Anthropic releases Fable 5, its first Mythos-class model, to the general public with safety guardrails in place

Anthropic announced the general release of Fable 5 on Tuesday, making it the first widespread release of a Mythos-tier model. Claude Mythos 5 is also being made available to vetted partners with existing Claude Mythos Preview access.

Fable 5 demonstrates improved performance in long-horizon memory management (an AI model's ability to track progress over extended, complex tasks), self-verification, and instruction following. Responses in high-risk areas such as biology and cybersecurity will be blocked and routed to Claude Opus 4.8, Anthropic's less powerful model released earlier this year. Early data shows at least 95% of Fable sessions run entirely on Fable's own responses rather than being routed to Opus.

Anthropic initially restricted Mythos-class models due to concerns about their ability to identify cybersecurity weaknesses. The company says it is now confident its safety guardrails are sufficient to prevent misuse, though acknowledged that some benign requests might initially be blocked.

Fable 5 and Mythos 5 are priced at $10 per million input tokens and $50 per million output tokens—twice the price of Claude Opus 4.8.