LightReasoner: Can Small Language Models Teach Large Language Models
  Reasoning?
Authors: name: Jingyuan Wang; name: Yankai Chen; name: Zhonghang Li; name: Chao Huang
Published: 2025-10-09 08:55:12

Abstract: Large language models (LLMs) have demonstrated remarkable progress in
reasoning, often through supervised fine-tuning (SFT). However, SFT is
resource-intensive, relying on large curated datasets, rejection-sampled
demonstrations, and uniform optimization across all tokens, even though only a
fraction carry meaningful learning value. In this work, we explore a
counterintuitive idea: can smaller language models (SLMs) teach larger language
models (LLMs) by revealing high-value reasoning moments that reflect the
latter's unique strength? We propose LightReasoner, a novel framework that
leverages the behavioral divergence between a stronger expert model (LLM) and a
weaker amateur model (SLM). LightReasoner operates in two stages: (1) a
sampling stage that pinpoints critical reasoning moments and constructs
supervision examples capturing the expert's advantage through expert-amateur
contrast, and (2) a fine-tuning stage that aligns the expert model with these
distilled examples, amplifying its reasoning strengths. Across seven
mathematical benchmarks, LightReasoner improves accuracy by up to 28.1%, while
reducing time consumption by 90%, sampled problems by 80%, and tuned token
usage by 99%, all without relying on ground-truth labels. By turning weaker
SLMs into effective teaching signals, LightReasoner offers a scalable and
resource-efficient approach for advancing LLM reasoning. Code is available at:
https://github.com/HKUDS/LightReasoner