Where does the bug appear (feature/product)?
Cursor IDE
Describe the Bug
GPT 5.1 in Cursor Chat fails when MCP servers are enabled.
Error: “Model returned error” + ConnectError: [invalid_argument].
Request ID: 211a7bca-5ddf-47d8-89ec-fa35f0093d38.
Workaround: disable MCP servers or switch model.
Note: GPT 5.1 works in Chat with MCP off; Compose unaffected.
Steps to Reproduce
Open Cursor IDE.
Ensure MCP servers are enabled.
Open Chat.
Select GPT 5.1.
Send any message.
Expected Behavior
Chat returns a normal model response.
Operating System
Windows 10/11
Current Cursor Version (Menu → About Cursor → Copy)
Version: 2.2.0 (user setup)
VSCode Version: 1.105.1
Commit: ea48594a09b2e8d1fd6fb5db8502d43815a1e1a0
Date: 2025-12-09T06:33:08.669Z
Electron: 37.7.0
Chromium: 138.0.7204.251
Node.js: 22.20.0
V8: 13.8.258.32-electron.0
OS: Windows_NT x64 10.0.19045
For AI issues: which model did you use?
GPT 5.1, GPT 5.1 Codex
For AI issues: add Request ID with privacy disabled
Request ID: 211a7bca-5ddf-47d8-89ec-fa35f0093d38
Additional Information
Key Observation:
GPT 5.1 works in Chat when MCP servers are disabled.
Compose does not show the issue.
Suspected Cause:
Likely an integration bug in Chat’s tool/agent pipeline when MCP tool schemas/parameters are passed to GPT 5.1, leading to a transport-level invalid_argument failure.
Impact:
Prevents using GPT 5.1 in Chat with MCP-enabled workflows.
Workaround:
Disable MCP servers when using GPT 5.1 in Chat.
Or use another model in Chat while keeping MCP enabled.
Does this stop you from using Cursor
No - Cursor works, but with this issue