During blood clot formation, the architecture of the growing aggregate is heterogeneous, with areas of densely and loosely packed cells called platelets.  The surface of activated platelets facilitates biochemical coagulation reactions, ultimately resulting in the formation of a stabilizing polymer network made of fibrin.  How platelet-plug microstructure and flow jointly govern the onset and development of the fibrin network is not completely understood.  Here, I will present a 2D computational framework that integrates a pre-adhered, discrete platelet aggregate, a reduced coagulation model on discrete platelet surfaces, and a fibrin polymerization model under physiologically relevant shear conditions.  By treating platelets as discrete surfaces rather than a continuum, we can investigate how the free fluid volume to platelet surface area ratio in the clot affects substrate uptake and binding/unbinding to platelet surfaces.  Our results suggest that early fibrin gelation dynamics are influenced by both flow and intraplug architecture and that chemical accumulation and transport within the aggregate determines the location of the initial fibrin gel.  This framework can provide a basis for future studies of platelet–coagulation interactions under flow and guide future microfluidic experiments.