The Emperor's New Reasoning: Format Imitation Overshadows Genuine Mathematical Understanding in SFT

Recent advances in large language models (LLMs) have yielded impressive gains on mathematical reasoning benchmarks via supervised fine-tuning (SFT). However, the brittleness of these models under input perturbations has cast doubt on whether such improvements reflect genuine reasoning abilities or merely superficial alignment with expected output formats. We investigate the mechanisms behind SFT improvements in small-scale LLMs, addressing four key questions: (1) Are performance gains primarily due to format alignment rather than reasoning? (2) Can high-quality supervision encourage genuine reasoning? (3) Does scaling data shift learning from format alignment to deeper reasoning? (4) Are format alignment gains consistent across model sizes and architectures? Through controlled experiments, we find that most performance improvements arise from format alignment rather than genuine reasoning enhancement. Moreover, SFT's effectiveness is strongly influenced by the alignment between the base model's inductive biases and the teacher model's output distribution, rather than the teacher's raw strength. Finally, scaling up training data offers diminishing returns and does not fundamentally alter the model's reasoning behavior. These findings suggest that current SFT practices may overestimate the reasoning abilities of LLMs and underscore the need for more rigorous evaluation methods.