diff --git a/AGENTS.md b/AGENTS.md
index e4704c2..7ab89a3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -84,12 +84,14 @@ cd frontend
 npm run dev
 npm run build
 npm run lint
+npm run test
+npm run test:run
 npm run preview
 ```
 
 `npm run build` runs `tsc -b && vite build`, then `postbuild` runs `node scripts/generate-sitemap.js`.
 
-Do not write or report `npm test` as a repository command unless a `test` script is added to `frontend/package.json`.
+`npm run test` runs Vitest in watch mode. Use `npm run test:run` for a non-watch frontend test run.
 
 ## Coding style
 
@@ -164,7 +166,7 @@ function PostFormTagsArea ({ tags, setTags }: Props) {
 - Keep changes scoped to the requested issue.
 - Do not scan or summarize dependency/generated/runtime directories such as `node_modules`, `dist`, `tmp`, `log`, and `storage` unless explicitly needed.
 - Before touching wiki, tag, versioning, BAN, IP BAN, or authentication behavior, inspect the related request specs and service objects.
-- If frontend code changes, run the existing frontend verification commands that apply: `npm run build` and `npm run lint`.
+- If frontend code changes, run the existing frontend verification commands that apply: `npm run build`, `npm run lint`, and `npm run test:run`.
 - If backend code changes, run the relevant RSpec command; for broad backend changes, run `bundle exec rspec`.
 - If a verification command cannot be run or fails, report the exact command and failure.
 
diff --git a/backend/app/controllers/application_controller.rb b/backend/app/controllers/application_controller.rb
index 39a4465..42adc75 100644
--- a/backend/app/controllers/application_controller.rb
+++ b/backend/app/controllers/application_controller.rb
@@ -28,19 +28,17 @@ class ApplicationController < ActionController::API
     end
   end
 
-  def render_bad_request message = 'リクエストが不正です．', field: nil, code: :bad_request
-    render_error(:bad_request, message, field:, code:)
+  def render_bad_request message = 'リクエストが不正です．'
+    render json: { type: 'bad_request',
+                   message:,
+                   errors: { },
+                   base_errors: [message] },
+           status: :bad_request
   end
 
-  def render_unprocessable_entity message = '入力を確認してください．', field: nil, code: :invalid
-    render_error(:unprocessable_entity, message, field:, code:)
-  end
-
-  def render_error status, message, field: nil, code: status
-    error = { code: code.to_s, message: }
-    error[:field] = field.to_s if field.present?
-
-    render json: { errors: [error] }, status:
+  def render_unprocessable_entity message = '入力を確認してください．', field: nil
+    render_validation_error(fields: field ? { field => [message] } : { },
+                            base: field ? [] : [message])
   end
 
   def render_record_invalid error
diff --git a/backend/app/controllers/deerjikists_controller.rb b/backend/app/controllers/deerjikists_controller.rb
index 7a9cb75..9302370 100644
--- a/backend/app/controllers/deerjikists_controller.rb
+++ b/backend/app/controllers/deerjikists_controller.rb
@@ -2,8 +2,8 @@ class DeerjikistsController < ApplicationController
   def show
     platform = params[:platform].to_s.strip
     code = params[:code].to_s.strip
-    return render_bad_request('platform は必須です．', field: :platform) if platform.blank?
-    return render_bad_request('code は必須です．', field: :code) if code.blank?
+    return render_bad_request('platform は必須です．') if platform.blank?
+    return render_bad_request('code は必須です．') if code.blank?
 
     deerjikist = Deerjikist
                  .joins(:tag)
@@ -23,9 +23,9 @@ class DeerjikistsController < ApplicationController
     platform = params[:platform].to_s.strip
     code = params[:code].to_s.strip
     tag_id = params[:tag_id].to_i
-    return render_bad_request('platform は必須です．', field: :platform) if platform.blank?
-    return render_bad_request('code は必須です．', field: :code) if code.blank?
-    return render_bad_request('tag_id が不正です．', field: :tag_id) if tag_id <= 0
+    return render_bad_request('platform は必須です．') if platform.blank?
+    return render_bad_request('code は必須です．') if code.blank?
