diff --git a/backend/app/controllers/nico_tags_controller.rb b/backend/app/controllers/nico_tags_controller.rb
index c897fe0..ea02a12 100644
--- a/backend/app/controllers/nico_tags_controller.rb
+++ b/backend/app/controllers/nico_tags_controller.rb
@@ -24,6 +24,7 @@ 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_name: :wiki_page })
     if name
       q = q.where(('external_tags.name LIKE ? ' +
                    "OR CONCAT(external_tags.platform, ':', external_tags.name) LIKE ?"),
diff --git a/frontend/src/components/DraggableDroppableTagRow.test.tsx b/frontend/src/components/DraggableDroppableTagRow.test.tsx
index 8469831..43a32aa 100644
--- a/frontend/src/components/DraggableDroppableTagRow.test.tsx
+++ b/frontend/src/components/DraggableDroppableTagRow.test.tsx
@@ -13,11 +13,14 @@ vi.mock ('@dnd-kit/core', () => dndKit)
 
 const tag = buildTag ({ id: 7, name: 'ドラッグ元', postCount: 3 })
 
-const renderRow = (activeDndId?: string) => {
+const renderRow = (
+    activeDndId?: string,
+    renderedTag = tag,
+) => {
   renderWithProviders (
       <DraggableDroppableTagRow
           activeDndId={activeDndId}
-          tag={tag}
+          tag={renderedTag}
           nestLevel={2}
           pathKey="cat-general-7"
           suppressClickRef={{ current: false }}/>,
@@ -72,4 +75,26 @@ describe ('DraggableDroppableTagRow', () => {
     renderRow ('tag-node:other')
     expect (tagBody ()).toHaveStyle ({ visibility: 'visible' })
   })
+
+  it ('disables drag and drop for external tags', () => {
+    const external = buildTag ({
+      id:       7,
+      name:     'nico:external',
+      category: 'nico',
+    })
+
+    renderRow (undefined, external)
+
+    expect (dndKit.useDraggable).toHaveBeenCalledWith (
+	expect.objectContaining ({
+	  disabled: true,
+	}),
+    )
+
+    expect (dndKit.useDroppable).toHaveBeenCalledWith (
+	expect.objectContaining ({
+	  disabled: true,
+	}),
+    )
+  })
 })
diff --git a/frontend/src/components/TagDetailSidebar.test.tsx b/frontend/src/components/TagDetailSidebar.test.tsx
new file mode 100644
index 0000000..e89683f
--- /dev/null
+++ b/frontend/src/components/TagDetailSidebar.test.tsx
@@ -0,0 +1,66 @@
+import { screen } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import TagDetailSidebar from '@/components/TagDetailSidebar'
+import { setClientTagRelationDisplayMode } from '@/lib/settings'
+import { buildPost, buildTag } from '@/test/factories'
+import { renderWithProviders } from '@/test/render'
+
+import type { ReactNode } from 'react'
+
+vi.mock ('@/components/TagSearch', () => ({
+  default: () => null,
+}))
+
+vi.mock ('@/components/DraggableDroppableTagRow', () => ({
+  default: ({ tag }: { tag: { name: string } }) => (
+      <span>{tag.name}</span>
+  ),
+}))
+
+vi.mock ('@dnd-kit/core', () => ({
+  DndContext: ({ children }: { children: ReactNode }) => <>{children}</>,
+  DragOverlay: ({ children }: { children: ReactNode }) => <>{children}</>,
+  MeasuringStrategy: { Always: 'always' },
+  MouseSensor: vi.fn (),
+  TouchSensor: vi.fn (),
+  pointerWithin: vi.fn (),
+  useDroppable: vi.fn (() => ({
+    setNodeRef: vi.fn (),
+    isOver:     false,
+  })),
+  useSensor: vi.fn (() => ({ })),
+  useSensors: vi.fn (() => []),
+}))
+
+describe ('TagDetailSidebar', () => {
+  beforeEach (() => {
+    localStorage.clear ()
