diff --git a/apps/web/src/lib/server/domains/posts/__tests__/post-merge.test.ts b/apps/web/src/lib/server/domains/posts/__tests__/post-merge.test.ts
index 863953d3d..96ec8697d 100644
--- a/apps/web/src/lib/server/domains/posts/__tests__/post-merge.test.ts
+++ b/apps/web/src/lib/server/domains/posts/__tests__/post-merge.test.ts
@@ -9,6 +9,7 @@ import type { PostId, PrincipalId } from '@opencoven-feedback/ids'
 const mockPostsFindFirst = vi.fn()
 const mockPrincipalFindFirst = vi.fn()
 const mockBoardsFindFirst = vi.fn()
+const mockDbSelect = vi.fn()
 const mockDbUpdate = vi.fn()
 const mockDbExecute = vi.fn()
 const createActivity = vi.fn()
@@ -21,6 +22,24 @@ function createUpdateChain() {
   return chain
 }
 
+type SelectChain = {
+  from: ReturnType<typeof vi.fn>
+  innerJoin: ReturnType<typeof vi.fn>
+  where: ReturnType<typeof vi.fn>
+  orderBy: ReturnType<typeof vi.fn>
+  limit: ReturnType<typeof vi.fn>
+}
+
+function createSelectChain(result: unknown[]): SelectChain {
+  const chain = {} as SelectChain
+  chain.from = vi.fn(() => chain)
+  chain.innerJoin = vi.fn(() => chain)
+  chain.where = vi.fn(() => chain)
+  chain.orderBy = vi.fn().mockResolvedValue(result)
+  chain.limit = vi.fn().mockResolvedValue(result)
+  return chain
+}
+
 vi.mock('@/lib/server/db', async () => {
   const { sql: realSql } = await vi.importActual<typeof import('drizzle-orm')>('drizzle-orm')
 
@@ -31,6 +50,7 @@ vi.mock('@/lib/server/db', async () => {
         principal: { findFirst: (...args: unknown[]) => mockPrincipalFindFirst(...args) },
         boards: { findFirst: (...args: unknown[]) => mockBoardsFindFirst(...args) },
       },
+      select: (...args: unknown[]) => mockDbSelect(...args),
       update: (..._args: unknown[]) => {
         mockDbUpdate(..._args)
         return createUpdateChain()
@@ -39,11 +59,11 @@ vi.mock('@/lib/server/db', async () => {
     },
     posts: { id: 'post_id', canonicalPostId: 'canonical_post_id' },
     votes: { principalId: 'principal_id', postId: 'post_id' },
-    boards: { id: 'board_id', slug: 'board_slug' },
+    boards: { id: 'board_id', slug: 'board_slug', isPublic: 'is_public' },
     principal: { id: 'principal_id', displayName: 'display_name' },
-    eq: vi.fn(),
-    and: vi.fn(),
-    isNull: vi.fn(),
+    eq: vi.fn((left, right) => ({ op: 'eq', left, right })),
+    and: vi.fn((...args) => ({ op: 'and', args })),
+    isNull: vi.fn((value) => ({ op: 'isNull', value })),
     sql: realSql,
   }
 })
@@ -84,7 +104,8 @@ vi.mock('@opencoven-feedback/ids', async (importOriginal) => {
 })
 
 // Import after mocks
-const { mergePost, unmergePost } = await import('../post.merge')
+const { mergePost, unmergePost, getPublicMergedPosts, getPublicPostMergeInfo } =
+  await import('../post.merge')
 
 const POST_A = 'post_aaa' as PostId
 const POST_B = 'post_bbb' as PostId
@@ -111,7 +132,7 @@ describe('mergePost', () => {
       return Promise.resolve(mockPost())
     })
     mockPrincipalFindFirst.mockResolvedValue({ displayName: 'Author' })
-    mockBoardsFindFirst.mockResolvedValue({ id: 'board_mock', slug: 'feedback' })
+    mockBoardsFindFirst.mockResolvedValue({ id: 'board_mock', slug: 'feedback', isPublic: true })
     // Default: vote count recalculation returns 5
     mockDbExecute.mockResolvedValue([{ unique_voters: 5 }])
   })
@@ -156,6 +177,18 @@ describe('mergePost', () => {
     )
   })
 
