UK AI Security Institute

The UK AI Security Institute (AISI) published an independent evaluation of Anthropic's Claude Mythos Preview on 15 April 2026, providing the first external confirmation that the model's attack-chaining capability is genuine. On isolated capture-the-flag tasks Mythos scored above 85%, with GPT-5.4, Claude Opus 4.6 and Codex 5.3 all within 5 to 10 percentage points, no single-task superiority claimed. The significant finding was in AISI's 32-step "Last Ones" benchmark: Mythos autonomously completed an operation the Institute estimates would require a trained human roughly 20 hours, confirming durable autonomous execution.

AISI was established in November 2023 after the first international AI Safety Summit at Bletchley Park, under the UK Department for Science, Innovation and Technology. Its mandate is to evaluate frontier models for safety risks before or after deployment, with privileged access to unreleased systems. The April 2026 Mythos evaluation tested a model Anthropic had withheld from public release, demonstrating that access. The Institute works alongside the National Cyber Security Centre and international counterparts including the US AI Safety Institute.

For this beat, the AISI finding closes a loop opened by the Bessent-Powell emergency meeting of 8 April, which Treasury and the Federal Reserve convened over AI cybersecurity risks they could not themselves verify. The 20-hour autonomous-operation benchmark is the first independent confirmation that the convening was warranted on substance. The UK has a standing independent evaluator publishing results; the US has ad hoc emergency convening with no public follow-up document.

AISI's Frontier AI Trends Report, published 1 May 2026, confirmed that GPT-5.5 cleared the same 32-step autonomous attack chain benchmark ('The Last Ones') that Claude Mythos passed in April, making it the second frontier model to do so in under five days. GPT-5.5 scored 71.4% on the expert cyber suite and completed the TLO benchmark end-to-end in 2 of 10 attempts. The Report's headline finding: frontier cyber capability is doubling roughly every four months. AISI has since rebranded operationally to the AI Security Institute (retaining AISI as its abbreviation), reflecting a shift in emphasis from safety evaluation to active security risk assessment. The Institute's track record of independent evaluations (Mythos in April, GPT-5.5 in May) makes it the primary source for the accelerating-capability thesis at the centre of the ai-jobs-power-money briefing. Those evaluations provided the technical basis on which the US Commerce Department subsequently acted: on 12 June 2026, Commerce Secretary Lutnick cited a jailbreak of Claude Mythos 5 in forcing Anthropic to disable it globally; the first US government intervention to suspend a commercial AI product under export-control authority.