Governance delegation agreements—international treaties allowing external actors legal authority within host states for fixed terms—succeed in simple and, under certain conditions, complex state-building tasks. These deals are well institutionalized and have input legitimacy because ratification requires sufficient domestic support from a ruling coalition. In order to obtain that input legitimacy, however, host states constrain external actors commensurate with their level of statehood: Stronger states delegate less legal authority. This article argues that these constraints, which produce joint rather than complete authority, require external actors to work within state structures rather than substituting for them, and thus make coordination of complex tasks more difficult. A quantitative overview of data on consent-based peacekeeping missions complements a qualitative analysis focused on comparative case studies in Melanesia and Central America to test the theory. The results support the theory and suggest that these deals hold promise particularly for accomplishing complex tasks in especially weak states.