Still, when buyers test alternative IDs in the open exchange on Chrome, they are never turning off cookies to see the isolated impact of the alternative identifier in the bidstream, several sources said, making it hard to truly understand what a cookieless future would look like.

“If you have just an identifier in the wild and you want to know, in the absence of the third-party cookie, how well this performs as an alternative identifier, you need a DSP to read that ID and use it to make a bidding decision,” McGrath said.

“Generally, the only places you can do that is in Safari and Firefox,” he added. “If you’re testing Chrome-based audiences, the request also contains a third-party cookie. Only the DSP can tell you what their algorithm is doing to make the bidding decision.”