+  it('throws ValidationError when boards have different public visibility', async () => {
+    mockPostsFindFirst
+      .mockResolvedValueOnce(mockPost({ id: POST_A, boardId: 'private_board' }))
+      .mockResolvedValueOnce(mockPost({ id: POST_B, boardId: 'public_board' }))
+    mockBoardsFindFirst
+      .mockResolvedValueOnce({ slug: 'internal', isPublic: false })
+      .mockResolvedValueOnce({ slug: 'feedback', isPublic: true })
+
+    await expect(mergePost(POST_A, POST_B, ACTOR)).rejects.toThrow(/same public visibility/)
+    expect(mockDbUpdate).not.toHaveBeenCalled()
+  })
+
   it('records activity on both posts after successful merge', async () => {
     mockPostsFindFirst
       .mockResolvedValueOnce(mockPost({ id: POST_A, title: 'Duplicate' }))
@@ -215,8 +248,8 @@ describe('mergePost', () => {
       .mockResolvedValueOnce(mockPost({ id: POST_A, title: 'Dup', boardId: 'board_a' }))
       .mockResolvedValueOnce(mockPost({ id: POST_B, title: 'Canon', boardId: 'board_b' }))
     mockBoardsFindFirst
-      .mockResolvedValueOnce({ slug: 'board-a' })
-      .mockResolvedValueOnce({ slug: 'board-b' })
+      .mockResolvedValueOnce({ slug: 'board-a', isPublic: true })
+      .mockResolvedValueOnce({ slug: 'board-b', isPublic: true })
 
     await mergePost(POST_A, POST_B, ACTOR)
 
@@ -238,10 +271,81 @@ describe('mergePost', () => {
   })
 })
 
+describe('public merge visibility helpers', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('filters merged-post summaries through public-board visibility', async () => {
+    const mergedAt = new Date('2026-06-06T12:00:00.000Z')
+    const selectChain = createSelectChain([
+      {
+        id: POST_A,
+        title: 'Public duplicate',
+        voteCount: 4,
+        authorName: 'Author',
+        createdAt: new Date('2026-06-05T12:00:00.000Z'),
+        mergedAt,
+      },
+    ])
+    mockDbSelect.mockReturnValueOnce(selectChain)
+
+    const result = await getPublicMergedPosts(POST_B)
+
+    expect(result).toEqual([
+      {
+        id: POST_A,
+        title: 'Public duplicate',
+        voteCount: 4,
+        authorName: 'Author',
+        createdAt: new Date('2026-06-05T12:00:00.000Z'),
+        mergedAt,
+      },
+    ])
+    expect(selectChain.innerJoin).toHaveBeenCalled()
+    expect(JSON.stringify(vi.mocked(selectChain.where).mock.calls[0]?.[0])).toContain(
+      '"left":"is_public","right":true'
+    )
+  })
+
+  it('hides public merge info when the canonical target is not public or is deleted', async () => {
+    const mergedAt = new Date('2026-06-06T12:00:00.000Z')
+    mockDbSelect
+      .mockReturnValueOnce(createSelectChain([{ canonicalPostId: POST_B, mergedAt }]))
+      .mockReturnValueOnce(createSelectChain([]))
+
+    await expect(getPublicPostMergeInfo(POST_A)).resolves.toBeNull()
+  })
+
+  it('returns public merge info only after both post queries pass public-board filters', async () => {
+    const mergedAt = new Date('2026-06-06T12:00:00.000Z')
+    const mergedPostQuery = createSelectChain([{ canonicalPostId: POST_B, mergedAt }])
+    const canonicalPostQuery = createSelectChain([
+      { id: POST_B, title: 'Canonical public post', boardSlug: 'feedback' },
+    ])
+    mockDbSelect.mockReturnValueOnce(mergedPostQuery).mockReturnValueOnce(canonicalPostQuery)
+
+    const result = await getPublicPostMergeInfo(POST_A)
+
+    expect(result).toEqual({
+      canonicalPostId: POST_B,
+      canonicalPostTitle: 'Canonical public post',
+      canonicalPostBoardSlug: 'feedback',
+      mergedAt,
+    })
+    expect(JSON.stringify(vi.mocked(mergedPostQuery.where).mock.calls[0]?.[0])).toContain(
+      '"left":"is_public","right":true'
+    )
+    expect(JSON.stringify(vi.mocked(canonicalPostQuery.where).mock.calls[0]?.[0])).toContain(
+      '"left":"is_public","right":true'
+    )
+  })
+})
+
 describe('unmergePost', () => {
   beforeEach(() => {
     vi.clearAllMocks()
-    mockBoardsFindFirst.mockResolvedValue({ id: 'board_mock', slug: 'feedback' })
+    mockBoardsFindFirst.mockResolvedValue({ id: 'board_mock', slug: 'feedback', isPublic: true })
     mockDbExecute.mockResolvedValue([{ unique_voters: 3 }])
   })
 
