What is Claude Mythos 5?

Claude Mythos 5 shares the same underlying model as Claude Fable 5, both belonging to Anthropic's Mythos tier, which sits above the Opus tier in capability.

The two differ mainly in safety posture: Fable 5 includes additional safety measures specific to biology, cybersecurity, and LLM R&D, while Mythos 5 itself is not currently available to the general public and is instead used by a small number of trusted organizations as part of Anthropic's Project Glasswing.

Public benchmark comparisons note that Claude Opus 5 trails Mythos 5 specifically on certain cybersecurity evaluations, even though it approaches Fable 5's general capability on many other tasks.

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