Drifting Field Policy: Wasserstein Gradient Flow on Policy Space for Offline-to-Online RL

We propose Drifting Field Policy (DFP), a non-ODE one-step generative policy built on the drifting model paradigm. Under online RL finetuning, we frame the policy update as a reverse-KL Wasserstein-2 gradient flow toward a soft target policy, realized by DFP as a steepest-descent step on probability space. By construction, this gradient step is decomposed into an ascent toward higher action-value regions and a score matching with the anchor policy as a trust region. We further derive a simple, tractable surrogate of the otherwise intractable update loss, akin to behavior cloning on top-K critic-selected actions. We find empirically that this mechanism uniquely benefits the drifting backbone owing to its non-ODE parameterization. With one-step inference, DFP achieves state-of-the-art performance on several manipulation tasks across Robomimic and OGBench, outperforming ODE-based policies.