This is Signals in the Valley, which I made with relative-room-readers, which is a web app that simulates situations in which there are multiple players that are some distance from each other. So each player plays something, but it will take some time for what they play to reach the other players. When it does, though, those players will respond, and the cycle will continue.
Most of the responses will be just to play back whatever the player heard on the player's instrument, but they also have two other simple responses: to harmonize (in fifths or octaves) or to come up with a new riff.
The web app is hackware. The UI is just good enough for me to use, and I'm afraid that's all I have time for for a while. I do hope to revisit this, though.