Text-to-CAD tips I wish I knew earlier

I've been using text-to-CAD tools most days for the past several months. Zoo.dev, CADAgent, a few others. I've generated hundreds of parts, ranging from mounting brackets I actually used to organic shapes that came back looking like something a toddler might sculpt from modelling clay if the toddler had access to a GPU. Along the way I've wasted plenty of time on mistakes that, in hindsight, were completely avoidable. These are the things I wish someone had told me on day one.

Always specify dimensions in millimeters#

This sounds obvious. It isn't, apparently, because I spent my first week typing things like "a bracket, 50 by 30, 3 thick" and getting parts back in what appeared to be an AI-generated unit system that corresponded to nothing on earth. Sometimes the numbers came back as millimeters. Sometimes they seemed to be in some scaled-up fantasy unit. Once I got a plate that was clearly interpreted as inches when I was thinking in metric, which meant my little sensor mount came back the size of a serving tray.

Now I write "50mm by 30mm, 3mm thick." Every dimension, every time, with the unit attached. The results got more consistent immediately. It's the kind of habit that feels redundant until you see what happens without it.

One part per prompt#

I tried, early on, to generate assemblies. "A box with a lid that fits on top, with four screw posts on the box and four clearance holes in the lid." The tool generated something that was technically box-shaped and lid-shaped, but the fit between them was fictional. The screw posts didn't align with the holes. The lid was 2mm too wide. The wall thicknesses differed between the two pieces.

Text-to-CAD tools generate one solid body per prompt. If you need two mating parts, generate them separately, each with explicit dimensions that account for clearance. A box that's 100mm by 60mm on the outside, with 2mm walls, means the lid needs to be about 96mm by 56mm on the inside if it's supposed to drop in. The AI won't figure out that relationship for you. You need to do that math and put it in the prompt.

This constraint is real and it won't change soon. Assemblies require understanding relationships between parts, and that's a fundamentally harder problem than generating a single shape from a description.

Name standard features instead of describing geometry#

"M4 clearance hole" works better than "4.3mm hole." "M3 counterbore" works better than "hole with a wider flat-bottomed recess at the top." The AI has been trained on engineering terminology, and using standard feature names gives it more context about what you're actually asking for.

I've found this pattern holds for most mechanical features. "Chamfer" produces a chamfer. "Angled cut on the edge" sometimes produces a chamfer and sometimes produces something that looks like the AI had a stroke mid-extrusion. "Fillet, 2mm radius" works. "Round off the corner" is a coin flip.

Standard names for standard features. It's like speaking the same language as the tool, which is a low bar, but one that the prompts need to clear.

Start simple, then add complexity#

My biggest time-waster was trying to describe the final part in one go. A full enclosure with bosses, ribs, vent slots, mounting tabs, and a cable routing channel. The result was always wrong in at least three ways, and figuring out which three took longer than generating five simple variants would have.

Now I start with the basic envelope: "Rectangular box, 100mm by 60mm by 40mm, 2mm wall thickness, open top." I export that, check it, and decide if the base geometry is worth building on. If it is, I add features in my CAD software. If I want the AI to handle a specific feature, I prompt for a simpler version of the part that includes just that feature.

This approach is less satisfying than typing one magnificent prompt and getting a perfect part back. It's also more productive by a wide margin.

Always export STEP, not STL#

I made this mistake exactly once. I exported an STL from Zoo, opened it in Fusion 360, and spent twenty minutes trying to figure out why I couldn't select individual faces. Because it was a mesh. Because STL is a mesh format. Because I'd exported the wrong format and was now staring at a blob of triangles that Fusion politely informed me was an "imported mesh body" rather than a solid.

STEP gives you real B-Rep geometry with selectable faces, measurable edges, and the ability to add fillets, cuts, and holes. STL gives you a triangle bag that's good for 3D printing and nothing else. Export STEP for engineering work. Always. If you need STL for a printer, export it from your CAD tool after you've verified and fixed the STEP import.

The text-to-CAD to STEP file post covers the STEP workflow in more detail, including what to check when you open the file.

Measure everything before trusting it#

This is the tip I repeat most often because it's the one that matters most.

The generated part looks right. The proportions seem correct. The holes are where they should be. And then you measure and discover that the bolt pattern is 2mm off, or the wall thickness is 3mm instead of the 2mm you asked for, or one of the four mounting holes is 0.5mm further from the edge than the other three.

I measure every critical dimension on every text-to-CAD import. It takes about two minutes and has caught errors that would have been embarrassing (prototyping) or expensive (anything headed to a machine shop). I use Fusion 360's Inspect tool. Select two faces, read the distance. Select a hole edge, read the diameter. Compare to the prompt.

