{"id":23664,"date":"2026-09-02T19:57:00","date_gmt":"2026-09-02T19:57:00","guid":{"rendered":"https:\/\/scannn.com\/introducing-claude-fable-5-1-and-claude-mythos-5-1-anthropic\/"},"modified":"2026-09-02T19:57:00","modified_gmt":"2026-09-02T19:57:00","slug":"introducing-claude-fable-5-1-and-claude-mythos-5-1-anthropic","status":"publish","type":"post","link":"https:\/\/scannn.com\/lv\/introducing-claude-fable-5-1-and-claude-mythos-5-1-anthropic\/","title":{"rendered":"Introducing Claude Fable 5.1 and Claude Mythos 5.1 \\ Anthropic"},"content":{"rendered":"\n
<\/p>\n

We\u2019re introducing Claude Fable 5.1 and Claude Mythos 5.1. They\u2019re the world\u2019s most advanced models for coding and knowledge work\u2014and their research capabilities offer an early glimpse of how AI models will contribute to scientific progress.<\/p>\n

Claude Fable 5.1 and Claude Mythos 5.1 are the same model, but with different levels of safeguards. Fable 5.1 is generally available, while Mythos 5.1 is available only through our trusted access programs; its safeguards are specifically designed to support work in cybersecurity and the life sciences.<\/p>\n

Alongside its increased capabilities, Fable 5.1 takes important steps towards addressing the feedback we\u2019ve received from customers on price, data retention, and safeguards.<\/p>\n

Price.<\/em><\/strong> Fable 5.1 will cost an estimated 25% less than Fable 5 for typical workloads, wherever usage is billed by token. This is because we\u2019re reducing our pricing on cache reads (where the model reads inputs that have already been processed and stored). For highly agentic work, the savings will often be much larger\u2014up to approximately 45%.<\/p>\n

Data retention. <\/em><\/strong>Our new system of Enterprise Frontier Safeguards (EFS) gives customers complete privacy (the same as a zero data retention policy) while still being state-of-the-art at preventing adversarial use. EFS works by storing data in cloud infrastructure controlled entirely by the customer, not Anthropic. It will be made available to enterprise customers in phases, beginning later this fall. Until EFS is available, eligible customers will be able to use Fable 5.1 with zero data retention.<\/p>\n

Safeguards. <\/em><\/strong>We\u2019ve improved our safeguards to reduce false positives (where the system flags benign content). In cybersecurity, our newest safeguards block 60% fewer false positives than before. In part, this is because Fable 5.1 can now be used to discover software vulnerabilities\u2014though not to develop exploits for them. In biology, we\u2019ve established an access program, developed in partnership with the US government, to enable access to Claude Mythos 5.1\u2019s advanced biology capabilities. We expect to open enrollment for scientists soon.<\/p>\n

A new performance frontier<\/h2>\n

Claude Fable 5.1 sets a new standard for coding, knowledge work, and long-running problem-solving tasks. The charts below show that Fable 5.1 is capable of much higher performance than its predecessor, Fable 5. And when set to Low or Medium effort, Fable 5.1 achieves results similar to or better than Fable 5\u2019s at a much lower cost. (Note that Fable 5.1 defaults to High effort in Claude Code, and to Medium in Claude Cowork and on Claude.ai.)<\/p>\n

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