On November 1, the code of Anthropic’s Mythos web page carried a label, “Mythos 5.5,” that was hidden. The find fed claims that some requests for Claude Opus 5.5 are hitting an unreleased mod
On November 1, the code of Anthropic’s Mythos web page carried a label, “Mythos 5.5,” that was hidden.
The find fed claims that some requests for Claude Opus 5.5 are hitting an unreleased model that users call Fable 5.5, which Anthropic has not announced.
An X user known as Chubby, Kim Isenberg, shared a screenshot of the page’s HTML on Nov. 1, citing an anonymous email tip. “Mythos 5.5 is already listed HTML source code of Anthropic’s official website,” Isenberg wrote.
Chubby’s post showing “Claude Mythos 5.5” in the code of Anthropic’s Mythos page, posted by @kimmonismus on X on November 1, 2026.
The X post has garnered over 3,600 likes. Pseudonymous leaker Chetaslua also noted that the label was mentioned six times in hidden code.
They were in an invisible headline and the alt text of the logo. The live page checked on November 2 only showed Mythos 5.1, the version that shipped with Fable 5.1 in September.
Fable is Anthropic’s flagship commercial Claude product, marketed for coding and office work. Mythos is a restricted AI model accessible to a select group of customers, like government and cybersecurity customers.
The routing talk began on October 1. Users posted what looked like stronger than usual outputs from Claude, still tagged Fable 5.1.
“Claude Opus 5.5 is routing to a new model,” Chetaslua wrote on October 11. The leaker said that the new model could be Fable 5.5 or a new Opus.
On October 13, a second hint was discovered. Following the general availability of “claude-fable-5-5” in Microsoft’s Azure catalog, the entry was automatically added to LiteLLM.
Builders use LiteLLM, an open-source tool, to connect with a variety of AI providers. Entry copied the limits and prices from Fable 5.1, but it was later removed.
Anthropic put out Fable 5 on June 9, but because of U.S. export controls, they had to take it offline for 18 days. Then came Fable 5.1 on September 1 and Opus 5.5 on September 22.
Anthropic says Opus 5.5 can do most of the same things as Fable 5.1 for a lot less money. “OpenAI is quietly A/B testing a new variant called GPT-6.2 Astra, and it’s already showing up in Codex usage analytics,” TokenGremlin wrote on November 1 on X.
A Codex screenshot shared on X showed that gpt-6.2-astra accounted for 19.8% of one account’s usage and gpt-6-astra for 80%.
OpenAI has not announced a model named 6.2. OpenAI released GPT-6 Astra on September 3.
Isenberg forecast a GPT-6.2 Astra launch “very soon, in response to Fable 5.5 and Anthropic’s IPO.”
On October 26, the company made its IPO filing public, under the Nasdaq ticker ANT, Cryptopolitan reported.
Anthropic’s revenue for the first half of 2026 was $16.29 billion, up from $1.16 billion a year earlier.
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