-x-auto">
+		    <table className="table-auto w-full min-w-[640px] whitespace-nowrap
+				      border-collapse text-gray-900 dark:text-gray-100">
+		      <thead className="border-b-2 border-black dark:border-white">
+			<tr>
+			  <th className="p-2 text-left">
+			    <SortHeader<HelpOrderField>
+				by="title"
+				label="タイトル"
+				currentOrder={order}
+				defaultDirection={defaultDirection}/>
+			  </th>
+			  <th className="p-2 text-left">
+			    <SortHeader<HelpOrderField>
+				by="updated_at"
+				label="有効開始年月日"
+				currentOrder={order}
+				defaultDirection={defaultDirection}/>
+			  </th>
+			</tr>
+		      </thead>
+		      <tbody>
+			{pageResults.map (entry => {
+			  const updatedAt = helpEditionDate (entry.edition)
+
+			  return (
+			      <tr key={entry.identifier}
+				  className="even:bg-gray-100 text-gray-900
+					     dark:even:bg-gray-700 dark:text-gray-100">
+				<td className="p-2">
+				  <PrefetchLink
+				      to={`/help/${encodeURIComponent (entry.identifier)}`}>
+				    {entry.title}
+				  </PrefetchLink>
+				</td>
+				<td className="p-2">
+				  {updatedAt != null && dateString (updatedAt, 'hour')}
+				</td>
+			      </tr>)
+			})}
+		      </tbody>
+		    </table>
+		  </div>
+		  {totalPages > 0 && <Pagination page={page} totalPages={totalPages}/>}
+		  {results.length === 0 && <p className="mt-4">該当する記事がありません．</p>}
+		</div>))}
+      </MainArea>)
+}
+
+
+export default HelpSearchPage

frontend/src/pages/wiki/WikiDetailPage.test.tsx

@@ -1,14 +1,19 @@
-import { screen } from '@testing-library/react'
-import { Route, Routes } from 'react-router-dom'
-import { describe, expect, it, vi } from 'vitest'
+import { act, fireEvent, screen, waitFor, within } from '@testing-library/react'
+import { Route, Routes, useLocation } from 'react-router-dom'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
 
+import { apiPost } from '@/lib/api'
+import { getHelpTitleIndex } from '@/lib/help'
+import { tagsKeys, wikiKeys } from '@/lib/queryKeys'
 import WikiDetailPage from '@/pages/wiki/WikiDetailPage'
-import { buildTag, buildWikiPage } from '@/test/factories'
+import WikiNewPage from '@/pages/wiki/WikiNewPage'
+import { buildTag, buildUser, buildWikiPage } from '@/test/factories'
 import { renderWithProviders } from '@/test/render'
 
 const wikiApi = vi.hoisted (() => ({
   fetchWikiPage:        vi.fn (),
   fetchWikiPageByTitle: vi.fn (),
+  fetchWikiPages:       vi.fn (),
 }))
 
 const tagsApi = vi.hoisted (() => ({
@@ -22,8 +27,140 @@ const postsApi = vi.hoisted (() => ({
 vi.mock ('@/lib/wiki', () => wikiApi)
 vi.mock ('@/lib/tags', () => tagsApi)
 vi.mock ('@/lib/posts', () => postsApi)
+vi.mock ('@/lib/help', () => ({ getHelpTitleIndex: vi.fn () }))
+vi.mock ('@/lib/api', async importOriginal => ({
+    ...await importOriginal<typeof import ('@/lib/api')> (), apiPost: vi.fn () }))
+vi.mock ('@/components/ui/use-toast', () => ({ toast: vi.fn () }))
+vi.mock ('react-markdown-editor-lite', () => ({
+    default: ({ value, onChange }: {
+	value: string
+	onChange: (event: { text: string }) => void }) => (
+	    <textarea aria-label="本文エディタ" value={value}
+		onChange={event => onChange ({ text: event.target.value })}/>) }))
+
+
+const Location = () => {
+  const location = useLocation ()
+  return <output data-testid="location">{location.pathname}{location.search}</output>
+}
+
 
