smlibpy

A Python library for 3D modeling on the SMLib NURBS kernel with an API inspired by cadquery and build123d.

It comes with an MCP server, so your agent can build parts too.

from smlibpy import *

plate = Box(80, 60, 10)
plate = plate.fillet(plate.edges().parallel_to(Z), 8)
plate -= GridLocations(60, 40, 2, 2) * Cylinder(4, 10)
plate -= Rectangle(30, 20).on(plate.faces().top()).extrude(-4)

plate.export("plate.step")
show(plate)

Parts are B-reps with exact surfaces: planes, cylinders, cones, spheres, tori and NURBS. Volumes, areas and inertia are integrated over those surfaces, and STEP files keep them as they are. You combine shapes with +, - and &, and every feature returns a new part. Faces and edges are picked by their geometry, for example part.faces().top() or part.edges().parallel_to(Z).concave(). The plate above builds in about 40 ms.

Features

Modeling

• Primitives: Box, Cylinder, Cone, Sphere, Torus, Wedge
• Holes: Hole, CounterBoreHole, CounterSinkHole, placed with Locations, GridLocations, PolarLocations, HexLocations
• Booleans: fuse, cut, intersect, which can also be written +, -, &
• Operations: extrude (also tapered), revolve, loft, sweep (also along helices), shell, offset, thicken, draft, mirror, split, section
• Fillets: circular, variable radius, and curvature-continuous (G2/G3); chamfers

Sheet metal

• fold and flange with exact bends (radius and K-factor)
• unfold, and flat_pattern with bend lines
• bends(), bend_allowance, bend_deduction, developed_length

Deformation

• twist, taper and bend work on finished parts. A tapered cylinder becomes an exact cone.
• Free-form deformation with a Lattice

Simplifying: remove_fillets and remove_faces take out fillets, holes and bosses, also on imported parts. faces().fillets() finds the fillet faces.

Molding: draft tilts walls, pockets and holes by a draft angle about a neutral plane. draft_analysis sorts each face for a pull direction into positive, insufficient, vertical or undercut. parting_line finds the parting line.

2D

• Profile is a pen: it draws lines and tangent arcs, turns, and adds fillets and chamfers at corners as it goes.
• Shapes: Rectangle, RoundedRectangle, Circle, Ellipse, Polygon, RegularPolygon, Triangle, Trapezoid, Slot, ArcSlot, Hull, and Stroke (a band along a path). The polygon shapes take fillet= and chamfer=.
• Exact 2D Booleans, offsets, and corner fillets and chamfers. Sketches can be extruded, revolved and swept.
• Text with exact font outlines (the Lato font is included)

Curves and surfaces

• Curves: Line, Polyline, PolarLine, CenterArc, ThreePointArc, RadiusArc, SagittaArc, TangentArc, EllipticalArc, Spline, Bezier, Helix, FilletPolyline
• Surfaces: surface through a grid of points, ruled, stitch (faces into a solid), to_nurbs

Selection

• By position (top(), bottom(), nearest(), …), by size (largest(), smallest()), and by geometry (lines(), circles(), planar(), cylinders(), parallel_to(), facing(), convex(), concave(), fillets(), …)
• filter_by, sort_by, group_by, set operators between lists, and neighbors() of a face

Measuring and checks

• volume, area, center and mass_properties (inertia, principal axes), computed from the exact surfaces to seven significant digits, with an error estimate
• distance_to, closest_point, intersection_curves, silhouette, project, outline, edge radius and convexity
• is_valid and heal(). Every kernel result is checked, and a fillet or shell that can't work raises an error that says what would fit.

Files

• STEP in and out: exact geometry, assemblies, units, names and colors (named(), colored())
• DXF in and out with exact arcs and splines; STL in and out; glTF/GLB and OBJ out
• SMLib's own format (save, load)

Pictures

• render() needs no GPU or browser:
  • shaded views, and sheets of 2 or 4 views
  • technical drawings in first-angle (ISO E) or third-angle (ISO A) projection, with dashed hidden lines and center lines
  • sections, x-ray, highlights, markers, a measurement overlay, zebra stripes and curvature maps
• diff_images compares two renders.
• show() opens a 3D viewer in the browser or a notebook.

