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.