[BUG] Thinking block modification error on v2.1.12 with fresh conversation

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?

Description

Still getting the thinking block modification error on latest version with a fresh conversation (not
resumed from older version).

API Error: 400
{"type":"error","error":{"type":"invalid_request_error","message":"messages.33.content.1: thinking or
redacted_thinking blocks in the latest assistant message cannot be modified. These blocks must remain as
they were in the original response."}}

## Environment

• Version: 2.1.12
• Platform: macOS
• Model: Opus 4.5

## Notes

• Conversation was started fresh within the last 1-2 days
• Error occurs repeatedly after some back-and-forth
• #12311 was closed as completed but issue persists on latest version
• Unable to comment on #12311 as it's locked

## Related

#12311, #12973, #12946

What Should Happen?

The convo should be able to continue as normal

Error Messages/Logs

API Error: 400                                                                                            
  {"type":"error","error":{"type":"invalid_request_error","message":"messages.33.content.1: 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

1. Start a new conversation with Opus 4.5 (thinking enabled by default)
2. Chat normally for extended back-and-forth
3. Error starts occurring repeatedly (likely when auto-compact triggers)

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

No idea

Claude Code Version

2.1.12

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_