Claude Fable 5.1 enforces strict conversation binding for thinking blocks

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Anthropic's Claude Fable 5.1 is the first Claude model to block requests where thinking blocks are returned in a modified conversation history, throwing a 400 error stating the block is 'bound to a different conversation.' Each thinking block now carries a cryptographic signature recording the exact conversation prefix, system prompt, tools array, and prior messages, verified byte-for-byte on every API call. This enforcement applies to all new API accounts created on or after August 31, 2026, across Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry. Older accounts currently receive a warning rather than a hard block, though Anthropic has indicated future models will enforce the check universally. Developers can work around prefix mismatches using the 'drop_block' setting via the 'thinking-binding-controls-2026-08-01' beta header, or by adopting append-only message patterns that preserve a warm prompt cache.

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