[BUG] API Error: 400 — thinking or redacted_thinking blocks in the latest assistant message cannot be modified.

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?

When working in an active session (mid-conversation, not at session start),
Claude Code throws an API 400 error:

"thinking or redacted_thinking blocks in the latest assistant message cannot
be modified. These blocks must remain as they were in the original response."

This has happened twice in the same workspace. Both times it occurred mid-session
after a response that used extended thinking. The error appears immediately
("Baked for 0s"), suggesting it fails on the next API call without retrying.

The workspace uses SessionStart and PreToolUse hooks that inject additionalContext
into the conversation. Suspected cause: a hook firing after a thinking-heavy
response
causes context reconstruction that alters or strips the immutable thinking blocks,
which the API then rejects.

Workaround: /clear resets the conversation and resolves it temporarily.

What Should Happen?

Claude Code should handle thinking blocks correctly when hooks inject context
▎ mid-session, and the conversation should continue without error.

Error Messages/Logs

Steps to Reproduce

"Use a workspace with SessionStart/PreToolUse hooks. Have an
extended conversation where thinking blocks appear. A hook fires mid-session. Next
API call throws 400."

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.154 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

• Steps to reproduce: "Use a workspace with SessionStart/PreToolUse hooks. Have an

extended conversation where thinking blocks appear. A hook fires mid-session. Next
API call throws 400."

• Expected: Session continues normally
• Actual: API Error 400, session broken
• Claude Code version: Run claude --version in terminal and paste the output
• OS: macOS (Darwin 25.5.0)