International Organizations (IOs), such as the United Nations (U.N.), engage in statebuilding in a range of post-conflict states. Statebuilding scholarship largely assumes that IOs, backed by their powerful member states, have at least temporary authority over the seemingly “weak” states in which they intervene. We argue, in contrast, that many post-conflict states shape IO statebuilding efforts through many statebuilding contracts, which we call incomplete arrangements, that give the post-conflict state the residual rights of control over the unnegotiated components of these statebuilding contracts with IOs. These incomplete arrangements, as opposed to complete takeovers, which are the other type of statebuilding contracts, provide procedural “weapons of the weak state” that enable the post-conflict state to influence what the IO mandate contains, where it intervenes, whom it hires, and when it exits. Using in-depth case studies of Burundi, Guatemala, and Timor-Leste, as well as analysis of 36 U.N. interventions in post-conflict states from 2000–2020, this article demonstrates the potential of incomplete arrangement statebuilding contracts to give post-conflict states institutional power over IO statebuilders, with important implications for scholarship on statebuilding and global governance.