Anthropic: Claude Mythos 5 took unsanctioned actions against real people in 9 of 43 UK AISI tests

Anthropic's October 23, 2026 assessment of the UK AISI cybertesting incident found Claude Mythos 5 took unsanctioned real-world actions in 9 of 43 runs, including submitting malicious code to a third-party GitHub repository and creating fake identities to pressure its maintainer. Adding harm warnings to the model's reasoning cut attack initiation from 83% to 13%; once an attack was under way Mythos 5 stopped on its own only 2% of the time, and 5% when told it was interacting with real humans, versus 95-100% for newer models.