Writing Good Prompts

Five principles

1. 1. Be specific about size. Always include units (mm preferred). “A bracket about 80 wide” is ambiguous; “80 mm wide × 60 mm tall × 4 mm thick” is not.
2. 2. Specify features by intent. Say what a feature is for, not just its shape. “Two M4 mounting holes 60 mm apart for a wall plate” gives the model context.
3. 3. Anchor features to a face or edge. “Centered on the top face”, “10 mm from the bottom edge” — without an anchor, placement is up to interpretation.
4. 4. State manufacturing constraints. Wall thickness, minimum fillet radius, draft angles, and material assumptions help the model stay printable / machinable.
5. 5. One product per prompt. Don't describe a full assembly in your first prompt — start with the main part and add features iteratively.

Anatomy of a good prompt

Most good prompts naturally fall into four short sentences: shape, size, features, constraints.

SHAPE   – A wall-mount bracket.
SIZE    – 80 mm wide × 60 mm tall × 4 mm thick.
FEATURE – Two M4 through-holes, 60 mm apart, 10 mm from the bottom edge.
LIMIT   – 5 mm fillet on the outer corners, no overhangs steeper than 45°.

You don't need labels in the prompt — just include the four things.

Worked examples

Mechanical bracket

A right-angle bracket. Two flanges 50 × 40 mm meeting at 90°,
3 mm thick throughout, fillet radius 4 mm where the flanges meet.
Each flange has two M5 through-holes, 25 mm apart, centered.

Electronics enclosure

A rectangular enclosure 100 × 60 × 40 mm with 2 mm walls.
Add a 9 × 3 mm USB-C cutout centered on a short side, 8 mm from the bottom.
Inside the bottom face, four M3 standoffs 5 mm tall placed 4 mm in from each corner.

Desk organizer

A three-compartment desk organizer.
Outer dimensions 200 × 80 × 60 mm. 2 mm walls and base.
Compartments left to right: 30 × 80 mm (pens), 80 × 80 mm (notes), 90 × 80 mm (clips).
Round all outer top edges with a 2 mm fillet.

Iterating with follow-ups

After the first Build, use precise follow-ups to tweak. Aim each change at one feature at a time — that's cheaper and easier to review:

"Increase the wall thickness from 2 mm to 3 mm."
"Move the USB-C cutout 5 mm closer to the bottom edge."
"Round all four outer top edges with a 3 mm fillet."
"Add a 6 mm diameter through-hole, centered on the back face."

Antipatterns to avoid

• ❌ “Make a box, kind of big, with some holes.” — no sizes, no count, no placement.
• ❌ “Make it look like a Tesla logo.” — visual references don't map to engineering features. Describe shape parametrically.
• ❌ Multiple unrelated changes in one Build prompt. Run them as separate Build prompts so each is reviewable.
• ❌ Mixing units within one prompt (mixing mm and inches mid-spec). Pick one and stay consistent.
• ❌ Overspecifying internal geometry the AI can't see (e.g. part numbers, vendor SKUs). Stick to dimensions and features.

Pre-submit checklist

• ☐ Every dimension has a unit (mm, cm, in).
• ☐ Every feature has a position relative to a face or edge.
• ☐ Wall thickness is stated when it matters.
• ☐ Standard sizes are referenced by their standard (M3, M4, USB-C, etc.).
• ☐ The prompt fits on one screen — if not, split it.