diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 48cf816..26e358e 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -140,10 +140,11 @@ class PostsController < ApplicationController
     original_created_from = params[:original_created_from]
     original_created_before = params[:original_created_before]
     parent_post_ids = parse_parent_post_ids
+    resized_thumbnail = thumbnail.present? ? Post.resized_thumbnail_attachment(thumbnail) : nil
 
     post = Post.new(title:, url:, thumbnail_base: nil, uploaded_user: current_user,
                     original_created_from:, original_created_before:)
-    post.thumbnail.attach(thumbnail) if thumbnail.present?
+    post.thumbnail.attach(resized_thumbnail) if resized_thumbnail
 
     ApplicationRecord.transaction do
       post.save!
@@ -156,8 +157,6 @@ class PostsController < ApplicationController
 
       sync_parent_posts!(post, parent_post_ids)
 
-      post.resized_thumbnail!
-
       PostVersionRecorder.record!(post:, event_type: :create, created_by_user: current_user)
     end
 
@@ -167,6 +166,8 @@ class PostsController < ApplicationController
     render_validation_error fields: { tags: 'ニコニコ・タグは直接指定できません．' }
   rescue Tag::DeprecatedTagNormalisationError
     render_unprocessable_entity '廃止済みタグは付与できません．', field: :tags
+  rescue MiniMagick::Error
+    render_validation_error fields: { thumbnail: ['サムネイル画像の変換に失敗しました．'] }
   rescue ArgumentError => e
     render_validation_error fields: { parent_post_ids: [e.message] }
   rescue ActiveRecord::RecordInvalid => e
diff --git a/backend/app/controllers/tags_controller.rb b/backend/app/controllers/tags_controller.rb
index f345a02..cb07ddd 100644
--- a/backend/app/controllers/tags_controller.rb
+++ b/backend/app/controllers/tags_controller.rb
@@ -245,10 +245,7 @@ class TagsController < ApplicationController
     return render_unprocessable_entity('カテゴリは必須です．', field: :category) if category.blank?
     return render_unprocessable_entity '廃止状態は必須です．', field: :deprecated unless params.key?(:deprecated)
 
-    if (name != tag.name &&
-        tag.in?([Tag.tagme, Tag.bot, Tag.no_deerjikist, Tag.video, Tag.niconico]))
-      return render_unprocessable_entity 'システム・タグの名称は変更できません．', field: :name
-    end
+    return unless validate_tag_rename(tag, name)
 
     alias_names = params[:aliases].to_s.split.uniq
     parent_names = params[:parent_tags].to_s.split.uniq
@@ -274,7 +271,7 @@ class TagsController < ApplicationController
       else
         tag.update!(category:, deprecated_at: deprecated ? Time.current : nil)
       end
-      tag.tag_name.update!(name:)
+      rename_tag_name!(tag, name) if name_changed
 
       alias_names << old_name if name_changed
       alias_names.delete(name)
@@ -310,6 +307,8 @@ class TagsController < ApplicationController
       return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
     end
 
+    return unless validate_tag_rename(tag, name)
+
     if tag.nico? || (category.present? && category == 'nico')
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
@@ -321,7 +320,7 @@ class TagsController < ApplicationController
       name_changed = name.present? && name != old_name
       wiki_page = tag.tag_name.wiki_page if name_changed
 
-      tag.tag_name.update!(name:) if name.present?
+      rename_tag_name!(tag, name) if name_changed
       tag.update!(category:) if category.present?
       if deprecated_given && tag.deprecated? != deprecated
         tag.update!(deprecated_at: deprecated ? Time.current : nil)
@@ -535,6 +534,67 @@ class TagsController < ApplicationController
         created_by_user:)
   end
 
