arXiv AI By Taehyeon Kim, Hyunsoo Lee, Youngsoo Jang, Moontae Lee

Efficiently Learning To Reason or Not to Reason: Root-token Policy Optimization for Adaptive Thinking

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The paper introduces Root-token Policy Optimization (RPO), a framework that lets large reasoning models decide when to engage in explicit reasoning by training only the initial root token. RPO uses group-relative rewards and advantages to focus on this branching point, dramatically cutting training overhead and VRAM usage. Experiments across various model families show that RPO achieves difficulty-aware adaptive thinking with roughly 2% of the compute required by previous adaptive-reasoning methods.

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