The advent of Multi-Disciplinary Analysis and Optimization environments (MDAO) has put increased emphasis on the automatic generation of configuration boundary representations directly from a feature tree (build recipe) and a set of externally-driven parameters. Many commercial computer-aided design systems exist to satisfy this need, but all currently suffer for several limitations: the build recipe is encoded in files with proprietary formats that are hard to generate and/or modify externally; the primitives are pre-defined and it is very difficult to create fully-parametric user-defined primitives; the build process is not explicitly "differentiated", thus making the gradients required by optimizers available only via finite differences; and, the systems are licensed in such a ways that distributed, simultaneous execution on many computers can be cost prohibitive. Described here is an open-source constructive solid modeler, named OpenCSM, that has been developed to circumvent these limitations. OpenCSM, which is built upon the OpenCASCADE geometry kernel and the EGADS geometry generation system, is freely available on virtually any computing system. The use of OpenCSM as part of NASA's OpenMDAO optimization system and as the basis for automatic overset grid generation is demonstrated.