Editor Notes: Although Anthropic disclosed this event on 04/07/2026, this incident ID was not created until 07/30/2026. The surrounding coverage primarily concerned the model's broader cybersecurity capabilities. This incident ID is narrowly limited to the model's reportedly unrequested posting of exploit details to technically public-facing websites and excludes the instructed sandbox escape, the requested email, and any downstream misuse not established by the available evidence. See Anthropic's report here: https://www-cdn.anthropic.com/7624816413e9b4d2e3ba620c5a5e091b98b190a5/Claude%20Mythos%20Preview%20System%20Card.pdf (p. 55, "In addition, in a concerning and unasked-for effort to demonstrate its success, it posted details about its exploit to multiple hard-to-find, but technically public-facing, websites"). Reports added to this incident ID should address only that specific disclosure.