API Error: thinking blocks cannot be modified during Write operations

Description

Intermittent API error occurs during Write tool operations, even across new CLI terminal sessions.

Error Message

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.1.content.71: `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_011CXVnWn4RVUr9hksVHoruj"}

Environment

• Claude Code version: 2.1.19
• Platform: Windows 11 (win32)
• Model: claude-opus-4-5-20251101

Steps to Reproduce

1. Start a conversation that involves writing files
2. Use the Write tool to create/modify files
3. Error occurs seemingly randomly during Write operations

Observed Behavior

• The Write tool output shows "Wrote 1 lines" even for multi-line files
• The file content is often successfully written despite the error
• Starting a new terminal session does not resolve the issue
• The error references messages.1.content.71 suggesting conversation history corruption

Expected Behavior

Write operations should complete without API errors about thinking block modification.

Additional Context

This appears to be a state management issue where thinking blocks in the conversation history are being modified between API calls, violating the API contract that thinking blocks must remain unchanged.