CRAFT: Conflict-Resolved Aggregation for Federated Training

The aggregation of conflicting client updates remains a fundamental bottleneck in federated learning (FL) over heterogeneous data distributions. Naive averaging, as used in FedAvg, often leads to destructive interference in which the global model improves on average but deteriorates significantly for specific clients. In this work, we propose CRAFT (Conflict-Resolved Aggregation for Federated Training), a new aggregation framework that treats the global update as a geometric correction problem. We formulate aggregation as finding the update closest to a reference direction while satisfying conflict-free constraints, ensuring non-negative alignment with the updates of participating clients. We derive a closed-form expression for the constrained optimization problem, avoiding the computational overhead of iterative solvers. Furthermore, we use a layer-wise adaptation to address conflicts at varying feature granularities. We provide a theoretical analysis showing that CRAFT promotes a common-descent structure and mitigates destructive interference through its projection geometry. Extensive experiments on non-IID and imbalanced benchmarks demonstrate that CRAFT significantly reduces performance disparity while maintaining competitive global accuracy against state-of-the-art baselines.