 describe ('WikiDetailPage', () => {
+  beforeEach (() => {
+    wikiApi.fetchWikiPageByTitle.mockReset ().mockResolvedValue (null)
+    wikiApi.fetchWikiPages.mockReset ().mockResolvedValue ([])
+    tagsApi.fetchTagByName.mockReset ().mockResolvedValue (null)
+    postsApi.fetchPosts.mockReset ().mockResolvedValue ({ posts: [], count: 0 })
+    vi.mocked (getHelpTitleIndex).mockReset ().mockResolvedValue ({ })
+    vi.mocked (apiPost).mockReset ()
+  })
+
+  it ('offers creation for an existing tag after the Help lookup completes', async () => {
+    let finish!: (index: Record<string, string[]>) => void
+    const pending = new Promise<Record<string, string[]>> (resolve => {
+      finish = resolve
+    })
+    vi.mocked (getHelpTitleIndex).mockReturnValueOnce (pending)
+    tagsApi.fetchTagByName.mockResolvedValue (buildTag ({ name: '既存タグ', hasWiki: false }))
+    renderWithProviders (
+	<Routes><Route path="/wiki/:title" element={<WikiDetailPage/>}/></Routes>,
+	{ route: `/wiki/${ encodeURIComponent ('既存タグ') }` })
+    await waitFor (() => expect (getHelpTitleIndex).toHaveBeenCalled ())
+    expect (screen.queryByText ('404')).not.toBeInTheDocument ()
+    expect (screen.queryByRole ('link', { name: '新規作成してください' }))
+	.not.toBeInTheDocument ()
+    await act (async () => finish ({ }))
+    expect (await screen.findByRole ('link', { name: '新規作成してください' }))
+	.toHaveAttribute ('href', `/wiki/new?title=${ encodeURIComponent ('既存タグ') }`)
+  })
+
+  it ('creates a tag description through the existing form and displays its content', async () => {
+    const page = buildWikiPage ({ title: '既存タグ', body: '説明文',
+	resourceType: 'tag_description' })
+    tagsApi.fetchTagByName.mockResolvedValue (buildTag ({ name: page.title, hasWiki: false }))
+    vi.mocked (apiPost).mockResolvedValueOnce (page)
+    const path = `/wiki/${ encodeURIComponent (page.title) }`
+    const { container, queryClient } = renderWithProviders (
+	<>
+	  <Location/>
+	  <Routes>
+	    <Route path="/wiki/new" element={<WikiNewPage user={buildUser ({ role: 'member' })}/>}/>
+	    <Route path="/wiki/:title" element={<WikiDetailPage/>}/>
+	  </Routes>
+	</>, { route: path })
+    fireEvent.click (await screen.findByRole ('link', { name: '新規作成してください' }))
+    await screen.findByRole ('heading', { name: '新規 Wiki ページ' })
+    expect (queryClient.getQueryData (wikiKeys.show (page.title, { }))).toBeNull ()
+    queryClient.setQueryData (wikiKeys.index ({ }), [])
+    wikiApi.fetchWikiPageByTitle.mockResolvedValue (page)
+    wikiApi.fetchWikiPages.mockResolvedValue ([page])
+    tagsApi.fetchTagByName.mockResolvedValue (buildTag ({ name: page.title, hasWiki: true }))
+    expect (screen.getAllByRole ('textbox')[0]).toHaveValue (page.title)
+    fireEvent.change (screen.getByRole ('textbox', { name: '本文エディタ' }), {
+	target: { value: page.body } })
+    fireEvent.click (screen.getByRole ('button', { name: '追加' }))
+    await screen.findByRole ('heading', { name: page.title })
+    expect (screen.getByTestId ('location')).toHaveTextContent (path)
+    expect (screen.queryByRole ('textbox', { name: '本文エディタ' })).not.toBeInTheDocument ()
+    expect (within (container.querySelector ('article')!).getByText (page.body))
+	.toBeInTheDocument ()
+    const form = vi.mocked (apiPost).mock.calls[0][1] as FormData
+    expect (form.get ('title')).toBe (page.title)
+    expect (form.get ('body')).toBe (page.body)
+    expect (apiPost).toHaveBeenCalledWith ('/wiki', form,
+	{ headers: { 'Content-Type': 'multipart/form-data' } })
+    expect (screen.queryByText ('404')).not.toBeInTheDocument ()
+    expect (queryClient.getQueryData (wikiKeys.show (page.title, { }))).toEqual (page)
+    await waitFor (() => expect (queryClient.getQueryData (wikiKeys.index ({ })))
+	.toEqual ([page]))
+    await waitFor (() => expect (queryClient.getQueryData (tagsKeys.show (page.title)))
+	.toMatchObject ({ hasWiki: true }))
+    expect (getHelpTitleIndex).toHaveBeenCalledTimes (1)
+  })
+
+  it ('keeps failed saves on the form without changing cached Wiki or tag state', async () => {
+    const title = '既存タグ'
+    const tag = buildTag ({ name: title, hasWiki: false })
+    const path = `/wiki/new?title=${ encodeURIComponent (title) }`
+    vi.mocked (apiPost).mockRejectedValueOnce (new Error ('save failed'))
+    const { queryClient } = renderWithProviders (
+	<><Location/><WikiNewPage user={buildUser ({ role: 'member' })}/></>, { route: path })
+    queryClient.setQueryData (wikiKeys.show (title, { }), null)
+    queryClient.setQueryData (wikiKeys.index ({ }), [])
+    queryClient.setQueryData (tagsKeys.show (title), tag)
+    fireEvent.change (screen.getByRole ('textbox', { name: '本文エディタ' }), {
+	target: { value: '保存しない本文' } })
+    await act (async () => fireEvent.click (screen.getByRole ('button', { name: '追加' })))
+    expect (apiPost).toHaveBeenCalledTimes (1)
+    expect (screen.getByTestId ('location')).toHaveTextContent (path)
+    expect (screen.getByRole ('heading', { name: '新規 Wiki ページ' })).toBeInTheDocument ()
+    expect (queryClient.getQueryData (wikiKeys.show (title, { }))).toBeNull ()
+    expect (queryClient.getQueryData (tagsKeys.show (title))).toEqual (tag)
+    expect (queryClient.getQueryState (wikiKeys.index ({ }))?.isInvalidated).toBe (false)
+  })
+
+  it ('shows NotFound when neither a tag nor a Help title exists', async () => {
+    renderWithProviders (
+	<Routes><Route path="/wiki/:title" element={<WikiDetailPage/>}/></Routes>,
+	{ route: '/wiki/missing' })
+    expect (await screen.findByText ('404')).toBeInTheDocument ()
+    expect (screen.queryByRole ('link', { name: '新規作成してください' }))
+	.not.toBeInTheDocument ()
+  })
+
+  it.each (['403', '500', 'offline']) ('keeps Wiki error %s distinct from missing content',
+      async message => {
+	wikiApi.fetchWikiPageByTitle.mockRejectedValue (new Error (message))
+	tagsApi.fetchTagByName.mockResolvedValue (buildTag ({ name: '既存タグ' }))
+	renderWithProviders (
+	    <Routes><Route path="/wiki/:title" element={<WikiDetailPage/>}/></Routes>,
+	    { route: `/wiki/${ encodeURIComponent ('既存タグ') }` })
+	expect (await screen.findByRole ('alert')).toHaveTextContent (message)
+	expect (getHelpTitleIndex).not.toHaveBeenCalled ()
+	expect (screen.queryByRole ('link', { name: '新規作成してください' }))
+	    .not.toBeInTheDocument ()
+      })
+
   it ('renders deprecated state outside the wiki title link', async () => {
     wikiApi.fetchWikiPageByTitle.mockResolvedValueOnce (buildWikiPage ({
       title:        '旧タグ',

