The upcoming Boston launch of a mobile app that helps drivers sell their city-owned street parking spaces­ already is running afoul of the city.

Baltimore’s Haystack Mobile Technologies bills its free app as empowering neighbors to exchange parking spots in real time and save emissions. But publicly owned parking spaces can’t be privately sold, a spokeswoman for Mayor Martin J. Walsh said.

“We encourage innovation, particularly relative to addressing our transportation challenges,” said Walsh spokeswoman Kate Norton, who said the city is in active discussions with potential partners to help people pay for parking and tickets through apps.

“Services like Haystack, however, artificially inflate the cost of parking and allow individuals to profit from public space,” she said. “Neither of these activities are in line with the city’s effort to keep parking as open and publicly accessible as possible. These spaces are publicly owned and cannot be privately sold.”

A spokeswoman for Haystack, which is having a Hub launch party Tuesday, said the founder was not available for comment.

It’s unclear if Haystack — which takes a cut of parking space sales — will dictate set prices for Boston parking spaces. In Baltimore, where it debuted in May, app users can sell their spaces for the market rate of $3 — with 75 cents of that pocketed by Haystack, according to published reports.

Users of the GPS-enabled app can offer their free or metered street spot, and those seeking parking can see how close those spots are. Both sides can communicate via an in-app chat feature, and the sellers can track the buyer’s movement and estimated arrival time. After a user confirms a successful spot swap, the fee is paid automatically from his or her Haystack account or on-file credit card

The app also has a “make-me-move” feature for those willing to leave a spot for the “right price,” which they can set.

City attorneys likely will be poring through regulations and laws to determine Haystack’s legality.

Attorney Brian Gaff of McDermott Will & Emery said, “It’s a new way of dealing with something that people really haven’t got their heads wrapped around. If there’s not a regulation prohibiting it … it would seem to maximize the use of metered spaces, which should benefit the city by increasing its revenue.”