Mythos was introduced in April as part of Project Glasswing, a partnership among top-tier tech organizations and Anthropic formed to find and fix vulnerabilities in internet infrastructure. It was restricted to only certain organizations because a tool that can find previously unknown vulnerabilities to fix them can also be used to find previously unknown vulnerabilities to exploit them.

Also: Apple, Google, and Microsoft join Anthropic's Project Glasswing to defend world's most critical software

Mythos and Glasswing are far more powerful than Anthropic's Claude Security tool, which is designed to run in Opus. Still, Claude Security can scan a codebase and help find some issues. But then, earlier this week, Anthropic announced and released Fable, technically "Fable 5," which is effectively a muzzled version of Mythos.

Anthropic was clear that Fable would not support certain risky avenues of research in cybersecurity, biology, and chemistry.

Also: Anthropic's new Claude Security tool scans your codebase for flaws - and helps you decide what to fix first

"Jailbreak-resistance claims should be viewed with appropriate caution," she says. The results "represent a point-in-time assessment. Attackers continuously adapt," Sally Vincent, a senior threat research engineer at Exabeam (a security analytics firm), said via email.