I study a mechanism design problem of allocating a single good to one of several agents. The mechanism is followed by an aftermarket, that is, a post-mechanism game played between the agent who acquired the good and third-party market participants. The designer has preferences over final outcomes, but she cannot redesign the aftermarket. However, she can influence its information structure by disclosing information elicited by the mechanism.

I introduce a class of allocation and disclosure rules, called cutoff rules, that disclose information about the buyer’s type only by revealing information about the random threshold (cutoff) that she had to outbid to win the object. A rule is implementable regardless of the form of the aftermarket and the underlying distribution of types if and only if it is a cutoff rule. Cutoff mechanisms are tractable, and admit an indirect implementation that often makes them easy to use in practice. I provide sufficient conditions for particularly simple designs, such as a second-price auction with disclosure of the price, to be optimal within the class of cutoff mechanisms.

The theory is illustrated with an application to the design of trading rules and post-transaction transparency in financial over-the-counter markets.