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)?

Version 26.901.51231

What subscription do you have?

ChatGPT Plus account

What platform is your computer?

WIN 10

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:

unexpected status 404 Not Found: The model gpt-5.5 does not exist or you do not have access to it., url: https://chatgpt.com/backend-api/codex/responses, cf-ray: a37c6c6c88bd5bcc-TUN, request id: 1986405c-ea34-4032-b49e-52653bdc1a3b

What steps can reproduce the bug?

Steps to reproduce:

Launch OpenAI Codex
Sign in with a ChatGPT Plus account.
Select model "gpt-5.5" (medium reasoning effort).
Start a new session.
Send a simple prompt

Result:

Codex fails and returns:

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

What is the expected behavior?

No response

Additional information

No response

Start by tracing model selection and the request to https://chatgpt.com/backend-api/codex/responses, then compare it with the model list shown by /status. Confirm whether gpt-5.5 is supported for ChatGPT Plus; done means a supported selection succeeds or an unavailable model is rejected with a clear message instead of a 404.