SolidCraft: The Free CAD App Where an AI Agent Builds a Part You Can Still Change
An open-source parametric modeller lets Claude or another assistant drive it command by command. The output is a real CAD part with a history, which is the thing every AI 3D tool so far has failed to give you.
Almost every AI tool that makes 3D objects hands you a sculpture. You type a description, you wait, and you get a shell of thousands of tiny triangles shaped like a phone stand. It looks right. Then you need the slot two millimetres wider, and there is nothing to grab. No slot exists in the file, only triangles arranged to look like one. So you describe it again and roll the dice.
SolidCraft does something stranger and more useful. It is a free CAD program, the kind engineers and product designers use to draw parts with exact dimensions, and it includes a plug that lets an AI agent operate it. The agent does not produce a shape. It presses the same buttons you would, in order: sketch a rectangle, set it to 60 by 40, pull it up 20, cut a hole, round the edges. When it gets the slot wrong, the slot is a step in a list with a number on it, and you change the number.
That difference sounds small. For anyone who makes physical things, it is the whole game.
Why a list of steps beats a better-looking shape
Here is the concept that makes the rest of this make sense. "Parametric" CAD keeps a timeline of every feature you added. A box, then a hole in the box, then a rounded edge on the hole. Each step remembers its numbers. Change the box from 60 to 80 millimetres wide and the program replays the whole list, and the hole and the rounded edge follow along.
Professional tools such as Fusion and SolidWorks work this way, and it is why a machinist or a manufacturer asks you for a STEP file. STEP is the industry's shared format for exact solid shapes. A triangle mesh, the kind most AI 3D generators export, is fine for a 3D printer and close to useless for a machine shop, because the curves have been chopped into flat facets and the design intent is gone.
SolidCraft exports STEP, along with IGES, 3MF, STL and OBJ, according to its README. It runs as a desktop app for Windows, Mac and Linux, or in your browser with nothing to install. It is released under MIT or Apache 2.0, your choice, which means you can use it for paid work and even ship modified versions. Its author, who goes by bherbruck on GitHub, posted it to Hacker News on October 12 in the US (just after midnight UTC on October 13), and it had about 1,300 GitHub stars by Tuesday morning.
The feature list is longer than you would expect from a project that young: sketches with constraints and driven dimensions, extrude, revolve, sweep, loft, fillet, chamfer, shell, draft, holes, threads, patterns and mirror. The project's roadmap marks sheet metal with unfolded flat patterns as done, and a set of plastic-enclosure features (bosses, ribs, snap fits, lips and grooves) as done too. Assemblies with joints and motion studies are in progress.
How the agent actually drives it
The plug is called MCP, short for Model Context Protocol. It is a standard way for an AI assistant to call tools inside another program. SolidCraft's version runs from its command-line companion, solidcraft-cli mcp, and the README gives the one-line setup for Claude Code:
claude mcp add solidcraft -- solidcraft-cli mcp
By default the agent works in an invisible session. The README describes a second mode that I would use every time: start the desktop app with solidcraft --control PORT and add --connect 127.0.0.1:PORT to the bridge, and the agent drives the window in front of you. You watch the part appear feature by feature. The README does not show those two pieces combined in one line, so treat that pairing as something to test on your machine.
What does the agent send? Every action in SolidCraft is a command with numbers attached. The README's script example is a single line:
{"command": "solid.extrude", "params": {"distance": 20}}
That is the whole trick. The toolbar, the scripting tool, the command line and the agent all speak this same language, so the agent is not doing anything a person could not do by hand. Its tools include measure, screenshot and export, plus undo and save. That means you can tell it to check its own work in two ways: by measuring the part it just built, and by looking at a picture of it.
This is where I think most first coverage will miss the point. People will frame SolidCraft as "a free Fusion" and judge it on feature count. The more interesting claim is that it gives an AI agent a way to fail cheaply. A generated mesh fails all at once. A command list fails at step seven, visibly, in a place you can point to.
The project grades itself, and the grades disagree
SolidCraft publishes its own report card. The project tests itself against public reference parts whose correct volume, surface area, centre of mass and bounding box are known, either from math or from the long-running open-source program FreeCAD. The README says 75 of 76 parts match and lists the remaining one as a tracked bug. The roadmap, updated October 11, says 76 of 76.
