[BUG] API 400 error: thinking/redacted_thinking blocks cannot be modified after Claude Opus 4.8

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?

Title:
API 400 error: thinking/redacted_thinking blocks cannot be modified after Claude Opus 4.8

Body:
I’m reporting a recurring API 400 error in Claude Code that prevents me from continuing or resuming a session.

Error message:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.3.content.101: `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_011CbZ1NFZYk1a2pwkUCuXMw"}

This has been happening constantly since Claude Opus 4.8.

When this error happens, I cannot properly continue, resume, or restart the session. The only workaround is to start a completely new session from scratch.

This is extremely frustrating because it wastes both time and usage limits. I’m subscribed to the Max 5 plan, but I end up having to spend additional quota just to redo work that was already in progress.

Environment:

• Product: Claude Code
• Plan: Max 5
• Model: Claude Opus 4.8
• Error type: API Error 400
• Impact: Session cannot be resumed or continued; a new session is required

Expected behavior:
The session should continue normally without failing due to unmodified thinking or redacted_thinking blocks.

Actual behavior:
Claude Code fails with API Error 400 and the current session becomes unusable.

Please investigate this issue, because it is blocking normal usage for paid users.

What Should Happen?

.

Error Messages/Logs

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.3.content.101: `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_011CbZ1NFZYk1a2pwkUCuXMw"}

Steps to Reproduce

.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

v2.1.158

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_