Bellman Policy Optimization

Hugging Face Daily Papers Papers

Summary

Bellman Policy Optimization (BPO) is a critic-free reinforcement learning method that reformulates Policy Mirror Descent using the Bellman equation for autoregressive generation with terminal rewards, improving mathematical reasoning in large language models.

Reinforcement learning with verifiable rewards (RLVR) improves the reasoning capabilities of large language models (LLMs). We introduce Bellman Policy Optimization (BPO), a critic-free method derived from Policy Mirror Descent (PMD). For autoregressive generation with terminal rewards, BPO uses the Bellman equations to reformulate PMD as a trajectory-level objective. The reformulation avoids estimating state values at intermediate states. We prove that it has the same unique optimal solution as the original PMD objective. We derive the practical BPO loss by approximating this objective. Its mismatch-correction weight is a smoothed ratio of complementary token probabilities. Experiments on mathematical reasoning benchmarks demonstrate the effectiveness of BPO.
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Source: https://huggingface.co/papers/2609.15987

Abstract

Reinforcementlearningwithverifiablerewards(RLVR)improvesthereasoningcapabilitiesoflargelanguagemodels(LLMs).WeintroduceBellmanPolicyOptimization(BPO),acritic-freemethodderivedfromPolicyMirrorDescent(PMD).Forautoregressivegenerationwithterminalrewards,BPOusestheBellmanequationstoreformulatePMDasatrajectory-levelobjective.Thereformulationavoidsestimatingstatevaluesatintermediatestates.WeprovethatithasthesameuniqueoptimalsolutionastheoriginalPMDobjective.WederivethepracticalBPOlossbyapproximatingthisobjective.Itsmismatch-correctionweightisasmoothedratioofcomplementarytokenprobabilities.ExperimentsonmathematicalreasoningbenchmarksdemonstratetheeffectivenessofBPO.

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