Anthropic's official website source code hid six instances of Mythos 5.5, wiped to zero hours later

On the afternoon of November 1, someone counted the page source of anthropic.com/claude/mythos and found that "Claude Mythos 5.5" appeared six times. The heading visible on the page was still at 5.1.

In the screenshot, the locations are visible: a hidden h1, the logo's alt text, and several SVG references. Normal visitors would not see these; they would only see the 5.1 introduction, pricing, and announcement list.

The person who posted this was Chubby (@kimmonismus). He said in the tweet that the clue came from an anonymous netizen. In the comments, someone checked Ctrl+U and confirmed the source code really said this, and added: this is more reliable than most model rumors; at least anyone can see it for themselves.

But the source code was not the only trace.

On October 13, there was a commit in the LiteLLM GitHub repository titled feat(azure): add azure_ai/claudia-fable-5-5 (#46201), which added claude-fable-5-5 to the Azure price and context window table.

The next day, October 14, another commit removed it. The reason was written plainly: Azure does not list. In other words, Azure did not have this entry on its side, so the sync had to be fixed.

It is still present today in MLflow's Azure model catalog: 1M context, $10 input and $50 output per million tokens, exactly the same price as 5.1.

However, a netizen looked through MLflow's build scripts and pointed out that this catalog is generated from LiteLLM's model_prices_and_context_window.json. So it does not count as a second independent sighting; it is a residual trace left after the October 13 entry was synced.

Chetslua's original tweet put it more clearly: "Not just Mythos 5.5. Fable 5.5 already hit Azure." Both models showed activity.

Anthropic's own page clearly explains the relationship between Fable and Mythos. Fable 5.1 and Mythos 5.1 are the same underlying model; the difference is that Fable comes with cybersecurity and biology-domain guardrails and can be used by a broader population. Mythos goes through a verification channel and is only provided to approved security teams and life-science research institutions.

So Fable 5.5 appearing in the price list means Mythos 5.5 is also on the way. This also explains why the hidden headings on both pages in the source code were changed together. Someone in the comments mentioned that the Fable page has the same hidden header, saying Fable 5, while the page itself is still at 5.1.

Then came the cleanup.

By 22:29 India time, someone checked again, and the Mythos page source had been restored to 5.1, with "Mythos 5.5" appearing 0 times. From discovery to complete cleanup, it took only a few hours.

Cryptobriefing also followed up on this news on November 1, with the headline Anthropic hints at Claude Mythos 5.5 for cybersecurity and biology research.

The comments split into two camps.

One camp saw it as a signal of an imminent release: no one would rush to scrub source code like this for a piece of fake news. The other camp poured cold water on it, saying hidden HTML and third-party price lists are only clues, and a withdrawn entry actually shows that Azure did not list it; it will not count until an official model ID or release notes land.

Both judgments make sense. The confirmable facts are: it was added to the price list on October 13 and removed on October 14; it appeared six times in the source code on November 1 and was cleared that night. As for the release date, Anthropic has still not issued any announcement.

There is one detail worth remembering. LiteLLM's price list usually runs ahead of release announcements. Someone in the comments said that when claude-fable-5-5 reappears in it, you will probably be able to see how much it costs per million tokens before the official word comes out.

The question now is whether this cleanup was so fast because a release is imminent, or because someone was patching a leak.