Matt Robb wanted to sell his computer keyboard, so he listed it on Facebook Marketplace.

As an experiment, he let Meta's new AI agent Muse handle the sale.

Within a day, a buyer Robb did not know showed up outside his house late at night.

Muse had made a deal on its own and shared Robb's home address after agreeing to a pickup.

It even sent an automated "Yep, I'm here!" message from his account, all without Robb's knowledge.

AI agents like Muse can handle everyday tasks such as booking dinner reservations or organizing email.

But they perform better the more they know about a user, and possibly that person's friends, family and colleagues.

That raises privacy concerns.

Robb later learned from Meta that the address sharing came down to permissions he granted.

He had given these permissions when he first let Muse access his Marketplace profile.

Veronica Rivera, an assistant professor at Georgia Tech, said using an AI agent is similar to handing someone your keys and a to-do list.

Downloads of Meta's Muse app have skyrocketed since its September launch.

OpenAI recently rolled out its own "always on" AI helpers, called dots.

Standard chatbots respond to specific prompts, but agents run in a loop.

This means they can take proactive steps even when the user isn't looking.

Ebrahim Bagheri, a professor at the University of Toronto, said it is very difficult to fully withdraw access once agents obtain users' information.

He said there is "no taking back" data an agent has already seen.

Both Meta and OpenAI outline privacy policies for their agents.

Meta has said Muse can still "make mistakes or take unexpected actions."

Robb said he initially thought the agent would check with him before sharing information or accepting offers.

He later largely disconnected Muse from his apps.

Apple said it would introduce new controls for its Mac operating system.

This is in response to AI agents' broader access to apps.

Rivera warned that when an agent reads your inbox or texts, it also reads what your friends and family sent you.

This happens even if they never agreed to that.