Title: SPARK: Stepwise Process-Aware Rewards for Reference-Free Reinforcement Learning
Primary area: foundation or frontier models, including LLMs
Keywords: Process Reward Models, Inference-time Scaling, Reference-free Reinforcement Learning, Mathematical Reasoning, Synthetic Verification

Abstract: Process reward models (PRMs) that provide dense, step-level feedback have shown promise for reinforcement learning, yet their adoption remains limited by the need for expensive step-level annotations or ground truth references. We propose SPARK--a three-stage framework where in the first stage a generator model produces diverse solutions and a verifier model evaluates them using parallel scaling (self-consistency) and sequential scaling (meta-critique). In the second stage, we use these verification outputs as synthetic training data to fine-tune generative process reward models, which subsequently serve as reward signals during training. We show that aggregating multiple independent verifications at the step level produces training data for process reward models that surpass ground-truth outcome supervision—achieving 67.5 F1 on ProcessBench (a benchmark for identifying erroneous steps in mathematical reasoning) compared to 66.4 for reference-guided training and 61.9 for GPT-4o. In the final stage, we apply our generative PRM with chain-of-thought verification (PRM-CoT) as the reward model in RL experiments on mathematical reasoning, and introduce format constraints to prevent reward hacking. Using Qwen2.5-Math-7B, we achieve 47.4\% average accuracy across six mathematical reasoning benchmarks, outperforming ground-truth-based RLVR (43.9\%). Our work enables reference-free RL training that exceeds ground-truth methods, opening new possibilities for domains lacking verifiable answers or accessible ground truth.

TL;DR: We train process reward models without ground truth by aggregating multiple verification attempts through inference-time scaling, achieving better performance than ground-truth-based approaches.