Claude Fable 5.1 thinking blocks only replay to the same or newer models

UpdateSep 1, 2026VerifiedAdded Sep 22, 2026

Thinking blocks from Claude Fable 5.1 and Mythos 5.1 can't be read by earlier models and are dropped if replayed to them. Fable 5.1 also checks that nothing before a replayed block has changed, which matters for agents that edit conversation history.

LLMs Developer tools

Read Anthropic's release notes

https://platform.claude.com/docs/en/release-notes/overview#september-1-2026

Summaries of vendors' own notes. Product names and logos belong to their owners; logos via logo.dev.

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