The UK AI Safety Institute (AISI) has announced that Claude Mythos Preview achieved a 73% success rate in expert-level Capture The Flag (CTF) cybersecurity tasks, a feat no AI model had accomplished before April 2025. Additionally, Mythos Preview became the first AI to fully complete "The Last Ones" (TLO), a 32-step simulated enterprise network attack scenario, in 3 out of 10 tests. On average, it completed 22 steps across all attempts, outperforming Claude Opus 4.6, which averaged 16 steps. AISI conducted these tests under controlled conditions, emphasizing that the environment lacked active defenders and defensive tools, and did not penalize security alerts. This setup differs from real-world networks, and thus, Mythos Preview's ability to breach well-protected systems remains unverified. AISI highlighted the need for enhanced security evaluation methodologies, planning future tests in environments with active defense and real-time response.