We develop a tool akin to the revelation principle for mechanism design
with limited commitment. We identify a canonical class of mechanisms
rich enough to replicate the payoffs of any equilibrium in a
mechanism-selection game between an uninformed designer and a privately
informed agent. A cornerstone of our methodology is the idea that a
mechanism should encode not only the rules that determine the
allocation, but also the information the designer obtains from the
interaction with the agent. Therefore, how much the designer learns,
which is the key tension in design with limited commitment, becomes an
explicit part of the design. We show how this insight can be used to
transform the designer's problem into a constrained optimization one: To
the usual truthtelling and participation constraints, one must add the
designer's sequential rationality constraint.