frontend/src/pages/wiki/WikiDetailPage.tsx

@@ -1,5 +1,3 @@
-import type { FC } from 'react'
-
 import { useQuery } from '@tanstack/react-query'
 import { useEffect, useMemo } from 'react'
 import { Helmet } from 'react-helmet-async'
@@ -13,10 +11,14 @@ import TabGroup, { Tab } from '@/components/common/TabGroup'
 import MainArea from '@/components/layout/MainArea'
 import { SITE_TITLE } from '@/config'
 import { WikiIdBus } from '@/lib/eventBus/WikiIdBus'
+import { getHelpTitleIndex } from '@/lib/help'
 import { fetchPosts } from '@/lib/posts'
-import { postsKeys, tagsKeys, wikiKeys } from '@/lib/queryKeys'
+import { helpKeys, postsKeys, tagsKeys, wikiKeys } from '@/lib/queryKeys'
 import { fetchTagByName } from '@/lib/tags'
 import { fetchWikiPage, fetchWikiPageByTitle } from '@/lib/wiki'
+import NotFound from '@/pages/NotFound'
+
+import type { FC } from 'react'
 
 import type { Tag } from '@/types'
 
@@ -33,15 +35,33 @@ const WikiDetailPage: FC = () => {
   const query = new URLSearchParams (location.search)
   const version = query.get ('version') || undefined
 
-  const { data: wikiPage, isLoading: loading } = useQuery ({
+  const { data: wikiPage, isLoading: loading, isSuccess, isFetching, error } = useQuery ({
       enabled:  Boolean (title) && !(/^\d+$/.test (title)),
       queryKey: wikiKeys.show (title, { version }),
       queryFn:  () => fetchWikiPageByTitle (title, { version }) })
 
+  const missingWiki = isSuccess && !(isFetching) && wikiPage == null
+  const { data: titleIndex, isPending: lookupPending,
+	  isFetching: lookupFetching, error: lookupError } = useQuery ({
+      enabled: missingWiki,
+      queryKey: helpKeys.titles ('ja'),
+      queryFn: () => getHelpTitleIndex ('ja'),
+      staleTime: Infinity })
+  const lookupTitle = title.trim ().normalize ('NFKC').toLowerCase ().replace (/^ヘルプ:/, '')
+  const identifiers = useMemo (() => (
+      (titleIndex != null && Object.hasOwn (titleIndex, lookupTitle))
+      ? titleIndex[lookupTitle] : []), [titleIndex, lookupTitle])
+
+  useEffect (() => {
+    if (missingWiki && !(lookupFetching) && lookupError == null && identifiers.length === 1)
+      navigate (`/help/${ encodeURIComponent (identifiers[0]) }`, { replace: true })
+  }, [missingWiki, lookupFetching, lookupError, identifiers, navigate])
+
   const effectiveTitle = wikiPage?.title ?? title
   const deprecated = wikiPage?.deprecatedAt != null
 
-  const { data: tag } = useQuery ({
+  const { data: tag, isPending: tagPending,
+	  isFetching: tagFetching, error: tagError } = useQuery ({
       enabled:  Boolean (effectiveTitle),
       queryKey: tagsKeys.show (effectiveTitle),
       queryFn:  () => fetchTagByName (effectiveTitle) })
@@ -73,10 +93,12 @@ const WikiDetailPage: FC = () => {
     if (!(/^\d+$/.test (title)))
       return
 
+    let active = true
     void (async () => {
       try
       {
 	const data = await fetchWikiPage (title, { })
+	if (active)
 	  navigate (`/wiki/${ encodeURIComponent (data.title) }`, { replace: true })
       }
       catch
@@ -84,8 +106,23 @@ const WikiDetailPage: FC = () => {
 	;
       }
     }) ()
+
+    return () => {
+      active = false
+    }
   }, [title, navigate])
 
