According to users on X, Anthropic is quietly testing unreleased versions of Mythos and Fable, while OpenAI is set to soon launch GPT 6.2 Astra.
However, neither side has confirmed these reports. The rumors primarily stem from a few demonstrations, a single directory entry, and a hidden tag on a website—all of which were removed shortly after appearing.
Fable is Claude's highest commercial tier and Anthropic's most expensive, flagship model available for sale to the market, primarily designed for software development and office task automation. Mythos is a restricted model offered by Anthropic to a limited number of clients, including government agencies and cybersecurity leaders, due to its enhanced capabilities.
Anthropic filed its IPO application on October 26, marking its first public offering of shares. The company disclosed revenue of $16.29 billion for the first half of 2026, compared to $1.16 billion in the same period last year.
Anthropic launched Fable 5 on June 9, then took it offline for 18 days due to U.S. export controls—government restrictions on who can use a particular technology—and released Fable 5.1 on September 1. Fable 5.5 was launched on September 22, with Anthropic stating that it performs comparably to Fable 5.1 on most tasks but at a fraction of the cost.
This discussion began on October 1, when a user posted screenshots showing that Claude was still labeled as Fable 5.1, yet its outputs appeared stronger. On October 11, the pseudonymous user Chetaslua—who is known for sharing clues about unreleased models, detecting output changes, logs, and other signals of new models—posted: “Claude Opus 5.5 is being routed to a new model,” adding that this new model could be either Fable 5.5 or a new Opus model.
"Routing" refers to requests being silently sent to another model in the background. Some users suspect this is an A/B test, in which Anthropic shows a new version to a subset of users for comparison with the current version, while the old name remains displayed on screen.
AI developer Shikhar—who is known for creating a Spider-Man game that went viral on social media using vibe coding—wrote on X: “Is this Fable 5.5? I saw someone post that Opus was routed to Fable 5.5, so I decided to use it to create a complex 2D animation.” He attached a video animation made with Opus, which clearly exceeds the model’s typical output quality.
Another reputable leaker in the AI community, Min Choi, included the video in a compilation of examples: “A 30-second, 2D animated clip in the Spider-Verse style: characters standing on a city street, then swinging across rooftops on spider webs.” Shikhar framed the matter as a question rather than a conclusion, but the timing still fueled FOMO sentiment.
He stated that this is an HTML file (with WebGL, GLSL shaders, and a 2D canvas) created when Opus was allegedly routed to Fable 5.5. He asked himself: “Is this Fable 5.5?”
On October 13, the open-source tool LiteLLM, used by developers to connect multiple AI providers, underwent an automated update that added an entry named "claude-fable-5-5," following its listing as generally available in the Microsoft Azure cloud directory. This entry copied pricing and limits directly from Fable 5.1; however, its note stated that no source confirmed the pricing, capabilities, or text length limits for Fable 5.5, after which the entry was removed.
“Mythos 5.5 has appeared in the HTML source code of Anthropic’s official website,” wrote Kim Isenberg—who also goes by the pseudonym “Chubby,” one of the most active and prominent accounts in the AI community—on Sunday beside a screenshot of the Anthropic Mythos page code. “Thanks to the anonymous person who sent me the tip via email,” the account later posted.
Mythos is a restricted version of Fable, both built on the same underlying model, but Mythos has fewer safety filters and is exclusively available to vetted cybersecurity and biology researchers.
According to Chetaslua, the tag appeared six times in the page’s hidden code, within an invisible heading and the alt text of a logo—alt text being descriptive text attached to images for screen readers. A real-time inspection of the page showed that only Mythos 5.1 remains on the page, the version Anthropic released in September alongside Fable 5.1.
OpenAI also has its own AI FOMO.
User TokenGremlin wrote on Sunday: “OpenAI is quietly running an A/B test on a new variant called GPT-6.2 Astra, which has already appeared in Codex usage analytics.” He added: “The model selector still shows GPT-6 Astra, but usage details for some accounts now list gpt-6.2-astra separately.”
Codex is OpenAI's programming assistant. A screenshot circulating on X shows that, for a certain account, gpt-6.2-astra accounts for 19.8%, while gpt-6-astra beside it accounts for 80%. OpenAI has not yet announced a 6.2 version, having released GPT-6 Astra on September 3.
Chubby wrote: "GPT-6.2 Astra has appeared in Codex usage analytics, as Token Gremlin mentioned." He predicts this version "will be released soon in response to Fable 5.5 and Anthropic's IPO."
"Next week will be exciting," Chubby wrote.
Anthropic’s S-1—the registration filing submitted to U.S. regulators before selling shares—lists the Nasdaq ticker symbol ANT and names Goldman Sachs and Morgan Stanley as the initial underwriters, the banks that will sell shares to investors. However, the number of shares and price range remain unspecified.
Because Anthropic initially submitted its application confidentially in June, SEC rules require related documents to be made public at least 15 days before the roadshow. The roadshow is the marketing tour during which banks present the stock to investors, so the earliest possible start date is November 10.