GPT-5.5 selected in Codex App but backend returns 404 Model not found

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

v0.137.0

What subscription do you have?

ChatGPT Plus

What platform is your computer?

Windows 11 x64

What issue are you seeing?

Codex App starts correctly and authenticates successfully.

Current configuration:

model = "gpt-5.5"
model_reasoning_effort = "medium"

The model appears in /status and can be selected successfully.

However, when sending any prompt, Codex returns:

unexpected status 404 Not Found: Model not found gpt-5.5

Additional output:

Falling back from WebSockets to HTTPS transport.
unexpected status 404 Not Found: Model not found gpt-5.5-codex-1p-exp-p-0604-811f2f-ev3-text-1-treatment

Request ID:
cdc4d6c6-d1e2-48fb-b350-3b2245db32fe

CF-Ray:
a083c4afceb93c1b-LIM

GPT-5.4 works correctly on the same installation and account.

Account type:
ChatGPT Plus

Codex version:
v0.137.0

Operating system:
Windows 11 x64

<img width="803" height="413" alt="Image" src="https://github.com/user-attachments/assets/b393850e-503e-418a-b373-2903fe46950f" />

<img width="1183" height="597" alt="Image" src="https://github.com/user-attachments/assets/c131a23f-06d6-40b3-8f5f-0be8d6cb2b86" />

What steps can reproduce the bug?

Feedback ID: 019ea4a5-6373-7932-b443-e29f94a3cc11

Steps to reproduce:

1. Launch OpenAI Codex App v0.137.0.
2. Sign in with a ChatGPT Plus account.
3. Select model "gpt-5.5" (medium reasoning effort).
4. Start a new session.
5. Send a simple prompt such as:

hola

Result:

Codex fails and returns:

unexpected status 404 Not Found: Model not found gpt-5.5

Additional output:

Falling back from WebSockets to HTTPS transport.
unexpected status 404 Not Found: Model not found gpt-5.5-codex-1p-exp-p-0604-811f2f-ev3-text-1-treatment

GPT-5.4 works correctly on the same installation and account.

What is the expected behavior?

When GPT-5.5 is selected, Codex should successfully process prompts and generate responses, just like GPT-5.4.

The model appears in /status and can be selected successfully, so prompts should execute without returning a 404 error.

Additional information

Codex version: v0.137.0

Subscription: ChatGPT Plus

Platform: Windows 11 x64

Request ID:
cdc4d6c6-d1e2-48fb-b350-3b2245db32fe

CF-Ray:
a083c4afceb93c1b-LIM

The issue occurs only with GPT-5.5.
GPT-5.4 works normally on the same machine and account.

A feedback report was already submitted through the Codex App.
Feedback ID:
019ea4a5-6373-7932-b443-e29f94a3cc11