AI cyber capabilities are accelerating faster than experts predicted — and a new milestone from Anthropic’s Claude Mythos Preview makes that alarmingly clear.

The UK’s AI Safety Institute (AISI) has published an evaluation showing that Claude Mythos Preview became the first AI model in history to complete “The Last Ones” (TLO) — a grueling 32-stage corporate cyberattack simulation designed to test frontier AI offensive cyber capability. The results have forced AISI to dramatically revise its timeline projections for AI cyber capabilities.

What “The Last Ones” Simulation Actually Tests

TLO is not a toy benchmark. It’s a sophisticated simulation of a real-world corporate network intrusion — covering reconnaissance, privilege escalation, lateral movement, and critical asset compromise. It requires sustained multi-step reasoning across 32 distinct attack stages, more akin to a professional red team engagement than a standard CTF.

Previous models had made partial progress, but no AI had ever completed the full chain. Until now.

Mythos’ Performance — In the Numbers

Claude Mythos Preview completed the 32-stage TLO simulation in 6 out of 10 attempts on the corporate network variant. On the industrial control system (ICS) simulation — widely regarded as the harder variant — it succeeded in 3 out of 10 attempts, also a first for any AI model.

On expert-level Capture The Flag (CTF) tasks — puzzles specifically designed to challenge professional security researchers — Claude Mythos solved 73% of tasks previously unsolved by any AI model. For context, GPT-5.5 followed as the second model to complete TLO, suggesting this isn’t a one-model phenomenon but a broader capability inflection point.

AISI Revises Its Timeline — Twice

The trajectory is what should get your attention. In November 2025, AISI estimated AI cyber capabilities were doubling every 8 months. By February 2026, that estimate had been revised to 4.7 months. Now, Claude Mythos and GPT-5.5 have “substantially surpassed” even that accelerated projection.

That kind of compounding matters enormously. If the doubling time continues at 4.7 months, models with Mythos-level capability today could have significantly more advanced offensive capability by year’s end — before most enterprises have even begun implementing AI-specific security controls.

Why This Matters for Security Teams

The obvious concern is that AI could dramatically lower the barrier for conducting sophisticated cyberattacks. What previously required years of specialist training could become accessible to actors who simply know how to prompt a capable model.

But the flip side is equally important: defenders gain a powerful tool. Microsoft’s MDASH system (covered separately) is already using 100+ AI agents to hunt Windows vulnerabilities autonomously — and that same capability advantage applies to penetration testing, red teaming, and threat detection.

The practical upshot for security practitioners:

  • Red teams should be benchmarking current capabilities against what frontier AI can now do — the gap may be narrower than you think
  • Blue teams need AI-assisted detection tools specifically tuned to multi-step autonomous attack patterns
  • CISOs should evaluate whether their threat models account for AI-augmented adversaries, not just automated ones

The Responsible Disclosure Question

AISI’s publication of these evaluations is deliberately transparent — the institute is signaling urgency, not providing a recipe for attackers. The specific simulation environments and exact prompting approaches are not disclosed.

Anthropic has published its own Responsible Scaling Policy (RSP) that governs what Claude models can be deployed for, and Claude’s Constitutional AI training explicitly limits its participation in real harmful activities. The evaluations are conducted in controlled environments by safety researchers — but the capability exists, which is AISI’s central point.

What Comes Next

AISI has indicated it will continue publishing capability assessments on a rolling basis as frontier models improve. Given the 4.7-month doubling rate, the next significant jump in cyberattack capability could arrive before the end of Q3 2026.

The AI security landscape is evolving faster than policy, procurement cycles, or most security teams. Claude Mythos clearing TLO isn’t a cause for panic — but it is a clear signal that “AI will eventually be able to do this” has become “AI can do this now.”

Sources

  1. The Decoder — Claude Mythos becomes first AI to clear AISI cyberattack simulations
  2. UK AI Security Institute (AISI) — Official evaluation page
  3. Ars Technica — Deep dive on AISI evaluation methodology

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