Bug Report: Claude Models Fail in Copilot for Xcode (HTTP 400 – Invalid Request Body)

Start with github-copilot-for-xcode.log and reproduce the Agent-mode failure with Claude and a non-Claude model. Compare the request and error details for both paths, then verify that Claude completes a file edit or multi-step task without the HTTP 400 error.

Describe the bug
Claude models consistently fail in GitHub Copilot for Xcode with an HTTP 400 Bad Request error. The error indicates an invalid_request_body caused by a malformed or empty thinking block in the request payload.
As a result, Claude models are effectively unusable in Agent mode and auto-edit workflows in Xcode.

This does not occur with non-Claude models.
Versions
Copilot for Xcode: Latest available version (at time of report)
Xcode: 26.2 (reproducible across recent versions)
macOS: 15.7.4+ (Sequoia)

Copilot Pro subscription is active and confirmed via github education plan.
Steps to reproduce

1. Open Xcode
2. Enable GitHub Copilot for Xcode
3. Select a Claude model (Sonnet 4.5 or Opus 4.5)
4. Enable Agent mode (or attempt auto-edit / multi-step tasks)
5. Ask Copilot to perform a file edit or multi-step task
6. Observe failure during or immediately after generation

Screenshots

Logs

github-copilot-for-xcode.log

Additional context
Issue only occurs with Claude models

Other Copilot-supported models work correctly in Agent mode

Reproduced after:

Full uninstall & reinstall of Copilot for Xcode

Clearing Copilot, Claude, and Xcode extension caches

Rebooting macOS

Re-authenticating GitHub

Strongly suggests a Copilot ↔ Claude integration issue, where Copilot sends malformed or empty thinking blocks that Claude strictly rejects

This appears to be a backend schema compatibility issue, not a local configuration problem

Impact: Claude models cannot be used for Agent workflows in Xcode, forcing manual copy-paste usage and degrading developer productivity.