Copilot Chat returns 400 error when using Claude Opus 4.8 — thinking.type.enabled not supported

[ErkanBarin]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Models

Body

Description
Selecting claude-opus-4.8 as the model in GitHub Copilot Chat (VS Code) consistently fails with a 400 error. The request never returns a response — it hangs briefly then errors out. All other Claude models (Opus 4.7, Sonnet 4.6, etc.) work fine.

Error from Copilot Chat extension logs
Server error: 400
{
"type": "error",
"error": {
"type": "invalid_request_error",
"message": ""thinking.type.enabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior."
}
}

Root cause
Claude Opus 4.8 changed the thinking API parameter format. It no longer accepts thinking.type: "enabled" — it requires thinking.type: "adaptive" with output_config.effort. The Copilot Chat extension still sends the old format, causing every Opus 4.8 request to be rejected by the API.

Environment
VS Code: 1.123.0 (remote SSH)
GitHub Copilot Chat extension: 0.51.0
OS: RHEL 9 (Linux 5.14.0)
Platform: VS Code Remote - SSH
Steps to reproduce
Open Copilot Chat in VS Code
Switch model to claude-opus-4.8
Send any message
Observe: request hangs, then fails with error
Expected behavior
Copilot Chat should send thinking.type: "adaptive" (with output_config.effort) when targeting Claude Opus 4.8, matching the current Anthropic API requirements for this model.

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