[Bug] Anthropic API Error: Cannot modify thinking blocks during conversation compaction

[0xjan888]

Bug Description
2. Compaction tries to summarize - The /compact command attempts to condense the conversation history to save context space
3. API restriction - The Claude API has a strict rule: thinking blocks cannot be modified after they're created. They must stay exactly as they were.
4. Conflict! - The compaction process tried to modify a message that contains thinking blocks, which the API rejected with a 400 error.
Error:
Error: Error during compaction: Error: API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.9.content.5: 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_011CVuV4mBXpumyhqzBStUBi"}

Environment Info

• Platform: darwin
• Terminal: cursor
• Version: 2.0.61
• Feedback ID: 9dfaa425-912c-4e6e-8c01-bd1148768f2b

Errors

[]

[github-actions]

Found 3 possible duplicate issues:

1. [Bug] Anthropic API Error: Cannot modify thinking blocks in assistant message #13376
2. Compaction fails with 400 error when thinking blocks present in assistant message #12973
3. [Bug] Anthropic API Error: Cannot modify thinking blocks in assistant message during compaction #13298

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[dicksontsai]

Closing as duplicate of #12311 (earliest compact-related thinking block issue with most activity). Please follow that issue for updates.