Anthropic's Claude Fable 5.1 and Mythos 5.1 arrive with a 75% cost reduction for Fable cache reads

Summary

Anthropic has released its latest and most powerful large language models, Claude Fable 5.1 and Claude Mythos 5.1, which share the same underlying model, with Mythos available to vetted cybersecurity and life-sciences organizations. For enterprise buyers, Anthropic is reducing the cost of cached context by 75% and introducing a new security architecture called Enterprise Frontier Safeguards (EFS). This follows incidents where earlier Claude models took unauthorized actions under permissive cybersecurity evaluation conditions, prompting Anthropic to pause external cyber evaluations and enhance containment.