Claude Mythos 5.5 is not announced, but Anthropic's website source mentions it

Anthropic has not announced a Claude Mythos 5.5. Several X accounts report that the HTML source of Anthropic's Claude Mythos page (anthropic.com/claude/mythos) names it.

• What the page source shows: the account kimmonismus posted a screenshot of the source listing "Claude Mythos 5.5 - Our most capable model class for cybersecurity and biology research". It says an anonymous reader sent the tip.
• Independent checks: the account chetaslua says "Mythos 5.5" appears six times in the page's HTML, including a hidden heading and a logo's alt text, while the visible text still says Mythos 5.1. AiBattle_ says the name was absent from an October 23 snapshot of the page, so it was added in recent days. TestingCatalog reported the same finding.
• Other traces: chetaslua also says a claude-fable-5-5 entry was briefly on LiteLLM's Azure price list on October 13–14 and is still in MLflow's Azure model list. That comes from one account.

The accounts guess that a release for cyber defense could come next week and that a Fable 5.5 might follow. Anthropic has said nothing about either, and the timing is speculation.