Anthropic released Claude Fable 5.1 and Mythos 5.1, calling them the world's most advanced models for coding and knowledge work. Fable 5.1 is generally available, while Mythos 5.1 ships through trusted-access programs with safeguards built for cybersecurity and the life sciences.

Fable 5.1 costs about 25% less than Fable 5 for typical token-billed workloads, with heavily agentic tasks saving up to roughly 45%, driven by cheaper cache reads. It also debuts Enterprise Frontier Safeguards, giving enterprises zero-data-retention privacy by keeping data on customer-controlled cloud infrastructure rather than Anthropic's.

On benchmarks it sets a new frontier, scoring 52.6% on Terminal-Bench-Science against Fable 5's 24.7%, and topping Opus 5 and GPT-5.6 on agentic coding and reasoning. In testing at hedge fund Millennium, it traced the root cause of a years-old crash that no engineer or rival model had cracked.

Anthropic also framed the launch as an early glimpse of AI-driven science: Mythos 5.1 designed protein binders with roughly 10 times higher affinity than the best prior competition entries, while Fable 5.1 built a sharper elevation map covering a third of Venus from decades-old NASA radar data.

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