Clues on Anthropic's website point to new Mythos model

A mention of Claude Mythos 5.5 has appeared on Anthropic's official Mythos page, according to TestingCatalog. The change looks new and affects only the version number. There are no new specifications and no announcement from Anthropic.

Everything in this story is based on TestingCatalog, which draws some of its information from posts on X. Anthropic has not confirmed anything.

Hidden text in the page code

According to a post on X, "Mythos 5.5" appears six times in the page's HTML, including in a hidden heading and in the alt text for the logo (the text that describes an image). The visible text still mentions version 5.1.

Another post quotes the page's text as describing Mythos 5.5 as Anthropic's most capable model class for cybersecurity and biological research. The mention does not appear in a snapshot of the page from October 23, and according to one post, it was therefore probably added within the past few days.

The post about the six instances of "Mythos 5.5" in the HTML also notes that "claude-fable-5-5" was added to LiteLLM's price list for Azure on October 13 and removed again on October 14. Several reports also say that Anthropic is A/B testing updated Fable 5.5 versions with select users. No release date has been confirmed.

Mythos and Fable

Anthropic previously restricted Mythos 5.1 to verified researchers in cybersecurity and life sciences. Fable 5.1 was the same underlying model, but with extra safeguards for general use.

If Anthropic repeats the strategy, Mythos 5.5 may initially be reserved for select cyber defenders, TestingCatalog says. Anthropic has previously released both variants at the same time, so a joint launch is plausible, according to TestingCatalog.

Rumors about launch and performance

TestingCatalog writes that a release could come as early as next week, but that timing and access are unconfirmed. One user on X guesses Tuesday and hopes for a price 20 percent lower than the previous generation.

Some anonymous users describe Fable 5.5 as state-of-the-art, and some claim it beats Google's unreleased Gemini 4 Carbon. The comparisons are anecdotal, and there are no public benchmarks.

The stock market in the background

Earlier Mythos announcements triggered drops in cybersecurity stocks because investors doubted the future of traditional security software. TestingCatalog believes the market may react differently this time, because recent cyberattacks and vulnerability disclosures show continued demand.

On October 26, Anthropic filed for a Nasdaq listing under the ticker ANT. According to TestingCatalog, a major model release ahead of the market debut could attract investor attention.