This paper describes the design of Mimi, a multi-modal interactive musical improvisation system that explores the potential and powerful impact of visual feedback in performermachine interaction. Mimi is a performer-centric tool designed for use in performance and teaching. Its key andnovel component is its visual interface, designed to providethe performer with instantaneous and continuous information on the state of the system. For human improvisation,in which context and planning are paramount, the relevantstate of the system extends to the near future and recentpast. Mimi’s visual interface allows for a peculiar blendof raw reflex typically associated with improvisation, andpreparation and timing more closely affiliated with scorebased reading. Mimi is not only an effective improvisationpartner, it has also proven itself to be an invaluable platformthrough which to interrogate the mental models necessaryfor successful improvisation.