ProFit: Leveraging High-Value Signals in SFT via Probability-Guided Token Selection

TL;DR

ProFit introduces a probability-guided token selection method that masks low-probability tokens during supervised fine-tuning of LLMs, reducing overfitting and improving performance on reasoning and math tasks.

Contribution

It reveals the link between token probability and semantic importance and proposes a novel masking strategy to enhance SFT without requiring multiple references.

Findings

ProFit outperforms traditional SFT on reasoning benchmarks.

Masking low-probability tokens reduces overfitting.

The approach improves model generalization in reasoning and math tasks.

Abstract

Supervised fine-tuning (SFT) is a fundamental post-training strategy to align Large Language Models (LLMs) with human intent. However, traditional SFT often ignores the one-to-many nature of language by forcing alignment with a single reference answer, leading to the model overfitting to non-core expressions. Although our empirical analysis suggests that introducing multiple reference answers can mitigate this issue, the prohibitive data and computational costs necessitate a strategic shift: prioritizing the mitigation of single-reference overfitting over the costly pursuit of answer diversity. To achieve this, we reveal the intrinsic connection between token probability and semantic importance: high-probability tokens carry the core logical framework, while low-probability tokens are mostly replaceable expressions. Based on this insight, we propose ProFit, which selectively masks low-probability tokens to prevent surface-level overfitting. Extensive experiments confirm that ProFit consistently outperforms traditional SFT baselines on general reasoning and mathematical benchmarks.