@@ -301,8 +405,8 @@ describe('unmergePost', () => {
       )
       .mockResolvedValueOnce(mockPost({ id: POST_B, title: 'Canon', boardId: 'board_b' }))
     mockBoardsFindFirst
-      .mockResolvedValueOnce({ slug: 'board-a' })
-      .mockResolvedValueOnce({ slug: 'board-b' })
+      .mockResolvedValueOnce({ slug: 'board-a', isPublic: true })
+      .mockResolvedValueOnce({ slug: 'board-b', isPublic: true })
 
     await unmergePost(POST_A, ACTOR)
 
diff --git a/apps/web/src/lib/server/domains/posts/post.merge.ts b/apps/web/src/lib/server/domains/posts/post.merge.ts
index 25363fdc3..f9da505de 100644
--- a/apps/web/src/lib/server/domains/posts/post.merge.ts
+++ b/apps/web/src/lib/server/domains/posts/post.merge.ts
@@ -107,6 +107,28 @@ export async function mergePost(
     )
   }
 
+  const [duplicateBoard, canonicalBoard] = await Promise.all([
+    db.query.boards.findFirst({
+      where: eq(boards.id, duplicatePost.boardId),
+      columns: { slug: true, isPublic: true },
+    }),
+    db.query.boards.findFirst({
+      where: eq(boards.id, canonicalPost.boardId),
+      columns: { slug: true, isPublic: true },
+    }),
+  ])
+
+  if (!duplicateBoard || !canonicalBoard) {
+    throw new NotFoundError('BOARD_NOT_FOUND', 'One or both post boards could not be found')
+  }
+
+  if (duplicateBoard.isPublic !== canonicalBoard.isPublic) {
+    throw new ValidationError(
+      'INCOMPATIBLE_MERGE_VISIBILITY',
+      'Posts can only be merged when their boards have the same public visibility.'
+    )
+  }
+
   // Mark the duplicate post as merged
   await db
     .update(posts)
@@ -153,33 +175,21 @@ export async function mergePost(
   })
 
   // Dispatch post.merged event for webhooks and integrations
-  const [dupBoard, canBoard] = await Promise.all([
-    db.query.boards.findFirst({
-      where: eq(boards.id, duplicatePost.boardId),
-      columns: { slug: true },
-    }),
-    db.query.boards.findFirst({
-      where: eq(boards.id, canonicalPost.boardId),
-      columns: { slug: true },
-    }),
-  ])
-  if (dupBoard && canBoard) {
-    dispatchPostMerged(
-      buildEventActor({ principalId: actorPrincipalId, userId: actorUserId }),
-      {
-        id: duplicatePostId,
-        title: duplicatePost.title,
-        boardId: duplicatePost.boardId,
-        boardSlug: dupBoard.slug,
-      },
-      {
-        id: canonicalPostId,
-        title: canonicalPost.title,
-        boardId: canonicalPost.boardId,
-        boardSlug: canBoard.slug,
-      }
-    )
-  }
+  dispatchPostMerged(
+    buildEventActor({ principalId: actorPrincipalId, userId: actorUserId }),
+    {
+      id: duplicatePostId,
+      title: duplicatePost.title,
+      boardId: duplicatePost.boardId,
+      boardSlug: duplicateBoard.slug,
+    },
+    {
+      id: canonicalPostId,
+      title: canonicalPost.title,
+      boardId: canonicalPost.boardId,
+      boardSlug: canonicalBoard.slug,
+    }
+  )
 
   return {
     canonicalPost: { id: canonicalPostId, voteCount: newVoteCount },
@@ -336,6 +346,46 @@ export async function getMergedPosts(canonicalPostId: PostId): Promise<MergedPos
   }))
 }
 
+/**
+ * Get public-board posts merged into a canonical post.
+ *
+ * Public portal responses must not expose private/internal-board merged post
+ * metadata, even if an older cross-visibility merge exists.
+ */
+export async function getPublicMergedPosts(canonicalPostId: PostId): Promise<MergedPostSummary[]> {
+  const mergedPosts = await db
+    .select({
+      id: posts.id,
+      title: posts.title,
+      voteCount: posts.voteCount,
+      createdAt: posts.createdAt,
+      mergedAt: posts.mergedAt,
+      authorName: sql<string | null>`(
+        SELECT m.display_name FROM ${principalTable} m
+        WHERE m.id = ${posts.principalId}
+      )`.as('author_name'),
+    })
+    .from(posts)
+    .innerJoin(boards, eq(posts.boardId, boards.id))
+    .where(
+      and(
+        eq(posts.canonicalPostId, canonicalPostId),
+        isNull(posts.deletedAt),
+        eq(boards.isPublic, true)
+      )
+    )
+    .orderBy(posts.mergedAt)
+
+  return mergedPosts.map((p) => ({
+    id: p.id,
+    title: p.title,
+    voteCount: p.voteCount,
+    authorName: p.authorName,
+    createdAt: p.createdAt,
+    mergedAt: p.mergedAt!,
+  }))
+}
+
 /**
  * Get merge info for a post that has been merged into another.
  * Returns null if the post is not merged.
@@ -376,6 +426,52 @@ export async function getPostMergeInfo(postId: PostId): Promise<PostMergeInfo |
   }
 }
 
