Attaching a file to a new ChatGPT chat fails with 404 conversation_inaccessible on /conversation/init (works in browser, works without attachment)

What version of the Codex App are you using (From “About Codex” dialog)?

26.915.31945

What subscription do you have?

ChatGPT (plan_type: Pro), active until 2026-09-25, personal account (single org, owner)

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 x64

What issue are you seeing?

In the Windows desktop app, attaching a file to a new ChatGPT chat makes conversation
creation fail. The identical chat sent without an attachment succeeds.

Failing (file attached) — POST /conversation/init returns 404:

2026-09-20T12:54:21.444Z warning [electron-message-handler] sa_server_request_failed
  errorCode=conversation_inaccessible
  routePattern=/conversation/init  status=404  url=/conversation/init
  detail: {"code":"conversation_inaccessible","can_retry":false,
           "conversation_id":"6aafd774-7b58-83e8-b2a4-4b5aba160765"}

Reproduced at 12:47, 12:49, 12:54 and 13:01 UTC, each time with a freshly generated
conversation_id, so it is not tied to any particular conversation.

Succeeding (same app, same account, same minutes, no attachment) at 13:17:33 UTC:

chatgpt_conversation_refetch_started   conversationId=6aafdce4-4ef8-83e8-b942-89c8dcdda27c
chatgpt_conversation_refetch_completed currentNodeApplied=true

Scope of the failure:

	Result
App, new chat with attachment	404 conversation_inaccessible
App, new chat without attachment	works
App, Work / Codex (thread/start -> turn/completed, 13:10 UTC)	works
Browser (chatgpt.com), same account, with attachment	works

So this is not app-wide connectivity, not an account or subscription problem, and not
specific to any project or conversation. It is specific to attaching a file from the
desktop app.

What steps can reproduce the bug?

Feedback ID: 01a0bef0-2bc1-7152-a088-b2f977ef527a

1. Sign in to the Windows desktop app.
2. Start a new ChatGPT chat. (Reproduces both inside a ChatGPT project and with no
   project selected.)
3. Attach any file and send.
   -> POST /conversation/init returns 404 conversation_inaccessible and the
   conversation is never created.
4. Start another new chat and send the same text without attaching a file.
   -> Works normally.
5. Do step 3 again in a browser at chatgpt.com with the same account.
   -> Works normally.

Reproduces every time.

What is the expected behavior?

The file is uploaded and the conversation is created, the same as it behaves in the
browser with the same account, and the same as the app behaves when no file is attached.

Additional information

Already tried, with no change:

1. Full sign-out / sign-in (auth refreshed 2026-09-20 12:52 UTC, subscription re-verified
   as active).
2. Uninstall + reinstall of the MSIX package.
3. Regenerated installation_id in %USERPROFILE%\.codex\.

Logs: %LOCALAPPDATA%\Packages\OpenAI.Codex_2p2nqsd0c76g0\LocalCache\Local\Codex\Logs\

No /files or upload endpoint appears in the main-process log, only the failing
/conversation/init, so the upload itself may be failing earlier in the renderer.

Possibly related: #31049, #27552, #45480, #45912

Reproduce the Windows desktop flow with and without an attachment, then inspect the renderer and main-process logs at %LOCALAPPDATA%\Packages\OpenAI.Codex_2p2nqsd0c76g0\LocalCache\Local\Codex\Logs. Trace the attachment path around POST /conversation/init and compare it with the browser behavior. Done means attached files upload and the new conversation is created without conversation_inaccessible.