+  const displayError = error ?? (missingWiki ? lookupError ?? tagError : null)
+  if (displayError != null)
+    return <MainArea><p role="alert">{displayError.message}</p></MainArea>
+
+  const waiting = loading || (isFetching && wikiPage == null)
+      || (missingWiki && (lookupPending || lookupFetching || tagPending
+			  || (tagFetching && tag == null) || identifiers.length === 1))
+
+  if (missingWiki && !(waiting) && (tag == null || version != null))
+    return <NotFound/>
+
   return (
       <MainArea>
 	<Helmet>
@@ -119,7 +156,7 @@ const WikiDetailPage: FC = () => {
 		     {...(version && { to: `/wiki/${ encodeURIComponent (title) }` })}/>
 	    {deprecated && <span>（廃止）</span>}
 	  </h1>
-	  {loading ? <div>Loading...</div> : <WikiBody title={title} body={wikiPage?.body}/>}
+	  {waiting ? <div>Loading...</div> : <WikiBody title={title} body={wikiPage?.body}/>}
 
 	  {(!(version) && posts.length > 0) && (
 	      <div className="not-prose">

frontend/src/pages/wiki/WikiHelpCompatibility.test.tsx

@@ -0,0 +1,144 @@
+import { act, fireEvent, screen, waitFor } from '@testing-library/react'
+import { Route, Routes, useLocation, useNavigate } from 'react-router-dom'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { apiGet } from '@/lib/api'
+import * as help from '@/lib/help'
+import HelpDetailPage from '@/pages/help/HelpDetailPage'
+import WikiDetailPage from '@/pages/wiki/WikiDetailPage'
+import { buildWikiPage } from '@/test/factories'
+import { helpFixture } from '@/test/helpFixtures'
+import { renderWithProviders } from '@/test/render'
+
+vi.mock ('@/lib/api', () => ({ apiGet: vi.fn () }))
+vi.mock ('@/lib/help', async () => (await import ('@/test/helpFixtures')).helpFixture.api)
+vi.mock ('@/lib/tags', () => ({ fetchTagByName: async () => null }))
+vi.mock ('@/lib/posts', () => ({ fetchPosts: async () => ({ posts: [], count: 0 }) }))
+
+
+const Location = () => {
+  const location = useLocation ()
+  const navigate = useNavigate ()
+  return (
+      <>
+	<output data-testid="location">{location.pathname}{location.search}</output>
+	<button onClick={() => navigate (`/wiki/${ encodeURIComponent ('該当なし') }`)}>
+	  switch title
+	</button>
+      </>)
+}
+
+
+const renderWiki = (title: string) => {
+  const route = `/wiki/${encodeURIComponent (title)}`
+  const { queryClient } = renderWithProviders (
+      <>
+	<Location/>
+	<Routes>
+	  <Route path="/wiki/:title" element={<WikiDetailPage/>}/>
+	  <Route path="/help/:identifier" element={<HelpDetailPage/>}/>
+	  <Route path="/404" element={<div>unexpected redirect</div>}/>
+	</Routes>
+      </>, { route })
+  return { route, queryClient }
+}
+
+
+describe ('Legacy Wiki to Help compatibility', () => {
+  beforeEach (() => {
+    helpFixture.reset ()
+    vi.mocked (apiGet).mockReset ()
+    vi.mocked (apiGet).mockImplementation (async path => {
+      if (path === '/wiki')
+	return []
+      throw { isAxiosError: true, response: { status: 404 } }
+    })
+  })
+
+  it ('keeps a successful Wiki result even when Help has the same title', async () => {
+    vi.mocked (apiGet).mockResolvedValueOnce (buildWikiPage ({
+	title: 'ホーム', body: 'Wiki の現行本文' }))
+    const { route } = renderWiki ('ホーム')
+    await screen.findByText ('Wiki の現行本文')
+    expect (screen.getByTestId ('location')).toHaveTextContent (route)
+    expect (screen.queryByText ('タグを検索する')).not.toBeInTheDocument ()
+    expect (helpFixture.loads.titles).not.toHaveBeenCalled ()
+  })
+
+  it.each (['ホーム', 'ヘルプ:ホーム', '旧ホーム', 'ヘルプ:旧ホーム', 'ホーム第十版']) (
+      'redirects a unique current or historical Japanese title: %s', async title => {
+	renderWiki (title)
+	await waitFor (() => {
+	  expect (screen.getByTestId ('location')).toHaveTextContent ('/help/guide')
+	})
+	expect (await screen.findByRole ('heading', { name: 'ホーム第十版' }))
+	    .toBeInTheDocument ()
+	Object.values (helpFixture.raw).forEach (load => expect (load).not.toHaveBeenCalled ())
+	expect (apiGet).toHaveBeenCalledWith (
+	    `/wiki/title/${encodeURIComponent (title)}`, expect.anything ())
+      })
+
+  it ('keeps the original URL and displays NotFound when no title matches', async () => {
+    const { route, queryClient } = renderWiki ('該当なし')
+    await screen.findByText ('404')
+    await waitFor (() => {
+      expect (helpFixture.loads.titles).toHaveBeenCalled ()
+      expect (queryClient.isFetching ()).toBe (0)
+    })
+    expect (screen.getByTestId ('location')).toHaveTextContent (route)
+    expect (screen.queryByText ('unexpected redirect')).not.toBeInTheDocument ()
+  })
+
+  it ('does not choose between articles with the same title', async () => {
+    helpFixture.assets.titles.default.ja['ホーム'] = ['guide', '設定 手順']
+    const { route, queryClient } = renderWiki ('ホーム')
+    await screen.findByText ('404')
+    await waitFor (() => {
+      expect (helpFixture.loads.titles).toHaveBeenCalled ()
+      expect (queryClient.isFetching ()).toBe (0)
+    })
+    expect (screen.getByTestId ('location')).toHaveTextContent (route)
+  })
+
+  it ('waits for title lookup and does not redirect a later Wiki URL using an old title',
+      async () => {
+	let finish!: (index: Record<string, string[]>) => void
+	const pending = new Promise<Record<string, string[]>> (resolve => {
+	  finish = resolve
+	})
+	const lookup = vi.spyOn (help, 'getHelpTitleIndex').mockReturnValueOnce (pending)
+	renderWiki ('ホーム')
+	await waitFor (() => expect (lookup).toHaveBeenCalled ())
+	expect (screen.queryByText ('404')).not.toBeInTheDocument ()
+	fireEvent.click (screen.getByRole ('button', { name: 'switch title' }))
+	await waitFor (() => expect (apiGet).toHaveBeenCalledWith (
+	    `/wiki/title/${ encodeURIComponent ('該当なし') }`, expect.anything ()))
+	await act (async () => finish ({ 'ホーム': ['guide'] }))
+	await screen.findByText ('404')
+	expect (screen.getByTestId ('location'))
+	    .toHaveTextContent (`/wiki/${ encodeURIComponent ('該当なし') }`)
+      })
+
+  it ('does not treat a Wiki network failure as a missing Wiki', async () => {
+    vi.mocked (apiGet).mockRejectedValueOnce (new Error ('offline'))
+    const { route } = renderWiki ('ホーム')
+    expect (await screen.findByRole ('alert')).toHaveTextContent ('offline')
+    expect (screen.getByTestId ('location')).toHaveTextContent (route)
+    expect (helpFixture.loads.titles).not.toHaveBeenCalled ()
+    expect (screen.queryByText ('404')).not.toBeInTheDocument ()
+    expect (screen.queryByRole ('link', { name: '新規作成してください' }))
+	.not.toBeInTheDocument ()
+  })
+
+  it.each ([403, 500]) ('does not redirect HTTP %s to Help', async status => {
+    vi.mocked (apiGet).mockRejectedValueOnce ({ isAxiosError: true, response: { status } })
+    const { route, queryClient } = renderWiki ('ホーム')
+    await waitFor (() => {
+      expect (apiGet).toHaveBeenCalled ()
+      expect (queryClient.isFetching ()).toBe (0)
+    })
+    expect (screen.getByTestId ('location')).toHaveTextContent (route)
+    expect (screen.queryByText ('タグを検索する')).not.toBeInTheDocument ()
+    expect (helpFixture.loads.titles).not.toHaveBeenCalled ()
+  })
+})

