Claude Fable 5 Jailbreak Exposes Limits of Anthropic’s Cyber Guardrails

Anthropic launched Claude Fable 5 as the public, safeguarded version of its new Mythos 5 model, describing both as configurations of the same underlying system and positioning Mythos 5 as its strongest cyber-capable model to date. In its system card, Anthropic said Fable 5 uses classifiers and fallback behavior to divert high-risk requests to Claude Opus 4.8, keeping its cyber performance roughly in line with the older model while Mythos 5 remains restricted to vetted partners through Project Glasswing. The company assessed Mythos 5 as a Tier 1 cyber offense risk, meaning it can materially assist offensive operations but does not autonomously conduct adaptive cyber campaigns.

Soon after release, researcher Pliny the Liberator reportedly bypassed Fable 5’s safeguards using a multi-agent jailbreak that combined Unicode obfuscation, long-context manipulation, narrative framing, and decomposition of harmful requests, allegedly eliciting exploit-development and chemistry-related guidance and exposing a leaked system prompt on GitHub. The incident undercut Anthropic’s claim that more than 1,000 hours of external testing had found no universal jailbreaks, while also intensifying criticism from security researchers who said Fable 5’s guardrails were already so broad that they blocked benign work such as reading security blogs, reviewing code, and writing secure software. Anthropic had not publicly responded to the jailbreak claims at the time of reporting.