The discourse of social organizing in projects centres on temporary-permanent relationships across projects and between projects and embedded organizations. Less explored are temporary-temporary relationships within project ecologies. We investigate the practice of collaborative consumption in project ecologies as a form of such relationships. Specifically, we focus on tensions arising from contradictory logics of cooperation and competition in these temporary and ad-hoc strategic interactions. Based on the case of the construction industry, our results elaborate how coopetitive tensions shape the extent and forms of these ad-hoc temporary interactions among stakeholders to guide innovation in project supply chains. Beyond the limitations of broad strategic collaborations, these innovations promote the prospects for joint specializations, collective commitments, strategic resource dependences and more permeable organizational boundaries. But organizing these ad-hoc strategic interactions requires additional regulatory frameworks, as well as alternative artifacts, such as a digital platform for brokering structural holes in information relating supply and demand. Our study extends the stakeholder discourse in project studies through empirical research into emerging forms of temporary ad-hoc stakeholder interactions rather than broad strategic collaborations. Understanding these ad-hoc interactions and associated coopetitive tensions instigates a wider range of social organizing in project ecologies.