In embodiments of extensions for USB driver interface functions, a set of USB driver interfaces are exposed by a USB core driver stack, and the USB driver interfaces include USB driver interface functions to interface with USB client function drivers that correspond to client USB devices. A composite device driver registers itself and requests a function handle for each function of a client USB device. The USB client function drivers are enumerated and the function handles generated for each function of the client USB device. A check first protocol is enforced that directs a USB client function driver to check for availability of a USB driver interface function before interfacing with the USB core driver stack via the USB driver interfaces. A contract version identifier is received that indicates a set of operation rules by which a USB client function driver interfaces with the USB core driver stack.