Constraint Checking is the process of verifying that a system, solution, or generated output satisfies a defined set of rules, conditions, or boundaries. In AI and optimization research, it is applied to ensure that model outputs remain within feasible solution spaces, that agentic systems follow prescribed operational boundaries, and that generated code or plans meet logical or structural requirements. It plays a central role in constrained optimization, formal verification, safe AI deployment, and solver-based reasoning pipelines.