<p>This study investigates the differential gains from regionalization by examining how institutional forms and border effects shape regional cooperation’s impact on interurban capital flows in China’s Yangtze River Delta (YRD). Drawing on dataset of cooperation news and investment events, it investigates whether, how, and for whom different modes of regional cooperation facilitates capital redistribution. The findings indicate that interprovincial mechanisms, particularly intercity alliances and official exchanges, significantly promote capital flows from core to peripheral cities. Yet the redistributive effects are highly uneven. Only a subset of strategically positioned border-zone cities that are empowered by delegated authority and embedded within cross-regional industrial alliances secure substantial capital inflows. This spatial selectivity does not indicate governance failure but reflects a state-led strategy of targeted spatial development. In contrast to the hopeful assumption of inclusive trickle-down benefits, regional cooperation in China follows a logic of experimental regionalism, privileging a limited number of institutionally prepared peripheral cities as pilot for cross-provincial integration for core-led capital spillovers. Over time, such selective inclusion may contribute to broader core–periphery convergence, but only through a phased, state-orchestrated process of institutional coordination.</p>