For as long as there have been ads on the internet, marketing has targeted human eyeballs. But as AI agents crowd onto the web, advertisers and publishers are beginning to deliver ads only AI can see, sparking consternation among some of the agents’ developers, who fear marketers could try to sway agents to take actions without their users’ knowledge.

The first publisher to experiment with these types of invisible ads was venerable Time magazine, which continues to generate a significant portion of its revenue from ads on Time.com. Those ads—for Ally Bank and Project Management Institute, a trade group for project managers—first showed up online in August and were developed in a partnership with Mobian, a New York–based ad tech startup.…