## Summary Claude models (Opus 4. 1, 4.5, and Sonnet) stopped working in **Agent mode** on GitHub Copilot for Xcode. The issue started around early January 2026 - it was working fine until the end of December. ## Error Message

[SwStudioAI]

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Sorry, an error occurred while generating a response. Details: unhandled status from server: 400 {"error": {"message":"messages.1.content.0.thinking: each thinking block must contain thinking","code":"invalid_request_body"}}
<img width="497" height="900" ## Summary
Claude models (Opus 4. 1, 4.5, and Sonnet) stopped working in Agent mode on GitHub Copilot for Xcode. The issue started around early January 2026 - it was working fine until the end of December.

Error Messagealt="Captura de pantalla 2026-01-10 a la(s) 11 45 17 p m" src="https://github.com/user-attachments/assets/5718a3aa-3644-44ca-8334-c839bc311126" />

[Devvekariya711]

It is not your fault. You didn't break anything.

This is a bug in the GitHub Copilot extension for Xcode. The extension is trying to talk to the Claude API using a specific feature (likely "Chain of Thought" or reasoning), but it is sending a "malformed" message. The server is rejecting it immediately.

⬇️ For more details see below ⬇️

[Devvekariya711]

Why did it start in January?
Since this started around early January 2026, GitHub likely pushed an update to the Copilot extension (or the backend router) that changed how they structure these data packets, and they messed up the formatting for the "Agent Mode" specifically.

Immediate Workarounds
Since this is a code-level bug in the extension, you cannot "fix" it yourself, but you can try these workarounds to keep working:

Turn off Agent Mode: Try using the standard Chat mode instead of Agent mode for now. The standard mode usually skips the complex "thinking block" structure that is causing the crash.

Switch Models (If possible): If Xcode allows you to toggle back to GPT-4o or an older model temporarily, that might bypass the specific API requirement that Claude is enforcing.

Check for Extension Updates: Go to your Xcode extension settings immediately. GitHub usually spots these 400 errors quickly and might have already pushed a hotfix (look for version updates released in the last 24 hours).