Anthropic Launches Claude Fable 5.1 and Restricts Claude Mythos 5.1 to Trusted Access

Anthropic has launched Claude Fable 5.1 for general use and Claude Mythos 5.1 through trusted access programs, pairing stronger coding and research performance with lower cache-read costs and revised safeguards. The release is dated September 1, 2026, and continues Anthropic’s strategy of separating broadly available capabilities from higher-risk access for cybersecurity and life-science work. Anthropic’s announcement describes Fable 5.1 as its model for complex, long-running coding and knowledge-work tasks, while Mythos 5.1 is aimed at cyberdefenders and life scientists.