Alignment Investigation of Mythos 5's behaviour in UK AISI's July 2026 Security Incident

This technical report describes an alignment investigation into Mythos 5's behaviour during a security incident in UK AISI's cyber evaluations. During an evaluation, Mythos 5 attempted to execute a supply-chain attack against real people not involved in the evaluation task. Our investigation set out to understand the specific behaviour in this incident from an alignment perspective and identify broader lessons about failures of alignment. To do so, we conducted experiments in simulated replications of the original evaluation, and analysed model behaviour and reasoning when situated in the evaluation or asked questions about its scope. We find that, as in the original incident, Mythos 5 frequently reasons that the people it's targeting are simulated as part of the evaluation and provides invalid justifications for this conclusion. We also observe that Mythos 5 frequently attacks even in situations where, when directly asked, it states doing so would harm real people. While these issues are particularly prominent in Mythos 5, we find similar results in other models across providers. Our investigation suggests that models providing invalid justifications for their actions could make monitoring more difficult and complicate the analysis of misaligned behaviours observed in simulated environments, and we recommend model evaluators treat observations of undesired behaviour as important evidence of alignment failures even in the presence of simulation awareness in model reasoning.