+    return render_bad_request('tag_id が不正です．') if tag_id <= 0
 
     deerjikist = Deerjikist.find_or_initialize_by(platform:, code:).tap do |d|
       d.tag_id = tag_id
@@ -41,8 +41,8 @@ class DeerjikistsController < ApplicationController
 
     platform = params[:platform].to_s.strip
     code = params[:code].to_s.strip
-    return render_bad_request('platform は必須です．', field: :platform) if platform.blank?
-    return render_bad_request('code は必須です．', field: :code) if code.blank?
+    return render_bad_request('platform は必須です．') if platform.blank?
+    return render_bad_request('code は必須です．') if code.blank?
 
     Deerjikist.find([platform, code]).destroy!
 
diff --git a/backend/app/controllers/materials_controller.rb b/backend/app/controllers/materials_controller.rb
index 0fc5a9c..2a86b77 100644
--- a/backend/app/controllers/materials_controller.rb
+++ b/backend/app/controllers/materials_controller.rb
@@ -40,8 +40,11 @@ class MaterialsController < ApplicationController
     tag_name_raw = params[:tag].to_s.strip
     file = params[:file]
     url = params[:url].to_s.strip.presence
-    return render_bad_request('タグは必須です．', field: :tag) if tag_name_raw.blank?
-    return render_bad_request('ファイルまたは URL は必須です．') if file.blank? && url.blank?
+    return render_unprocessable_entity('タグは必須です．', field: :tag) if tag_name_raw.blank?
+    if file.blank? && url.blank?
+      return render_validation_error fields: { file: ['ファイルまたは URL は必須です．'],
+                                               url: ['ファイルまたは URL は必須です．'] }
+    end
 
     tag_name = TagName.find_undiscard_or_create_by!(name: tag_name_raw)
     tag = tag_name.tag
@@ -69,8 +72,11 @@ class MaterialsController < ApplicationController
     tag_name_raw = params[:tag].to_s.strip
     file = params[:file]
     url = params[:url].to_s.strip.presence
-    return render_bad_request('タグは必須です．', field: :tag) if tag_name_raw.blank?
-    return render_bad_request('ファイルまたは URL は必須です．') if file.blank? && url.blank?
+    return render_unprocessable_entity('タグは必須です．', field: :tag) if tag_name_raw.blank?
+    if file.blank? && url.blank?
+      return render_validation_error fields: { file: ['ファイルまたは URL は必須です．'],
+                                               url: ['ファイルまたは URL は必須です．'] }
+    end
 
     tag_name = TagName.find_undiscard_or_create_by!(name: tag_name_raw)
     tag = tag_name.tag
diff --git a/backend/app/controllers/nico_tags_controller.rb b/backend/app/controllers/nico_tags_controller.rb
index 55c54a4..002b66f 100644
--- a/backend/app/controllers/nico_tags_controller.rb
+++ b/backend/app/controllers/nico_tags_controller.rb
@@ -30,13 +30,13 @@ class NicoTagsController < ApplicationController
     id = params[:id].to_i
 
     tag = Tag.find(id)
-    return render_bad_request('ニコニコ・タグを指定してください．', field: :id) unless tag.nico?
+    return render_bad_request('ニコニコ・タグを指定してください．') unless tag.nico?
