Crowdfunding through donations has become a significant tool for inclusive financing and digital philanthropy. However, prior research has focused on donation intention rather than realized donation behavior. To address this gap, this study investigates the determinants of actual donation behavior using an Activity Theory perspective, conceptualizing crowdfunding participation as a socio-technical activity system embedded within institutional contexts. Using survey data from 284 crowdfunding users (160 from Japan and 124 from Indonesia) and applying structural equation modeling (SEM-PLS), this study investigates how platform trust, crowd familiarity, upstream reciprocity, and downstream reciprocity influence crowdfunding readiness and actual donation outcomes. The findings reveal a readiness–action gap in Indonesia, where crowdfunding readiness does not significantly translate into actual donation behavior, whereas readiness significantly predicts realized donations in Japan. Trust and downstream reciprocity positively influence readiness in both countries, while upstream reciprocity shows contextual variation. These results suggest that the transformation of readiness into action depends not only on individual motivation but also on institutional maturity, governance transparency, and systemic alignment. From a sustainable development perspective, the findings show how improving economic accessibility, regulatory clarity, and trust infrastructure can strengthen crowdfunding's contribution to SDGs 1, 10, and 17.