[Windows][ChatGPT Desktop 26.901.20858] Codex returns 404 on /backend-api/codex/responses while web works

What version of the Codex App / ChatGPT Desktop are you using?

ChatGPT Desktop 26.901.20858 (released Sep 3, 2026).

What subscription do you have?

ChatGPT Pro.

What platform is your computer?

Windows x64.

Which model were you using?

GPT-5.6 Sol (High reasoning).

What issue are you seeing?

Codex in the ChatGPT Windows desktop app cannot send prompts. Every request retries 5/5 times and then fails with:

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

The important A/B result is that Codex works normally on chatgpt.com using the same ChatGPT account, the same Windows computer, and the same network connection. Only the Windows desktop app fails.

This makes the problem look specific to the Windows desktop client / bundled Codex runtime rather than an account-wide, network-wide, or service-wide outage.

Steps to reproduce

1. Open ChatGPT Desktop 26.901.20858 on Windows.
2. Open Codex.
3. Start a new Codex conversation.
4. Select GPT-5.6 Sol (High reasoning).
5. Send any prompt.
6. Observe repeated reconnect attempts up to 5/5.
7. The request ultimately fails with HTTP 404 from /backend-api/codex/responses.
8. Open chatgpt.com on the same machine and use Codex with the same account.
9. The web version works normally.

Troubleshooting already attempted

• Fully restarted ChatGPT Desktop.
• Windows App Repair.
• Windows App Reset.
• Signed out and signed back in.
• Tested a fresh Codex conversation.
• Confirmed web Codex works normally on the same account/machine/network.

The desktop app still returns the same 404.

Expected behavior

Codex requests in ChatGPT Desktop should work the same way as Codex on chatgpt.com.

Actual behavior

Every Codex request in the Windows desktop app fails with HTTP 404 after repeated reconnect attempts.

Related reports

• #28756 — same /backend-api/codex/responses 404 error family.
• #42532 — Windows + GPT-5.6 Sol + /backend-api/codex/responses 404 after the current update, but reported from CLI rather than ChatGPT Desktop.

This report is specifically for ChatGPT Windows Desktop 26.901.20858, where web works while the desktop app fails.

Start by reproducing the failure in ChatGPT Desktop 26.901.20858 on Windows and inspect requests to /backend-api/codex/responses, then compare them with the working chatgpt.com request. Use reports #28756 and #42532 as related cases. Done means Codex prompts succeed in the desktop app without the repeated 404 retries.