Following a deliberative shift towards public–private partnership networks in global environmental governance, the multi-stakeholder framework is increasingly advocated for engaging multiple actors in collective decision-making. As this arrangement relies on proper participatory conditions in order to include all relevant stakeholders, input legitimacy is crucial to achieving legitimate outcomes. However, ‘stakeholding’ implies that actors—recast into a specific institutional context—are sorted into new formal or informal categories. This paper scrutinizes the clean development mechanism (CDM) under the Kyoto Protocol to interrogate the problematic issue of ‘stakeholding’—i.e. the ‘sorting’ of actors—in enacting the multi-stakeholder framework. Based on an analysis of 25 CDM projects that provides insight into the widest range of participation opportunities for civil society regarding specific projects, this paper considers how certain institutional context of the Mechanism’s stakeholder framework affects the involvement of civil society actors and the implications of this for balanced and fair input legitimacy. The findings suggest that, in practice, the informal corporate-induced sorting of actors into internal and external stakeholders keeps civil society actors outside the CDM’s inner circle, forcing them to voice their concerns regarding specific projects via CDM insiders or through irregular channels. Furthermore, the absence of a clear definition of stakeholder in local consultations results in the inclusion of unsorted actors, destabilizing the distribution of participation opportunities. The paper concludes that recasting the deliberative principles of openness and plurality into the CDM’s corporate-inspired stakeholding creates a specific institutional context that imposes more than one set of perhaps incompatible stakeholder categories while impairing input legitimacy.