In embodiments of driver interface functions to interface client function drivers, a set of serial communication protocol driver interfaces are exposed by a core driver stack, and the serial communication protocol driver interfaces include driver interface functions to interface with client function drivers that correspond to client devices configured for data communication in accordance with the serial communication protocol. A client function driver can check for the availability of a driver interface function before interfacing with the core driver stack via the serial communication protocol driver interfaces. A contract version identifier can also be received from the client function driver via an extension of the driver interface functions, where the contract version identifier indicates a set of operation rules by which the client function driver interfaces with the core driver stack.