API Error 400 - thinking blocks modified when resuming agent

Description

When running a Task agent, I encountered this error:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.11.content.1: `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_011CXbbEmykjoXGsScGqtu7C"}

Expected behavior

The agent should either:

1. Successfully resume without corrupting thinking blocks, or
2. Gracefully handle the error and offer to start fresh

Environment

• OS: macOS (Darwin 25.2.0)
• Model: claude-opus-4-5-20251101

Additional context

This appears to happen when conversation history gets out of sync, possibly due to context management or an interrupted request.