+/**
+ * Get merge info for a public portal post without exposing private canonical
+ * metadata. Returns null unless both the merged post and canonical target are
+ * on public boards and not deleted.
+ */
+export async function getPublicPostMergeInfo(postId: PostId): Promise<PostMergeInfo | null> {
+  const mergedPost = await db
+    .select({
+      canonicalPostId: posts.canonicalPostId,
+      mergedAt: posts.mergedAt,
+    })
+    .from(posts)
+    .innerJoin(boards, eq(posts.boardId, boards.id))
+    .where(and(eq(posts.id, postId), isNull(posts.deletedAt), eq(boards.isPublic, true)))
+    .limit(1)
+
+  const post = mergedPost[0]
+  if (!post?.canonicalPostId || !post.mergedAt) {
+    return null
+  }
+
+  const canonicalPost = await db
+    .select({
+      id: posts.id,
+      title: posts.title,
+      boardSlug: boards.slug,
+    })
+    .from(posts)
+    .innerJoin(boards, eq(posts.boardId, boards.id))
+    .where(
+      and(eq(posts.id, post.canonicalPostId), isNull(posts.deletedAt), eq(boards.isPublic, true))
+    )
+    .limit(1)
+
+  if (!canonicalPost[0]) {
+    return null
+  }
+
+  return {
+    canonicalPostId: canonicalPost[0].id,
+    canonicalPostTitle: canonicalPost[0].title,
+    canonicalPostBoardSlug: canonicalPost[0].boardSlug,
+    mergedAt: post.mergedAt,
+  }
+}
+
 /**
  * Result of a merge preview — simulates what the canonical post would look like after merging.
  */
diff --git a/apps/web/src/lib/server/domains/posts/post.public.detail.ts b/apps/web/src/lib/server/domains/posts/post.public.detail.ts
index 397e6b312..e21b8d182 100644
--- a/apps/web/src/lib/server/domains/posts/post.public.detail.ts
+++ b/apps/web/src/lib/server/domains/posts/post.public.detail.ts
@@ -147,7 +147,10 @@ export async function getPublicPostDetail(
         SELECT ${postUuid}::uuid
         UNION ALL
         SELECT p.id FROM ${posts} p
-        WHERE p.canonical_post_id = ${postUuid}::uuid AND p.deleted_at IS NULL
+        INNER JOIN ${boards} b ON b.id = p.board_id
+        WHERE p.canonical_post_id = ${postUuid}::uuid
+          AND p.deleted_at IS NULL
+          AND b.is_public = true
       )
       ${options?.includePrivateComments ? sql`` : sql`AND c.is_private = false`}
       GROUP BY c.id, m.display_name, m.avatar_key, m.avatar_url, scf.name, scf.color, sct.name, sct.color
diff --git a/apps/web/src/lib/server/functions/portal.ts b/apps/web/src/lib/server/functions/portal.ts
index 7b713ddd5..cc2094421 100644
--- a/apps/web/src/lib/server/functions/portal.ts
+++ b/apps/web/src/lib/server/functions/portal.ts
@@ -25,7 +25,7 @@ import {
   getVotedPostIdsByUserId,
 } from '@/lib/server/domains/posts/post.public'
 import { getPublicPostDetail } from '@/lib/server/domains/posts/post.public.detail'
-import { getPostMergeInfo, getMergedPosts } from '@/lib/server/domains/posts/post.merge'
+import { getPublicPostMergeInfo, getPublicMergedPosts } from '@/lib/server/domains/posts/post.merge'
 import { listPublicStatuses } from '@/lib/server/domains/statuses/status.service'
 import { listPublicTags } from '@/lib/server/domains/tags/tag.service'
 import { getSubscriptionStatus } from '@/lib/server/domains/subscriptions/subscription.service'
@@ -202,10 +202,10 @@ export const fetchPublicPostDetail = createServerFn({ method: 'GET' })
     // Fetch merge info for this post
     const postId = data.postId as PostId
     const [mergeInfo, mergedPostsList] = await Promise.all([
-      getPostMergeInfo(postId).then((info) =>
+      getPublicPostMergeInfo(postId).then((info) =>
         info ? { ...info, mergedAt: toISOString(info.mergedAt) } : null
       ),
-      getMergedPosts(postId),
+      getPublicMergedPosts(postId),
     ])
 
     return {