GrowBrep: Growing Closed B-Rep Shells One Face at a Time under Grammar Constraints

Boundary representation (B-Rep) is the native format of computer-aided design (CAD), and generating it means producing continuous geometry and discrete topology at the same time, with topology as a hard constraint. A B-rep is usable downstream only if it closes into a solid, yet existing generators can only check closure after generation. We present GrowBrep, an autoregressive B-rep generator that makes closure a property of its sequence grammar. GrowBrep serializes a solid as a shell that grows one face at a time along a frontier of open edges. Each edge is introduced exactly once and closed exactly once by a second half-edge, and the sequence cannot end while any edge remains open, so every sequence the model is allowed to finish describes a closed, consistently oriented shell. The grammar does not constrain geometry, which the model still has to learn. Geometry enters the sequence through per-primitive autoencoders with a finite scalar quantization bottleneck. A decoder-only transformer generates the sequence, conditioned on a complexity token and optionally on an input point cloud, and illegal tokens are masked out at every decoding step. On ABC-1M, 81.7% of GrowBrep's generation attempts yield valid watertight solids, versus 49.0% for the strongest baseline, under one strict validity checker applied to every attempt of every method without rejection sampling. On the distribution-level metrics (COV, MMD, and JSD) it is comparable to the strongest baselines. Conditioned on point clouds, it produces a valid solid for 80.0% of the inputs, compared with 51.6% for the closest prior method on its own published test set, at $65\times$ lower cost per cloud. Drawing five independent samples per cloud and keeping the first valid solid raises validity to 95.8%.