On the Interplay of Pre-Training, Mid-Training, and RL on Reasoning Language Models
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We build controlled experimental setups to disentangle how each training stage—pre-training, mid-training, and RL—contributes to a model’s reasoning ability. Our findings challenge common assumptions: RL only generates true capability gains when operating at the model’s competence boundary; mid-training is a powerful yet overlooked driver of generalization under fixed compute; and process-aware rewards curb reward hacking while enhancing reasoning fidelity. Together, this work provides a clear blueprint for building more reliable, reasoning-centric language models.

