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smlibpy 0.1·python 3.10 – 3.14


smlibpy is a Python library for 3D modeling on the SMLib NURBS kernel. It comes with an MCP server, so your agent can build parts too.

agent, look at smlibpy.classcad.ch and install the mcp

Your agent finds the steps in /llms.txt. If you only want the library: pip install smlibpy.

Or add it by handClaude Code · Claude Desktop · Codex · VS Code · Cursor · OpenCode

in a terminal

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

reads STEP USD DXFwrites STEP USD DXF STL glTFMIT + Apache 2.0no account

An agent is told: agent, look at smlibpy.classcad.ch and install the mcp. It reads llms.txt and adds the smlibpy server, which runs with uvx and needs no account. In a new session it is asked for a pipe flange: a Ø120 × 12 mm plate with a Ø70 neck, 40 mm tall, bored Ø50, with eight Ø10 holes on a Ø95 circle. It reads the guides, revolves a half section, adds the holes, a fillet and a chamfer, checks the volume (157 979.5 mm³), looks at a snapshot of four views and saves flange.step. The session's live view comes up beside the conversation with the part in it, and the part is turned.

01Python

Your script builds the model. Each line creates a shape or changes it: combine solids with + and −, round edges, cut a pocket. Hover over a line to see the part at that step.

plate.pyline 3
  1. from smlibpy import *
  3. plate = Box(80, 60, 10)
  4. plate = plate.fillet(plate.edges().parallel_to(Z), 8)
  5. plate -= GridLocations(60, 40, 2, 2) * Cylinder(4, 10)
  6. plate -= Rectangle(30, 20).on(plate.faces().top()).extrude(-4)
  8. plate.export("plate.step")
a box, centered on the origin48 000 mm³ · 6 faces

builds in 22 msvolume 43 040 mm³19 faces · validSTEP with the exact geometry

02Features

173 features. Each drawing shows the call under it on a small example, built with smlibpy and drawn from the part it returned. Red is what the call made.

0 features

Parts

Solids from primitives, from sketches and from other solids. Every call returns a new part, and parts combine with +, - and &, or merge into one part of cells. Parts →

smlibpy or ClassCAD?

smlibpy builds a part once, from a script. A part that rebuilds when a value changes, with a feature history, constrained sketches and assemblies, is what ClassCAD is for. ClassCAD →

03Install

There are wheels for Linux, macOS and Windows, for Python 3.10 to 3.14. The kernel is compiled in, so there is nothing else to set up.

In Python

terminal · part.py
  1. $ pip install smlibpy
  3. from smlibpy import *
  5. part = Box(40, 30, 10) - Cylinder(6, 20)
  6. print(part.volume)
  7. part.export("part.step")
  8. show(part)

show() opens the part in your browser, or inline in a notebook. render() saves a PNG instead, for scripts and CI. Getting started has more.

For your agent

terminal
  1. $ claude plugin marketplace add awv-informatik/smlibpy
  2. $ claude plugin install smlibpy@smlibpy

The plugin installs the MCP server and a skill with a guide for every function. The server runs on your machine with uvx and shows the session in Buerligons, the ClassCAD app, after every script; what you select there reaches the agent. Setup for other hosts is at the top of this page and in the docs.

04JavaScript

smlibwasm is smlibpy for JavaScript and TypeScript: the same API on the same kernel, compiled to WebAssembly. It runs in the browser and in Node, without a server and without dependencies.

Install it with npm

terminal · part.js
  1. $ npm install @classcad/smlibwasm
  3. import { Box, Cylinder, toStep } from '@classcad/smlibwasm'
  5. const part = Box(40, 30, 10).cut(Cylinder(6, 20))
  6. console.log(part.volume)
  7. const step = toStep(part)

Names are camelCase, and operators are methods: a.cut(b) for Python’s a - b. The JavaScript docs have the whole API in TypeScript.

1.8 MB over the wire

The kernel, smlibwasm.wasm
1.5 MB
The library
0.21 MB
The font of Text
46 kB
The loader, smlibwasm.mjs
22 kB
Together
1.8 MB

Compressed with brotli, as web servers send it. Browsers cache the kernel, so it loads once. Importing the package waits for the kernel; every call after that is synchronous.

