Claude Mythos 5.1

The restricted sibling of Claude Fable 5.1, announced Sep 1, 2026. It is the same underlying model as Fable 5.1 but ships with more permissive safeguards for vetted users, available only through Anthropic's trusted-access programs — the Cyber Verification Program (defensive security work) and the Life Sciences Verification Program (professional biology R&D, developed with the US government). Anthropic reports it has the strongest cyber capabilities of any model it has released (still within the lower risk tier of its Frontier Compliance Framework) and scores 60.9% on Terminal-Bench 4.0 (vs 55.8% for safeguarded Fable 5.1). Also powers Claude Security's codebase vulnerability scanning. Access is currently limited to a set of US organizations. Not publicly token-billed, so list pricing is not recorded.