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On November 1, 2026, TestingCatalog reported that a hidden mention of Claude Mythos 5.5 was discovered on Anthropic's official website, with the related description being "the strongest model category for cybersecurity and biology research." The post cited Chubby's claim that Mythos 5.5 had already appeared in the website's HTML source code; based on this, TestingCatalog speculated whether a Mythos 5.5 aimed at cyber defense would be launched next week, and surmised that Fable 5.5 might also arrive alongside it. Currently there is only this one report; Anthropic has not responded, and there have been no further developments.
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Hidden mention of Claude Mythos 5.5 appears on Anthropic's official website
Source:Testing Catalog
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Nov 2
36Kr · AI
Claude Mythos 5.5 and GPT-6.2 Astra both surface, AI world may see a wave of flagship model updates in November
The string "Claude Mythos 5.5" has appeared in the HTML source code of Anthropic's official website, and OpenAI's unannounced GPT-6.2 Astra has also shown up in some users' Codex usage statistics and API response records. Circulating configurations suggest that Fable 5.5 may support a 1 million token input context and a maximum output of 128,000 tokens, at $10 per million input tokens and $50 per million output tokens; screenshots of GPT-6.2 Astra, meanwhile, show a 1M window. The related demo videos and API call records do not include complete prompts or logs, and the models' identities still await official confirmation.
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This post contributes to heat; it is not a formal report or confirmed fact.
Nov 1
Kol TregaskesRelated discussion
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This post contributes to heat; it is not a formal report or confirmed fact.
Testing Catalog
Claude Mythos 5.5 Appears in Anthropic's Official Website Source Code
TestingCatalog claims to have found a hidden mention of Claude Mythos 5.5 on Anthropic's official website, described as "the strongest model category for cybersecurity and biology research." The post cites Chubby's claim that Mythos 5.5 has already appeared in the website's HTML source code; based on this, the author speculates whether a Mythos 5.5 aimed at cyber defense will be released next week, and surmises that Fable 5.5 may also arrive alongside it.
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