Claude Mythos 5

Claude Mythos 5, released June 9, 2026 alongside Claude Fable 5, is the unrestricted half of a pair: the same Claude 5 frontier model offered to vetted organisations without the dual-use safety measures that let Fable 5 ship to everyone. Where Fable 5's classifiers flag a request touching cybersecurity, biology and chemistry, or model distillation and hand it to a less capable model, Mythos 5 answers it. It succeeded Claude Mythos Preview, which Anthropic disclosed on April 7, 2026 and put in the hands of roughly forty organisations under Project Glasswing — a group that had grown to about 150 across more than fifteen countries by June 2.

Anthropic published no separate benchmark table for the model. In testing by the UK AI Security Institute the Mythos line ranked first for cyber capability, ahead of Claude Opus 4.6 and a tie between GPT-5.4 and GPT-5.3-Codex, and Anthropic reported vulnerabilities found across every major operating system and browser, including 271 in Firefox that Mozilla confirmed. Access moved faster than capability: on June 12, 2026 the US government barred both Mythos 5 and Fable 5 to non-US nationals on national-security grounds, a restriction the Department of Commerce lifted on June 30. On August 21, 2026 Anthropic opened Mythos 5 to Claude Enterprise customers as scans inside Claude Security, with integrations into partner security products to follow — the same deliver-the-output-not-the-model posture OpenAI took with Daybreak and Google with the CodeMender pilot. Claude Mythos 5.1 succeeded it on September 1, 2026, again as the Glasswing-only twin of that day's Fable release.