Through the Valley: Path to Effective Long CoT Training for Small
  Language Models
Authors: name: Renjie Luo, name: Jiaxi Li, name: Chen Huang, name: Wei Lu

Long chain-of-thought (CoT) supervision has become a common strategy to
enhance reasoning in language models. While effective for large models, we
identify a phenomenon we call Long CoT Degradation, in which small language
models (SLMs; <=3B parameters) trained on limited long CoT data experience
significant performance deterioration. Through extensive experiments on the
Qwen2.5, LLaMA3 and Gemma3 families, we demonstrate that this degradation is
widespread across SLMs. In some settings, models trained on only 8k long CoT
examples lose up to 75% of their original performance before fine-tuning.
Strikingly, we further observe that for some particularly small models, even
training on 220k long CoT examples fails to recover or surpass their original
performance prior to fine-tuning. Our analysis attributes this effect to error
accumulation: while longer responses increase the capacity for multi-step
reasoning, they also amplify the risk of compounding mistakes. Furthermore, we
find that Long CoT Degradation may negatively impacts downstream reinforcement
learning (RL), although this can be alleviated by sufficiently scaled
supervised fine-tuning (SFT). Our findings challenge common assumptions about
the benefits of long CoT training for SLMs and offer practical guidance for
building more effective small-scale reasoning models.