Claude Fable 5.1 Tư duy bảo toàn: Khắc phục lỗi The Block Is Bound to a Different Conversation

Claude Fable 5.1's thinking block has a new requirement to prevent users from accidentally breaking conversations. If an agent harness is moved into Claude Fable 5.1 and a thinking block is bound to a different conversation, the code may have edited the conversation history. Claude Fable 5.1 is the first Claude model to enforce this check.

The problem, messages.5.content.0: Invalid `signature` in `thinking` block, occurs when a thinking block is no longer bound to the previous conversation. The API checks that the prefix matches exactly, including the system prompt, tools array, and all previous messages. This verification helps prevent tampering and protects the prompt cache.

Two main impacts of this change are:

1. Agents using the agent harness might stop working due to the thinking block mismatch error.

2. The token usage cost for reading from the prompt cache is reduced, saving approximately 0.25 USD per million tokens read from cache.

The affected platforms include Claude Code, Claude Managed Agents, Claude Agent SDK, and any API key accounts created on or after August 31, 2026. The change does not affect accounts created before this date, except for those requiring prefix_mismatch_behavior.

To fix the issue, set the prefix_mismatch_behavior to drop_block in the beta header. This will drop the misbound thinking block and any subsequent blocks, allowing the API to continue processing the request. An alternative is to update the system prompt or tools between requests.