+    vi.clearAllMocks ()
+  })
+
+  it ('keeps internal and external tags with the same numeric id in flat mode', () => {
+    setClientTagRelationDisplayMode ('flat')
+
+    const internal = buildTag ({
+      id:       7,
+      name:     'internal_collision',
+      category: 'general',
+    })
+    const external = buildTag ({
+      id:       7,
+      name:     'nico:external_collision',
+      category: 'nico',
+    })
+
+    renderWithProviders (
+        <TagDetailSidebar
+            post={buildPost ({
+                tags: [internal, external],
+            })}/>,
+    )
+
+    expect (screen.getByText ('internal_collision')).toBeInTheDocument ()
+    expect (screen.getByText ('nico:external_collision')).toBeInTheDocument ()
+  })
+})
diff --git a/frontend/src/components/TagLink.test.tsx b/frontend/src/components/TagLink.test.tsx
index e6a073c..8922265 100644
--- a/frontend/src/components/TagLink.test.tsx
+++ b/frontend/src/components/TagLink.test.tsx
@@ -57,4 +57,31 @@ describe ('TagLink', () => {
     expect (screen.getByText ('正式名')).toBeInTheDocument ()
     expect (screen.queryByRole ('link')).not.toBeInTheDocument ()
   })
+
+  it ('does not show a missing-information marker for external tags', () => {
+    renderWithProviders (
+	<TagLink
+	    tag={buildTag ({
+		id:             7,
+		name:           'nico:external',
+		category:       'nico',
+		hasWiki:        false,
+		materialId:     null,
+		hasDeerjikists: false,
+	    })}
+	    withCount={false}/>,
+    )
+
+    expect (
+	screen.getByRole ('link', { name: 'nico:external' }),
+    ).toBeInTheDocument ()
+
+    expect (
+	screen.queryByRole ('link', { name: '!' }),
+    ).not.toBeInTheDocument ()
+
+    expect (
+	screen.queryByTitle ('nico:external Wiki が存在しません．'),
+    ).not.toBeInTheDocument ()
+  })
 })
diff --git a/frontend/src/lib/posts.test.ts b/frontend/src/lib/posts.test.ts
index 600df31..972775d 100644
--- a/frontend/src/lib/posts.test.ts
+++ b/frontend/src/lib/posts.test.ts
@@ -116,4 +116,25 @@ describe ('posts API functions', () => {
         { params: { page: 2, limit: 50 } },
     )
   })
+
+  it ('maps an explicit external tag history filter to external_tag', async () => {
+    api.apiGet.mockResolvedValueOnce ({ versions: [], count: 0 })
+
+    await fetchPostChanges ({
+      externalTag: '7',
+      page:        2,
+      limit:       50,
+    })
+
+    expect (api.apiGet).toHaveBeenCalledWith (
+	'/posts/versions',
+	{
+	  params: {
+	    external_tag: '7',
+	    page:         2,
+	    limit:        50,
+	  },
+	},
+    )
+  })
 })
diff --git a/frontend/src/lib/prefetchers.test.ts b/frontend/src/lib/prefetchers.test.ts
index 8a341a3..b147c79 100644
--- a/frontend/src/lib/prefetchers.test.ts
+++ b/frontend/src/lib/prefetchers.test.ts
@@ -137,4 +137,19 @@ describe ('prefetchForURL', () => {
     expect (tagsApi.fetchTags).not.toHaveBeenCalled ()
     expect (wikiApi.fetchWikiPages).not.toHaveBeenCalled ()
   })
+
+  it ('prefetches external tag post history without treating it as an internal tag', async () => {
+    await prefetchForURL (
+	qc (),
+	'http://localhost/posts/changes?external_tag=12&page=2&limit=50',
+    )
+
+    expect (postsApi.fetchPostChanges).toHaveBeenCalledWith ({
+      externalTag: '12',
+      page:        2,
+      limit:       50,
+    })
+
