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

arXiv:2611.10030v1 Announce Type: new Abstract: 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 sim