Codex Desktop on macOS returns 404 on /backend-api/codex/responses while web works

Environment

• Platform: macOS desktop Codex app
• Subscription: ChatGPT Pro
• Region/edge observed in error: LAX
• Models tested: GPT-5.6 Sol and GPT-5.5
• ChatGPT/Codex web experience works normally on the same account

Issue

Codex Desktop cannot complete even a brand-new trivial conversation. The app repeatedly reconnects and returns:

unexpected status 404 Not Found: Unknown error, url: https://chatgpt.com/backend-api/codex/responses

Example cf-ray:

• a355a4b0e8a31e11-LAX

When the app attempts automatic context compaction it also fails with:

Error running remote compact task: unexpected status 404 Not Found: Unknown error, url: https://chatgpt.com/backend-api/codex/responses

Example cf-ray:

• a355a7ab3b62cbaf-LAX

Reproduction

1. Open Codex Desktop on macOS.
2. Create a brand-new conversation.
3. Send a trivial message such as OK.
4. App shows Reconnecting 5/5 and the /backend-api/codex/responses request returns 404.
5. Switch model from GPT-5.6 Sol to GPT-5.5 and retry.
6. The same 404 occurs.

Troubleshooting already tried

• Restarted the Mac.
• Opened a different/new Codex conversation.
• Tested a trivial new conversation unrelated to the project.
• Switched from GPT-5.6 Sol to GPT-5.5.
• Same account works in the browser/web experience.

Expected behavior

Codex Desktop should successfully send a request and return a response, matching the working web experience for the same account.

Additional context

This blocks active development work in Codex Desktop. The failure is reproducible outside any specific repository or long-running thread, so it does not appear to be caused by project state or conversation size.

Related reports may include #42468, #38323, and #29794, but this case also reproduces in a brand-new trivial conversation and specifically affects macOS Desktop while web works.

Start by reproducing the macOS Desktop request to /backend-api/codex/responses and compare it with the working web experience. Review related reports #42468, #38323, and #29794; the issue is resolved when a new Desktop conversation and remote compaction receive responses instead of 404 errors.