Skip to content

b123d-print

Vibe CAD for 3D printing. Tell your AI assistant what you need, in plain words, with a photo and a few caliper readings. It designs the part, looks at it from every side, checks it for printing problems and hands you a file for your slicer. No CAD skills needed, and it works with any assistant that supports MCP: Claude, Gemini CLI, Cursor, VS Code Copilot, Codex and more.

What you can make

Ask the way you would ask a friend who owns calipers and a CAD program. These were designed this way, and printed:

  • "A box for this IR blaster board: the LED pointing out the top, a screw-on lid, two tabs to mount it on the wall." A two-part case, checked against a stand-in of the board so it fits.
  • "An outdoor case for this motion sensor. It should slide off a wall plate and keep the rain out." Dovetail rails, a click-in detent, a drain slot, and a cover that prints without supports.
  • "An adapter plate: 2-gang box behind, 1-gang smart switch in front." Counterbored screw holes, and a wall check added after the first print broke through a 0.05 mm skin no render showed.

Or remix a model you downloaded (STL, 3MF or STEP) and change it:

  • "Here's an L bracket STL I downloaded: turn its two mounting holes into slots and add gussets." The original is loaded as a solid and changed, the checks tell the original's flaws from new ones, and a before/after view shows exactly what moved. (A demo for the docs, not printed yet.)

Before a long print, test the fit with a fit-test template: a thin, quick print cut from the real part.

  • "Make a fit-test template of the mounting face so I can check the hole pattern." The assistant shows you the face in red, asks you to confirm, and gives you a 1.2 mm plate with the part's exact holes: minutes to print instead of hours.

And the everyday things: a spacer, a knob, a bracket, a cable clip, a replacement for a broken part you can measure. Then keep talking to it: "make it 5 mm taller", "add a hole for an M4 screw", "the lid is too tight". Every dimension stays a named number in a short build123d script, so each change is exact, and you get STEP files to keep editing in other CAD if you want.
ESP IR blaster box, assembly with board stand-ins Section through the LED: base, lid, board and LED stand-ins
Two-part box for an IR transceiver board, with the board stand-ins Section through the LED: lid cone, board, clearances
Outdoor motion sensor case, arch cover Wall plate section through the counterbores
Outdoor sensor case: dovetail back plate + support-free cover Wall plate: section just past the face, where the wall check looks
Remix: the downloaded L bracket and the version with slots and gussets Remix: the original colored where the remix differs
Remix: a downloaded bracket, before and after Where the remix differs: red cut, grey added

Get started in 2 minutes

The MCP server is a plug-in that gives your AI assistant CAD tools. It runs on your own computer. The easy way: paste this into your AI assistant (Claude Code, Claude Desktop, Cursor, Gemini CLI, Codex, …):

Install the b123d-print MCP server for me. It runs with
`uvx --from b123d-print b123d-print-mcp --workspace <folder>`
where <folder> is a folder for my parts, such as ~/parts.
Install uv first if it is missing (https://docs.astral.sh/uv/).
Add the server to your MCP settings, tell me when to restart you,
and after the restart check that its tools (list_parts, new_part,
build_part, ...) are available.

It will probably ask to run a few commands: that is expected. Prefer to do it yourself, or want to see what it does? Quick start has the three manual steps. Then ask: "Design a 40 mm round spacer, 10 mm tall, with an M3 hole."

Scope: parts for FDM 3D printing, nothing else. Everything is built around a printed part: print orientation with the bed at Z=0, hole compensation, mating clearance, calibration coupons, and print checks built on sourced FDM design rules. It is not a general CAD framework.

What's in it

  • b123d-print package + CLI — export, 6-view renders, colored multi-part cross-sections, and print checks: overlap between mating parts, overhang and bridges in print orientation, thin features and wall thickness around cuts, holes (round tops that sag), bed-edge fillets, gaps between moving parts, probes ("is the roof closed?"). See The checks and CLI and Python.
  • b123d-print-mcp — an MCP server, so it works from Claude Desktop, Claude Code, Gemini CLI, Cursor, VS Code, Codex and other MCP clients. Part scripts are model-written code: they run in a sandbox with a timeout and no network.
  • compare — measure a part against a reference STL, 3MF or STEP: size, volume and surface differences, a 0–100 score, pass/fail, and a deviation view.
  • Remix — start from someone else's STL, 3MF or STEP: import_model makes a mesh a solid (coplanar triangles merged), the checks tell the original's problems from the ones the remix added, and a before/after view shows the change.
  • Fit-test templates — a quick thin print of a face, a slice or a region of the real part (same holes and clearances) to check the hole pattern and the fits before the full print (fit-test templates).
  • The workflow, model-neutral — MCP prompts plus an AGENTS.md any agent can read. The rules come from real prints; one exists because a part broke through a 0.05 mm skin that no render showed.
  • Examples — four parts designed this way and printed, plus calibration coupons for holes, fits, and overhangs and bridges.

Where to start

Platforms and status

0.x — early. The part-script convention may still change. CI tests macOS, Ubuntu and Windows, the built package via uvx, and the Docker image on every push.

Platform Local (stdio) OS sandbox for part scripts SSH mode HTTP mode
macOS yes sandbox-exec server and client server and client
Linux yes bubblewrap (install it) server and client server and client
Windows 10/11 yes, natively none: process isolation + audit hook (Sandbox) client only client; the server runs too, without an OS sandbox

On Windows, run the server in WSL2 (with bubblewrap) or Docker when the part scripts come from a model you don't watch, or from other people. As a client of an SSH or HTTP server elsewhere, Windows needs nothing extra: b123d-print pull uses the OpenSSH client built into Windows (ssh.exe).