Claude Mythos 5.5 showed up on Anthropic's own site, then vanished

Anthropic hasn't announced Mythos 5.5, but its website briefly did. An Internet Archive snapshot of anthropic.com/claude/mythos taken at 21:49 UTC on October 31 contains "Claude Mythos 5.5" six times: in a hidden page header and in the alt text of both the desktop and mobile wordmarks. The visible copy on that same snapshot still talks about Mythos 5.1. The next snapshot, at 20:05 UTC on November 1, has no 5.5 at all, and the live page today only says 5.1.

So the name was in the page template and then got pulled. The first public report came from @kimmonismus on November 1, citing an anonymous tip, followed by testingcatalog, which also quoted a tagline, "Our most capable model class for cybersecurity and biology research". That tagline is not in the archived HTML we checked, so treat it as their claim.

Why it matters: Mythos is Anthropic's gated, highest-capability tier, and so far Mythos and the Fable line have shipped in pairs. Fable 5.5 has been a rumor since a cloud catalog sighting in mid October. A 5.5 wordmark in Anthropic's own production HTML is the strongest evidence yet that the next step is close, and it says the naming is decided.

What it does not tell us is anything about capability, pricing or access. Until Anthropic posts, this is a spotted name, not a launch. Archived page: https://web.archive.org/web/20261031214926/https://www.anthropic.com/claude/mythos