Anthropic's New Claude Comes in Two Versions, and Only One Is Public
Anthropic released two versions of the same language model on Sept. 1: Claude Fable 5.1, generally available, and Claude Mythos 5.1, which carries looser safeguards and is offered only to vetted cybersecurity and life-science organizations in the United States.
The company describes the two as the same model with different levels of safeguards. Mythos 5.1 is offered through a Cyber Verification Program for defensive security work and a Life Sciences Verification Program that Anthropic says it built in partnership with the U.S. government and has already enrolled its first participants in. Access is currently limited to a set of U.S. organizations.
Pricing moved on one line. Anthropic says cache reads, where the model reuses context it has already processed, now cost 75% less, at $0.25 per million tokens, while input and output prices stay at $10 and $50 per million tokens. The company estimates that this reduces the cost of typical workloads by about 25%, and by up to about 45% for context-heavy, tool-heavy work; the estimate comes from its own measurement of four weeks of usage in August 2026.
Fable 5.1 may now be used to identify software vulnerabilities but not to develop exploits for them; penetration testing, exploit generation and binary-based vulnerability scanning are still redirected to Anthropic's Opus models. Claude Code users should see around 60% fewer cyber safeguard interventions per session than with Fable 5.
Every performance figure published with the release is Anthropic's own, measured in Anthropic's setup, and no independent evaluation of either model exists. The company reports 52.6% for Fable 5.1 on the Terminal-Bench-Science 0.1 benchmark against 24.7% for Fable 5, and 55.8% for Fable 5.1 and 60.9% for Mythos 5.1 on Terminal-Bench 4.0.
Fable 5.1 is available on Anthropic's API and on Amazon Web Services, Google Cloud and Microsoft Azure. A data-retention scheme announced alongside it, Enterprise Frontier Safeguards, keeps customer data on the customer's own cloud infrastructure and begins rolling out in phases this fall.
