One day after Anthropic launched Claude Fable 5, cybersecurity professionals are publicly complaining that its guardrails are over-broad and blocking legitimate security work. IBM X-Force researcher Valentina Palmiotti documented that Fable "rejects any request that could be tangentially cyber related" โ including reading security blog posts and reviewing code. Tolmo AI founder Matt Suiche confirmed the model "assumes it is cybersecurity related work instead of software engineering best practices," blocking routine dev tasks. When the classifier triggers, the model pauses the session with a warning citing "safety measures flagged this message for cybersecurity or biology topics," then silently downgrades to Claude Opus 4.8 without disclosing the switch. Anthropic's mitigation is a Cyber Verification Program (similar to OpenAI's Trusted Access for Cyber) that grants approved professionals fewer restrictions โ but this requires pre-registration, meaning researchers without clearance hit walls immediately on a model Anthropic is billing as its most powerful public release. The research community is particularly frustrated because Fable was launched specifically as the accessible version of Mythos-class capability, yet its guardrails make it less usable for the security use case Anthropic's own marketing emphasises.