Neglected Free Lunch from Post-training: Progress Advantage for LLM Agents

• URL: http://arxiv.org/abs/2606.26080v1
• Date: Wed, 24 Jun 2026 17:54:08 GMT
• Title: Neglected Free Lunch from Post-training: Progress Advantage for LLM Agents
• Abstract summary: We show that reinforcement learning post-training already provides the ingredients for effective step-level scoring.<n>We derive an implicit advantage under a general Markov decision process.<n>We validate the effectiveness of the progress advantage across three different applications.
• Score: 15.278397068400205
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Process reward models enable fine-grained, step-level evaluation of LLMs, yet building them for agentic settings remains prohibitively difficult: long-horizon interactions, irreversible actions, and stochastic environment feedback make both human annotation and Monte Carlo estimation infeasible at scale. In this work, we show that reinforcement learning (RL) post-training already provides the ingredients for effective step-level scoring, eliminating the need for dedicated reward model training altogether. Concretely, we derive an implicit advantage under a general stochastic Markov decision process, which we term progress advantage -- log-probability ratio between the RL-trained policy and its reference policy exactly recovers the optimal advantage function. This formulation makes the resulting signal annotation-free, domain-agnostic, and available as a byproduct of the standard RL post-training pipeline. We validate the effectiveness of the progress advantage across three different applications: test-time scaling, uncertainty quantification, and failure attribution on five benchmarks and four model families. Across all settings, it consistently outperforms confidence-based baselines and, despite requiring no task-specific training, surpasses dedicated trained reward models. We complement these results with deeper analyses on characteristics of progress advantage, offering practical guidance for adoption in real-world agentic systems.

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