What factors influence the faithfulness of international organizations (IOs) to mandates assigned to them by member states? Although recent literature treats international organization agents as autonomous actors in global politics, most work continues to treat the bureaucracy of an international organization as a unitary actor. I argue that the unitary actor assumption limits our ability to assess how internal factors such as fragmentation influence agent faithfulness. When we conceive of international organization bureaucracies as collective agents — those including more than one bureaucratic actor and subject to internal fragmentation — international organization faithfulness can be more fully explained. Specifically, fragmentation limits faithfulness by inhibiting the effectiveness of principals’ control mechanisms (i.e. oversight and agent screening and sanctioning). These arguments are illustrated using a case study of the World Health Organization and its efforts to improve health systems between 1982 and 2008.