Claude Fable 5.1 and Claude Mythos 5.1

Anthropic has introduced Claude Fable 5.1 and Claude Mythos 5.1, the world's most advanced models for coding and knowledge work. These models are designed to offer cutting-edge research capabilities and contribute to scientific progress. Claude Fable 5.1 is now generally available, while Mythos 5.1 is accessible only through trusted access programs, with safeguards tailored for cybersecurity and life sciences applications.

The new Fable 5.1 comes at an estimated 25% lower cost than Fable 5 for typical workloads, thanks to reduced pricing on cache reads and significant savings (up to 45%) for highly agentic work. In terms of data retention, Enterprise Frontier Safeguards (EFS) allow customers full privacy, similar to a zero data retention policy, while still maintaining advanced ad-hoc use protection. EFS stores data in customer-controlled cloud infrastructure and will be gradually rolled out to enterprise customers beginning later this fall.

Improvements in safeguards have reduced false positives for benign content, with cybersecurity models blocking 60% fewer false positives compared to previous versions. In biology, Claude Mythos 5.1 has been granted access through a US government partnership to enable advanced biology capabilities. Anthropic expects to open enrollment for scientists to utilize this model soon.

Fable 5.1 is a significant upgrade over its predecessor, Fable 5, delivering superior performance in coding, knowledge work, and long-running problem-solving tasks. Benchmarks demonstrate its ability to achieve higher results than Fable 5 at a much lower cost, with the option to select lower effort levels in certain applications.

Fable 5.1 is designed to avoid shortcuts that compromise work quality and is capable of identifying root causes of software issues. For example, Millennium's testing revealed that Fable 5.1 successfully identified the cause of a rare crash in its internal systems, a problem that had eluded engineers for years.