The harness packages each record into a task with a hidden answer. The AI returns a design. The physics grader meshes it, runs the same solver with the same loads and boundary conditions, and checks the same spec limit. Open solvers only: CalculiX for structural and thermal, OpenFOAM for flow.
A baseline agent that reads CAD, runs the simulation, edits geometry and retries sets the bar, so you can see how far a raw model is from a working loop.