[BUG]

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?

I have had it hit the invalid request warning box: 25 times in the past two days. This is not something I've ever had happen with 4.6 or 4.7, or in the months that I have been working with Claude code, but since Opus 4.8 dropped, I hit this invalid request every couple of hours on routine things.

What Should Happen?

I mean, it shouldn't just crash with an invalid request where I can't continue the conversation or do anything. I lose hours of work because I completely lose the conversation with an invalid request box that pops up.

Error Messages/Logs

API Error: 400 messages.7.content.8: `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

I mean, this happens in the middle of every Claude Code session of any sort of reasonable size, I manage my usage. I keep an eye on the context window, but nowhere near the context ceiling. I just get slapped with an invalid request, and then I have to completely start over.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

1.9659.2

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_