Title: ST-PPO: Stabilized Off-Policy Proximal Policy Optimization for Multi-Turn Agents
Primary area: foundation or frontier models, including LLMs
Keywords: Reinforcement Learning, Multi-turn task, LLM agent, off policy

Abstract: Proximal policy optimization (PPO) has been widely adopted for training large language models (LLMs) at the token level in multi-turn dialogue and reasoning tasks. However, its performance is often unstable and prone to collapse. Through empirical analysis, we identify two main sources of instability in this setting: (1) token-level importance sampling, which is misaligned with the natural granularity of multi-turn environments with distinct turn-level stages, and (2) inaccurate advantage estimates from off-policy samples, where the critic has not learned to evaluate certain state-action pairs, resulting in high-variance gradients and unstable updates. To address these challenges, we introduce two complementary stabilization techniques: (1) turn-level importance sampling, which aligns optimization with the natural structure of multi-turn reasoning, and (2) clipping-bias correction, which normalizes gradients by downweighting unreliable, highly off-policy samples. Depending on how these components are combined, we obtain three variants: Turn-PPO (turn-level sampling only), \textbf{S-PPO} (clipping-bias correction applied to token-level PPO), and \textbf{ST-PPO} (turn-level sampling combined with clipping-bias correction). In our experiments, we mainly study ST-PPO and S-PPO, which together demonstrate how the two stabilization mechanisms address complementary sources of instability. Experiments on multi-turn search tasks show that ST-PPO and S-PPO consistently prevent the performance collapses observed in large-model training, maintain lower clipping ratios throughout optimization, and achieve higher task performance than standard token-level PPO. These results demonstrate that combining turn-level importance sampling with clipping-bias correction provides a practical and scalable solution for stabilizing multi-turn LLM agent training.