[BUG] API Error 400: thinking blocks must remain as original response - claude-opus-4-8 incompatibility

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?

API returns 400 error on thinking/redacted_thinking block handling.
Claude Code appears to be modifying or not preserving thinking blocks
correctly when sending them back in subsequent API calls.

What Should Happen?

Opus 4.8 should work without errors, same as Sonnet 4.6.

Error Messages/Logs

Steps to Reproduce

Opus 4.8 should work without errors, same as Sonnet 4.6.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.156

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_