Mythos 5.5 appeared in Anthropic’s page code. Then the name changed back.

An Internet Archive capture of Anthropic’s Mythos page from 31 October contains “Claude Claude Mythos 5.5” in a hidden heading and image alt text. We inspected the archived HTML itself. The rest of the page still describes Mythos 5.1, including its access and pricing.

@kimmonismus reported the name on 1 November, crediting an anonymous tip; TestingCatalog amplified it and separately credited @SergioOSINT. A later archive capture, from 1 November at 20:05 UTC, reads Mythos 5.1 again. So does the live page checked on 3 November.

This is a concrete clue to a possible successor in Anthropic’s restricted research-model family. It is not evidence that 5.5 is available, better, or arriving on a particular day. A planned name, a publishing mistake and an abandoned change remain possible explanations; only Anthropic can settle which one applies.