arXiv Machine Learning
Oct 22

EPIC: Rethinking Trust Regions for Autoregressive LLM Policies via Exact-KL Prefix Importance-Ratio Clipping

EPIC introduces a new PPO-style clipping method that operates at the prefix level rather than token level, using the exact reverse-KL coordinate to regulate policy drift during autoregressive generation. By reformulating generation as a finite-horizon Markov decision process, the authors derive a performance-difference identity that identifies the cumulative likelihood ratio of the generated prefix as the exact importance weight for each policy-improvement step. Experiments on off‑policy reinforcement learning with a sparse mixture‑of‑experts language model demonstrate that EPIC consistently outperforms standard and sequence‑level clipping baselines on mathematical‑reasoning benchmarks while remaining competitive with routing‑replay alternatives.

By Chengxi Li, Hanlin Wu, Zhicheng Cai, Huan-ang Gao, Wei-Ying Ma, Jingjing Liu, Hao Zhou