Claude Fable 5.1 launches; Mythos 5.1 stays restricted
Anthropic announced Fable 5.1 and Mythos 5.1 on September 1. They are the same model with different safeguards and access.
Two names, one model
Fable 5.1 is generally available with Anthropic's production safeguards, on the Claude API, AWS, Google Cloud and Microsoft Azure. Mythos 5.1 is limited to vetted cybersecurity and life-sciences organisations in restricted-access programs, currently in the US; Anthropic says it is working with the US government to expand access.
Performance and price
On Anthropic's published evaluations, Fable 5.1 more than doubles Fable 5 on agentic scientific research and nearly doubles it on business workflows. Pricing stays at $10 per million input tokens and $50 per million output tokens, while cache reads are 75% cheaper.
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