International organizations play an important role in policy implementation. As member states do not necessarily resolve political disagreements before delegating tasks, this article focuses on how individual member states seek to influence policy implementation by international organizations. It argues that the institutional context in which delegation takes place affects the opportunities for such unilateral influence. Particularly when the agent has considerable autonomy, implementation is likely to be a contested process. The article presents evidence on the implementation of peacebuilding policy by three international organizations in Kosovo after independence in 2008. Despite the fact that the member states within the UN, OSCE and EU fundamentally disagree on the legal status of Kosovo, the organizations have deployed substantial peacebuilding missions. The UN, OSCE and EU have, however, different institutional designs: implementing agents in the UN and OSCE have, by default, more autonomy than those in the EU. We analogously observe variation in how and to what extent member states exert unilateral influence during implementation of peacebuilding policy on the ground in Kosovo.