I've been using zoo and its KCL language with some success for boundary-representation CAD writing. If I understood correctly, µcad serves the same purpose. Comparing code samples between both of them, I personally enjoy KCL's pipelined approach more.
My main beef with zoo is the fact that they are promoting vendor lock-in by forcing users to use their cloud-hosted geometry kernel with absolutely no local alternative. It's not clear to me how µcad solves this problem.
Hm, at least build123d (which I had never heard of, thanks!) can export STEP, which I believe is becoming a necessity if one wants to assemble real-world models with FreeCAD (and nicely also slicers are picking up support to it). I'm on the edge though if I'd really like a proper DSL instead of building it on top of Python, although I can see that too has its benefits (e.g. library access).
Does anyone have idea about the STEP export support status for KCL/µcad? To me it looks like KCL cannot, and I cannot find information on it about µcad. The one tool I'm familiar within this space is OpenSCAD and it cannot. While FreeCAD is able to (sometimes?) convert from STL to STEP, it seems actually working with such models in FreeCAD is quite compute-inefficient.
Converting from STL to STEP is essentially always a case-by-case thing.
If you import, sew, check geometry, refine, etc., you can eliminate unnecessary triangles in planar faces. But you can't magically make a 120-sided-polygon that should be a circle a circle; you're going to have to do that yourself.
And FreeCAD is not the best tool for correcting broken meshes, which are commonplace. MeshLab and the vaguely-on-life-support AutoCAD MeshMixer are better at it.
STEP/IGES is as you say more or less essential if you want to work with code-CAD output in any other serious context. STL is not enough: it is like converting to JPEG in the middle of a photo editing flow, not at the end. It destroys information — in this case geometry.
Yeah I hate messing with STLs for that reason. It's an output format. Similar to when you make an image in Photoshop with a load of layers and then export to jpeg where it's flattened and all the layer info is lost.
Obj is a bit better but for real design modification you kinda need the original file and the program it was designed in. At least in my experience.
I've never come across STEP though, is that a real design parameter preserving file format?
STEP doesn't preserve all of your design intent (sketches etc.) but it does preserve the geometry (edges, vertexes, faces). So it is a lot easier to work with it because it is lossless and precise. Curves are curves, not quantised/faceted.
So in our image processing analogy, it is still in a sense a "flattened" representation of the layers, but it's a vector format. The best way to think of it is broadly like a 3D SVG, I guess.
3D printer slicers (except maybe Cura without a paid add-on) can usually load STEP now, but they are still internally meshing before slicing. I think Orca/Bambu/PrusaSlicer all give you some control over that meshing (they all use OpenCascade to do it, in fact).
In FreeCAD you can do things like defeaturing, so if you have a hole in a rounded plate, you can delete the hole, you can delete corner-rounding. You can also break STEP files down to faces and use the surface/curves tool to work on them, or use them as a BaseFeature for a Part Design Body, etc.
This is a pretty useful video for showing the differences:
If you're talking about CAD in general I can see your point.
For FreeCAD specifically, there looks to be an OpenSCAD import process directly. I don't have experience with how it works, but that may be better than going through STL
I know the process is there, but I really, really doubt it does anything else than just uses OpenSCAD code to render the mesh, as FreeCAD can also deal with meshes. I think it would be pretty unlikely that the FreeCAD devs would have implemented such a rendering system based on OpenSCAD.
From my quick tests, it seems to rebuild the OpenSCAD objects using FreeCAD part workbench primitives.
But I also quite quickly found examples that don't work. I don't know if that's the import not being very good, me finding weird examples, needing to install more libraries, or PEBKAC.
My main beef with zoo is the fact that they are promoting vendor lock-in by forcing users to use their cloud-hosted geometry kernel with absolutely no local alternative. It's not clear to me how µcad solves this problem.
[1] Lego brick in KCL: https://zoo.dev/docs/kcl-samples/lego
[2] Lego brick in µcad: https://microcad.xyz/index.php/2025/11/12/lego-bricks/
[3] Gear in KCL: https://zoo.dev/docs/kcl-samples/spur-gear
[4] Gear in µcad: https://microcad.xyz/index.php/2025/11/12/gears/