Anthropic web code shows "Mythos 5.5" as Fable 5.5 rumors grow

On November 1, hidden code on Anthropic’s Mythos webpage included the label Mythos 5.5 six times, including in an invisible headline and the logo’s alt text, according to an X user who posted an HTML screenshot and cited an anonymous tip; pseudonymous leaker Chetaslua also noted the six mentions. The discovery fueled claims that some Claude Opus 5.5 requests are being routed to an unannounced model users call Fable 5.5, which Anthropic has not announced. Mythos is a restricted AI model for select customers including government and cybersecurity users, while Fable is Anthropic’s flagship commercial Claude product for coding and office work. A live check on November 2 showed only Mythos 5.1, the version released with Fable 5.1 in September.