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This paper introduces $R^3$, a novel approach that enables vision-language models (VLMs) to reason in natural language, enhancing robotic manipulation tasks that require complex decision-making. By mid-training VLMs on expert-generated reasoning traces and refining them through single-step reinforcement learning, the authors demonstrate significant improvements in exploration and generalization for long-horizon tasks. The results show that free-form language reasoning can effectively guide low-level policies, outperforming traditional imitation learning methods in controlled environments like Language Table and simulated bimanual grocery packing.
Free-form language reasoning transforms VLMs into powerful robotic reasoners, significantly boosting performance in complex manipulation tasks.
Reasoning in language allows foundation models to spend more test-time compute on hard problems, such as those requiring decomposition, constraint tracking, and prediction of future consequences. Whether this mechanism can improve robotic manipulation remains unclear, where long-horizon tasks require tracking partial progress, reasoning about object relations, recovering from mistakes, and steering noisy low-level policies. In this paper, we study whether VLMs can be trained to reason directly in natural language to guide low-level manipulation policies. We introduce $R^3$, a simple post-training recipe that turns off-the-shelf VLMs into robotic reasoners: it first mid-trains a VLM on expert-generated reasoning traces to initialize the desired reasoning style, then improves the reasoner with single-step rubric-based RL from offline action data. Unlike prior robotic reasoning methods that mostly use structured traces as auxiliary supervision, $R^3$ trains free-form language reasoning to produce test-time guidance for action. We instantiate $R^3$ on Language Table and simulated bimanual grocery packing, two controlled testbeds for studying robotic reasoning and long-horizon manipulation. $R^3$ improves exploration and generalization across unseen tasks and significantly outperforms instruction-only imitation learning baselines on both benchmarks. Our analyses suggest that free-form language reasoning can function as a test-time compute mechanism for steering low-level policies. Our project page is available at https://robotic-reasoner.github.io/.
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