# Claude model is unable **URL:** \ **Category:** Bug Reports\ **Tags:** anthropic\ **Created:** [May 31, 2026, 9:23am UTC](https://forum.cursor.com/t/claude-model-is-unable/162019 "2026-05-31T09:23:58Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![wande](https://avatars.discourse-cdn.com/v4/letter/w/e274bd/32.png) [@wande](https://forum.cursor.com/u/wande)\ **Post date:** [May 31, 2026, 9:23am UTC](https://forum.cursor.com/t/claude-model-is-unable/162019/1 "2026-05-31T09:23:58Z")
### 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: ```auto 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 **Root Cause (suspected)** Cursor is sending tool definitions to the Anthropic API in an outdated format that is missing the required `type` field.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 ---
**Author:** ![deanrie](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/deanrie/32/1263_2.png) [@deanrie](https://forum.cursor.com/u/deanrie)\ **Post date:** [May 31, 2026, 9:53am UTC](https://forum.cursor.com/t/claude-model-is-unable/162019/5 "2026-05-31T09:53:06Z")
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. ---
**Author:** ![system](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/system/32/99069_2.png) [@system](https://forum.cursor.com/u/system)\ **Post date:** [June 30, 2026, 9:53am UTC](https://forum.cursor.com/t/claude-model-is-unable/162019/6 "2026-06-30T09:53:08Z")
Closed on June 30, 2026, 9:53am UTC This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.