Procedures for the ex ante assessment of public policies are currently in vogue across the OECD. Their design is typically informed by a rational-instrumental model of problem solving, which assumes that knowledge is collected, evaluated, and then translated straightforwardly into ‘better policies’. But this model has been little affected by more than three decades of academic research which has demonstrated that the reality of everyday policy making is far messier. In this paper we analyse whether the uptake of ex ante assessment of policies is nonetheless capable of creating opportunities for policy deliberation and learning informed by new assessment knowledge. Drawing on an analysis of policy assessment procedures in three countries and the European Union, we find that there are several ways in which assessment knowledge is used in the policy process. Moreover, we argue that policy learning occurs despite, rather than because of, the instrumental design of new assessment procedures, which tends to act as a barrier to open deliberation and knowledge utilisation.