frontend/src/pages/wiki/WikiNewPage.tsx

@@ -1,10 +1,10 @@
-import type { FC } from 'react'
-
+import { useQueryClient } from '@tanstack/react-query'
 import MarkdownIt from 'markdown-it'
 import { useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import MdEditor from 'react-markdown-editor-lite'
 import { useLocation, useNavigate } from 'react-router-dom'
+import 'react-markdown-editor-lite/lib/index.css'
 
 import FieldError from '@/components/common/FieldError'
 import FormField from '@/components/common/FormField'
@@ -12,12 +12,13 @@ import MainArea from '@/components/layout/MainArea'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { apiPost } from '@/lib/api'
+import { tagsKeys, wikiKeys } from '@/lib/queryKeys'
 import { canEditContent } from '@/lib/users'
 import { inputClass } from '@/lib/utils'
 import { useValidationErrors } from '@/lib/useValidationErrors'
 import Forbidden from '@/pages/Forbidden'
 
-import 'react-markdown-editor-lite/lib/index.css'
+import type { FC } from 'react'
 
 import type { User, WikiPage } from '@/types'
 
@@ -38,6 +39,7 @@ const WikiNewPage: FC<Props> = ({ user }) => {
 
   const location = useLocation ()
   const navigate = useNavigate ()
+  const qc = useQueryClient ()
 
   const query = new URLSearchParams (location.search)
   const titleQuery = query.get ('title') ?? ''
@@ -58,8 +60,13 @@ const WikiNewPage: FC<Props> = ({ user }) => {
     {
       const data = await apiPost<WikiPage> ('/wiki', formData,
 				  { headers: { 'Content-Type': 'multipart/form-data' } })
+      await qc.cancelQueries ({ queryKey: wikiKeys.root })
+      qc.setQueryData (wikiKeys.show (data.title, { }), data)
+      await Promise.all ([
+	  qc.invalidateQueries ({ queryKey: wikiKeys.root }),
+	  qc.invalidateQueries ({ queryKey: tagsKeys.root })])
       toast ({ title: '投稿成功!' })
-      navigate (`/wiki/${ data.title }`)
+      navigate (`/wiki/${ encodeURIComponent (data.title) }`)
     }
     catch (e)
     {

frontend/src/pages/wiki/WikiSearchPage.test.tsx

@@ -1,62 +1,58 @@
 import { fireEvent, screen, waitFor } from '@testing-library/react'
-import { describe, expect, it, vi } from 'vitest'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
 
