They asked the AI model to choose a treatment from 19 options in a spreadsheet. A veterinary surgeon first performed a craniotomy on the mouse while the AI agent observed. The agent selected the therapy, brain region, and injection coordinates, then controlled the robotic arms and injector needle, as well as all the parameters, such as volume and speed.
The team initially tested Anthropic’s Claude Opus 5.5 model, but it refused to guide the needle into the mouse’s brain, suggesting it might stop midway through the surgery, which could endanger the animal.
“He did the experimental planning, but when it was time to inject, he said, ‘something on my output is telling me I shouldn’t do this,’” Santero says, referring to the agent. “That was very interesting.”
The team switched to using Claude Sonnet 5, which agreed to proceed. Opus is a heavy-reasoning intelligence specialist best for complex multistep problem-solving, according to Anthropic, while Sonnet is better suited for everyday tasks, though it may be a stretch to call mouse brain surgery a common chore. (Anthropic did not respond to a request for comment.)
Frontier labs have been planning for more widespread physical manifestations of AI, and researchers have already begun enlisting agents to do other things in the physical world. In August, Anthropic put out a framework for how its models can interact with the real world. More recently, AI engineers put GPT, Claude, and Grok in charge of a car to pick up lunch at In-N-Out Burger. Only one of the models succeeded.
The brain injection procedure wasn’t without hiccups either. The researchers had established safeguards, including requiring the AI to propose an injection plan for human approval and installing an emergency button a human could hit that would disconnect the rig’s motors. They also restricted the agent’s access to the underlying equipment code to prevent it from modifying its own operating rules.