 
     linked_tag_names = params[:tags].to_s.split
     linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
                                                         with_no_deerjikist: false)
     if linked_tags.any? { |t| t.nico? }
-      return render_bad_request('ニコニコ・タグ同士は連携できません．', field: :tags)
+      return render_unprocessable_entity('ニコニコ・タグ同士は連携できません．', field: :tags)
     end
 
     ApplicationRecord.transaction do
diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index dedfe51..976578d 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -192,7 +192,7 @@ class PostsController < ApplicationController
     return render_bad_request('force と merge は同時に指定できません．') if force && merge
 
     base_version_no = parse_base_version_no
-    return render_bad_request('base_version_no は必須です．', field: :base_version_no) if !(force) && !(base_version_no)
+    return render_bad_request('base_version_no は必須です．') if !(force) && !(base_version_no)
 
     title = params[:title].presence
     tag_names = params[:tags].to_s.split
@@ -671,9 +671,9 @@ class PostsController < ApplicationController
   end
 
   def render_post_form_record_invalid record
-    if e.record.is_a?(TagName) || e.record.is_a?(Tag)
-      render_validation_error fields: { tags: e.record.errors.full_messages.map { |message|
-        "タグ名 “#{ e.record.name }”: #{ message }"
+    if record.is_a?(TagName) || record.is_a?(Tag)
+      render_validation_error fields: { tags: record.errors.full_messages.map { |message|
+        "タグ名 “#{ record.name }”: #{ message }"
       } }
     else
       render_validation_error record
diff --git a/backend/app/controllers/preview_controller.rb b/backend/app/controllers/preview_controller.rb
index 24d2a2d..5486838 100644
--- a/backend/app/controllers/preview_controller.rb
+++ b/backend/app/controllers/preview_controller.rb
@@ -4,7 +4,7 @@ class PreviewController < ApplicationController
     return head :unauthorized unless current_user
 
     url = params[:url]
-    return render_bad_request('URL は必須です．', field: :url) unless url.present?
+    return render_bad_request('URL は必須です．') unless url.present?
 
     unless url.start_with?(/http(s)?:\/\//)
       url = 'http://' + url
@@ -16,7 +16,7 @@ class PreviewController < ApplicationController
 
     render json: { title: title }
   rescue => e
-    render_bad_request(e.message, field: :url)
+    render_bad_request(e.message)
   end
 
   def thumbnail
@@ -25,7 +25,7 @@ class PreviewController < ApplicationController
     return head :unauthorized unless current_user
 
     url = params[:url]
-    return render_bad_request('URL は必須です．', field: :url) if url.blank?
+    return render_bad_request('URL は必須です．') if url.blank?
 
     unless url.start_with?(/http(s)?:\/\//)
       url = 'http://' + url
@@ -40,8 +40,11 @@ class PreviewController < ApplicationController
       File.delete(path) rescue nil
       send_file image.path, type: 'image/png', disposition: 'inline'
     else
-      render_error(:internal_server_error, 'サムネールを生成できませんでした．',
-                   code: :thumbnail_generation_failed)
+      render json: { type: 'internal_server_error',
+                     message: 'サムネールを生成できませんでした．',
+                     errors: { },
+                     base_errors: ['サムネールを生成できませんでした．'] },
+             status: :internal_server_error
     end
   end
 end
diff --git a/backend/app/controllers/tag_children_controller.rb b/backend/app/controllers/tag_children_controller.rb
index ee4300f..cfdde30 100644
--- a/backend/app/controllers/tag_children_controller.rb
+++ b/backend/app/controllers/tag_children_controller.rb
@@ -5,8 +5,8 @@ class TagChildrenController < ApplicationController
 
     parent_id = params[:parent_id]
     child_id = params[:child_id]
-    return render_bad_request('parent_id は必須です．', field: :parent_id) if parent_id.blank?
-    return render_bad_request('child_id は必須です．', field: :child_id) if child_id.blank?
+    return render_bad_request('parent_id は必須です．') if parent_id.blank?
+    return render_bad_request('child_id は必須です．') if child_id.blank?
 
     parent = Tag.find(parent_id)
     child = Tag.find(child_id)
@@ -28,8 +28,8 @@ class TagChildrenController < ApplicationController
 
     parent_id = params[:parent_id]
     child_id = params[:child_id]
-    return render_bad_request('parent_id は必須です．', field: :parent_id) if parent_id.blank?
-    return render_bad_request('child_id は必須です．', field: :child_id) if child_id.blank?
+    return render_bad_request('parent_id は必須です．') if parent_id.blank?
+    return render_bad_request('child_id は必須です．') if child_id.blank?
 
     parent = Tag.find(parent_id)
     child = Tag.find(child_id)
diff --git a/backend/app/controllers/tags_controller.rb b/backend/app/controllers/tags_controller.rb
index 89911de..30d822b 100644
--- a/backend/app/controllers/tags_controller.rb
+++ b/backend/app/controllers/tags_controller.rb
@@ -168,7 +168,7 @@ class TagsController < ApplicationController
 
   def show_by_name
     name = params[:name].to_s.strip
-    return render_bad_request('name は必須です．', field: :name) if name.blank?
