Gemini models return 400 Bad Request in Copilot CLI

[sammallon]

Request ID: 8B9B:2E3309:6C8CC:19684A:6A593988
Timestamp: 2026-07-16T13:05:40Z
Error: CAPIError: 400 Bad Request

Reproduction

1. Using Copilot with Gemini model selection (tried gemini-3.1-pro-preview and gemini-3.5-flash)
2. Sent plain text prompt with no attachments/tools
3. All advanced options at defaults (no custom reasoning/context overrides)
4. Extensions/tool-calling disabled

Observed Behavior

• Request rejected with HTTP 400 before reaching model inference
• Error appears to be request-shape validation (not model timeout or API availability)
• Other models (GPT, Claude) work fine in same session

Expected Behavior

Gemini models should accept and process the request like other available models

Environment

• Copilot CLI (latest via app)
• Windows

Note: It appears to work if switched to "explore" mode

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Request the backend to trace this request ID to identify which field/parameter was rejected.