macOS Codex: gpt-5.5 returns 404 Model not found while gpt-5.4 works

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

26.602.40724

What subscription do you have?

Plus

What platform is your computer?

Darwin 22.3.0 arm64 arm

What issue are you seeing?

When using Codex on macOS with ChatGPT authentication, gpt-5.5 is listed locally as an available model but real requests fail server-side.

Observed errors:

• unexpected status 404 Not Found: Model not found gpt-5.5-codex-1p-exp-p-0604-811f2f-ev3-text-1-treatment
• unexpected status 404 Not Found: Model not found gpt-5.5

Working model:

• gpt-5.4

Failing model:

• gpt-5.5

This happens in the Codex app and is also reproducible from the bundled CLI while using the same ChatGPT login.

What steps can reproduce the bug?

1. Log into Codex with ChatGPT authentication.
2. Confirm the local model catalog lists gpt-5.5.
3. Run:

codex exec --skip-git-repo-check -m gpt-5.5 'Reply with exactly: OK'

1. Observe websocket prewarm failure for the treatment model:

gpt-5.5-codex-1p-exp-p-0604-811f2f-ev3-text-1-treatment

1. Observe HTTP fallback and final 404 for gpt-5.5.
2. Run:

codex exec --skip-git-repo-check -m gpt-5.4 'Reply with exactly: OK'

1. Observe that gpt-5.4 works normally.

Also reproduced with:

• codex exec --skip-git-repo-check -c 'model_reasoning_effort="low"' -m gpt-5.5 'Reply with exactly: OK'
• codex exec --skip-git-repo-check -c 'model_reasoning_effort="medium"' -m gpt-5.5 'Reply with exactly: OK'
• codex exec --skip-git-repo-check -c 'model_reasoning_effort="high"' -m gpt-5.5 'Reply with exactly: OK'
• codex exec --skip-git-repo-check -m gpt-5.5-2026-04-23 'Reply with exactly: OK' which returns: The 'gpt-5.5-2026-04-23' model is not supported when using Codex with a ChatGPT account.

Request IDs captured:

• 7463da3a-4876-4a94-9015-c47e07e0d2b6
• acf1e346-e322-4f72-8a19-7770b2897d06
• be50d67b-cc94-494e-90e3-028989eb1bc9
• b247b419-dd98-4892-a392-2ec1877ba820

What is the expected behavior?

If gpt-5.5 is listed as available locally for a ChatGPT Plus account in Codex, selecting or invoking gpt-5.5 should work normally instead of failing with a 404 model-not-found error.

Additional information

Environment:

• macOS 13.2.1 (Apple Silicon)
• Codex App 26.602.40724
• Bundled CLI 0.137.0-alpha.4
• Auth mode: ChatGPT

What was ruled out:

• Old client version
• Local default model misconfiguration
• Thread-specific state
• Reasoning-effort-specific failure

This looks like a ChatGPT-auth entitlement or backend routing mismatch for gpt-5.5 in Codex on this account.