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[{"h":"CONFIRMED","p":"Anthropic launched Claude Fable 5.1 and Claude Mythos 5.1 as two access tiers for the same model. Fable 5.1 is generally available. Mythos 5.1 is reserved for trusted access programs in cybersecurity and life sciences. The central change combines stronger performance for coding, knowledge work and long tasks with a more concrete dispute over price, privacy and who may operate less restrictive safeguards."},{"h":"Lower cost for long running agents","p":"In typical use, Fable 5.1 costs about 25% less than Fable 5 because cache reads are cheaper. In highly agentic work, the savings can reach roughly 45%. For a small or midsize business or an automation professional, the practical effect appears when an agent revisits documents, instructions and history over many stages. The gain is not merely a cheaper answer. It is the ability to keep longer workflows running without repeated context becoming the largest line on the bill."},{"h":"Privacy controlled by the customer","p":"Anthropic also introduced Enterprise Frontier Safeguards, or EFS, a privacy architecture intended to provide protection equivalent to zero data retention while keeping data on infrastructure controlled by the customer rather than Anthropic. The rollout will happen in phases starting in the Northern Hemisphere autumn and will begin with enterprise customers. Until then, eligible customers can use Fable 5.1 with zero data retention. That is a meaningful promise, but not yet a capability fully available to the whole market."},{"h":"Less restrictive safeguards, not no limits","p":"Cybersecurity safeguards have improved, with 60% fewer false positives according to Anthropic. Fable 5.1 can discover software vulnerabilities in defensive work, but it cannot develop exploits. Mythos 5.1 remains behind trusted access and concentrates more advanced capabilities in cybersecurity and biology. In biology, Anthropic developed an access program in partnership with the United States government. Enrollment for scientists is expected to open soon, without turning Mythos into an open product."},{"h":"The impact on automation teams","p":"The promise of a new bar for coding, knowledge work and long tasks matters more to teams that measure cost per task than to people comparing isolated answers. At Low or Medium effort, Fable 5.1 can match or outperform Fable 5 on suitable tasks at lower cost. Defaults vary by product: High effort in Claude Code and Medium in Claude Cowork and Claude.ai. The sensible decision is to test a representative workload, measure quality, latency and cost, and only then expand the migration."},{"h":"The real limit","p":"Mythos 5.1 is not available to every company, and EFS has not yet been fully delivered. Those two limits prevent the announcement from being read as a universal upgrade. Anthropic is offering general access to Fable 5.1 and conditional access to Mythos 5.1 while moving part of its privacy promise into a future implementation. The launch matters, but the gap between announced capability and accessible capability remains the point buyers must separate from marketing."},{"h":"Sources","p":"Anthropic, Introducing Claude Fable 5.1 and Claude Mythos 5.1
Anthropic, Enterprise Frontier Safeguards"}]