Sonnet 4 stoped working

Support Bug Reports
1

Describe the Bug

I am getting this in Agent mode. Lost a lot of tokens

Connection failed. If the problem persists, please check your internet connection or VPN
Serialization error in aiserver.v1.StreamUnifiedChatRequestWithTools [internal]
(Request ID: 8f298ee6-4b2a-4f99-9123-97849f64b08e)

Haiku works ok

Steps to Reproduce

Just ask to do something

Operating System

Windows 10/11

Current Cursor Version (Menu → About Cursor → Copy)

Version: 1.3.4 (user setup)
VSCode Version: 1.99.3
Commit: bfb7c44bcb74430be0a6dd5edf885489879f2a20
Date: 2025-07-29T03:59:10.762Z
Electron: 34.5.1
Chromium: 132.0.6834.210
Node.js: 20.19.0
V8: 13.2.152.41-electron.0
OS: Windows_NT x64 10.0.19045

2

Hey, thanks for the report. Does creating a new chat resolve the issue?

3

Hello

No, the same
I narrowed the prompt down just to check linters errors (php backend)

It starts generating text response in the chat and fails after 2-3 lines.

Other prompts seem to work fine