+  def validate_tag_rename tag, name
+    return true if name.blank? || name == tag.name
+
+    if tag.in?([Tag.tagme, Tag.bot, Tag.no_deerjikist, Tag.video, Tag.niconico])
+      render_unprocessable_entity 'システム・タグの名称は変更できません．', field: :name
+      return false
+    end
+
+    target_tag_name = TagName.with_discarded.find_by(name:)
+    return true if target_tag_name.nil?
+    return true if target_tag_name.canonical_id?
+
+    render_unprocessable_entity 'その名前は既に使はれてゐます．', field: :name
+    false
+  end
+
+  def rename_tag_name! tag, name
+    return if name == tag.name
+
+    current_tag_name = tag.tag_name
+    target_tag_name = TagName.with_discarded.find_by(name:)
+
+    if target_tag_name.nil?
+      current_tag_name.update!(name:)
+      return
+    end
+
+    promote_tag_alias!(
+        tag,
+        current_tag_name:,
+        promoted_tag_name: target_tag_name)
+  end
+
+  def promote_tag_alias! tag, current_tag_name:, promoted_tag_name:
+    old_owner_tag = promoted_tag_name.canonical&.tag
+
+    if old_owner_tag && old_owner_tag != tag
+      TagVersioning.ensure_snapshot!(old_owner_tag, created_by_user: current_user)
+    end
+
+    promoted_tag_name.undiscard! if promoted_tag_name.discarded?
+    promoted_tag_name.update!(canonical: nil)
+
+    TagName.with_discarded
+           .where(canonical_id: current_tag_name.id)
+           .where.not(id: promoted_tag_name.id)
+           .find_each do |alias_tag_name|
+      alias_tag_name.update!(canonical: promoted_tag_name)
+    end
+
+    current_tag_name.wiki_page&.update!(tag_name: promoted_tag_name)
+    tag.update!(tag_name: promoted_tag_name)
+    current_tag_name.association(:wiki_page).reset
+    current_tag_name.association(:tag).reset
+    current_tag_name.reload.update!(canonical: promoted_tag_name)
+
+    return unless old_owner_tag && old_owner_tag != tag
+
+    record_tag_version!(old_owner_tag.reload, event_type: :update, created_by_user: current_user)
+  end
+
   def update_aliases! tag, alias_names
     alias_names = alias_names.uniq
 
diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index d354314..b2835b0 100644
--- a/backend/app/models/post.rb
+++ b/backend/app/models/post.rb
@@ -1,5 +1,19 @@
 class Post < ApplicationRecord
   require 'mini_magick'
+  require 'stringio'
+
+  def self.resized_thumbnail_attachment(upload)
+    upload.rewind
+    image = MiniMagick::Image.read(upload.read)
+    image.resize '180x180'
+    image.format 'jpg'
+
+    { io: StringIO.new(image.to_blob),
+      filename: 'resized_thumbnail.jpg',
+      content_type: 'image/jpeg' }
+  ensure
+    upload.rewind
+  end
 
   belongs_to :uploaded_user, class_name: 'User', optional: true
 
@@ -87,12 +101,7 @@ class Post < ApplicationRecord
   def resized_thumbnail!
     return unless thumbnail.attached?
 
-    image = MiniMagick::Image.read(thumbnail.download)
-    image.resize '180x180'
-    thumbnail.purge
-    thumbnail.attach(io:            File.open(image.path),
-                     filename:      'resized_thumbnail.jpg',
-                     content_type:  'image/jpeg')
+    thumbnail.attach(self.class.resized_thumbnail_attachment(StringIO.new(thumbnail.download)))
   end
 
   private
diff --git a/backend/app/models/tag_implication.rb b/backend/app/models/tag_implication.rb
index a629764..888713c 100644
--- a/backend/app/models/tag_implication.rb
+++ b/backend/app/models/tag_implication.rb
@@ -6,6 +6,7 @@ class TagImplication < ApplicationRecord
   validates :parent_tag_id, presence: true
 
   validate :parent_tag_mustnt_be_itself
+  validate :parent_tag_mustnt_create_cycle
 
