SpatIO is an extended reality (XR) toolkit for prototyping spatial interactions where the precise three-dimensional placement of sensors and actuators is crucial. The system allows designers to first build a fully virtual prototype within a virtual environment (SpatIO Environment) and then gradually transition it into a physical prototype using a 3D-printed module system (SpatIO Module). It also features a three-dimensional flow-based programming interface that supports spatial code placement and continuous code execution (SpatIO Code). Throughout this process, SpatIO provides a seamless workflow from initial virtual design to final physical implementation.