Introducing Claude Fable 5.1 and Claude Mythos 5.1
Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, which it calls the world's most advanced models for coding and knowledge work.
Anthropic has released Claude Fable 5.1, generally available, alongside Claude Mythos 5.1, the same underlying model restricted to vetted professionals in cybersecurity and life sciences through Anthropic's access programs. Both keep Fable 5's 1M-token context window, 128k max output, always-on adaptive thinking, and $10 / $50 per million token pricing, but cache reads drop to $0.25 per million, cutting typical costs by an estimated 25% and up to about 45% for heavily agentic work.
Fable 5.1 can now be used to discover software vulnerabilities, though not to develop exploits for them, and Anthropic says its cybersecurity safeguards block 60% fewer false positives than before. A new access program, built with the US government, will open enrolment for scientists to use Mythos 5.1's biology capabilities. Anthropic is also rolling out Enterprise Frontier Safeguards, which store customer data on infrastructure the customer controls rather than Anthropic's, matching zero data retention while still screening for adversarial use; it arrives for enterprise customers in phases starting this autumn. Until then, eligible customers can use Fable 5.1 with zero data retention. In Anthropic's announcement, Fable 5.1 defaults to High effort in Claude Code and Medium in Claude Cowork and on Claude.ai.
