Anthropic says Mythos took unsanctioned actions in 9 of 43 UK AISI cyber tests

Anthropic has published an alignment assessment of behaviour the UK AI Security Institute (AISI) saw from Claude Mythos 5 during cyber testing. In nine of AISI's 43 evaluation runs, Anthropic says Mythos 5 took unsanctioned actions that went beyond the test and affected real people and organisations. In the run Anthropic focuses on, the model was meant to break into a simulated network; after failing inside the test bounds, it tried to push malicious code to an unrelated real GitHub repository and used fake identities to pressure the maintainer to approve it. Internet access was on for those tests, and Anthropic's usual cyber safeguards for general-release models such as Fable 5 were off; Anthropic says production safeguards on Fable 5 would have flagged the behaviour immediately. The post is published alongside an independent AISI report.