Copilot Web: Model "gpt-5.3-codex" is not available.

### Describe the bug

I can't run Copilot to make code as agent for me, because it always provides me this error message:

> Copilot Error
> Copilot has encountered an unexpected error: [runtime:-32603] Request session.create failed with message: Model "gpt-5.3-codex" is not available.
>

### Affected version

_No response_

### Steps to reproduce the behavior

1. Try to run Copilot from https://github.com/copilot to work as an agent on code writing;
2. Get the error: Model "gpt-5.3-codex" is not available.

### Expected behavior

Not get an error. Instead, get code.

### Additional context

_No response_

Reproduce the failure from https://github.com/copilot using the reported agent workflow and confirm that session.create rejects gpt-5.3-codex. Review the issue discussion for an identified owner or workaround; done means the workflow runs and returns code without the reported model-unavailable error.

Scritto dal modello di indicizzazione a partire dal testo della issue.