Claude Mythos 5.1

Claude Mythos 5.1 is Anthropic's restricted-access counterpart to Claude Fable 5.1, announced on 1 September 2026 as "our newest Mythos-class model, with gains in cybersecurity and biology". Anthropic states plainly that the two are the same underlying model: Fable ships with safeguards for cybersecurity and biology and is generally available, while Mythos runs with those safeguards reduced for authorised users.

Access is not something a buyer can purchase. It runs through trusted-access programmes — the Cyber Verification Program for cyberdefenders and an invite-only Life Sciences Verification Program — and is currently limited to a set of US organisations, with expansion described as in progress. No credit card or usage tier moves an account into either. Pricing is $10 per million input tokens and $50 per million output, and a 30-day data-retention policy for safety monitoring applies by default.

Anthropic describes gains on cybersecurity, biology and healthcare benchmarks but publishes no figures on the page cited, and states no context window. Neither is recorded here rather than being inferred from Fable 5.1.