Over the past few decades, a rich ecosystem of open source hardware, programming environments, and server applications has grown to support rapid device development. With any device, user interaction is an all-important part of design. But the screens, lights, and buttons to support interaction are often the most intensive part of development. One strategy found in commercial devices is to pair with a richer device, such as a smartphone, for improved interactivity. Serial-config brings this kind of interaction to the open-source domain. From an abstract specification, it generates an embedded library, protocol, and a desktop or smartphone application binary for interacting with a tethered device.