A minimal, LLM-friendly CAD library in Python.

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Documentation: https://llmcad.org

Source Code: https://github.com/llmcad/llmcad

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llmcad is a Python CAD library designed from the ground up to be LLM-friendly. It wraps OpenCASCADE (via OCP) with a minimal, explicit API that makes 3D modeling predictable and debuggable — whether you're writing code yourself or letting an AI agent do it.

 Developed by and powering BuildCAD AI — the #1 Text-to-CAD tool.

Why llmcad?

LLMs struggle with 3D spatial reasoning. Traditional CAD APIs make this worse with large surface areas, implicit state, and global coordinate math.

llmcad fixes this with three design principles:

• Named geometry: Every face and edge has a semantic name (top, front, left_edge). No guessing indices or filtering by coordinates.
• Face-local coordinates: Position things relative to faces using intuitive local offsets (offset, inset). No global coordinate math.
• Minimal API: ~28 core concepts. If an LLM can't hold the entire API in context, the API is too big.

Key Features

• Shapes — Box, Cylinder, Sphere centered at origin with named faces
• Sketches — Rect, Circle, Ellipse, Polygon, Text, and turtle-style Sketch builder
• Operations — extrude, revolve, loft, sweep, fillet, chamfer, shell, split, mirror
• Assembly — place parts on faces, Assembly for multi-part models
• Booleans — + (union), - (cut), & (intersect) — operator syntax, no method chains
• Visual debugging — Multi-view snapshot() renders for LLM feedback loops
• Export — export_step, export_stl, export_glb for CAD interchange, 3D printing, and web viewers
• Immutable — Boolean ops return new bodies. No hidden mutation.

Installation

pip install llmcad

Requires Python 3.10+. Dependencies (cadquery-ocp, numpy, Pillow, vtk) are installed automatically.

Quick Example

A mounting plate with a boss, through-hole, filleted edges, and corner mounting holes — in 15 lines:

from llmcad import Box, Rect, Circle, extrude, fillet

# Base plate
plate = Box(100, 60, 10, color="steel")

# Raised boss on top
boss = extrude(Rect(30, 30).place_on(plate.top), amount=20)
plate = plate + boss

# Through-hole in boss
hole = extrude(Circle(12).place_on(plate.top), through=True)
plate = plate - hole

# Fillet top edges
plate = fillet(plate.top.edges, radius=3)

# Corner mounting holes
for corner in plate.bottom.corners:
    pos = corner.inset(dx=8, dy=8)
    h = extrude(Circle(5).place_on(plate.bottom, at=pos), through=True)
    plate = plate - h

How It Works

Named Faces

Every shape comes with semantic face names. No need to filter by normal direction or index into face lists.

box = Box(60, 40, 20)

box.top      # +Z face
box.bottom   # -Z face
box.front    # -Y face
box.back     # +Y face
box.left     # -X face
box.right    # +X face

Face-Local Positioning

Place sketches and holes relative to faces using local coordinates. offset(dx, dy) moves in the face's local frame — dx is always "right" and dy is always "up", regardless of which face you're on.

# Hole offset 20mm to the right on the top face
pos = plate.top.center.offset(dx=20)
hole = extrude(Circle(5).place_on(plate.top, at=pos), through=True)

# Holes at each corner, inset 8mm from edges
for corner in plate.top.corners:
    pos = corner.inset(dx=8, dy=8)  # direction toward center is automatic

Boolean Operations

Combine bodies with Python operators. Each operation returns a new body — the originals are unchanged.

base = Box(80, 60, 10)
boss = extrude(Rect(30, 30).place_on(base.top), amount=20)

result = base + boss   # union
result = base - boss   # cut
result = base & boss   # intersect

Visual Debugging

Render multi-view snapshots for visual verification — essential for LLM feedback loops.

from llmcad import snapshot

snapshot(plate, "plate")  # saves plate.png with front, right, top, and iso views

More Examples

Box	Cylinder	Sphere
Plate with Hole	Filleted Edges	Chamfered Edges
Boolean Subtraction	Corner Holes	Stacked Boxes

See the full examples gallery for all 18 examples with code.

Export

Export to STEP (CAD interchange), STL (3D printing), or GLB (web viewers):

from llmcad import Box, export_step, export_stl, export_glb

box = Box(50, 50, 50, color="steel")

export_step(box, "box.step")  # exact B-Rep geometry
export_stl(box, "box.stl")    # triangulated mesh
export_glb(box, "box.glb")    # binary glTF 2.0 with PBR material

API at a Glance

Category	Symbols
Shapes	Box, Cylinder, Sphere
Sketches	Rect, Circle, Ellipse, Polygon, Text, Sketch
Operations	extrude, revolve, loft, sweep, fillet, chamfer, shell, split, mirror
Assembly	place, Assembly, RevoluteJoint
Booleans	+ (union), - (cut), & (intersect)
Debug	snapshot, show, show_faces, show_edges, measure
Export	export_step, export_stl, export_glb
Types	Body, FaceRef, EdgeRef, Position

Full API reference: llmcad.org/api-reference

Dependencies

llmcad builds on:

• OCP — Python bindings for OpenCASCADE, the industry-standard open-source CAD kernel
• NumPy — array operations for tessellation and rendering
• Pillow — image output for snapshots
• VTK — offscreen multi-view rendering

License

This project is licensed under the terms of the MIT license.

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  Developed by and powering BuildCAD AI — the #1 Text-to-CAD tool.