Where does the bug appear (feature/product)?
Background Agent (GitHub, Slack, Web, Linear)
Describe the Bug
Hi Cursor team,
I’m experiencing issues while using the Cursor web app. Whenever I try to work with selected models (e.g., gpt-5 and claude-4.1-opus), I see console errors like:
POST https://cursor.com/api/auth/notifyBackgroundComposerShown 500 (Internal Server Error)
Failed to fetch detailed diff
cannot decode message aiserver.v1.GetOptimizedDiffDetailsResponse from JSON
Additional details:
- Affected
bcId:bc-badc14fd-2c77-46d9-a5a8-50165140b449 - I’m using only the Cursor web app; no active agents.
- I’ve tried reloading the page and clearing cache, but the issue persists.
- Browser: Chrome
- OS: Windows 10 ( MiniOS10 )
Thank you in advance for your help.
Steps to Reproduce
Open Cursor web app.
Select one or more models (gpt-5, claude-4.1-opus).
Observe the console logs for errors (500, Failed to fetch detailed diff, etc.).
Attempt to use Background Composer features — the request fails.
Expected Behavior
The web app should successfully fetch diffs and start Background Composer without returning 500 errors. Streaming or polling for updates should work as expected.
Operating System
Windows 10/11
Current Cursor Version (Menu → About Cursor → Copy)
I am not having.
For AI issues: which model did you use?
Any model
For AI issues: add Request ID with privacy disabled
Background Agent: bc-badc14fd-2c77-46d9-a5a8-50165140b449
Additional Information
The error occurs with any selected model.
No active agents are used.
I’ve tried clearing cache and reloading the page; issue persists.
Cursor is unusable in this state.
Does this stop you from using Cursor
Yes - Cursor is unusable