Sketchpad 3D

An MCP (Model Context Protocol) server that lets anyone create, iterate on, and export 3D models through natural conversation with an LLM. Works with any MCP-compatible client — Claude, ChatGPT, Codex, and more. Zero setup — OpenSCAD is bundled via WebAssembly, together with the BOSL2 library and fonts.

Published on npm as sketchpad-3d-model-mcp (formerly @khromov/simple-3d-modeling-mcp). Based on mazzanfar/simple-3d-modeling-mcp.

Features

• Zero setup — npx sketchpad-3d-model-mcp and go. No OpenSCAD install, no PATH configuration.

• Fast — uses OpenSCAD's Manifold geometry backend; booleans that took over a minute with CGAL finish in well under a second. Repeat renders of the same code are cached.

• Previews the model can reason about — every render returns the image and the model's size, bounds, volume, triangle count, echo() output and warnings, so the AI can check “is it really 80 mm wide?”.

• Real error messages — OpenSCAD's own errors (with line numbers) plus hints, so the AI can fix its code.

• Multi-view grids — labelled front / right / top (orthographic) + perspective views in one image.

• Turntable animations — 360° animated previews for the user.

• Colours — color() shows up in previews, the viewer and 3MF exports. Previews also show transparency, as in color("red", 0.3).

• BOSL2 and text() — include <BOSL2/std.scad> and text("…") work out of the box.

• Live browser viewer — rotate/zoom/pan, model history, dimensions, STL/3MF download, open in your slicer. Runs locally, works offline.

• Real files — export STL, 3MF, OBJ, OFF (and SVG/DXF for 2D designs) to a folder you choose; save and render .scad files with relative include/use.

Related MCP server: OpenSCAD MCP Server

Quick Start

This server speaks the standard Model Context Protocol over stdio, so it works with any MCP-compatible client.

Claude Desktop

Easiest: download sketchpad-3d.mcpb and double-click it. The extension settings let you pick the export folder or turn off the automatic browser viewer.

Or add it to claude_desktop_config.json manually:

{
  "mcpServers": {
    "sketchpad-3d": {
      "command": "npx",
      "args": ["-y", "sketchpad-3d-model-mcp"]
    }
  }
}

Claude Code

claude mcp add sketchpad-3d -- npx -y sketchpad-3d-model-mcp

ChatGPT Desktop (macOS)

Go to Settings → MCP Servers → Add Server, then enter:

• Name: sketchpad-3d

• Command: npx -y sketchpad-3d-model-mcp

OpenAI Codex CLI

codex mcp add sketchpad-3d -- npx -y sketchpad-3d-model-mcp

Other MCP Clients (Cursor, Windsurf, etc.)

Any client that supports MCP over stdio can use this server:

npx -y sketchpad-3d-model-mcp

Requires Node.js 22, 24 or 26 (LTS).

Tools

All model tools accept either code or file (an absolute path to a .scad file; relative include/use/import paths resolve from its folder), plus params for variable overrides (like OpenSCAD's -D), e.g. {"width": 40, "size": [10, 20, 5]}.

A render response looks like this (plus the image):

Size: 80 × 45 × 62.5 mm (X × Y × Z)
Bounds: X -40 … 40 · Y -22.5 … 22.5 · Z 0 … 62.5
Volume: 41.27 cm³ · Surface: 210.33 cm² · 1,536 triangles

Geometry evaluated in 0.31s.

ECHO output:
"lip height", 12

Interactive viewer: http://localhost:51386

Live Browser Viewer

On the first render an interactive viewer opens in your browser (set OPENSCAD_AUTO_OPEN_VIEWER=false to only print its URL):

• Live updates — refreshes on every render; your camera position is kept while you iterate

• History — every version, named after the render's title

• Dimensions — size, true volume and triangle count

• Code — the OpenSCAD source of each version, read-only, with syntax highlighting, -D overrides, copy and a full-screen view

• Colours — color() is shown; preset swatches or a colour picker set the colour of uncoloured parts

• Export — download STL or 3MF, or open the model in Bambu Studio, OrcaSlicer, PrusaSlicer, Cura or Creality Print (sent as a 3MF, so Bambu Studio 2.5+ maps its colours to filaments)

• Controls — rotate (drag), zoom (scroll), pan (right- or middle-drag), edges, wireframe, auto-rotate, grid

• Themes — a light "drawing on paper" theme and a dark blueprint theme; follows your OS setting until you pick one

The viewer listens on 127.0.0.1 only and rejects requests from other web pages (Host and Origin checks).

