[BUG] API issue: `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?

Sometimes when I run out of context and claude code attempts to auto-compact I get an error like this:
⎿ Error: Error during compaction: Error: API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.3.content.91: 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_011CVkUHwyt9SbRZuxMRLEnV"}
I cannot manually compact or rewind, it's a blocking issue. I'm not doing anything on my side to the payloads nor do I have anything running on my machine that is doing anything to intercept or modify API calls.

What Should Happen?

Auto-compact should not throw blocking API errors, it should continue on.

Error Messages/Logs

⎿ Error: Error during compaction: Error: API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.3.content.91: `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_011CVkUHwyt9SbRZuxMRLEnV"}

Steps to Reproduce

This is an intermittent failure, I have no idea what causes it, but it's happened twice before auto-compact. It also won't allow manual context compaction nor rewinding to try and compact. If it helps, claude is helping me write lean code to formally prove a conjecture.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Version: 2.0.55

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_