MDPSim is a client/server-style simulator for Markov decision processes (MDPs). It is used to evaluate planning systems in the Probabilistic Track of the International Planning Competition.

It reads PPDDL problem specifications and sends information about the initial state to a client. The client sends an action as response and the simulator executes the actions sent by a client. After executing an action, a new state is sent to the client. Client-server communication is in XML format.