Image shows "Mythos 5.5" in official website source
An image was posted on X purporting to show that the name "Claude Mythos 5.5," seen as the next version of "Claude Mythos," which Anthropic calls its most capable model for cyber defense and life sciences research, appeared in the HTML (the code underlying the page) of the company's official website. The AI news account kimmonismus posted it on Nov. 1 (12:23 a.m. Nov. 2 Japan time), saying the information came in an anonymous email. According to FourWeekMBA, the image showed the source of the Mythos product page, with "Claude Claude Mythos 5.5" written in a heading that is not displayed on screen and in the alternative text of the logo image. Anthropic has made no announcement, and there is no information on performance, pricing or release timing.
Mythos 5.5 is already listed HTML source code of Anthropic's official website.
Official page still shows the current "5.1"
The content of the image has not been confirmed on the official page. When FourWeekMBA retrieved the page twice at 5:31 p.m. on Nov. 1 (UTC; 2:31 a.m. Nov. 2 Japan time), "Mythos 5.1" appeared 43 times, while "Mythos 5.5" did not appear once. When ORIENS retrieved it at 11:47 a.m. Nov. 3 Japan time, both the heading and the alternative text read "Claude Mythos 5.1," and there was no mention of "Mythos 5.5."
On the other hand, developer chetaslua posted on Nov. 1 that "Mythos 5.5" appeared six times in the HTML, and another user wrote that Anthropic had removed the relevant text (according to China's 36Kr). According to U.S.-based TestingCatalog, a version of the page saved on Oct. 23 contained no mention of 5.5. It remains possible that it was displayed for only a few hours, but whether the image captured the live page cannot be confirmed.
Mythos and Fable are the same model, released the same day both times
Mythos is offered only to vetted organizations as a model strong in defending against cyberattacks and in life sciences research. Anthropic does not release it to the public because of the risk of misuse, and instead offers "Claude Fable," the same model with added safeguards for the cyber and biological fields, to the general public. It released Mythos 5 and Fable 5 on the same day on June 9, and Mythos 5.1 and Fable 5.1 on the same day on Sept. 1. If Mythos 5.5 exists, past practice would mean the public "Fable 5.5" comes out on the same day.
- Mythos's existence revealed through a leaked blog draftReport
- Mythos Preview announced and offered to a limited set of companiesOfficial
- Mythos 5 and Fable 5 released simultaneouslyOfficial
- Mythos 5.1 and Fable 5.1 released simultaneouslyOfficial
- "claude-fable-5-5" appears in LiteLLM settings, removed the next dayLeak
- Post says Fable 5.5 will be released the following week. It was notRumor
- "Fable 5.5" listed as co-author of external codeLeak
- Anthropic files registration statement to go publicOfficial
- Image shows "Mythos 5.5" in source of Mythos pageLeak
Name of the public Fable 5.5 also circulated early
The name Fable 5.5 has already been spotted in several places. On Oct. 13, "claude-fable-5-5" appeared in the settings of LiteLLM, open software for calling multiple AI models together, and was removed the next day (according to FourWeekMBA). According to Decrypt, the entry was added automatically based on a list from Microsoft's Azure cloud, and its pricing and limits copied the values for Fable 5.1. On Oct. 20, a post said that the name "Fable 5.5" had been left as a co-author in the public code of an external development tool.
Speculation about the release timing has already missed once. On Oct. 18, a post said that a test version of Fable 5.5 had been distributed to partner companies and would be released the following Tuesday or Wednesday, but there was no release that week. This time, chetaslua has also posted expecting a release on Tuesday, Nov. 3, but the only basis is the name's appearance in the site's code.
Pricing fell to $10 for input with Mythos 5
Mythos pricing has dropped sharply from the first version. According to U.S.-based DataCamp, Mythos Preview's developer API pricing was $25 for input and $125 for output per million tokens. The current Mythos 5.1 starts at $10 for input and $50 for output, and Fable 5.1, the same model, is also priced at $10 for input and $50 for output. "Claude Opus 5.5," positioned for everyday work, costs $4 for input and $20 for output.
The Fable 5.5 pricing that briefly appeared in LiteLLM was the same as Fable 5.1, at $10 for input and $50 for output. chetaslua posted expecting pricing 20% lower than the previous generation, but Anthropic has not disclosed pricing for 5.5.
Fable 5.1 is available on the individual Pro, Max and Ultra plans and the business Team and Enterprise plans, and is offered by Amazon Web Services, Google Cloud and Microsoft Foundry in addition to Anthropic. Mythos 5.1 is limited to vetted cyber defense organizations and life sciences researchers, and requires consent to 30-day data retention for safety monitoring.
- Claude Mythos Preview
- Claude Mythos 5.1, Fable 5.1
- Claude Opus 5.5
Mid-IPO, and cyber stocks sold off in March
Anthropic filed a registration statement (S-1) to go public with the U.S. Securities and Exchange Commission on Oct. 26. According to Decrypt, revenue for January-June 2026 was $16.29 billion, up sharply from $1.16 billion a year earlier. Its Nasdaq ticker is "ANT," and under the rules, the earliest it can begin investor presentations is Nov. 10.
At rival OpenAI, a post on the same day, Nov. 1, said that an unannounced model name, "gpt-6.2-astra," had appeared in usage data for its AI coding tool Codex (according to Decrypt). OpenAI has not announced a new model either.
Reports about Mythos have moved cybersecurity stocks before. On March 27, when Mythos's existence was reported, CrowdStrike fell 7% and Tenable fell 9% (according to Forbes).
