How do leaders address the world in service of international cooperation, and do their messages enhance or detract from the global agenda? Leaders increasingly appear at the United Nations General Assembly (UNGA), a deliberative forum meant for diplomacy, not politics. At the same time, the UNGA has long undergone revitalization efforts, and leaders’ contributions could potentially ensure more meaningful cooperation. Building on theories of leader communication, we argue that, in contrast to other country representatives, heads of state have more leeway to deviate from the assembly’s priorities; as politicians, they have incentives to discuss their own, more particularistic, topics. Drawing from novel data on leaders’ speeches in the UNGA, we use text analytics to show that leaders tend to depart from the policy agenda of the session. Furthermore, national political leaders speak more plainly, centering speech on themselves and departing from the general conventions of diplomatic debate. This suggests that, while leaders’ attendance potentially generates more publicity and visibility for the UNGA, their contributions may also undermine the general debate. Our findings shed light on the ways in which leaders can politicize mulitlateral cooperation, and we give evidence for the role of individuals in the vitality of international organizations.