05How it works

Parts are B-reps with exact surfaces: planes, cylinders, cones, spheres, tori and NURBS. Volumes and areas are integrated over those surfaces, so they are accurate to about seven digits.

smlibpy checks what the kernel returns. If a fillet is too large for its edges, you get an error that tells you the largest size that fits.

smlibpy reads and writes STEP itself and keeps the exact geometry. We compared the reader with OpenCASCADE on 430 files.

The MCP server runs scripts in a separate process. If a script hangs or the kernel crashes, the server restarts it and replays the session.

  1. Your scriptPython you write, or that an agent writes and runs through the MCP server.
  2. MCP serverRuns the scripts, keeps the session, shows it live in Buerligons and renders pictures.
  3. smlibpyShapes, selections, features, result checks, and the STEP reader and writer.
  4. SMLibThe C++ kernel: topology, Booleans, fillets, offsets, sweeps and mass properties.

06Examples

Each one is a short script in the repository. The drawings and the numbers come from running those scripts.

07smlibpy and ClassCAD

Both use SMLib. ClassCAD has been built on it for twenty years.

ClassCAD

A CAD system.

ClassCAD stores how a part was made. Sketches have constraints and dimensions, and features reference the faces and edges they were built on. When you change a dimension, ClassCAD recalculates only what depends on it.

It also has assemblies with mates, a PDM for versions and releases, and Buerligons, the CAD app in the browser.

history graph · references · constrained sketches · assemblies · PDM · reuse · incremental recalculation

smlibpy

A library.

In smlibpy, a part is what your script produces. Nothing is stored besides the script. To change the part, you edit the script and run it again, which takes milliseconds for most parts.

That works well for variants, batch jobs, notebooks, CI checks and agents. It is also the quickest way to try out SMLib.

scripts · exact geometry · STEP in and out · notebooks · agents

STEP works between the two: a part made with smlibpy opens in ClassCAD as a solid. An agent with both MCP servers can save a part with smlibpy and load it into Buerligons.

08SMLib and OpenCASCADE

OpenCASCADE has been the open-source B-rep kernel for more than twenty years. Now SMLib is open source too. This is how they compare.

SMLibOpenCASCADE
Where fromA commercial NURBS kernel that has been in production use for decades; ClassCAD has used it for twenty years.Matra Datavision’s CAS.CADE, open source since 1999 and developed by Open Cascade SAS.
LicenseApache 2.0: you can use, change and ship it, also in closed-source products.LGPL 2.1 with an exception for linking; changes to OCCT itself stay open.
ScopeA modeling kernel: geometry, topology, Booleans, blends, offsets, sweeps, tessellation, healing.A platform: the kernel plus data exchange, meshing, visualization and an application framework.
GeometryNURBS throughout; planes, cylinders and the other analytic surfaces stay analytic.Analytic surfaces, B-splines, offsets and more, each with its own algorithms.
FilletsCircular, variable and curvature-continuous (G2, G3) blends.Rolling-ball fillets, tangent-continuous, constant or variable.
Pythonsmlibpy, and SMLib’s own low-level bindings.pythonOCC and OCP, with CadQuery, build123d and others on top.

09Questions

Is it free?
Yes. smlibpy is MIT licensed and SMLib comes with it under Apache 2.0. Both allow commercial use.
Do I need a GPU, a CAD system or an account?
No. It is a Python package with the kernel compiled in. Pictures are rendered in software, and the viewer is a page in your browser.
How exact is it?
The geometry is exact. Triangles only come in when you export a mesh. Volumes, areas and centers are accurate to about seven digits by default, and mass_properties(accuracy=…) can go further.
Who makes it?
AWV Informatik in St. Gallen, the people behind ClassCAD, buerli and Buerligons. We have built CAD on SMLib for twenty years.
Is it ready?
It is new, version 0.1. Modeling, measuring and STEP work and are tested. Assemblies with constraints, 2D drawings and text are not there yet.