arXiv AI By Haodong Liang, Yanhao Jin, Krishnakumar Balasubramanian, Lifeng Lai

Learning Style, Forgetting Semantics: A Case Study of SFT and RFT on Classification Tasks

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The paper investigates why supervised fine-tuning (SFT) causes more forgetting than reinforcement fine-tuning (RFT) on classification tasks where tokens share a semantic class but differ in style. By deriving a linear-softmax policy decomposition, the authors show that SFT and RFT have parallel semantic updates but diverge in style dynamics, with SFT exhibiting off‑axis style drift that leads to a positive lower bound on forgetting. Theoretical results are supported by simulations over task sequences.

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