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 introdu