[BUG] API Error: 400

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 a multi-turn conversation in the VSCode extension, Claude Code throws an API Error 400 indicating that thinking blocks were modified, blocking further interaction.

Error:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.9.content.33: thinking or redacted_thinking blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response."},"request_id":"req_011CXaKk27Fnc85C7KskBgyB"}

This appears to be an internal state management issue where the conversation history becomes corrupted, causing thinking blocks to be incorrectly serialized when sent back to the API.

What Should Happen?

The conversation should continue without errors. Thinking blocks should be preserved and managed internally by Claude Code without user intervention.

Error Messages/Logs

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.9.content.33: thinking or redacted_thinking blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response."},"request_id":"req_011CXaKk27Fnc85C7KskBgyB"}

Steps to Reproduce

1. Start a conversation in Claude Code (VSCode extension)
2. Have a multi-turn conversation (5+ exchanges)
3. At some point, the error appears and blocks further interaction

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.12

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_