build123d is a Python-based parametric boundary representation modeling framework built on the Open Cascade geometric kernel. The library provides a clean, fully Pythonic interface for creating precise 2D and 3D models suitable for 3D printing, CNC machining, and laser cutting. It offers expressive algebraic modeling with operator-driven design logic.

Designed for modern CAD-as-code workflows, build123d features minimal internal state, explicit geometry classes, and standards-compliant code with rich type hints. The framework supports extensibility through subclassing without monkey patching and includes deep Python integration with natural conversions and iterable operations. Users can construct complex models using algebraic operators like obj += sub_obj and Plane.XZ * Pos(X=5) * Rectangle(1, 1).

Two interfaces are available: Algebra Mode for stateless, explicit modeling and Builder Mode for design-history-like workflows with tracked state. The library handles multiple geometry types including Edges, Wires, Curves, Faces, Shells, Sketches, Solids, and Parts. It supports import/export of various formats like STL, STEP, and SVG for data interchange with CAD tools and manufacturing workflows. build123d is actively developed on GitHub with comprehensive documentation and examples.