StepOPSD: Step-Aware Online Preference Distillation for Agent Reinforcement Learning
This work addresses the credit assignment mismatch between sparse trajectory rewards and critical local actions in multi-turn agent reinforcement learning. It proposes StepOPSD, a novel framework that refines credit assignment to the level of individual agent steps for the first time. StepOPSD guides GRPO policy updates through step-level trajectory decomposition, hindsight-augmented teacher context re-scoring, sign-preserving advantage shaping, and normalized credit budgeting. Additionally, it introduces a dual-parameter control mechanism—α_clip and λ_mix—to modulate learning dynamics. The method achieves state-of-the-art performance on ALFWorld (e.g., 79.1% on Heat, 95.0% on PickTwo) and Search-QA (61.6% on TriviaQA). Empirical analysis further reveals that α_clip stabilizes local trust regions, while λ_mix exhibits task-dependent tuning characteristics.