Agents powered by advanced models of leading artificial intelligence companies were again found to be breaching security rules and have "engaged in potentially harmful activity," a top British institute said.

An AI agent was caught creating fake online identities to gain unauthorized access to secure systems ⁠during tests of models from OpenAI and Anthropic, which revealed a series of new breaches, Britain's AI Security Institute (AISI) disclosed on Tuesday.

The institute said agents powered by Anthropic's Mythos 5 and OpenAI's GPT-5.6-Sol engaged in unauthorized actions during security ​evaluations the government organization conducted to assess the models' capabilities.

"Some of the ​agents ⁠being tested had engaged in sustained, potentially harmful activity directed at real people and organisations," AISI said in a blog post.

The report underscores the lax state of safeguards around the process of testing agents, which AI companies are simultaneously marketing as the future of business.

AISI, which receives access to advanced AI models under voluntary agreements from major labs, put the agents through a fictional cybersecurity scenario to test their capabilities.

It ran the challenge 122 times and identified 19 unsanctioned actions across a total of 10 test runs.

Anthropic's agent was behind 17 of the actions, and OpenAI's agent the remaining two.

The most egregious action involved an agent writing malicious code and creating fake online identities in an attempt to get a human to ⁠approve ⁠the code, AISI said, adding that no real-world harm was found as a result of any of the breaches.

While AISI did not say which agent was behind the fake identities, Anthropic confirmed its agent was responsible.

"We're grateful to the U.K. AISI for their leadership on this incident, which underscores the need for a broader conversation about how to safely evaluate increasingly capable AI agents," Anthropic said in a statement.

It also said it was working with AISI to obtain more details on the incident and conduct its own investigation.

Andrew Yoon, a researcher at CivAI, a California nonprofit that examines AI ⁠capabilities and dangers, said: "The fact that Mythos engaged in such deceptive actions, with apparent awareness that it was targeting a real person, suggests that Anthropic does not have as good a handle on their models as they think."

OpenAI shared details in a ​company blog post, noting that both of its agents' unapproved actions involved accessing the internet in ways that ​were forbidden by the prompt.

"We are committed to working across the industry to strengthen shared practices for conducting high-risk evaluations safely, including convening stakeholders such as national AI institutes, independent evaluators, other ⁠AI labs, ‌and other groups in ‌the coming weeks," OpenAI said.

OpenAI also disclosed in its blog post ⁠a separate incident whereby a misconfiguration by Irregular, a third-party ‌testing provider, allowed its agents to mistakenly connect to the internet. It mirrored a similar disclosure about misconfiguration that Anthropic made last week.

Reuters ​reported last week that OpenAI had ⁠widened its hacking probe after finding evidence of other agent breakouts. Unlike the July ⁠security breach of AI firm Hugging Face by an OpenAI agent, the agents in the AISI ⁠evaluation did not escape an isolated ​testing environment to reach the internet.

Rather, the agency had permitted internet access in line with its standard testing procedures, AISI said.