tl;dr: Anthropic's newly released Fable model, a public version of its Mythos cybersecurity model, is drawing complaints from security researchers who say its guardrails are overly aggressive and keyword-based, blocking even benign requests like code reviews or reading blog posts. When triggered, Fable falls back to Claude Opus 4.8, citing flagged "cybersecurity or biology topics." Researchers can apply to Anthropic's Cyber Verification Program for looser restrictions, similar to OpenAI's Trusted Access for Cyber.