Claude model is unable

Support Bug Reports
1

Where does the bug appear (feature/product)?

BugBot

Describe the Bug

Where does the bug appear?
Cursor IDE – Agent mode with Claude models

Describe the Bug
When using Claude models (claude-sonnet / claude-opus) in Agent mode, every request fails with the following error:

Failed to deserialize the JSON body into the target type: tools[0]: missing field `type`

Other models (GPT-5.5, Gemini, etc.) work fine in the same setup. Only Claude models are affected.

Root Cause (suspected)
Cursor is sending tool definitions to the Anthropic API in an outdated format that is missing the required type field. Anthropic’s API now requires each tool object to include "type": "custom". Other providers use the OpenAI tool format, which Cursor handles correctly.

Steps to Reproduce

  1. Open Cursor with latest version
  2. Set model to any Claude model (sonnet / opus)
  3. Use Agent mode with any prompt that triggers tool use
  4. Error appears immediately

Expected Behavior
Claude models should work in Agent mode the same way other models do.

Request IDs

  • 77c80736-bab1-4860-a29d-22cf4d4fc882
  • 77145b7d-bbf8-45fd-9fdd-a23e50e2921d
  • 8092f759-5f03-4296-a3f7-c31df2109041

OS: Windows
Cursor Version: Latest

Steps to Reproduce

As mentioned above

Expected Behavior

As mentioned above

Operating System

Windows 10/11

Version Information

Version: 3.6.21 (user setup)
VS Code Extension API: 1.105.1
Commit: e7a7e93f4d75f8272503ecf33cedbaae10114a10
Date: 2026-05-28T21:45:36.072Z
Layout: glass
Build Type: Stable
Release Track: Default
Electron: 39.8.1
Chromium: 142.0.7444.265
Node.js: 22.22.1
V8: 14.2.231.22-electron.0
xterm.js: 6.1.0-beta.220
OS: Windows_NT x64 10.0.26200

For AI issues: which model did you use?

As mentioned above

For AI issues: add Request ID with privacy disabled

As mentioned above

Additional Information

As mentioned above

Does this stop you from using Cursor

No - Cursor works, but with this issue

5

Hey, thanks for the detailed report and the Request ID, that really helped. From the logs there’s an interesting detail: when the model is set to default, requests succeed, but when you explicitly pick Claude (Opus/Sonnet), they fail with this error. GPT-5.5 works for you too.

That pattern usually points to a custom config. Can you please check:

  1. In Cursor Settings → Models, do you have a custom Anthropic API key set?
  2. Is the Base URL overridden (OpenAI override)?

If yes, try temporarily removing the custom key or URL, then run Claude using Cursor’s built-in access and tell me if the error still happens. This will help us figure out which side has the tool definition format issue.

If you don’t have any custom key or override set up and you’re on the default Cursor access, let me know and we’ll dig deeper.