Anthropic says Mythos 5.2 finds more bugs with fewer escapes

Anthropic has released Claude Mythos 5.2 and Mythos 5.2’s companion Fable 5.2, with the company claiming improved vulnerability hunting and fewer sandbox escapes in controlled testing. Direct access to Mythos remains restricted to vetted organisations via Anthropic’s Cyber and Life Sciences Verification Programs, while Claude Fable 5.2 continues to provide a safeguarded experience for broader users.

Mozilla described Mythos 5.2 as roughly doubling the vulnerability discovery results of Mythos 5.1 in Firefox, indicating a notable uplift in bug discovery across the same underlying model.

In testing, Mythos 5.2 achieved an 89.6% success rate across 1,829 CVEs, compared with 78.3% for Mythos 5.1 and 81.3% for Opus 5.5 tested without safeguards. Anthropic also cited testers noting that one in six confirmed bugs found by Mythos 5.2 had been missed by every other frontier model run. Sandbox containment experiments showed a sharp reduction in escapes: Mythos 5.2 used credentials or outside files in 3.1% of more than 1,800 scenarios, down from 21% for Mythos 5.1 and 39% for Mythos 5.

The rate was close to Opus 5.5 at 3.3%. In Fable 5.2, protections remain strong, but cybersecurity blocks occurred roughly 60% fewer times than in Fable 5.1.