CSPO: Constraint-Sensitive Policy Optimization for Safe Reinforcement Learning

Safe reinforcement learning (Safe RL) aims to maximize expected return while satisfying safety constraints, typically modeled as constrained Markov decision processes. While primal-dual methods scale well to deep RL, they often suffer from delayed constraint correction, leading to oscillatory behavior and prolonged safety violations. In this paper, we propose Constraint-Sensitive Policy Optimization (CSPO), a first-order primal-dual method that incorporates local constraint sensitivity into policy updates. CSPO augments the primal objective with a constraint-sensitive correction derived from the shortest signed distance to the safety boundary, enabling smarter recovery steps back to safety, compensating for delayed Lagrange multiplier updates, and reducing oscillations near the boundary, while preserving the KKT solutions of the original constrained problem. Extensive experiments on navigation and locomotion benchmarks demonstrate that CSPO achieves faster safety recovery and high reward preservation, resulting in higher constrained returns (+15.6\% average improvement) compared to state-of-the-art primal-dual and penalty-based methods.