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

It's only the first day of September 2026, but the month and fall season are already off to the races in AI land, as Anthropic has just released its latest and most powerful large language models yet — Claude Fable 5.1 and Claude Mythos 5.1. The two names refer to the same underlying model. Fable 5.1 is the generally available version, with Anthropic’s production safeguards in place. Mythos 5.1 is available through restricted-access programs for vetted cybersecurity and life-sciences organizations that need capabilities normally constrained by those safeguards.For enterprise buyers, however, the release is about more than another round of benchmark gains. Anthropic is simultaneously changing the economics of running persistent agents, reducing the cost of cached context by 75%, and introducing a new security architecture called Enterprise Frontier Safeguards, or EFS, designed to let organizations retain monitoring data inside infrastructure they control.Those changes arrive at a particularly consequential moment. Over the past several weeks, Anthropic and the U.K. AI Security Institute have disclosed incidents in which earlier Claude models, running under unusually permissive cybe…