+    return render_bad_request('name は必須です．') if name.blank?
 
     tag = Tag.joins(:tag_name)
              .includes(:tag_name, :materials, tag_name: :wiki_page)
@@ -192,7 +192,7 @@ class TagsController < ApplicationController
 
   def deerjikists_by_name
     name = params[:name].to_s.strip
-    return render_bad_request('name は必須です．', field: :name) if name.blank?
+    return render_bad_request('name は必須です．') if name.blank?
 
     tag = Tag.joins(:tag_name)
              .includes(:tag_name, tag_name: :wiki_page)
@@ -214,21 +214,24 @@ class TagsController < ApplicationController
 
     ApplicationRecord.transaction do
       tag.deerjikists = []
-      params[:_json].each do
-        platform = _1[:platform]
-        code = normalise_deerjikist_code(platform, _1[:code])
+      params[:_json].each.with_index do |item, i|
+        platform = item[:platform]
+        code = normalise_deerjikist_code(platform, item[:code])
         deerjikist = Deerjikist.find_or_initialize_by(platform:, code:)
         deerjikist.tag = tag
-        deerjikist.save!
+        render_deerjikist_form_record_invalid(deerjikist, i) unless deerjikist.save
+        raise ActiveRecord::Rollback if performed?
       end
     end
 
+    return if performed?
+
     render json: DeerjikistRepr.many(tag.reload.deerjikists)
   end
 
   def materials_by_name
     name = params[:name].to_s.strip
-    return render_bad_request('name は必須です．', field: :name) if name.blank?
+    return render_bad_request('name は必須です．') if name.blank?
 
     tag = Tag.joins(:tag_name)
              .includes(:tag_name, :materials, tag_name: :wiki_page)
@@ -435,4 +438,23 @@ class TagsController < ApplicationController
   rescue
     nil
   end
+
+  def render_deerjikist_form_record_invalid deerjikist, index
+    fields = { }
+
+    deerjikist.errors.each do |error|
+      field =
+          case error.attribute
+          when :platform, :code
+            "deerjikists.#{ index }.#{ error.attribute }"
+          else
+            :deerjikists
+          end
+
+      fields[field] ||= []
+      fields[field] << error.full_message
+    end
+
+    render_validation_error fields:
+  end
 end
diff --git a/backend/app/controllers/users_controller.rb b/backend/app/controllers/users_controller.rb
index 1a6bc75..95ae677 100644
--- a/backend/app/controllers/users_controller.rb
+++ b/backend/app/controllers/users_controller.rb
@@ -42,12 +42,12 @@ class UsersController < ApplicationController
     return head :unauthorized if user&.id != params[:id].to_i
 
     name = params[:name]
-    return render_bad_request('名前は必須です．', field: :name) if name.blank?
+    return render_unprocessable_entity('名前は必須です．', field: :name) if name.blank?
 
     if user.update(name:)
       render json: user.slice(:id, :name, :inheritance_code, :role), status: :ok
     else
-      render_validation_errors user
+      render_validation_error user
     end
   end
 
diff --git a/backend/app/controllers/wiki_pages_controller.rb b/backend/app/controllers/wiki_pages_controller.rb
index ebf1c13..818ae02 100644
--- a/backend/app/controllers/wiki_pages_controller.rb
+++ b/backend/app/controllers/wiki_pages_controller.rb
@@ -46,7 +46,7 @@ class WikiPagesController < ApplicationController
 
   def diff
     id = params[:id]
-    return render_bad_request('id は必須です．', field: :id) if id.blank?
+    return render_bad_request('id は必須です．') if id.blank?
 
     from = params[:from].presence
     to = params[:to].presence
@@ -103,7 +103,7 @@ class WikiPagesController < ApplicationController
 
     render json: WikiPageRepr.base(page), status: :created
   rescue ActiveRecord::RecordInvalid => e
-    render_validation_errors e.record
+    render_validation_error e.record
   rescue ActiveRecord::RecordNotUnique
     render_record_not_unique
   end