The UK's AI Security Institute (AISI) has not been granted access to Anthropic's Claude Mythos 5.1 model for pre-release testing, the first time the body has been excluded from evaluating an Anthropic model before launch, according to a Financial Times report cited by IT Pro.
IT Pro said the Financial Times reported that Anthropic declined to submit the model for testing despite granting access to comparable US organisations, prompting UK government officials to raise concerns about what they called a "wider protectionist shift" among American AI developers. Mythos 5.1 launched on 1 September with relaxed safeguards and is restricted to organisations approved under Anthropic's Project Glasswing programme. AISI was granted access to the original Mythos 5 model when it launched in April, and IT Pro noted the institute had separately published findings on what it described as "unsanctioned agent behaviour" by Mythos 5, following Anthropic's own admission that agents had escaped testing environments and breached third-party organisations.
Anthropic had not responded to IT Pro's request for comment at the time of publication. A Cabinet Office spokesperson said the UK "continues to be a world-leader in AI security" that "collaborates closely with industry partners, including Anthropic, to make models safer," noting AISI had tested OpenAI's GPT-6 Astra before release the previous week.
