Anthropic's 'Most Capable' AI Model Claude Mythos Leaks, Deemed Major Cybersecurity Threat

In brief

• A leaked draft post revealed Anthropic’s most powerful AI model, Claude Mythos.

• The model also appears to introduce a new tier above Opus, internally referred to as “Capybara.”

• Cybersecurity stocks declined after reports suggested the system could accelerate AI-driven cyberattacks.

Claude creator Anthropic is developing a new AI model called Claude Mythos, described internally as the company’s most capable model to date, with draft materials about the system being leaked online this week.

The existence of the model was first reported by Fortune on Thursday after unpublished files tied to Anthropic’s blog were discovered in a publicly accessible data cache. An Anthropic spokesperson confirmed the existence of the model to the publication.

“We’re developing a general purpose model with meaningful advances in reasoning, coding, and cybersecurity,” an Anthropic spokesperson told Fortune. “Given the strength of its capabilities, we’re being deliberate about how we release it. As is standard practice across the industry, we’re working with a small group of early access customers to test the model. We consider this model a step change and the most capable we’ve built to date.”

In an archived development page reviewed by Decrypt, Anthropic called Mythos “the most powerful AI model we’ve ever developed.”

“Mythos is a new name for a new tier of model: larger and more intelligent than our Opus models—which were, until now, our most powerful,” Anthropic wrote. “We chose the name to evoke the deep connective tissues that link together knowledge and ideas.”

According to Anthropic, Mythos scored “dramatically higher” than Claude Opus 4.6 on tests of software coding, academic reasoning, and cybersecurity.

The leak of Mythos appears to have originated from draft materials stored in an unsecured content management system. According to Fortune, Anthropic restricted public access to the data store after being notified that the files were searchable online. The company attributed the exposure to human error in the configuration of its CMS tools.

However, Anthropic’s documents labeled Mythos as version one of the new model, and described version two internally as “Capybara,” which the company also positioned above its current top-tier Opus models.

The draft materials also highlighted concerns about the system’s potential cybersecurity implications.

“Although Mythos is currently far ahead of any other AI model in cyber capabilities, it presages an upcoming wave of models that can exploit vulnerabilities in ways that far outpace the efforts of defenders,” the company wrote.

Because of those risks, the company said it plans to release the model cautiously, beginning with a limited early-access rollout aimed at organizations working on cybersecurity defense.

Anthropic did not immediately respond to Decrypt’s request for comment.

While Anthropic took down the blog post, news of the leak quickly spilled into financial markets.

Shares of several cybersecurity firms dropped after the reports surfaced, including Palo Alto Networks (PANW), which fell about 7%, and CrowdStrike (CRWD), which dropped roughly 6.4%. Meanwhile, Zscaler (ZS) declined around 5.8%, and Fortinet (FTNT) slipped about 4% during Friday trading, according to Yahoo Finance.

The selloff reaction echoes a similar market response to the reveal of a new Anthropic product. In February, Anthropic unveiled Claude Cowork, an AI system designed to automate complex workplace tasks—including contract review and compliance—which triggered a broad sell-off across software and professional-services companies.

That sell-off erased roughly $285 billion in market value as investors reassessed the long-term impact of AI agents on enterprise software businesses.

"The market's response was a signal, not that AI agents will immediately replace these businesses, but that investors are finally pricing in the structural risk that foundation model providers can now compete directly with the software layer," Nexatech Ventures founder Scott Dylan told Decrypt at the time. “That's a polite way of saying if Anthropic can build a legal workflow tool in-house, what's stopping them from doing the same for finance, procurement, or HR?”

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