You could return the set of moves, or you could -not- do that, and just run the state machine in the client. That way, you don't have to complicate the server

You could return the set of moves, or you could -not- do that, and just run the state machine in the client. That way, you don't have to complicate the server by making it calculate future moves and write yet another client which is purposefully less featured, but still intelligent enough to parse the returned moves and map them to whichever piece is selected and you have less code to maintain. You can keep on gambiting around the problem with "concessions" which is easily solved by activaing code you've already written on the client side but that's no more than a mental exercise and has little to do with efficiency in problem solving.