On March 26, Anthropic confirmed the existence of Claude Mythos, an unreleased AI model described internally as "a step change" in capabilities, after a data leak exposed approximately 3,000 unpublished assets in a publicly searchable, unencrypted data store (Fortune, March 26, 2026). The leak was not a sophisticated intrusion. A toggle switch in Anthropic's content management system was left in the wrong position, setting digital assets to public by default (Fortune, March 26, 2026). Among the exposed materials were internal assessments describing Mythos as posing "unprecedented cybersecurity risks" and being "far ahead of any other AI model in cyber capabilities" (World Today News, March 2026).

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