Copilot Agent fails with Grok Code Fast 1 (invalid_request_body)

[igovet]

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Bug

Copilot Feature Area

Copilot Agent Mode

Body

When using the Copilot Agent with the Grok Code Fast 1 model selected, requests fail even with a simple message like "test". The error occurs without any additional references or context. The Agent works correctly when using other models (e.g., GPT-4o). I have not tested this in the standard Chat mode.

Error Message:

Sorry, your request failed. Please try again.
Reason: Request Failed: 400 {"error":{"message":"Client specified an invalid argument","code":"invalid_request_body"}}

Request IDs:

• Copilot Request ID: e458966a-07e2-4f95-a7db-882de6c06d00
• GH Request ID: B41C:1AE73E:251012C:2C97FB1:69943CA7

Environment:

• OS: Linux
• VS Code version: Version: 1.109.3
  Commit: b6a47e94e326b5c209d118cf0f994d6065585705
  Date: 2026-02-12T17:54:48.470Z
  Electron: 39.3.0
  ElectronBuildId: 13168319
  Chromium: 142.0.7444.265
  Node.js: 22.21.1
  V8: 14.2.231.22-electron.0
  OS: Linux x64 6.17.0-14-generic snap
• GitHub Copilot extension version: Identifier github.copilot-chat Version 0.37.6
• Mode: Agent
• Model: Grok Code Fast 1 (Built-in)

Steps to Reproduce:

1. Open Copilot Chat and switch to Agent mode.
2. Select the Grok Code Fast 1 model.
3. Send the message: test
4. Result: Receive 400 invalid_request_body error.

Expected Behavior:
The Agent should process the request and provide a response, even for short queries.

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[midiakiasat]

This is almost certainly schema incompatibility between Agent mode and the Grok Code Fast 1 backend, not a user-side issue.

Key signal:

• Same prompt works on GPT-4o
• Fails immediately with invalid_request_body
• No context, no tools invoked
• Agent mode only

Agent mode sends a structured payload (tools, planning state, conversation metadata).
A 400 invalid_request_body strongly indicates the Grok Code Fast 1 endpoint is rejecting the Agent-mode request schema.

This is a model–agent contract mismatch, not malformed user input.

Recommended actions:

1. Confirm whether Grok Code Fast 1 officially supports Agent mode.
2. Reproduce in plain Chat mode (to isolate Agent wrapper).
3. Include the full request capability set (tools enabled, workspace context size, etc.) if support asks.

This requires backend alignment between Copilot Agent and the Grok model integration, not something fixable via extension config.

[igovet]

I just found the problem, started to disable the tools again, and the problem turned out to be ClickUp MCP Tools; if you disable it, everything starts working.