+import { apiGet } from '@/lib/api'
 import WikiSearchPage from '@/pages/wiki/WikiSearchPage'
 import { buildWikiPage } from '@/test/factories'
 import { renderWithProviders } from '@/test/render'
 
-const api = vi.hoisted (() => ({
-  apiGet: vi.fn (),
-}))
+vi.mock ('@/lib/api', () => ({ apiGet: vi.fn () }))
 
-vi.mock ('@/lib/api', () => api)
 
-describe ('WikiSearchPage', () => {
+describe ('Wiki resource identity', () => {
+  beforeEach (() => vi.mocked (apiGet).mockReset ())
+
   it ('loads initial wiki pages and renders links', async () => {
-    api.apiGet.mockResolvedValueOnce ([buildWikiPage ({ title: '虹夏' })])
-
+    vi.mocked (apiGet).mockResolvedValueOnce ([buildWikiPage ({ title: '虹夏' })])
     renderWithProviders (<WikiSearchPage/>)
-
-    expect (await screen.findByRole ('link', { name: '虹夏' })).toHaveAttribute (
-        'href',
-        '/wiki/%E8%99%B9%E5%A4%8F',
-    )
-    expect (api.apiGet).toHaveBeenCalledWith ('/wiki', { params: { title: '' } })
+    expect (await screen.findByRole ('link', { name: '虹夏' }))
+	.toHaveAttribute ('href', '/wiki/%E8%99%B9%E5%A4%8F')
+    expect (apiGet).toHaveBeenCalledWith ('/wiki', { params: { title: '' } })
   })
 
   it ('searches by title on submit', async () => {
-    api.apiGet
-      .mockResolvedValueOnce ([])
+    vi.mocked (apiGet).mockResolvedValueOnce ([])
 	.mockResolvedValueOnce ([buildWikiPage ({ title: '検索結果' })])
-
     renderWithProviders (<WikiSearchPage/>)
-
+    await waitFor (() => expect (apiGet).toHaveBeenCalledTimes (1))
     fireEvent.change (screen.getAllByRole ('textbox')[0], { target: { value: '検索' } })
     fireEvent.click (screen.getByRole ('button', { name: '検索' }))
-
-    await waitFor (() => {
-      expect (api.apiGet).toHaveBeenLastCalledWith (
-          '/wiki',
-          { params: { title: '検索' } },
-      )
-    })
+    await waitFor (() => expect (apiGet).toHaveBeenLastCalledWith (
+	'/wiki', { params: { title: '検索' } }))
     expect (await screen.findByRole ('link', { name: '検索結果' })).toBeInTheDocument ()
   })
 
   it ('marks deprecated wiki tags in the result title', async () => {
-    api.apiGet.mockResolvedValueOnce ([
-        buildWikiPage ({
-            title:        '旧タグ',
-            deprecatedAt: '2026-06-01T00:00:00.000Z',
-        }),
-    ])
+    vi.mocked (apiGet).mockResolvedValueOnce ([buildWikiPage ({
+	title: '旧タグ', deprecatedAt: '2026-06-01T00:00:00.000Z' })])
+    renderWithProviders (<WikiSearchPage/>)
+    expect (await screen.findByRole ('link', { name: '旧タグ' })).toBeInTheDocument ()
+    expect (screen.getByText ('（廃止）').closest ('a')).toBeNull ()
+  })
 
+  it ('renders colliding description and group IDs with stable distinct keys', async () => {
+    const errors = vi.spyOn (console, 'error').mockImplementation (() => {})
+    const description = buildWikiPage ({ id: 51, title: '説明', resourceType: 'tag_description' })
+    const group = buildWikiPage ({ id: 51, title: 'タググループ:一覧', resourceType: 'tag_group' })
+    vi.mocked (apiGet).mockResolvedValue ([description, group])
     renderWithProviders (<WikiSearchPage/>)
 
-    const link = await screen.findByRole ('link', { name: '旧タグ' })
-    const marker = screen.getByText ('（廃止）')
-
-    expect (link).toBeInTheDocument ()
-    expect (marker.closest ('a')).toBeNull ()
+    await screen.findByRole ('link', { name: '説明' })
+    expect (screen.getByRole ('link', { name: 'タググループ:一覧' }))
+	.toHaveAttribute ('href', `/wiki/${ encodeURIComponent (group.title) }`)
+    await waitFor (() => {
+      expect (errors.mock.calls.some (args => args.some (value =>
+	  String (value).includes ('same key')))).toBe (false)
+    })
   })
 })

