There has been considerable debate about the delegation of power to international organizations, but few studies compare national public administrations with international organizations. In the meantime, international and national bureaucrats are important actors in world politics since they represent states in the international arena. Sometimes executive agents attempt to bypass control by member states and to overreach their delegated authority (agency slack), while at other times they do not. How and under what conditions do agents engage in slack? To answer this question, the article builds on principal–agent theories by comparing different forms of power delegation at the national and international levels. It argues that the institutional design of delegation contract and oversight mechanisms have an impact on the extent of agency slack. In developing this argument, it compares the delegation of power from the legislature (the US Congress) to a national public administration (the United States Trade Representative) and from an intergovernmental organization (the Council of Ministers) to a supranational organization (the European Commission) in the negotiation of international trade agreements. The findings show that agency slack is less likely when the institutional design of the delegation mandate is rule-based and principals have a combination of police-patrol and fire-alarm oversight mechanisms at their disposal to control their agents.