[BUG] Persistent API Error: 400 for 12+ hours across multiple networks and locations

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

During active conversation sessions, Claude Code intermittently returns API Error: 400 responses.
The error occurs repeatedly and unpredictably — sometimes mid-conversation after several successful tool calls, sometimes when invoking tools. No specific trigger pattern identified; it happens across different tool types (Read, Edit, Bash, Agent, etc.).

What Should Happen?

Tool calls and responses should complete without 400 errors.

Error Messages/Logs

Searched for 2 patterns (ctrl+o to expand)  
  ⎿  API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"Output blocked      by content filtering policy"},"request_id":"req_011CZPmqiwEeMGypdajPk47w"}

Steps to Reproduce

1. Start a conversation with a large codebase (~100+ files)
2. Perform multi-step tasks involving several tool calls (Read, Edit, Agent, etc.)
3. At some point, responses fail with API Error: 400
4. Retrying the same prompt sometimes works, sometimes doesn't

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.83

Platform

Other

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_