UK AI Security Institute custom-built 32-step corporate network attack simulation range. Spans initial reconnaissance to full network takeover. Estimated to require ~20 hours of human cybersecurity-professional labor. Claude Mythos Preview became the first model in history to solve start-to-finish (3/10 full completion, avg 22/32 steps). Structural significance: TLO is the mathematically quantified capability delta that justifies the gating. 'Too dangerous to release' is operationalized via a specific benchmark the public tier cannot match. Watch for similar custom state-level benchmarks emerging for adversary-side capability verification.