While a wide range of managerial practices suggest that coopetition plays a crucial role in advancing firm innovation, how this effect occurs and the boundary conditions remain unclear. The literature revealing the specific mechanisms by which inter-firm coopetition affects firm innovation, including mediating mechanisms and boundary conditions, is still insufficient. By integrating the resource dependence theory and the capability view, this study explores how firm agility links inter-firm coopetition and open innovation within industrial clusters. In addition, based on conceptualizing coopetition as a concept containing three elements (cooperation, constructive conflict, and destructive conflict), this study examines government intervention in industrial clusters as a boundary factor and explores how it affects the relationship between inter-firm coopetition and firm agility. Based on the analysis of a sample of 181 industrial cluster firms in China, the results of this study show that firm agility mediates the relationship between cooperation, constructive conflict, and open innovation, respectively, and that government intervention diminishes both the facilitating effect of constructive conflict on firm agility and the negative effect of destructive conflict on firm agility. The findings contribute to the understanding of how and when coopetition affects open innovation and provide a theoretical basis for firms to utilize coopetition to innovate successfully.