Waymo and Uber have ended their robotaxi partnership in Phoenix, the companies confirmed to TechCrunch.

According to TechCrunch, the arrangement quietly wound down in May after running for nearly three years. As a result, Waymo's self-driving robotaxis are no longer available to book through Uber's ride-hailing app in Phoenix, Arizona.

The partnership was an unusual one. It paired Waymo, the autonomous-driving company spun out of Google's parent Alphabet, with Uber, whose core business is the very ride-hailing service that driverless cars could eventually disrupt. For a stretch, riders opening the Uber app in Phoenix could be matched with one of Waymo's fully autonomous vehicles.

Uber is not exiting the driverless market in the city, however. Reporting by Sean O'Kane for TechCrunch notes that Uber says it is preparing to launch a separate autonomous vehicle partnership in Phoenix, signaling that the company still sees a future for self-driving rides on its platform — just with a different collaborator.

Neither the sources nor the companies detailed exactly why the Waymo tie-up concluded, and the wind-down drew little public announcement at the time.

Why it matters: Phoenix has been a key proving ground for driverless taxis, and the split shows how quickly alliances in the fast-moving autonomous vehicle industry can form and dissolve as rivals jockey to control how riders actually hail a self-driving car.