frontend/src/pages/wiki/WikiSearchPage.tsx

@@ -81,7 +81,8 @@ const WikiSearchPage: FC = () => {
 	    </thead>
 	    <tbody>
 	      {results.map (page => (
-		  <tr key={page.id} className="even:bg-gray-100 dark:even:bg-gray-700">
+		  <tr key={`${ page.resourceType }:${ page.id }`}
+		      className="even:bg-gray-100 dark:even:bg-gray-700">
 		    <td className="p-2">
 		      <PrefetchLink to={`/wiki/${ encodeURIComponent (page.title) }`}>
 			{page.title}

frontend/src/test/compileMdx.ts

@@ -0,0 +1,37 @@
+import * as jsxRuntime from 'react/jsx-runtime'
+import { ModuleKind, ScriptTarget, transpileModule } from 'typescript'
+
+import { useMDXComponents } from '@/mdx-components'
+import viteConfig from '../../vite.config'
+
+import type { ComponentType } from 'react'
+
+type Transform = (source: string, id: string) => Promise<{ code: string } | null>
+
+
+export const transformMdx = async (source: string, suffix = '.mdx') => {
+  const plugins = viteConfig.plugins?.flat () as { name: string; transform: Transform }[]
+  const plugin = plugins.find (entry => entry.name === '@mdx-js/rollup')
+  if (plugin == null)
+    throw new Error ('The application MDX compiler is not registered')
+  return plugin.transform.call (plugin, source, `/test/help-table${suffix}`)
+}
+
+
+export const compileMdx = async (source: string): Promise<ComponentType> => {
+  const result = await transformMdx (source)
+  if (result == null)
+    throw new Error ('MDX was not compiled')
+  const code = transpileModule (result.code, {
+      compilerOptions: { module: ModuleKind.CommonJS, target: ScriptTarget.ES2022 } }).outputText
+  const exports = { } as { default: ComponentType }
+  const requireFixture = (name: string) => {
+    if (name === 'react/jsx-runtime')
+      return jsxRuntime
+    if (name === '@/mdx-components')
+      return { useMDXComponents }
+    throw new Error (`Unexpected MDX dependency: ${name}`)
+  }
+  new Function ('require', 'exports', code) (requireFixture, exports)
+  return exports.default
+}

frontend/src/test/factories.ts

@@ -59,6 +59,7 @@ export const buildUser = (overrides: Partial<User> = {}): User => ({
 })
 
 export const buildWikiPage = (overrides: Partial<WikiPage> = {}): WikiPage => ({
+  resourceType: 'tag_description',
   id:            1,
   title:         'テストWiki',
   deprecatedAt:  null,

