GenCAD

I would even argue that for basic modelling majority of tools/features in CAD operate at the abstraction closer to CSG for describing what and B-rep is only treated as how. Just like good chunk of code based CAD use combination of CSG for what and triangle mesh based geometry engine for how. That's assuming you consider standard 2d->3d operations (extrude, revolve, sweep along arbitrary 2d profile) as valid primitives for CSG.

User comes into direct contact of B-rep in very specific situations: 1 doing operations like fillets/chamfers/draft/thickness based on intermediate geometry, 2 attaching sketches or other features to generated geometry or using generated edges (instead of new sketches) for guiding operations like complex sweeps, 3 surface based modelling workflows where you build up the the solid from individual faces typically including complex curved surfaces.

In case of 1 and 2 the the dependency on b-rep based representation is only marginal, in theory you could select edges in triangle mesh based underlying representation but the final result but quality of result wouldn't be as nice and TNP issues for parametric model editing would likely be even bigger than it is for existing CAD. That's not really CSG territory anymore but isn't exclusive to B-rep either, and involves a bunch of work that's outside the scope of B-rep. In non parametric mesh modellers with more destructive editing workflows like blender chamfers and fillets work fine. And if anything for reliable parametric models you often want to limit dependencies on intermediate geometry as that depends on CAD keeping track of where each edge/face originated from outside the b-rep and increases the chance of TNP issues.

3 is critical for industrial design containing large amount of complex curved surfaces like cars and other consumer products, but there are also many more technical parts where it can be completely ignored. Cad tutorials for beginner tutorials almost completely ignore this category of cad modelling. The part about not being exclusive to b-rep also applies for surface modelling part.

I’ve seen several vibe coded attempts at a geometric kernel (including several of my own) and this happens every time.

Vibe coding a geometric kernel is practically impossible because sooner or later* the LLM inevitably takes the tessellation shortcut and if you don’t catch it, the codebase is completely compromised. At the end of the day, there isn’t enough training data in architecture or algorithms (opencascade solvespace and truck being the only real examples, all significantly worse than commercial kernels like Parasolid or ACIS).

* usually as soon as you ask it to do anything non-trivial. If you’re lucky you’ll get a naive Newton marching algorithm on analytical bodies, which is slightly better but has the same problem with degenerate and pathological cases (coincidence, tangents, parallels)

https://youtube.com/@thang010146

There are mechanical engineers out there who can literally model objects nearly as fast and they can 'think' about the layout of said object. If you look at the complexity of, say, a CAD model from a real, highly complex aluminum casting section of an automotive subframe, or the living-room-sized cross-fuselage spar forging of a fighter aircraft, with hundreds of ribs and fillets and features- and compare that to the simple model you are trying to make in OpenSCAD, you should quickly realize the parallels in difficulty you are trying to express (similar to the person without knowledge of C++ or Python watching someone be able to build applications by typing code from their fingertips as if they already knew what to type...)

You are struggling for a few reasons- 1., it is a knowledge hurdle of an entire field you are trying to surmount- again, go watch someone actually model a real, complex part and watch the speed, they can do so in a tool like Solidworks, CATIA, NX, etc... at a rate that is far different because they have experience that it can honestly take even good people years to accumulate - and 2. they are using professional tools - you mention OpenSCAD, like it is CAD, but it isn't. It is programmatic mesh generation, and it turns out that programmatically typing out how to generate complex things is much more difficult than a combination of a graphical GUI and graph-based generator that all big CAD programs figured out starting in the 1980s. If those tools you use were really the best way to make complex models 'paramaterized', then why do we design our fighter jets, Formula 1 cars, or Space X rockets in Dassault's CATIA or Siemen's NX ?

You want a LLM to take a sketch to your CAD, but what I'm saying is, there are people out there that can skip the sketch and build the CAD as fast as you can likely hand draw the first sketch, and these are skills you can actually learn, but you may just be using the wrong tools and have not had the practice necessary.