fix: classify malformed thread metadata

README.md

@@ -1,7 +1,8 @@
 # Twitter Lite
 
-Twitter Lite is an intentional, read-only X reader. It stays empty until you
-enter a user handle, profile URL, or search query.
+Twitter Lite is an intentional, read-only X reader. It shows content only after
+you enter a user handle, profile URL, or search query, follow a deliberate post
+detail link, or manually open a valid `/status/:tweetId` URL.
 
 ## Scope
 

docs/superpowers/specs/2026-07-13-twitter-lite-design.md

@@ -258,6 +258,8 @@ External links open in a new tab with `rel="noreferrer noopener"`. The app never
 Server functions translate library failures into a small discriminated error model:
 
 - `invalid-input`;
+- `post-not-found`;
+- `post-unavailable`;
 - `user-not-found`;
 - `user-unavailable`;
 - `relay-config`;
@@ -335,7 +337,9 @@ Bird's README and changelog document the search product option. No AGENTS.md or
 The work is complete when:
 
 1. Bird supports Top and Latest through its public library and CLI contracts while preserving Latest as the default.
-2. Twitter Lite shows no posts before an explicit user submission.
+2. Twitter Lite shows no posts before an explicit user action: submitting a
+   target or query, following a post detail link, or manually opening a valid
+   status URL.
 3. A valid handle or profile URL loads that user's posts.
 4. Search supports Top, Latest, and the follows filter.
 5. Infinite scrolling continues cursor pages without a fixed limit and without duplicate posts.

src/features/posts/post-service.test.ts

@@ -153,6 +153,49 @@ describe('loadThreadPage', () => {
     expect(client.getThreadPaged).not.toHaveBeenCalled()
   })
 
+  it('classifies malformed Bird conversation metadata as an upstream failure', async () => {
+    const client = reader()
+    const privateMetadata = 'FINAL_PRIVATE_CONVERSATION_MARKER_71c9'
+    vi.mocked(client.getTweet).mockResolvedValue({
+      success: true,
+      tweet: {
+        id: '123',
+        text: 'focal',
+        conversationId: privateMetadata,
+        author: { username: 'focus', name: 'Focus' },
+      },
+    })
+    const errorSpy = vi.spyOn(console, 'error').mockImplementation(() => {})
+
+    try {
+      const result = await loadThreadPage(client, { tweetId: '123' })
+
+      expect(result).toEqual({
+        ok: false,
+        error: {
+          code: 'upstream',
+          message: 'X から投稿を取得できませんでした。',
+          retryable: true,
+        },
+      })
+      expect(client.getTweet).toHaveBeenCalledWith('123')
+      expect(client.getThreadPaged).not.toHaveBeenCalled()
+      expect(errorSpy).toHaveBeenCalledTimes(1)
+      expect(errorSpy).toHaveBeenCalledWith(
+        'Unexpected Bird failure:',
+        expect.objectContaining({
+          message: '投稿 ID を確認してください。',
+        }),
+      )
+      expect(JSON.stringify(result)).not.toContain(privateMetadata)
+      expect(errorSpy.mock.calls.flat().join(' ')).not.toContain(
+        privateMetadata,
+      )
+    } finally {
+      errorSpy.mockRestore()
+    }
+  })
+
   it('rejects an invalid ID without a Bird call', async () => {
     const client = reader()
 

src/features/posts/post-service.ts

@@ -133,9 +133,13 @@ export async function loadThreadPage(
         return upstreamFailure(focal.error ?? 'Tweet not found', 'post')
       }
       focalPost = publicPost(focal.tweet)
-      conversationId = normalizeTweetId(
-        focal.tweet.conversationId ?? focal.tweet.id,
-      )
+      try {
+        conversationId = normalizeTweetId(
+          focal.tweet.conversationId ?? focal.tweet.id,
+        )
+      } catch (error) {
+        return upstreamFailure(error, 'post')
+      }
     }
 
     const result = await reader.getThreadPaged(conversationId, {