AR interoperability is the capacity for augmented reality content, spatial anchors, and experiences to function consistently across different hardware devices, operating systems, and rendering engines. It depends on shared data formats for spatial maps and anchors, common coordinate reference conventions, and standardised APIs for camera, tracking, and rendering access, as pursued by efforts such as the ETSI Augmented Reality Framework. Without it, AR content authored for one platform typically requires substantial rework to run on another, fragmenting the content ecosystem and raising development cost. It is a prerequisite for shared, persistent AR experiences that multiple users on different devices can view and manipulate together.