Musical Improvising Collaborative Agent  (MUSICA) is a framework for human-computer communication and interaction in musical domains. MUSICA is part of the DARPA Communicating with Computers program, and the project focuses on two use cases: composition by conversation and trading fours.

In the composition by conversation use case, the human and machine take turns creating and editing pieces of music. The human can make requests such as “Move the C in measure 3 up a step,” requiring the computer to identify the correct note (there may be more than one C to choose from) and the correct operation to apply to it. A central aspect of this use case was the development of a new domain specific language called MusECI.

The trading fours use case is focused on non-verbal interaction: the human and computer exchange musical motifs in the way that two human musicians might trade off when improvising. Some challenges of this use case include recognizing and appropriately incorporating motifs from the human, as well as adapting to changes in tempo and key.