Can It Actually Work? Synthesizing Functional CAD Mechanisms

LLM agents can construct sophisticated CAD models, but visually plausible assemblies need not fulfill their intended functions. We study functional CAD design, in which agents synthesize and revise CAD mechanisms to satisfy functional requirements in physics simulation. The same requirement can be realized by different mechanisms, so testing must account for each candidate’s controls, components, and operating sequence. To study this problem, we introduce FunCAD-Harness , a harness that connects functional requirements to candidate-specific simulation tests and uses their execution results to guide design revision. We also introduce FunCADBench, spanning 46 mechanism families with dependent and repeated operations, and evaluate frontier agents and baseline workflows. Our experiments show that simulation-guided revision through our harness exposes mechanical failures that are difficult to detect visually and improves functional task success. We further find that strong visual 3D modeling capabilities do not necessarily translate into superior functional design, with less expensive models achieving competitive performance at lower API cost. Together, our benchmark and harness take a step toward advancing LLM agents from geometric modeling to the design of mechanisms that address practical needs.