SCOPE: Compress Mathematical Reasoning Steps for Efficient Automated Process Annotation
To address the prohibitively high annotation cost of Process Reward Models (PRMs) for mathematical reasoning, this paper proposes a compression-driven automated annotation framework. It converts natural-language reasoning steps into AST-normalized code, identifies and merges semantically equivalent steps, and constructs an equivalence-step prefix tree to enable compression-aware high-quality sample generation. By replacing inefficient Monte Carlo sampling, the method reduces annotation complexity from *O(NMK)* to *O(N)*, achieving a 20× speedup—generating a 196K-sample high-quality dataset using only 5% of the computational budget. On both Best-of-N and ProcessBench benchmarks, it outperforms all existing automated annotation methods. Crucially, it is the first approach to jointly improve PRM training data quality and annotation efficiency, establishing a new paradigm for low-cost, high-fidelity alignment of mathematical reasoning.