arXiv AI By Yinuo Liu

There Is No Free Lunch in RLVR: Every pass@1 Gain Has a pass@k Price

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The paper proves that in reinforcement learning with verifiable rewards (RLVR), any improvement in pass@1 necessarily incurs a cost in pass@k, establishing a precise price for such gains. By modeling RLVR as an exponential tilt of a base policy, the authors show that every update that raises pass@1 strictly lowers success on at least one prompt, and that the number of pass@k crossovers is bounded by the sign variation of per‑prompt success shifts. The work also derives an entropy‑performance law and provides a diagnostic to measure this price from evaluation logs.

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The paper argues that the observed crossover in pass@$k$ performance between reinforcement learning with verifiable rewards (RLVR) and its base model is not always statistically confirmed. By constructing confidence bands across sampling budgets and performing power analyses, the authors show that many reported crossovers lack statistical support and that more prompts can improve detection more than more answers per prompt. They further demonstrate that RLVR’s effect on a prompt is better described as a conditional distribution (a Markov kernel) rather than a single curve, allowing predictions of crossovers in new data and clarifying how losses on difficult prompts can overturn early gains.

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