   private
 
@@ -14,4 +15,27 @@ class TagImplication < ApplicationRecord
       errors.add :parent_tag_id, '親タグは子タグと同一であってはなりません．'
     end
   end
+
+  def parent_tag_mustnt_create_cycle
+    return if tag_id.blank? || parent_tag_id.blank?
+    return if errors[:parent_tag_id].present?
+
+    seen = { }
+    stack = [parent_tag_id]
+
+    until stack.empty?
+      current_id = stack.pop
+      next if seen[current_id]
+
+      seen[current_id] = true
+
+      if current_id == tag_id
+        errors.add :parent_tag_id, '親タグに子孫タグを指定すると循環します．'
+        errors.add :base, 'タグの親子関係が循環します．'
+        return
+      end
+
+      stack.concat(TagImplication.where(tag_id: current_id).pluck(:parent_tag_id))
+    end
+  end
 end
diff --git a/backend/spec/models/tag_implication_spec.rb b/backend/spec/models/tag_implication_spec.rb
new file mode 100644
index 0000000..a29cc1c
--- /dev/null
+++ b/backend/spec/models/tag_implication_spec.rb
@@ -0,0 +1,36 @@
+require 'rails_helper'
+
+RSpec.describe TagImplication, type: :model do
+  it 'rejects a parent tag that would create a cycle' do
+    child = create(:tag, name: 'tag_implication_cycle_child')
+    parent = create(:tag, name: 'tag_implication_cycle_parent')
+
+    described_class.create!(tag: child, parent_tag: parent)
+
+    implication = described_class.new(tag: parent, parent_tag: child)
+
+    expect(implication).not_to be_valid
+    expect(implication.errors[:parent_tag_id]).to include(
+      '親タグに子孫タグを指定すると循環します．'
+    )
+    expect(implication.errors[:base]).to be_present
+  end
+
+  it 'terminates even when existing data already contains a cycle' do
+    child = create(:tag, name: 'tag_implication_existing_cycle_child')
+    parent = create(:tag, name: 'tag_implication_existing_cycle_parent')
+    ancestor = create(:tag, name: 'tag_implication_existing_cycle_ancestor')
+
+    described_class.create!(tag: parent, parent_tag: ancestor)
+    described_class.insert_all!(
+      [
+        { tag_id: ancestor.id, parent_tag_id: parent.id,
+          created_at: Time.current, updated_at: Time.current }
+      ]
+    )
+
+    implication = described_class.new(tag: child, parent_tag: parent)
+
+    expect(implication).to be_valid
+  end
+end
diff --git a/backend/spec/models/tag_spec.rb b/backend/spec/models/tag_spec.rb
index 014cdde..1b2549c 100644
--- a/backend/spec/models/tag_spec.rb
+++ b/backend/spec/models/tag_spec.rb
@@ -54,7 +54,17 @@ RSpec.describe Tag, type: :model do
       first = create(:tag, name: 'expand_cycle_first')
       second = create(:tag, name: 'expand_cycle_second')
       TagImplication.create!(tag: first, parent_tag: second)
-      TagImplication.create!(tag: second, parent_tag: first)
+      now = Time.current
+      TagImplication.insert_all!(
+        [
+          {
+            tag_id: second.id,
+            parent_tag_id: first.id,
+            created_at: now,
+            updated_at: now
+          }
+        ]
+      )
 
       expect(described_class.expand_parent_tags([first])).to contain_exactly(first, second)
     end
diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index 18d5b73..b640f1a 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -1,4 +1,5 @@
 require 'rails_helper'
+require 'base64'
 require 'set'
 
 include ActiveSupport::Testing::TimeHelpers
@@ -8,6 +9,11 @@ RSpec.describe 'Posts API', type: :request do
   # resized_thumbnail! が MiniMagick 依存でコケやすいので request spec ではスタブしとくのが無難。
   before do
     allow_any_instance_of(Post).to receive(:resized_thumbnail!).and_return(true)
+    allow(Post).to receive(:resized_thumbnail_attachment).and_return(
+      io: StringIO.new('dummy'),
+      filename: 'resized_thumbnail.jpg',
+      content_type: 'image/jpeg'
+    )
   end
 
   def create_nico_tag!(name)
@@ -19,6 +25,18 @@ RSpec.describe 'Posts API', type: :request do
     Rack::Test::UploadedFile.new(StringIO.new('dummy'), 'image/jpeg', original_filename: 'dummy.jpg')
   end
 