frontend/src/test/helpFixtures.tsx

@@ -0,0 +1,126 @@
+import { createElement } from 'react'
+import { ModuleKind, ScriptTarget, transpileModule } from 'typescript'
+import { vi } from 'vitest'
+
+import source from '@/lib/help.ts?raw'
+
+import type { ComponentType } from 'react'
+
+import type * as Help from '@/lib/help'
+
+export const editionRows = [
+    ['guide', 'ja', '2026-10-01', '旧ホーム', '過去限定本文'],
+    ['guide', 'ja', '2026-10-02', 'ホーム', 'タグを検索する'],
+    ['guide', 'ja', '2026-10-02.2', 'ホーム第二版', '第二版本文'],
+    ['guide', 'ja', '2026-10-02.3', 'ホーム第三版', '第三版本文'],
+    ['guide', 'ja', '2026-10-02.10', 'ホーム第十版', 'タグを検索する 第十版本文'],
+    ['guide', 'ja', '2026-10-02.11', '未公開ホーム', '未公開限定本文'],
+    ['guide', 'en', '2026-10-02', 'Home', 'English body'],
+    ['設定 手順', 'ja', '2026-10-03', '設定', 'タグを設定する']] as const
+
+const compiled = transpileModule (source, {
+    compilerOptions: { module: ModuleKind.CommonJS, target: ScriptTarget.ES2022 } }).outputText
+
+
+// Supply generated virtual modules, executing the actual runtime Help module.
+export const createHelpFixture = () => {
+  const articles: Record<string, Help.ArticleMeta> = {
+      guide: { identifier: 'guide', canonicalLanguage: 'ja', languages: {
+	  ja: { currentEdition: '2026-10-02.10', publishedEditions: [
+	      '2026-10-01', '2026-10-02', '2026-10-02.2', '2026-10-02.3', '2026-10-02.10'] },
+	  en: { currentEdition: '2026-10-02', publishedEditions: ['2026-10-02'] } } },
+      '設定 手順': { identifier: '設定 手順', canonicalLanguage: 'ja', languages: {
+	  ja: { currentEdition: '2026-10-03', publishedEditions: ['2026-10-03'] } } } }
+  const published = editionRows.filter (([id, language, edition]) =>
+      articles[id].languages[language].publishedEditions.includes (edition))
+  const raw = Object.fromEntries (editionRows.map (([id, language, edition, title, body]) => [
+      `../../help/${id}/editions/${language}/${edition}.mdx`,
+      vi.fn (async () => `# ${title}\n\n${body}`)]))
+  const moduleLoads = Object.fromEntries (published.map (([id, language, edition, title, body]) => [
+      `../../help/${id}/editions/${language}/${edition}.mdx`,
+      vi.fn (async (): Promise<{ default: ComponentType }> => ({
+	  default: () => createElement ('div', null,
+	      createElement ('h1', null, title), createElement ('p', null, body)) }))]))
+  // Dynamic imports reuse a module namespace even when called more than once.
+  const caches: Record<string, { promise?: Promise<{ default: ComponentType }> }> = { }
+  const modules = Object.fromEntries (Object.entries (moduleLoads).map (([path, load]) => {
+    caches[path] = { }
+    return [path, vi.fn (() => caches[path].promise ??= load ())]
+  }))
+  const texts = Object.fromEntries (published.map (([id, language, edition, title, body]) => [
+      `${ id }/${ language }/${ edition }`,
+      vi.fn (async () => ({ default: { title, body: `\n${ body }` } }))]))
+  const components = Object.fromEntries (published.map (([id, language, edition]) => [
+      `${ id }/${ language }/${ edition }`,
+      modules[`../../help/${ id }/editions/${ language }/${ edition }.mdx`]]))
+  const sources = Object.fromEntries (published.map (([id, language, edition]) => [
+      `${ id }/${ language }/${ edition }`,
+      async () => ({ default: await raw[
+	  `../../help/${ id }/editions/${ language }/${ edition }.mdx`] () })]))
+  const search: Record<string, Help.HelpSearchEntry[]> = { }
+  const changes: Record<string, Help.HelpChange[]> = { }
+  const titles: Record<string, Help.HelpTitleIndex> = { }
+  published.forEach (([id, language, edition, title, body]) => {
+    const editions = articles[id].languages[language].publishedEditions
+    const index = editions.indexOf (edition)
+    const searchRows = search[language] ??= []
+    if (articles[id].languages[language].currentEdition === edition)
+	searchRows.push ({ identifier: id, edition, title, body: `\n${ body }` })
+    const rows = changes[language] ??= []
+    rows.push ({ identifier: id, edition, title,
+	pred: editions[index - 1] ?? null,
+	change: index === 0 ? 'new' : 'update', date: edition.split ('.')[0] })
+    const lookup = (titles[language] ??= { })
+    const identifiers = lookup[title.trim ().normalize ('NFKC').toLowerCase ()] ??= []
+    if (!(identifiers.includes (id)))
+      identifiers.push (id)
+  })
+  Object.values (changes).forEach (rows => rows.sort ((a, b) => {
+    const [dateA, revisionA = '1'] = a.edition.split ('.')
+    const [dateB, revisionB = '1'] = b.edition.split ('.')
+    return dateB.localeCompare (dateA) || Number (revisionB) - Number (revisionA)
+	|| a.identifier.localeCompare (b.identifier)
+  }))
+  const assets = {
+      articles: { default: articles }, search: { default: search },
+      changes: { default: changes }, titles: { default: titles } }
+  const snapshot = JSON.stringify (assets)
+  const loaders = { components, sources, texts }
+  const loads = {
+      articles: vi.fn (() => assets.articles), search: vi.fn (() => assets.search),
+      changes: vi.fn (() => assets.changes), titles: vi.fn (() => assets.titles),
+      loaders: vi.fn (() => loaders) }
+  const api = { } as typeof Help
+  const requireFixture = (name: string) => {
+    const key = name.replace ('virtual:help/', '') as keyof typeof loads
+    if (Object.hasOwn (loads, key))
+      return loads[key] ()
+    throw new Error (`Unexpected Help dependency: ${name}`)
+  }
+  new Function ('require', 'exports', compiled) (requireFixture, api)
+  const reset = () => {
+    Object.assign (assets, JSON.parse (snapshot))
+    editionRows.forEach (([id, language, edition, title, body]) => {
+      const path = `../../help/${ id }/editions/${ language }/${ edition }.mdx`
+      raw[path].mockReset ().mockResolvedValue (`# ${ title }\n\n${ body }`)
+    })
+    published.forEach (([id, language, edition, title, body]) => {
+      const path = `../../help/${ id }/editions/${ language }/${ edition }.mdx`
+      caches[path].promise = undefined
+      moduleLoads[path].mockClear ()
+      modules[path].mockReset ().mockImplementation (() =>
+	  caches[path].promise ??= moduleLoads[path] ())
+      texts[`${ id }/${ language }/${ edition }`].mockReset ().mockResolvedValue ({
+	  default: { title, body: `\n${ body }` } })
+    })
+    loads.articles.mockReset ().mockImplementation (() => assets.articles)
+    loads.search.mockReset ().mockImplementation (() => assets.search)
+    loads.changes.mockReset ().mockImplementation (() => assets.changes)
+    loads.titles.mockReset ().mockImplementation (() => assets.titles)
+    loads.loaders.mockReset ().mockImplementation (() => loaders)
+  }
+  return { api, assets, loads, raw, modules, moduleLoads, texts, reset }
+}
+
+
+export const helpFixture = createHelpFixture ()

frontend/src/types.ts

@@ -452,6 +452,7 @@ export type ViewFlagBehavior = typeof ViewFlagBehavior[keyof typeof ViewFlagBeha
 
 export type WikiPage = {
     id:             number
+    resourceType:   'tag_description' | 'tag_group'
     title:          string
     deprecatedAt:   string | null
     createdUserId:  number