Cross-sector development partnerships are project-based collaborative arrangements between business, government, and civil society organizations in support of international development goals such as sustainability, health education, and economic development. Focusing on public private partnerships in development cooperation, we examine different constellations of bridging agents and their effects in the formation of single CSDP projects and longer-term alliances. We conceptualize bridging agency as a collective process involving both internal partner representatives and external intermediaries in initiating and/or supporting roles. We find that the (...) involvement of external intermediaries eases the formation of single projects and longer-term alliances. However, when projects are initiated by external intermediaries they tend to be repetitive and narrow in scope, whereas projects initiated by internal partner representatives often explore novel agendas and embody a greater potential for social innovation. Yet, the longer-term pursuit of these agendas beyond single projects may require external intermediaries in supporting roles. Findings help better understand micro-processes and collective practices of brokerage and alliance formation in transnational governance contexts and beyond. (shrink)