Agents

• The MCP server smlibpy-mcp has a live viewer, snapshots, function search and a tested guide for every function. People who want parametric apps that rebuild from sketches and features are sent to the ClassCAD MCP.
• A Claude Code plugin, installed from this repository (see below)

Install

pip install smlibpy

There are wheels for Linux (x86_64, aarch64), macOS (arm64, x86_64) and Windows, for Python 3.10 to 3.14, with the kernel compiled in. They are 5 to 7 MB, and numpy is the only dependency. For agents, pip install "smlibpy[mcp]" adds the MCP server; see AI agents.

To build from source you need CMake and a C++17 compiler (about a minute on a laptop):

git clone --recursive https://github.com/awv-informatik/smlibpy
cd smlibpy
pip install .

For agents

The MCP server lets Claude and other agents model with smlibpy. In Claude Code:

claude plugin marketplace add awv-informatik/smlibpy
claude plugin install smlibpy@smlibpy

Other hosts start uvx --from "smlibpy[mcp]" smlibpy-mcp; see AI agents. The agent works in four steps:

1. A skill tested on the real kernel: every function has a guide, written by agents that ran each call and wrote down what they found. The server hands the guides out, and its search knows CAD synonyms.
2. One script per step: the agent writes a whole step as a Python script and runs it. The session keeps the model between scripts; a kernel crash costs a restart, and the session is rebuilt from the scripts that made it.
3. A renderer to look with: snapshot draws the model in software, as technical drawings with hidden lines, sections, x-ray, highlighted selections, markers, zebra stripes or curvature maps.
4. Loops until it is right: after every step the agent checks numbers and pictures, fixes what is off, compares before and after on a pinned frame, and goes back to a checkpoint when a step fails.

A short tour

Sketches

Sketches are 2D regions. Place them on a plane or a face, then extrude, revolve, loft or sweep them.

profile = Polygon((25, 0), (60, 0), (60, 12), (35, 12), (35, 40), (25, 40))
flange = profile.on(Plane.front).revolve(Z)
flange -= PolarLocations(47.5, 8) * Cylinder(5, 30)

Text

Lettering is a sketch with the exact outlines of a font; size is the height of the capitals.

tag = Box(60, 20, 3)
tag -= Text("SMLIB 01", 8).on(tag.faces().top()).extrude(-0.5)

Selecting edges and faces

Selectors chain and always return lists. A list with one element works wherever a single shape is expected.

flange = flange.fillet(flange.edges().concave(), 4)
flange = flange.chamfer(flange.faces().top().edges().circles(), 1)

Fillets

Fillets can be circular, vary in radius, or be curvature continuous.

box = Box(30, 20, 10)
soft = box.fillet(box.edges(), 3, continuity="G2")
ramp = box.fillet(box.edges().parallel_to(Z).first, (1, 4))

Surfaces

Open curves make surfaces. You can stitch them, thicken them, or start from a grid of points.

import math

grid = [[(x, y, 4 * math.sin(x / 15) * math.cos(y / 15)) for x in range(0, 60, 10)] for y in range(0, 50, 10)]
panel = surface(grid).thicken(2)

Measuring

mp = flange.mass_properties(density=7.85e-3)   # steel, g/mm³
mp.mass, mp.center, mp.inertia

Files

STEP, DXF, STL, glTF/GLB, OBJ and SMLib's own exact format.

export_step([flange, panel.named("panel")], "parts.step")
export_glb(flange, "flange.glb")
export_dxf(flange.section(Plane.XZ), "flange-profile.dxf")

Documentation

• Getting started
• Concepts: shapes, planes, locations, selections
• Sketches, parts, features, surfaces
• Measuring and analysis, files and viewer
• Examples
• The kernel: how smlibpy uses SMLib, accuracy, limitations
• AI agents: the MCP server, so Claude and other agents can model with smlibpy

Status

smlibpy is new (version 0.2). It covers solid and surface modeling, text, and reads and writes STEP and DXF with the exact geometry. Assemblies with product structure and dimensioned 2D drawings are not there yet.

License

smlibpy is MIT licensed. It builds and bundles the SMLib kernel from NVIDIA-Omniverse/usd-brep, © NVIDIA, under the Apache License 2.0; see NOTICE.