+  def real_thumbnail_upload
+    gif =
+        Base64.decode64(
+            'R0lGODdhAQABAIABAP///wAAACwAAAAAAQABAAACAkQBADs=')
+
+    Rack::Test::UploadedFile.new(
+      StringIO.new(gif),
+      'image/gif',
+      original_filename: 'thumbnail.gif'
+    )
+  end
+
   def post_write_params params = { }
     { parent_post_ids: '' }.merge(params)
   end
@@ -696,6 +714,66 @@ RSpec.describe 'Posts API', type: :request do
       expect(json['tags'][0]).to have_key('name')
     end
 
+    it '201 when posting manually with a thumbnail' do
+      sign_in_as(member)
+      allow(Post).to receive(:resized_thumbnail_attachment).and_call_original
+
+      post '/posts', params: post_write_params(
+        title: 'thumbnail post',
+        url: 'https://example.com/thumbnail-post',
+        tags: 'spec_tag',
+        thumbnail: real_thumbnail_upload
+      )
+
+      expect(response).to have_http_status(:created)
+      post_record = Post.find(json.fetch('id'))
+      expect(post_record.thumbnail).to be_attached
+      expect(post_record.thumbnail.blob.content_type).to eq('image/jpeg')
+      expect { post_record.thumbnail.download }.not_to raise_error
+    end
+
+    it 'resizes the thumbnail before the create transaction begins' do
+      sign_in_as(member)
+
+      open_transactions = []
+      baseline_open_transactions = Post.connection.open_transactions
+      allow(Post).to receive(:resized_thumbnail_attachment) do |_upload|
+        open_transactions << Post.connection.open_transactions
+        { io: StringIO.new('dummy'),
+          filename: 'resized_thumbnail.jpg',
+          content_type: 'image/jpeg' }
+      end
+
+      post '/posts', params: post_write_params(
+        title: 'transaction post',
+        url: 'https://example.com/transaction-post',
+        tags: 'spec_tag',
+        thumbnail: dummy_upload
+      )
+
+      expect(response).to have_http_status(:created)
+      expect(open_transactions).to eq([baseline_open_transactions])
+    end
+
+    it 'returns 422 and does not create a post when thumbnail resize fails' do
+      sign_in_as(member)
+      allow(Post).to receive(:resized_thumbnail_attachment).and_raise(MiniMagick::Error)
+
+      expect {
+        post '/posts', params: post_write_params(
+          title: 'broken thumbnail post',
+          url: 'https://example.com/broken-thumbnail-post',
+          tags: 'spec_tag',
+          thumbnail: dummy_upload
+        )
+      }.not_to change(Post, :count)
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json.fetch('errors')).to include(
+        'thumbnail' => ['サムネイル画像の変換に失敗しました．']
+      )
+    end
+
     it '201 and creates post + tags when member and tags have aliases' do
       sign_in_as(member)
 
diff --git a/backend/spec/requests/tag_children_spec.rb b/backend/spec/requests/tag_children_spec.rb
index 8e1b91b..69bb10a 100644
--- a/backend/spec/requests/tag_children_spec.rb
+++ b/backend/spec/requests/tag_children_spec.rb
@@ -56,6 +56,17 @@ RSpec.describe "TagChildren", type: :request do
 
         expect(response).to have_http_status(:no_content)
       end
+
+      it 'returns 422 and does not create relation when the new link makes a cycle' do
+        TagImplication.create!(tag: parent, parent_tag: child)
+
+        expect {
+          do_request
+        }.not_to change(TagImplication, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(TagImplication.where(tag: child, parent_tag: parent)).not_to exist
+      end
     end
 
