Users on X say some Claude Opus 5.5 requests are being routed to an unreleased model they call Fable 5.5. Anthropic has not announced the model, and the company’s Mythos webpage continues to list version 5.1.

A hidden “Mythos 5.5” label appeared in the source code of Anthropic’s Mythos page on November 1. On October 13, an entry called “claude-fable-5-5” was added to the LiteLLM catalog, with specifications copied from Fable 5.1. Both the hidden label and the catalog entry were later removed.

Separately, OpenAI’s “gpt-6.2-astra” appeared in Codex usage data for some accounts on November 1. OpenAI has not announced a GPT-6.2 model. The reports come as Anthropic prepares for its initial public offering, whose roadshow cannot begin before November 10.

The claims remain unconfirmed. They are based on user reports, a small number of showcase demonstrations, a catalog entry and a website label that appeared briefly. Each reported version number would represent an update to a line the companies already ship: Fable 5.1, Mythos 5.1 and GPT-6 Astra. Fable is Claude’s top commercial tier and Anthropic’s most expensive flagship model for software development and office work. Mythos is a restricted model offered to a select group of clients, including government and cybersecurity organizations, because of its capabilities.

Reports of an unreleased Claude model

Anthropic filed for an IPO—the first sale of its shares to the public—on October 26. The company reported $16.29 billion in revenue for the first half of 2026, compared with $1.16 billion a year earlier, a roughly fourteenfold increase.

Anthropic launched Fable 5 on June 9, then took it offline for 18 days under U.S. export controls, which are government restrictions on who may use certain technologies. The company released Fable 5.1 on September 1. Opus 5.5 followed on September 22, and Anthropic says that model performs at the level of Fable 5.1 on most tasks at a fraction of the price. The span from Fable 5 to Opus 5.5 covered less than four months.

The speculation began on October 1, when users posted screenshots showing Claude labeled Fable 5.1 alongside outputs they considered stronger than usual. On October 11, the pseudonymous user Chetaslua—known for sharing information about unreleased models and identifying changes in outputs, logs and other signals before official announcements—wrote that “Claude Opus 5.5 is routing to a new model.” The account added that the model could be Fable 5.5 or a new Opus model.

“🚨 Claude Opus 5.5 is routing to a new model < this can be fable 5.5 or new opus i have no idea > Drew every frame of this animation using JS. The life of a Human Two Prompts : literally 4 lines Time : 2 Hours Tokens : 27 Miliion Thinking Effort : Max” — Chetaslua (@chetaslua), October 11, 2026: https://x.com/chetaslua/status/2109244702235275680?ref_src=twsrc%5Etfw

In this context, routing means that a request is sent to a different model behind the scenes. Some users suspect Anthropic may be conducting an A/B test, showing a new version to a portion of users while retaining the existing model name in the interface.

AI developer Shikhar, who is known for creating a Spiderman game through “vibe coding” that went viral on social media, also raised the possibility. “Is this Fable 5.5? Saw posts that Opus is routed to Fable 5.5, so I decided to make a complex 2D animation with it,” Shikhar wrote on X alongside an animation produced with Opus. The user said the result was of higher quality than the model’s usual output.

AI community account Min Choi included the clip in a roundup of examples, describing it as “A 30-second Spider-Verse-style 2D animation: a character on a city street, then a web-swing over the rooftops.” The post said the animation was contained in one HTML file using WebGL, GLSL shaders and 2D Canvas, and was made with Opus while it was reportedly routing to Fable 5.5. Shikhar presented the question as speculation rather than a confirmed finding.

“10. A 30-second Spider-Verse-style 2D animation: a character on a city street, then a web-swing over the rooftops. He says it's one HTML file (WebGL, GLSL shaders and 2D Canvas), made with Opus while it was reportedly routing to Fable 5.5. He asks "Is this Fable 5.5?" himself.… ” — Min Choi (@minchoi), November 2, 2026: https://x.com/minchoi/status/2117363258269315182?ref_src=twsrc%5Etfw

Mythos 5.5 label appears in source code

On October 13, an automated update to LiteLLM, an open-source tool that developers use to connect to multiple AI providers, added “claude-fable-5-5” after Microsoft’s Azure cloud catalog listed the model as generally available. The LiteLLM entry copied its price and limits from Fable 5.1. Its notes said that no source had provided a price, capability or text-length limit for Fable 5.5. The entry was later removed. The related LiteLLM change is available here: https://github.com/BerriAI/litellm/pull/46201

Kim Isenberg, who also uses the pseudonym “Chubby” and is one of the most active and widely followed accounts in the AI community, reported on Sunday that “Mythos 5.5” had appeared in the HTML source code of Anthropic’s official website. The account said the information came from an anonymous tip sent by email.

“Mythos 5.5 is already listed HTML source code of Anthropic's official website Thanks to the anonymous person who sent me the tip via email 🙂” — Chubby♨️ (@kimmonismus), November 1, 2026: https://x.com/kimmonismus/status/2116914141017370972?ref_src=twsrc%5Etfw

Mythos is described as the restricted counterpart to Fable: the same underlying model with lighter safety filters, reserved for vetted cybersecurity and biology researchers. Chetaslua said the “Mythos 5.5” label appeared six times in the hidden code, including in an invisible headline and in the logo’s alt text, the description attached to an image for screen readers. A check of the live page on Monday showed only Mythos 5.1, which Anthropic released alongside Fable 5.1 in September.

GPT-6.2 Astra appears in Codex data

OpenAI has also become the subject of reports about an unannounced model. “OpenAI is quietly A/B testing a new variant called GPT-6.2 Astra, and it’s already showing up in Codex usage analytics,” a user known as TokenGremlin wrote on Sunday. “The model picker still says GPT-6 Astra, but the usage breakdown for some accounts now lists gpt-6.2-astra separately.”

“OpenAI is quietly A/B testing a new variant called GPT-6.2 Astra, and it's already showing up in Codex usage analytics. The model picker still says GPT-6 Astra, but the usage breakdown for some accounts now lists gpt-6.2-astra separately. Multiple users are seeing the exact same…” — Token Gremlin (@TokenGremlin), November 1, 2026: https://x.com/TokenGremlin/status/2116834891744600291?ref_src=twsrc%5Etfw

Codex is OpenAI’s coding assistant. A screenshot circulating on X showed gpt-6.2-astra accounting for 19.8% of one account’s usage, compared with 80% for gpt-6-astra. OpenAI released GPT-6 Astra on September 3 and has not announced a 6.2 model.

“GPT-6.2 Astra is already showing up in codex usage analytics, as per Token Gremlin,” Chubby wrote, predicting a release “very soon, in response to Fable 5.5 and Anthropic’s IPO.” The account added, “Next week is going to be exciting.”

Anthropic’s IPO timetable

Anthropic’s S-1 registration statement, which companies file with U.S. regulators before selling shares, lists the Nasdaq ticker ANT. It names Goldman Sachs and Morgan Stanley first among its underwriters, the banks responsible for selling shares to investors. The filing does not yet specify the number of shares or the price range.

Anthropic initially filed confidentially in June. Under SEC rules, the paperwork must be public for 15 days before the roadshow, the marketing period during which banks present the offering to prospective investors. That requirement places the earliest possible start of Anthropic’s roadshow on November 10. For now, the two traces at the center of the reports—the LiteLLM entry and the source-code label—have both been removed, and any confirmation would have to come from Anthropic or OpenAI directly.

Original source: https://decrypt.co/382390/claude-mythos-fable-5-5-gpt-6-2-astra-leaks

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