I think this API is over-engineered here is what I would do: 1 global integer called board (9*2 bits, enough to represent every square on the board as 0=vacant

I think this API is over-engineered here is what I would do: 1 global integer called board (9*2 bits, enough to represent every square on the board as 0=vacant, 1=X, 2=O), and a list of agent callbacks. The program loops through the agents calling each in turn. The agent does whatever it wants, but generally computes the next state from the current state (either via AI, GUI, or network) and sets the global board variable. It is the agents responsibily to operate correctly. If the GUI or AI wants to cheat, undo moves, call in a pinch-hitter, or change their opponents agent mid-game, let them (all they need do is set the state and callbacks appropriately). Both the GUI and AI should be designed to be stateless in this regard: display or compute off the current state, whatever it is. There's no need for the API to detect when the game is over, nor enforce rules. The AI plays it correctly or is rewritten. The GUI echos the users desire, so let it do whatever it wants. If you want to prevent cheating, just add a cheat-detector callback between each agent. If you want to know when the game ends, have your GUI detect it and declare the winner. The AI doesn't even need to know when the game ends because you'd obviously have a GUI to display the battle -- let the gui determine who has won and when. (Before you laugh, consider that this design would reduce the API size from [assumably] ~150 loc (plus comments) to about 5 simple, flexible lines of code: 1 global variable, 1 global array of callbacks, and 1 function to unendingly iterate over the callbacks.)