A reference to an unannounced Claude Mythos 5.5 has been spotted on Anthropic's official Mythos webpage, TestingCatalog reported on November 1. The line describes it as “Our most capable model class for cybersecurity and biology research.” The mention is not present in the page's October 23 snapshot, so it was added in the last few days — but it is only a version number: no specifications, pricing, release date or announcement accompany it.

Alongside the sighting, multiple reports say Anthropic has been A/B testing updated Claude Fable 5.5 checkpoints with selected users, with early impressions described as state-of-the-art. Those comparisons are anecdotal and no public benchmarks exist. Anthropic released the previous Mythos and Fable variants together — Mythos 5.1 restricted to verified cybersecurity and life-sciences researchers, Fable 5.1 the same underlying model with extra safeguards for general use — so a paired launch is plausible, and some testers speculate a launch could come as early as Tuesday. That timing is speculation, not a schedule.

This stays a rumor: Anthropic has confirmed nothing, and a website string is not a launch. It is, however, the strongest new data point yet for the Wire's long-running Fable 5.5 story, first logged on October 13 from Azure and LiteLLM listings — that rumor remains unresolved. The sighting also lands a week after Anthropic filed for a Nasdaq listing under the ticker ANT on October 26, when a major model release would draw extra attention.