If you take one habit away from this post, let it be this one. Measure before you trust.

Learn which parts are worth generating#

After months of trial and error, I've developed an instinct for which parts to generate and which to model by hand. The dividing line is roughly this: if I can describe every feature with a dimension and a position, and the features don't depend on each other in complex ways, it's worth trying text-to-CAD. If the part has features that reference other features, complex curvature, or relationships that require manufacturing awareness, I model it from scratch.

Parts I generate: mounting plates, brackets, standoffs, spacers, adapter plates, simple enclosure shells, cable clips, sensor mounts. Parts I don't bother generating: gears, snap-fit enclosures, sheet metal parts, anything with lofted surfaces, anything that mates with another part in a tight tolerance assembly.

The time savings come from generating the boring parts fast, not from attempting the complex ones and spending twice as long fixing them.

Position features with absolute references#

"Two holes near the top" gives the AI creative license to put them wherever it wants. "Two holes centered 10mm from the top edge, spaced 30mm apart, centered on the part width" gives it coordinates. The second prompt produces more accurate output almost every time.

I've noticed that the AI handles absolute positioning better than relative positioning. "15mm from the left edge" works better than "one-third of the way across." "Centered on the 80mm dimension" works better than "in the middle." The more you can anchor features to specific numbers, the less room the AI has to improvise, and improvisation is not what you want from a tool that's supposed to produce dimensioned geometry.

Don't prompt for manufacturing details#

Draft angles, bend allowances, thread specifications, surface finishes, tolerance callouts. None of the current tools handle these. I wasted time early on trying to specify "1-degree draft on all vertical faces" and getting back geometry that had ignored the instruction entirely. Same with "M4x0.7 tapped hole." The tool doesn't model threads. It doesn't know what a K-factor is. It doesn't understand that a sheet metal part needs to unfold.

Save manufacturing details for your CAD software. Use text-to-CAD for the basic geometry, then add draft, threads, bend reliefs, and tolerances in Fusion 360 or SolidWorks where the tools actually exist for those operations.

Re-prompt instead of salvaging bad geometry#

When the generated part is significantly wrong, the temptation is to fix it. Move this hole, adjust that dimension, patch the missing feature. Sometimes that works. Often, especially when multiple features are wrong, you end up spending more time editing imported geometry than it would take to re-prompt with better wording or just model the part from scratch.

My rule: if I need to fix more than two things, I re-prompt. If I need to fix more than three things, I model it by hand. The breakeven point where fixing is slower than starting over arrives faster than you'd expect, because imported geometry doesn't have a parametric history. Every fix is a direct edit on a dumb solid, which means you can't roll back, you can't change your mind easily, and downstream edits get progressively messier.

Keep a prompt library#

This one took me embarrassingly long to figure out. When a prompt produces a good result, save it. I keep a text file with prompts that worked, organized by part type. When I need a similar part, I start from a working prompt and modify the dimensions rather than writing from scratch.

"L-bracket, 3mm thick, 50mm tall leg, 40mm base leg, two M4 clearance holes on the base spaced 25mm apart and centered, one M4 clearance hole centered on the tall leg at 35mm height."

That prompt produced a good bracket. Next time I need a bracket, I change the dimensions and feature count. I don't try to rephrase it from memory, because the specific wording that works is not always the wording I'd naturally write. Prompt engineering is a skill, and a library of working prompts is the most practical form of that skill.

The best prompts for text-to-CAD post collects more of these, and the text-to-CAD prompt engineering post explains the principles behind why certain phrasings work.

The meta-tip#

The biggest lesson from months of text-to-CAD is that the tool rewards precision and punishes ambiguity. Every minute you spend making your prompt more specific saves you several minutes of fixing the output. Every dimension you leave out is a dimension the AI invents. Every feature you describe vaguely is a feature that comes back wrong in a way you'll need to fix by hand.

Text-to-CAD is a shortcut, not a replacement. It's a first-draft generator for simple geometry, and it's good at that job when you meet it halfway with clear, dimensioned, specific descriptions. The text-to-CAD for beginners guide is a good starting point if you're just getting into this, and the text-to-CAD guide covers the full picture.

I still model complex parts by hand. I still verify every dimension on generated parts. But for the twenty-odd simple brackets, plates, and mounts I generate each month, these tips have cut my cleanup time roughly in half. Which is not a miracle, but it's an hour of my life back each month that I used to spend arguing with holes that refused to be where I put them.