Anthropic Releases Fable 5.1 and Mythos 5.1

Summary

Anthropic has launched Claude Fable 5.1 and Claude Mythos 5.1, sharing the same underlying model but offering different access and safety controls. Fable 5.1 is broadly available, while Mythos 5.1 is restricted to approved organizations. The models improve agentic science, coding, knowledge work and long-running tool use. Prompt-cache read prices fell 75%, potentially lowering highly agentic workloads by up to 45%, though maximum-reasoning tasks may cost more due to longer outputs. Anthropic also presented applications in root-cause debugging, protein design, Venus mapping and GPU optimization, positioning the release around deployable high-value work rather than low-cost general chat.