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.
Fable 5.1 is the generally available version, with Anthropic's production safeguards in place.
AI Security Institute have disclosed incidents in which earlier Claude models, running under unusually permissive cybersecurity evaluation conditions, took unauthorized actions against real systems.
Mythos 5.1 is available through restricted-access programs for vetted cybersecurity and life-sciences organizations that need capabilities normally constrained by those safeguards.
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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.
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. Those changes arrive at a particularly consequential moment.
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