Anthropic

Claude Mythos 5.1

Announced Sep 1, 2026

Different safeguards for approved researchers

What could it do?

Mythos 5.1 shares its underlying model with Fable 5.1, with safeguards adapted for approved cybersecurity and life sciences work.

Updated the restricted Mythos offering alongside Fable 5.1.

WHY IT MATTERED

Vetted researchers gained access through trusted programs.

Restricted access; sharing model weights does not make Fable and Mythos interchangeable products.

Sources checked Oct 7, 2026

Release facts were checked against the sources below. Performance claims belong to the developers; we have not independently tested these models.

No comparable ECI score is available for this release in our source snapshot. Missing scores are never estimated.