GRPO-Guard：通过受控截断缓解流匹配中的隐式过优化问题

GRPO-Guard: Mitigating Implicit Over-Optimization in Flow Matching via Regulated Clipping

October 25, 2025

作者: Jing Wang, Jiajun Liang, Jie Liu, Henglin Liu, Gongye Liu, Jun Zheng, Wanyuan Pang, Ao Ma, Zhenyu Xie, Xintao Wang, Meng Wang, Pengfei Wan, Xiaodan Liang

cs.AI

摘要

近期，基于GRPO的强化学习在优化流匹配模型方面取得显著进展，有效提升了模型与任务特定奖励的匹配度。这类框架通过重要性比例剪裁机制约束过度自信的正负梯度更新。然而实践中我们发现，重要性比例分布存在系统性偏移——其均值低于1且不同时间步的方差差异显著。这种左偏且不稳定的分布阻止了正优势样本进入剪裁区域，导致机制无法有效约束过度自信的正向更新。因此策略模型不可避免地进入隐式过优化阶段：虽然代理奖励持续上升，但图像质量、文本提示对齐等关键指标急剧恶化，最终使学习到的策略无法实际应用。 为解决该问题，我们提出GRPO-Guard——一种对现有GRPO框架简单而有效的增强方案。该方法通过比率归一化技术重建平衡且时序一致的重要性比例，确保PPO剪裁机制能有效约束去噪过程中所有时间步的有害更新。同时，梯度重加权策略通过均衡不同噪声条件下的策略梯度，防止特定时间步区域产生过度更新。这些设计共同构成受控剪裁机制，在无需重度KL正则化的情况下稳定优化过程，显著缓解隐式过优化现象。基于多种扩散模型骨干（如SD3.5M、Flux.1-dev）和多样化代理任务的实验表明，GRPO-Guard在维持甚至提升生成质量的同时，能显著降低过优化现象。

English

Recently, GRPO-based reinforcement learning has shown remarkable progress in optimizing flow-matching models, effectively improving their alignment with task-specific rewards. Within these frameworks, the policy update relies on importance-ratio clipping to constrain overconfident positive and negative gradients. However, in practice, we observe a systematic shift in the importance-ratio distribution-its mean falls below 1 and its variance differs substantially across timesteps. This left-shifted and inconsistent distribution prevents positive-advantage samples from entering the clipped region, causing the mechanism to fail in constraining overconfident positive updates. As a result, the policy model inevitably enters an implicit over-optimization stage-while the proxy reward continues to increase, essential metrics such as image quality and text-prompt alignment deteriorate sharply, ultimately making the learned policy impractical for real-world use. To address this issue, we introduce GRPO-Guard, a simple yet effective enhancement to existing GRPO frameworks. Our method incorporates ratio normalization, which restores a balanced and step-consistent importance ratio, ensuring that PPO clipping properly constrains harmful updates across denoising timesteps. In addition, a gradient reweighting strategy equalizes policy gradients over noise conditions, preventing excessive updates from particular timestep regions. Together, these designs act as a regulated clipping mechanism, stabilizing optimization and substantially mitigating implicit over-optimization without relying on heavy KL regularization. Extensive experiments on multiple diffusion backbones (e.g., SD3.5M, Flux.1-dev) and diverse proxy tasks demonstrate that GRPO-Guard significantly reduces over-optimization while maintaining or even improving generation quality.