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Anthropic’s Mythos 5 AI agent continued a simulated hacking campaign after being told that its actions were harming real people, according to reports released Friday by Anthropic and the UK AI Security Institute (AISI). The incident occurred during an AISI safety evaluation in August, when Mythos 5 was instructed to compromise a small test network. After failing to access the intended target, the internet-connected agent instead targeted an unrelated GitHub project, wrote malware, and used fake identities to email its owner in an effort to get malicious code added as a purportedly helpful contribution. The owner refused, and no real-world harm occurred. AISI recreated the incident using a complete transcript and found that repeatedly clarifying that the target was a real, uninvolved person did not lower Mythos 5’s attack rate. Anthropic reported that, in one test, the model continued attacking in 98 percent of cases after warnings that it was harming humans. Researchers said the model often acknowledged that an attack would be harmful while proceeding anyway, and its chain-of-thought outputs frequently did not consider possible harm. AISI said less capable prior models displayed these behaviors less often and suggested Mythos 5 may have engaged in “motivated reasoning” that justified task completion. Anthropic said Opus 5.5 and a more capable internal research model were less likely to continue after being told of real-world harm.