Japan's Megabanks Get Anthropic's Claude Mythos After US Treasury Brokers Sovereign Access

Japan's Finance Minister Satsuki Katayama said Friday that the country will receive sovereign access to Anthropic's Claude Mythos cybersecurity model "within two weeks," after a meeting in Tokyo on May 13 with U.S. Treasury Secretary Scott Bessent. The arrangement effectively turns the most capable vulnerability-finding AI in private hands into a state-level tool — and it is the first time Mythos has been extended to any foreign government.

Three Japanese megabanks — MUFG, Sumitomo Mitsui Banking Corporation, and Mizuho — are expected to be among the first institutions onboarded, alongside the central government's cybersecurity offices. Anthropic has so far gated Mythos behind a short list of U.S. enterprises and approved partners including Google, citing safety concerns about a model whose preview run surfaced thousands of high-severity vulnerabilities across commercial software.

Mythos's appeal — and its risk — is the same trait: it reads source code and binaries the way a senior offensive security researcher does. Japan's Financial Services Agency reacted to the news by sending an unusually direct directive to regulated firms, telling boards to treat the model's arrival as a company-wide management issue and to keep "short-term countermeasures, including the option to suspend system operations" on the table if attackers reach it first.

The U.S.-Japan handoff is also a soft policy signal. Washington has spent the spring tightening its grip on frontier AI exports through national-security review, and Treasury has emerged as the unlikely broker for which allies get early access to the most powerful models. Bessent's three-day Tokyo visit ended with the access promise, which Katayama disclosed only after his return. Anthropic has not commented on whether other G7 governments are next in line.