anthropic disclosed it withheld a model called mythos from public release because researchers showed it could break through strong cybersecurity defenses and find thousands of vulnerabilities in open-source code; it released a deliberately weakened public version, claude fable 5, and restricted mythos's ability to self-improve. a study reports mythos compresses the time to weaponize known-but-unpatched flaws from weeks to hours. the structurally unusual part is a frontier lab publicly stating its own model is too capable at offensive cyber to release as-is.