Q-Steer: Action-Value Guidance for Molecular Policy Optimization
This work addresses the myopic optimization problem in molecular design, where sparse rewards are only available upon complete molecule generation, making it difficult for policy-based methods to evaluate the contribution of intermediate actions. To overcome this, the authors propose Q-Steer, a general framework that guides token selection during rollout by leveraging an offline-trained and frozen Prefix-Action Value Scorer (PAVS-Q). This scorer estimates the expected future reward for each candidate SMILES token given the current prefix, and the normalized value advantage is incorporated into the sampling logits to steer generation. Without modifying the online optimizer or increasing the oracle query budget, Q-Steer consistently improves performance across diverse policy and language model backbones. On the PMO23 benchmark with a fixed budget of 10,000 oracle calls, it achieves gains in mean valid unique score ranging from +0.033 to +0.049 across all eight backbone–optimizer combinations, outperforming baselines in 18–20 out of 20 tasks per setting.