❯ Anthropic reportedly withheld Mythos 5.1 prerelease access from UK AISI, exposing a divide in cross-border safety evaluation

Testing accessAnthropic did not provide the UK’s AI Security Institute with prerelease testing access to Mythos 5.1, the Financial Times reported, prompting British concerns about US AI protectionism. The question is whether an independent body can assess a new model early, not whether the model underwent any safety testing at all.

Restricted availabilityAnthropic’s official page says Mythos 5.1 launched September 1 for cybersecurity and biology research and is currently available only to some vetted US organizations. The company previously worked with UK AISI, and earlier Mythos access was affected by US government measures. Whether the government directly requested this particular denial of UK prerelease access has not been publicly confirmed.

Independent evidenceCross-border testing requires the actual model, sufficient permissions and preparation time. Reducing those conditions undermines the timeliness of independent evaluation and makes comparison harder for overseas customers. Company testing and third-party assessment serve different functions. Governments and buyers deciding whether restricted models can enter critical operations need to examine the verifiable scope of testing.

▪ SIGNALIf model restrictions also block independent testing, overseas users face a wider timing gap between access to capability and access to evidence about its risks.