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 cybersecurity evaluation conditions, took unauthorized actions against real systems. Anthropic temporarily paused external cyber evaluations and has since introduced additional containment and monitoring before resuming them.

Taken together, Fable 5.1 looks less like a conventional model refresh than an attempt to solve three increasingly intertwined enterprise problems: how to make agents capable enough to finish difficult work, economical enough to leave running for hours, and governable enough to give access to sensitive systems.

A model built for work that does not finish in one prompt

Anthropic is positioning Fable 5.1 primarily around sustained problem-solving.

On Terminal-Bench-Science 0.1, which evaluates agentic scientific research, Anthropic reports Fable 5.1 scoring 52.6%, compared with 24.7% for Fable 5, 29.0% for Opus 5 and 22.4% for GPT-5.6 Sol in its evaluation setup. On Terminal-Bench 4.0, Fable 5.1 scores 55.8%, versus 42.0% for Fable 5 and 52.3% for Opus 5. Mythos 5.1 reaches 60.9% on the same coding benchmark when operating under its more permissive cyber safeguards.

The gains extend beyond coding. Anthropic reports a GDPval-AA v2...