Anthropic left UK regulator out of Mythos 5.1 preview

Anthropic did not give the UK's AI Security Institute pre-release access to Mythos, a restricted-access sibling of Fable 5.1, according to the Financial Times. Vetted organizations in the US got that access, per IT Pro's writeup of the report.

It's the first time AISI has been excluded from an Anthropic frontier release. AISI exists to test models before labs release them widely, catching dangerous capabilities early instead of finding out about them later. Skip that step once and nobody outside Anthropic knows whether it's a policy shift or a one-off.

The UK Cabinet Office has ordered an urgent assessment of what happened. Some UK officials suspect the exclusion traces back to pressure from Washington, though that's unconfirmed. Neither the Financial Times report nor IT Pro's coverage says what changed between the models Anthropic gave AISI before this one. It's also unclear whether the decision came from Anthropic alone.

Watch what Anthropic gives AISI on its next restricted release. A repeat exclusion would confirm a harder line on which governments get early access. A return to the old pattern would point to something specific about Mythos. A pre-release testing arrangement only has value if it holds every time, not most times.