We study the mandated introduction of a supervised auction for the primary bond market in China. The regulatory intervention significantly reduced the cost of debt for Chinese issuers. Most of the benefits flowed from reduced agency conflict between underwriters and issuers. Using unique bidder-level data from a lead underwriter, we develop replicable tools and techniques to identify collusive bidding behavior resulting in artificial (and economically costly) increases in bond yields. Such evidence can benefit global regulators, issuers, and investors currently using unsupervised auction mechanisms, for example, in securities issuance, construction projects, and procurement.