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

[abbinavv]

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.