Trajectory Bellman Residual Minimization: A Simple Value-Based Method for LLM Reasoning (2505.15311v1)

Abstract: Policy-based methods currently dominate reinforcement learning (RL) pipelines for LLM reasoning, leaving value-based approaches largely unexplored. We revisit the classical paradigm of BeLLMan Residual Minimization and introduce Trajectory BeLLMan Residual Minimization (TBRM), an algorithm that naturally adapts this idea to LLMs, yielding a simple yet effective off-policy algorithm that optimizes a single trajectory-level BeLLMan objective using the model's own logits as $Q$-values. TBRM removes the need for critics, importance-sampling ratios, or clipping, and operates with only one rollout per prompt. We prove convergence to the near-optimal KL-regularized policy from arbitrary off-policy data via an improved change-of-trajectory-measure analysis. Experiments on standard mathematical-reasoning benchmarks show that TBRM consistently outperforms policy-based baselines, like PPO and GRPO, with comparable or lower computational and memory overhead. Our results indicate that value-based RL might be a principled and efficient alternative for enhancing reasoning capabilities in LLMs.