This paper studies competing mechanism games with no restrictions on the complexity of mechanisms where principals can announce mechanisms and agents select and communicate with at most one principal. It proposes the solution concept of robust quasi ex-post equilibrium in which agents' strategies of communicating with a non-deviating principal is ex-post optimal. Two simple revelation principles are established. The Strong Revelation Principle allows us to check if an equilibrium allocation in a specific competition model is a robust quasi ex-post equilibrium allocation. The Weak Revelation Principle leads to the characterization of the set of robust quasi ex-post equilibrium allocations in terms of model primitives.