     context "when Tag.find raises (invalid ids)" do
diff --git a/backend/spec/requests/tags_spec.rb b/backend/spec/requests/tags_spec.rb
index 927b4a7..0055364 100644
--- a/backend/spec/requests/tags_spec.rb
+++ b/backend/spec/requests/tags_spec.rb
@@ -581,6 +581,58 @@ RSpec.describe 'Tags API', type: :request do
         expect(response).to have_http_status(:ok)
         expect(wiki_page.reload.wiki_versions.count).to eq(before_wiki_version_count)
       end
+
+      it 'full update で旧名 alias が残った tag を PATCH で旧名へ戻せる' do
+        put "/tags/#{ tag.id }", params: {
+          name: 'patch_roundtrip_target',
+          category: 'general',
+          aliases: 'unko',
+          parent_tags: '',
+          deprecated: '0',
+        }
+
+        expect(response).to have_http_status(:ok)
+        expect(TagName.find_by!(name: 'spec_tag').canonical).to eq(tag.reload.tag_name)
+
+        patch "/tags/#{ tag.id }", params: { name: 'spec_tag' }
+
+        expect(response).to have_http_status(:ok)
+
+        tag.reload
+
+        expect(tag.name).to eq('spec_tag')
+        expect(tag.tag_name.canonical_id).to be_nil
+        expect(TagName.find_by!(name: 'patch_roundtrip_target').canonical).to eq(tag.tag_name)
+      end
+
+      it '別 tag の正規名には変更できない' do
+        wiki_page =
+            Wiki::Commit.create_content!(
+                tag_name: tag.tag_name,
+                body: 'patch collision wiki',
+                created_by_user: member_user,
+                message: 'init')
+
+        patch "/tags/#{ tag.id }", params: { name: 'unknown' }
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to include(
+          'name' => ['その名前は既に使はれてゐます．']
+        )
+
+        expect(tag.reload.name).to eq('spec_tag')
+        expect(tag.tag_name.aliases.map(&:name)).to contain_exactly('unko')
+        expect(wiki_page.reload.tag_name).to eq(tag.tag_name)
+      end
+
+      it 'system tag の name は変更できない' do
+        system_tag = Tag.bot
+
+        patch "/tags/#{ system_tag.id }", params: { name: 'patch_system_tag_renamed' }
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(system_tag.reload.name).to eq('bot操作')
+      end
     end
   end
 
@@ -745,7 +797,17 @@ RSpec.describe 'Tags API', type: :request do
       )
       TagImplication.create!(tag: first, parent_tag: root_material)
       TagImplication.create!(tag: second, parent_tag: first)
-      TagImplication.create!(tag: first, parent_tag: second)
+      now = Time.current
+      TagImplication.insert_all!(
+        [
+          {
+            tag_id: first.id,
+            parent_tag_id: second.id,
+            created_at: now,
+            updated_at: now
+          }
+        ]
+      )
 
       get '/tags/with-depth', params: { parent: root_material.id }
 
@@ -1029,6 +1091,42 @@ RSpec.describe 'Tags API', type: :request do
         expect(versions.second.created_by_user_id).to eq(member_user.id)
       end
 
+      it '同じ tag の旧 alias へ戻しても rename できる' do
+        put "/tags/#{ tag.id }", params: {
+          name: 'put_roundtrip_b',
+          category: 'general',
+          aliases: 'unko',
+          parent_tags: '',
+          deprecated: '0',
+        }
+
+        expect(response).to have_http_status(:ok)
+
+        put "/tags/#{ tag.id }", params: {
+          name: 'spec_tag',
+          category: 'general',
+          aliases: 'unko',
+          parent_tags: '',
+          deprecated: '0',
+        }
+
+        expect(response).to have_http_status(:ok)
+
+        tag.reload
+
+        expect(tag.name).to eq('spec_tag')
+        expect(TagName.find_by!(name: 'put_roundtrip_b').canonical).to eq(tag.tag_name)
+        expect(tag.tag_name.aliases.map(&:name)).to contain_exactly('put_roundtrip_b', 'unko')
+        expect(tag.tag_name.aliases.map(&:name)).not_to include('spec_tag')
+        expect(alias_tn.reload.canonical).to eq(tag.tag_name)
+
+        version = tag.tag_versions.order(:version_no).last
+
+        expect(version.event_type).to eq('update')
+        expect(version.name).to eq('spec_tag')
+        expect(version.aliases.split).to contain_exactly('put_roundtrip_b', 'unko')
+      end
+
       it 'parent tag の snapshot も作成する' do
         old_parent = Tag.create!(
           tag_name: TagName.create!(name: 'put_snapshot_old_parent'),
@@ -1153,6 +1251,48 @@ RSpec.describe 'Tags API', type: :request do
         )
       end
 
