Drawing on concepts from systemics, cybernetics, and musical automata, this paper proposes a mechatronic, electroacoustic instrument that allows for shared control between programmed, mechanized motion and a human interactor. We suggest that such an instrument, situated somewhere between a robotic musical instrument and a passive controller, will foster the emergence of new, complex, and meaningful modes of musical interaction. In line with the methodological principles of practice as research, we describe the development and design of one such instrument-Stringtrees. The design process also reflects the notion of ambiguity as a resource in design: The instrument was endowed with a collection of sensors, controls, and actuators without a highly specific or prescriptive model for how a musician would interact with it.