Beyond Objective Equivalence: Constraint Injection for LLM-Based Optimization Modeling on Vehicle Routing Problems

In three linesVRPCoder, an 8B model, translates natural-language vehicle routing scenarios into Gurobi code. Authors propose constraint injection to verify constraints are neither silently omitted nor spuriously added. With GRPO, VRPCoder reaches 93% Pass@1, outperforming Claude-Sonnet-4.5 by 28 points on VRP benchmarks.

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