Exports

export writes to the path you give (absolute, or ~/…), or to the export folder (default: ~/Downloads, configurable). If a file already exists, a numbered name is used unless overwrite: true. The format can come from the path's extension (~/Desktop/stand.3mf). Exports reuse the geometry from the last render of the same code, so they are near-instant.

Engine

OpenSCAD runs as its official WebAssembly build (a pinned nightly snapshot) with Manifold, BOSL2 and DejaVu fonts (Liberation Sans maps to DejaVu Sans). Each job runs in a worker thread with a timeout, and previews come from the built-in renderer. An installed OpenSCAD is never used. get_version shows the OpenSCAD version and settings.

Configuration

Compatibility Notes

Tip: even if a client doesn't render inline images, the AI still gets the dimensions, and the browser viewer (open_viewer) works everywhere.

Example Conversation

You: Make me a phone stand that holds the phone at 60 degrees

AI: calls render — Here's a phone stand with a 60° viewing angle. It's 80 × 70 × 62 mm with a 12 mm lip. The 3D viewer is open in your browser too. Want me to adjust anything?

You: Make it thicker and add a cable slot in the back

AI: modifies code, calls render_multiview — Updated: walls are now 4 mm and there's a 12 mm cable slot, visible in the back view.

You: Perfect! Export it for my Bambu printer

AI: calls export(format="3mf", title="phone stand") — Saved to ~/Downloads/phone-stand.3mf (42 KB). You can also open it in Bambu Studio from the viewer.

Development

npm install
npm run build        # fetches bundled assets (OpenSCAD WASM, fonts, BOSL2), builds the viewer page and compiles TypeScript
npm test             # unit, engine, renderer, viewer and MCP-level tests
npm run typecheck    # sources, tests and the viewer (svelte-check)
npm run dev          # watch mode
npm run dev:viewer   # viewer UI with hot reload, against sample models
npm run build:mcpb   # Claude Desktop extension bundle

npm run assets populates assets/ (git-ignored): OpenSCAD's WebAssembly build and BOSL2, both downloaded at pinned versions and verified against SHA-256 checksums, and DejaVu fonts from the dejavu-fonts-ttf package.

The browser viewer is a Vite + Svelte 5 app in viewer/ (an npm workspace). npm run build bundles it, three.js included, into dist/viewer-ui, which the server serves locally.

Dev container

.devcontainer/devcontainer.json sets up Node 22 and Bun (for the bun --bun vitest run suite CI also runs) on Debian, with the Svelte, Vitest and OpenSCAD VS Code extensions. Open the repository in VS Code and choose Reopen in Container, or use the devcontainer CLI:

devcontainer up --workspace-folder .
devcontainer exec --workspace-folder . npm test

Creating the container runs npm ci && npm run build, which needs network to download the OpenSCAD WASM build and BOSL2. After that npm test, npm run typecheck, npm run build:mcpb, npm run dev:viewer and the MCP server (node dist/index.js) work as-is.

• VS Code forwards port 5173 (npm run dev:viewer). The devcontainer CLI forwards no ports, so open it from inside the container or use VS Code.

• The live viewer started by the MCP server listens on a random 127.0.0.1 port, and no browser opens by itself inside the container. VS Code offers to forward the port when it appears. The viewer only accepts a localhost or 127.0.0.1 Host on that same port number, so it works when VS Code forwards it to the same local port, but not on a different local port or on a Codespaces *.app.github.dev URL. The npm run dev:viewer page also needs a localhost URL, though any port works.

• Exports go to ~/Downloads inside the container. Set OPENSCAD_OUTPUT_DIR to a folder under /workspaces to see them on the host.

Releasing

Releases are automated with release-please. Use Conventional Commits (feat:, fix:, feat!: …) on main; release-please keeps a release PR open with the next version and changelog. Merging it tags the release, publishes to npm with trusted publishing (OIDC, with provenance — no npm token is stored), and attaches sketchpad-3d.mcpb to the GitHub release.

License

GPL-2.0 (required by the bundled OpenSCAD). Bundled third-party assets keep their own licenses, included alongside them: OpenSCAD's WebAssembly build (GPL, see openscad/openscad), BOSL2 (BSD-2-Clause), DejaVu fonts (Bitstream Vera / public domain). The viewer page bundles three.js and Svelte (MIT), Lucide icons (ISC) and the Jost and IBM Plex Mono fonts (OFL-1.1); their license texts ship in dist/viewer-ui/THIRD-PARTY-LICENSES.md.

Credits

This project is based on mazzanfar/simple-3d-modeling-mcp.