Claude Fable 5.1 has a twin — and only one is public.

Anthropic's September 1 launch has one unusual detail: Claude Fable 5.1 and Claude Mythos 5.1 are the same underlying model, while the safeguards and access rules differ. 1

Fable 5.1 is generally available. Mythos 5.1 is limited to trusted programs for vetted cybersecurity and life-sciences work. 2 Anthropic also cut Fable's cache-read price by 75%, to $0.25 per million tokens. Anthropic estimates about 25% lower cost on typical workloads and up to about 45% lower cost on highly agentic work. 13

The performance figures come from Anthropic's own evaluations, with production safeguards enabled. The research examples are early company-reported results. The clearest product fact is the access split: one model, two safeguard envelopes.