Neither number is the author lying. One file is probably just newer than the other. But it is a useful reminder of how to read a young project: the author's self-assessment is a starting point, and it can drift between two pages of the same repository in a weekend. The README itself adds the important caveat: "These checks do not establish complete geometric or workflow equivalence." Matching a reference part's volume tells you the maths agrees. It does not tell you the tool feels right when you are fighting a fillet at 11 at night.
One more detail in the roadmap tells you how this was built. It estimates the remaining work to its first usable early version as "≈ 82 agent hours." The project measures its own to-do list in hours of AI agent time. Whatever you think of that, it explains the speed, and it is a good reason to test carefully rather than trust breadth.
Put this into practice
Start in the browser. Open the web app, adding ?sample to the address to load the sample part, and spend twenty minutes learning the feature timeline by hand. You will brief an agent much better once you know what a sketch, a constraint and a feature are in this program.
Then try one small, real part. A wall bracket, a cable clip, a spacer for a lamp. Install the desktop app, start it with the control port, connect the agent, and write your request the way you would brief a junior designer:
- Give every dimension a name and a number. "Base plate 60 by 40 by 4 millimetres. Two mounting holes, 5 mm, 45 mm apart on centre."
- Say what must stay true when things change. "The holes stay centred if the plate gets wider."
- Ask for a check after each feature. "After every step, measure the part and take a screenshot before moving on."
- Name the export. "Save as STEP for the machinist and 3MF for my printer."
Now the important test, the one that tells you whether you got a real part. Change one number yourself. Make the plate 80 millimetres wide and watch the timeline rebuild. If the holes move with it, the agent built the part the way a designer would. If something breaks, the timeline shows you exactly which step, and you have learned more about the tool than any review could tell you.
One habit worth keeping: units. SolidCraft works in millimetres and degrees by default, and its getting-started guide shows you can write values like "1 in" or "30 deg" to override. If you think in inches, say so in your brief, every time.
Where it breaks
Simple parts is the author's own ceiling. The README calls SolidCraft "early but usable for simple parts." Believe it. The roadmap lists fillet and corner cases that still fail, some combinations of curved faces that can break when you join or cut solids, and same-plane joins still in progress.
No 2D drawings yet. Many machine shops want a dimensioned drawing alongside the STEP file. Drawings are deferred "until further notice," along with machining toolpaths, simulation and rendering. You will need another program for the drawing.
AI meshes do not become editable parts. If you generate a model in an image-to-3D tool and import the STL or 3MF, SolidCraft brings it in as a mesh body. You can measure and move it, but the solid features cannot touch it. The README says its Convert Mesh tool rebuilds only supported closed parts with a constant profile, and the roadmap still lists that work as in progress.
Threads are approximate. Modelled threads are cut as faceted surfaces, accurate to within 1/200 of the pitch per the roadmap. Fine for most prints. Check it before you send a threaded part to a shop.
Imported assemblies flatten. STEP files with several components arrive as plain bodies, and some freeform surfaces are not read yet.
Downloads are not signed. Windows and macOS will warn you on first launch. That is normal for small open-source projects. It is still worth knowing before you put it on a client's machine.
There is no published example of a good agent session. The README documents the tools but not a sample conversation. Expect to spend your first hour learning how to describe parts so the agent does not guess. And the Hacker News thread had nine comments when I checked, so there is not much community experience to borrow yet.
The part you still own
AI 3D generation has spent two years getting better at the look of things. SolidCraft is a small bet on a different idea: that the valuable output is the recipe, not the dish. A recipe can be checked, corrected, scaled and handed to someone else. A dish can only be eaten or thrown away.
You do not have to believe the agent is a good designer to find that useful. You only have to believe that correcting a wrong number is faster than describing a whole object again. Pick the smallest part you actually need this month, brief it carefully, then change one dimension and see what survives. That five-minute test will tell you more about where AI fits in your shop than any amount of reading.
Medium metadata
Title: SolidCraft: The Free CAD App Where an AI Agent Builds a Part You Can Still Change
Subtitle: An open-source parametric modeller lets Claude or another assistant drive it command by command and export STEP files. Instead of a mesh you cannot edit, you get a feature history where a mistake is one number.
Tags: 3D Design, CAD, 3D Printing, Artificial Intelligence, Product Design
Estimated read time: 8 minutes