Exclusive Leak
Anthropic’s Next Frontier Model ‘Mythos’ Exposed in Data Breach — Company Calls It ‘A Step Change’
Nearly 3,000 unpublished assets, including a draft blog post for an unreleased model codenamed “Capybara,” were discoverable in an unsecured data cache — triggering Anthropic’s first public acknowledgment that Mythos exists.
Anthropic finds itself disclosing its most capable model earlier than planned after Fortune reported that nearly 3,000 unpublished internal assets — including a complete draft blog post for an unreleased frontier model — were accessible in an unsecured data store. The company acknowledged the existence of Claude Mythos, also referred to internally as “Capybara,” confirming that training has been completed and that early access testing is currently underway with select customers.
An Anthropic spokesperson described Mythos as representing “a step change” in AI performance and “the most capable we’ve built to date.” Leaked documents indicate the model significantly outperforms its predecessors in software coding, academic reasoning, and — most consequentially — cybersecurity tasks. The company acknowledged in internal materials that Mythos is “currently far ahead of any other AI model in cyber capabilities” and expressed concern that it could enable large-scale cyberattacks if deployed without care.
Anthropic says it is “being deliberate about how we release it” precisely because of these cybersecurity risks. The leaked materials also surfaced plans for an exclusive CEO summit in Europe at which unreleased Claude capabilities were to be demonstrated to business leaders. No public launch date has been announced. The incident highlights a deepening tension in frontier AI development: as models grow more capable, the gap between completion and public release widens — and unsecured data stores become a liability the entire industry must address.