On the Design of KL-Regularized Policy Gradient Algorithms for LLM
  Reasoning
Authors: name: Yifan Zhang; name: Yifeng Liu; name: Huizhuo Yuan; name: Yang Yuan; name: Quanquan Gu; name: Andrew C Yao
Published: 2025-05-23 06:01:21

Abstract: Policy gradient algorithms have been successfully applied to enhance the
reasoning capabilities of large language models (LLMs). Despite the widespread
use of Kullback-Leibler (KL) regularization in policy gradient algorithms to
stabilize training, the systematic exploration of how different KL divergence
formulations can be estimated and integrated into surrogate loss functions for
online reinforcement learning (RL) presents a nuanced and systematically
explorable design space. In this paper, we propose regularized policy gradient
(RPG), a systematic framework for deriving and analyzing KL-regularized policy
gradient methods in the online RL setting. We derive policy gradients and
corresponding surrogate loss functions for objectives regularized by both
forward and reverse KL divergences, considering both normalized and
unnormalized policy distributions. Furthermore, we present derivations for
fully differentiable loss functions as well as REINFORCE-style gradient
estimators, accommodating diverse algorithmic needs. We conduct extensive
experiments on RL for LLM reasoning using these methods, showing improved or
competitive results in terms of training stability and performance compared to
strong baselines such as GRPO, REINFORCE++, and DAPO. The code is available at
https://github.com/complex-reasoning/RPG.