CAPIError: 400 Unrecognized request argument 'reasoning_effort' when compacting after switching models #2553

Describe the bug

When running /compact in Copilot CLI after switching from the Claude Opus 4.6 model to the GPT-4.1 model, I receive the following error:

• If /compact is run automatically by Copilot:
  • Execution failed: CAPIError: 400 Unrecognized request argument supplied: reasoning_effort (Request ID: ID)
• If /compact is run manually by me:
  • Compaction Failed: CAPIError: 400 Unrecognized request argument supplied: reasoning_effort

Affected version

1.0.19

Steps to reproduce the behavior

1. Start a session using Claude Opus 4.6 (or any model supporting reasoning_effort).
2. Switch to GPT-4.1.
3. Run /compact (either automatically or manually).
4. Observe the error above.

Expected behavior

Copilot CLI should clear or ignore unsupported parameters when switching models, allowing /compact to run without errors.

Additional context

• The error does not occur if I switch back to a model supporting reasoning_effort.
• This appears to be a session state compatibility issue when switching models.