Misleading model_not_supported error after content-filter response #5309

Describe the bug

Related to #2661

Copilot CLI reports  Execution failed: 400 The requested model is not supported  even though the same session successfully makes requests with Claude Opus 5.5. I also reproduced the error with additional models, including Auto. Resetting subagent preferences and restarting with debug logging did not resolve it.

In the debug session, the Opus 5.5 request immediately before the failure was recorded with  finishReason: content_filter . The subsequent request returned  model_not_supported  ( invalid_request_error , param  model ). This suggests the 400 may be a misleading follow-on error after a content-filter response, rather than the selected model being unavailable. The CLI should ideally surface the original cause or recover without reporting a model support error.

The error shown is: Execution failed: 400 The requested model is not supported. (Request ID: 6231:23F1C:1A9683F:1CCEB35:6AE5217E)

Request ID:  6231:23F1C:1A9683F:1CCEB35:6AE5217E 
Environment: Copilot CLI 1.0.105, Windows; Auto, Claude Opus 5.5, and other models

Affected version

1.0.105

Steps to reproduce the behavior

Type a prompt that that gets filtered for cybersecurity risk

Expected behavior

Error message should clearly state that the request was filtered. This is what used to hapen.

Additional context

Windows 11, x86-64, Windows Terminal