arXiv AI By Shiye Lei, Zhihao Cheng, Dacheng Tao

Stabilizing Off-policy LLM Optimization with Prefix Importance Ratio

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The paper addresses instability in off‑policy reinforcement learning for large language models (LLMs) caused by large policy lag. It shows that the correct correction for the prefix‑marginal policy gradient is the cumulative prefix importance ratio, which token‑level weighting fails to capture. To improve stability, the authors propose Minimum Prefix Ratio (MinPRO), a surrogate objective that uses the minimum token‑level ratio in the preceding prefix, and demonstrate its effectiveness on dense and mixture‑of‑experts LLMs across mathematical reasoning benchmarks.

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