+    expect (tagsApi.fetchTag).not.toHaveBeenCalled ()
+  })
 })
diff --git a/frontend/src/pages/posts/PostHistoryPage.test.tsx b/frontend/src/pages/posts/PostHistoryPage.test.tsx
new file mode 100644
index 0000000..44b2ee5
--- /dev/null
+++ b/frontend/src/pages/posts/PostHistoryPage.test.tsx
@@ -0,0 +1,44 @@
+import { waitFor } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import PostHistoryPage from '@/pages/posts/PostHistoryPage'
+import { renderWithProviders } from '@/test/render'
+
+const postsApi = vi.hoisted (() => ({
+  fetchPostChanges: vi.fn (),
+  updatePost:       vi.fn (),
+}))
+
+const tagsApi = vi.hoisted (() => ({
+  fetchTag: vi.fn (),
+}))
+
+vi.mock ('@/lib/posts', () => postsApi)
+vi.mock ('@/lib/tags', () => tagsApi)
+
+describe ('PostHistoryPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    postsApi.fetchPostChanges.mockResolvedValue ({
+      versions: [],
+      count:    0,
+    })
+  })
+
+  it ('filters by external_tag without resolving the id as an internal tag', async () => {
+    renderWithProviders (
+        <PostHistoryPage/>,
+        { route: '/posts/changes?external_tag=7' },
+    )
+
+    await waitFor (() => {
+      expect (postsApi.fetchPostChanges).toHaveBeenCalledWith ({
+        externalTag: '7',
+        page:        1,
+        limit:       20,
+      })
+    })
+
+    expect (tagsApi.fetchTag).not.toHaveBeenCalled ()
+  })
+})
diff --git a/frontend/src/pages/tags/TagListPage.test.tsx b/frontend/src/pages/tags/TagListPage.test.tsx
index 3d0e9ae..a2d6763 100644
--- a/frontend/src/pages/tags/TagListPage.test.tsx
+++ b/frontend/src/pages/tags/TagListPage.test.tsx
@@ -1,4 +1,4 @@
-import { fireEvent, screen, waitFor } from '@testing-library/react'
+import { fireEvent, screen, waitFor, within } from '@testing-library/react'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import TagListPage from '@/pages/tags/TagListPage'
@@ -127,4 +127,49 @@ describe ('TagListPage', () => {
       )
     })
   })
+
+  it ('keeps colliding internal and external tags on their own routes', async () => {
+    const internal = buildTag ({
+      id:       7,
+      name:     'internal_collision',
+      category: 'general',
+    })
+    const external = buildTag ({
+      id:       7,
+      name:     'nico:external_collision',
+      category: 'nico',
+    })
+
+    tagsApi.fetchTags.mockResolvedValueOnce ({
+      tags:  [internal, external],
+      count: 2,
+    })
+
+    renderWithProviders (<TagListPage/>, { route: '/tags' })
+
+    const internalLink =
+	await screen.findByRole ('link', { name: 'internal_collision' })
+    const externalLink =
+	screen.getByRole ('link', { name: 'nico:external_collision' })
+
+    expect (internalLink).toHaveAttribute ('href', '/tags/7')
+    expect (externalLink).toHaveAttribute (
+	'href',
+	'/tags/nico?name=nico%3Aexternal_collision',
+    )
+
+    const internalRow = internalLink.closest ('tr')
+    const externalRow = externalLink.closest ('tr')
+
+    expect (internalRow).not.toBeNull ()
+    expect (externalRow).not.toBeNull ()
+
+    expect (
+	within (internalRow!).getByRole ('link', { name: '耕作履歴' }),
+    ).toHaveAttribute ('href', '/posts/changes?tag=7')
+
+    expect (
+	within (externalRow!).getByRole ('link', { name: '耕作履歴' }),
+    ).toHaveAttribute ('href', '/posts/changes?external_tag=7')
+  })
 })