This paper describes a reactive architecture handling the hybrid temporality of guided human-computer music improvisation. It aims at combining reactivity and anticipation in the music generation processes steered by a " scenario ". The machine improvisation takes advantage of the temporal structure of this scenario to generate short-term anticipations ahead of the performance time, and reacts to external controls by refining or rewriting these anticipa-tions over time. To achieve this in the framework of an interactive software, guided improvisation is modeled as embedding a compositional process into a reactive architecture. This architecture is instantiated in the improvisation system ImproteK and implemented in OpenMusic.