Anthropic Website Source Code Hides Mythos 5.5, While the Page Text Still Says 5.1

A hidden Claude Mythos 5.5 has appeared in the source code of the /claude/mythos page on Anthropic's official website.

TestingCatalog's post showed screenshots: the top navigation on the page is normal, but the page copy still says Claude Mythos 5.1. The first things noticed were the hidden h1 and the alt text of the wordmark image; both say Claude Mythos 5.5. The page title and meta description still remain at Claude Mythos. Some users said that when they open the page they only see Claude Mythos, and 5.5 exists only in the hidden h1. This suggests the web rendering layer is not exposing it.

The news was reposted by TestingCatalog; the clue came from @SergioOSINT and was first posted by Chubby. Some joked that an anonymous clue sounds mysterious, but in fact you can inspect the source code with Ctrl+U, which already makes it more solid than most model rumors. Other users noted that the Fable page has the same kind of hidden header, reading Fable 5, while the actual page is Fable 5.1. This counterexample means the clue should be discounted: it could represent pre-release staging, or it could simply be a template that wasn't cleaned up properly.

The page copy describes Mythos 5.5 as the strongest model class for cybersecurity and biological research. The existing Mythos 5.1 announcement, pricing, and access methods are still on the page. Mythos 5.1 is available only to vetted cybersecurity defenders and life sciences researchers; security teams can apply for advanced cyber capabilities through the Cyber Verification Program, with pricing starting at $10 per million input tokens and $50 per million output tokens, and a default requirement of 30-day data retention for security monitoring. Fable 5.1 and Mythos 5.1 are the same underlying model, with cybersecurity and biological safeguards; biological safeguard interventions occur 85% less often than in Fable 5.

So the appearance of 5.5 in the official website's source code can indicate a release clue, but it cannot prove that it will open to ordinary users next week. The Mythos name has always been tied to restricted access. If 5.5 really is coming, three things matter more: who is eligible to apply, how the capability boundaries are drawn, and whether pricing and retention policies will change. For ordinary users, in the short term it is more likely that we will first see movement around Fable 5.5. For developers and security teams, the hidden title in the official website source code is only an appetizer; the threshold of the verification program is the main course.

One user paired it with a GIF of a black cat using a computer, joking that Inspect Element has become the new AI release calendar.

Source: TestingCatalog's post