+      it 'wiki を持つ tag を旧 alias へ戻しても wiki を新 canonical へ移す' do
+        wiki_page =
+            Wiki::Commit.create_content!(
+                tag_name: tag.tag_name,
+                body: 'wiki body before',
+                created_by_user: member_user,
+                message: 'init')
+
+        expect {
+          put "/tags/#{ tag.id }", params: {
+            name: 'put_wiki_roundtrip_b',
+            category: 'general',
+            aliases: 'unko',
+            parent_tags: '',
+            deprecated: '0',
+          }
+
+          put "/tags/#{ tag.id }", params: {
+            name: 'spec_tag',
+            category: 'general',
+            aliases: 'unko',
+            parent_tags: '',
+            deprecated: '0',
+          }
+        }
+            .to change(TagVersion, :count).by(3)
+            .and change(WikiVersion, :count).by(2)
+
+        expect(response).to have_http_status(:ok)
+
+        tag.reload
+
+        expect(wiki_page.reload.tag_name).to eq(tag.tag_name)
+        expect(TagName.find_by!(name: 'put_wiki_roundtrip_b').wiki_page).to be_nil
+        expect(TagName.find_by!(name: 'put_wiki_roundtrip_b').canonical).to eq(tag.tag_name)
+
+        versions = wiki_page.wiki_versions.order(:version_no).last(2)
+
+        expect(versions.map(&:event_type)).to eq(['update', 'update'])
+        expect(versions.map(&:title)).to eq(['put_wiki_roundtrip_b', 'spec_tag'])
+      end
+
       it '別名を他 tag から奪った場合、奪はれた側の tag version も作成する' do
         old_owner = Tag.create!(
           tag_name: TagName.create!(name: 'put_alias_old_owner'),
@@ -1191,9 +1331,49 @@ RSpec.describe 'Tags API', type: :request do
         expect(old_owner_versions.second.aliases.split).not_to include('put_stolen_alias')
       end
 
-      it 'parent_tags に指定すると循環する tag は 422 にする' do
-        pending '#332 で対応予定'
+      it '別 tag の alias 名を rename で奪へる' do
+        old_owner = Tag.create!(
+          tag_name: TagName.create!(name: 'put_alias_collision_owner'),
+          category: :general
+        )
+        stolen_alias = TagName.create!(
+          name: 'put_alias_collision_name',
+          canonical: old_owner.tag_name
+        )
+        wiki_page =
+            Wiki::Commit.create_content!(
+                tag_name: tag.tag_name,
+                body: 'put collision wiki',
+                created_by_user: member_user,
+                message: 'init')
 
+        put "/tags/#{ tag.id }", params: {
+          name: 'put_alias_collision_name',
+          category: 'general',
+          aliases: 'unko',
+          parent_tags: '',
+          deprecated: '0',
+        }
+
+        expect(response).to have_http_status(:ok)
+
+        tag.reload
+        old_owner.reload
+        stolen_alias.reload
+
+        expect(tag.name).to eq('put_alias_collision_name')
+        expect(stolen_alias.canonical_id).to be_nil
+        expect(TagName.find_by!(name: 'spec_tag').canonical).to eq(tag.tag_name)
+        expect(old_owner.tag_name.aliases.map(&:name)).not_to include('put_alias_collision_name')
+
+        old_owner_versions = old_owner.tag_versions.order(:version_no)
+
+        expect(old_owner_versions.last.event_type).to eq('update')
+        expect(old_owner_versions.last.aliases.split).not_to include('put_alias_collision_name')
+        expect(wiki_page.reload.tag_name).to eq(tag.tag_name)
+      end
+
+      it 'parent_tags に指定すると循環する tag は 422 にする' do
         child = Tag.create!(
           tag_name: TagName.create!(name: 'put_cycle_child'),
           category: :general