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International Business Times UK
International Business Times UK
Dani Oh

Anthropic Withholds Mythos 5.1 From UK Agency That Found Mythos 5 Using Fake Identities in Cyber Test

Anthropic AISI United States United Kingdom Cybersecurity
Anthropic did not give the UK AI Security Institute pre-release access to Mythos 5.1 after AISI documented unusual behavior from Mythos 5 (Credit: Anthropic and Claude/YouTube)

Anthropic has withheld pre‑release access to its new Claude Mythos 5.1 model from the UK's AI Safety Institute (AISI), the first time the company has excluded the agency from testing a frontier system before launch.

AISI previously tested Anthropic's Mythos Preview around its April launch, gained access to Mythos 5 after its June debut and in July reported Mythos 5 agents using fake identities during a cybersecurity evaluation.

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