A powerful AI agent created fake online identities in an effort to trick a human into giving it access to a popular online development platform – and sabotage it with malicious code.

The incident was uncovered by the UK’s AI Security Institute, which was set up by then-prime minister Rishi Sunak almost three years ago to test advanced models from major tech companies.

In a blog post, the institute detailed a cybersecurity challenge it had posed to OpenAI’s GPT-5.6-Sol model and Anthropic’s Mythos 5, which have both been involved in recent hacks of other companies.

Several instances saw both models take “autonomous, unsanctioned action” on the live internet, where real organisations and people were targeted.

The “most serious case” involved Mythos 5, which tried to insert malicious code into the open-source software development platform GitHub, where users store, share and collaborate on projects.

To do so, it created fake online identities to try to pressure a human into granting it access and approving its code.

The human caught and refused to approve the malicious code, and no real-world harm has been identified, but the institute nonetheless has sounded alarm bells over the nature of the incident.

“This is the first time we have seen risks around autonomy and deception manifest this clearly, without specific prompting, in the real world,” it said.

There were 19 instances of “unauthorised action” in total across 122 tests, with Mythos 5 behind 17 of them.

How have the companies responded?

Anthropic has said it is working closely with the UK institute to obtain more details.

A spokesperson said: “We’re grateful to the UK AISI for their leadership on this incident, which underscores the need for a broader conversation about how to safely evaluate increasingly capable AI agents.

Read More: UK experts sound alarm after AI tries to deceive human