<p>This article examines a polycentric polity’s ability to deal with external and decision costs in a context of radical uncertainty and deep pluralism. I extend Buchanan and Tullock’s constitutional political economy framework to account for how disagreement about externalities assessments shapes constitutional choices. While polycentricity effectively aggregates dispersed knowledge about external costs, it generates its own cross-jurisdictional externalities, particularly through mobility rights between jurisdictions. Depending on context, free exit and entry are either a public good or a public bad for the broader polity. I argue that anarchistic interpretations of polycentricity tend to underestimate these second-order externalities. A polycentric polity needs both a state legitimate enough to mitigate cross-jurisdictional externalities and jurisdictional autonomy sufficient to leverage local knowledge and diverse perspectives.</p>