Codex Desktop 26.901.20858 - persistent 404 Unknown error on /backend-api/codex/responses

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

Powered by Codex & OWL Version 26.901.20858 Released Sep 2, 2026

What subscription do you have?

Plus

What platform is your computer?

Windows, Codex Desktop / ChatGPT desktop app

What issue are you seeing?

Codex Desktop consistently fails with the following error:

unexpected status 404 Not Found: Unknown error, url: https://chatgpt.com/backend-api/codex/responses, cf-ray: a35594097e46eca6-MAD

The issue persists across sessions and also occurs after completely reinstalling the desktop application.

App version:

Powered by Codex & OWL
Version 26.901.20858
Released Sep 2, 2026

Model: gpt-5.6-sol
Reasoning effort: xhigh
Platform: Windows

There is no custom openai_base_url configured in my .codex/config.toml.

What steps can reproduce the bug?

Open Codex Desktop on Windows.
Open an existing project/session or create a new one.
Send any prompt.
Wait for Codex to generate a response.
The request fails with:

unexpected status 404 Not Found: Unknown error

https://chatgpt.com/backend-api/codex/responses

I have already tried:

Restarting Codex
Restarting Windows
Signing out and back in
Creating new sessions
Checking .codex/config.toml
Reinstalling the desktop application

The problem remains unchanged.

What is the expected behavior?

Codex should successfully send the request to the backend and return a normal response.

Instead, the backend endpoint returns HTTP 404 and Codex cannot complete the request.

Additional information

Codex Desktop version:

Powered by Codex & OWL
Version 26.901.20858
Released Sep 2, 2026

Configuration:

model = "gpt-5.6-sol"
model_reasoning_effort = "xhigh"
service_tier = "default"

Windows sandbox: elevated

CF-Ray:

a35594097e46eca6-MAD

The issue persists after a complete reinstall, which suggests that this may be a backend, account entitlement, model routing, or server-side issue rather than a corrupted local installation.

A similar issue reporting the same endpoint and the same 404 "Unknown error" exists here:

https://github.com/openai/codex/issues/28756

No repository file or test is identified in the report. Start by reviewing the related issue 28756 and the reported /backend-api/codex/responses request details; the issue is resolved when Codex Desktop can send a prompt and receive a normal response instead of HTTP 404.