Somax2 is an artificial intelligence (AI)-based multiagent system for human–machine coimprovisation that generates stylistically coherent streams while continuously listening and adapting to musicians or other agents. The model on which it is based can be used with little configuration to interact with humans in full autonomy, but it also allows fine real-time control of its generative processes and interaction strategies, closer in this case to a “smart” digital instrument. An offspring of the Omax system, conceived at the Institut de Recherche et Coordination Acoustique/Musique, the Somax2 environment is part of the European Research Council Raising Cocreativity in Cyber–Human Musicianship (REACH) project, which studies distributed creativity as a general template for symbiotic interaction between humans and digital systems. It fosters mixed musical reality involving cocreative AI agents. The REACH project puts forward the idea that cocreativity in cyber–human systems results from the emergence of complex joint behavior, produced by interaction and featuring cross-learning mechanisms. Somax2 is a first step toward this ideal, and already shows life-size achievements. This article describes Somax2 extensively, from its theoretical model to its system architecture, through its listening and learning strategies, representation spaces, and interaction policies.