这篇论文发现RL训练只需调优一层Transformer就能接近全参数效果，还揭秘中间层才是关键，刷新认知。
本文基于Qwen3和Qwen2.5共7个模型，使用GRPO、GiGPO、Dr. GRPO三种RL算法，在数学推理、代码生成和智能体决策任务上实验。研究发现训练单层Transformer即可恢复全参数RL训练大部分甚至全部改进。作者引入“层贡献”指标量化单层训练相比全参数训练的收益比例。结果显示RL收益高度集中于少数中间层，输入输出层贡献显著较低。
Is One Layer Enough? Training A Single Transformer Layer Can Match Full-Parameter RL Training
Reinforcement learning (RL) has become a central component of post-training large language models (LLMs), yet little is understood about how RL adaptation is distributed across transformer layers. Existing approaches typically update all model parameters uniformly, implicitly assuming that every layer contributes similarly to the gains obtained during RL post-training. In this work, we challenge this assumption through a systematic layer-wise study of RL training. Surprisingly, we find that training a single transformer layer can recover most of the gains achieved by full-parameter RL training, and in some cases even surpass it. To quantify this phenomenon, we introduce the quantity layer contribution, which measures the fraction of full RL improvement recovered by training a layer in isolation. Across seven models spanning two model families (Qwen3, Qwen2.5), three RL algorithms (GRPO, GiGPO, Dr. GRPO), and multiple task domains including mathematical reasoning, code generation, and agentic decision-making, we observe a remarkably stable pattern: RL gains are highly concentrated in a small subset of, and in many cases even a single, transformer layers. More strikingly, the same structural pattern consistently emerges: high-contribution layers concentrate in the middle of the transformer stack, while layers near the input and output ends contribute substantially less. The resulting layer rankings remain strongly correlated across datasets, tasks, model families, and RL algorithms.