[BUG] API Error: 400 messages.7.content.12: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.

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?

<img width="1048" height="264" alt="Image" src="https://github.com/user-attachments/assets/d7a54f68-da8c-4a27-986c-917648ee055b" />

What Should Happen?

This SHOULDN't happen at all

Error Messages/Logs

API Error: 400 messages.7.content.12: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks
     must remain as they were in the original response.

Steps to Reproduce

Just try to run opus 4.8 normally

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.156

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_