Write a CAD brief your AI can actually check.

Start with a brief, not just an object name.

“Make a bracket” leaves the agent to invent dimensions, orientation, and construction. Those guesses may produce a plausible picture without producing the part you intended. Give the assistant a short specification with a way to determine whether it succeeded.

Goal: [what this model is for]
Units: millimeters
Envelope: [width × depth × height]
Orientation: [axes, origin, and base plane]
Parts: [named, separate objects]
Editable values: [dimensions you will change later]
Required details: [holes, spacing, profiles]
Build order: [first stage, then later details]
Checks: [dimensions, placement, closure]
Deliverables: FORM JSON plus [STL, OBJ, or 3DM]
If an operation is unsupported, explain before substituting.

Download the reusable CAD brief. These prompts are suggested workflows; model behavior varies with the assistant and its settings.

Example: a plate with a measurable hole pattern.

Start with the body, so there is only one thing to verify:

In FORM, create a 120 × 80 × 5 mm plate. Use X for width, Y for depth, and Z for thickness. Center X and Y at zero, and put the bottom at Z = 0. Name the part “Mounting plate.” Inspect its bounds and wait before adding detail.

Then specify the holes in the same coordinate system:

Add four circular through-holes, each 5 mm in diameter, with centers at X = ±50 mm and Y = ±30 mm. Cut fully through the 5 mm thickness along Z. Keep the plate’s outside dimensions unchanged. Inspect the resulting object and report any failed operation.

The centers form a 100 × 60 mm rectangular pattern, 10 mm from each outside edge. The 5 mm diameter is an example dimension, not a specified fastener fit. On a faceted solid, the circular openings and their measurements are approximations.

Say what changes and what stays fixed.

“Make it larger” is ambiguous. It could scale the holes, thicken the plate, or move the origin. State the specific dimension and the relationships you want preserved.

Increase only the plate width from 120 mm to 140 mm. Keep its depth at 80 mm and thickness at 5 mm. Keep each hole center 10 mm from its nearest X edge; preserve the Y coordinates and 5 mm hole diameter. Show the new bounds and hole-center coordinates.

That revision implies X centers at ±60 mm and Y centers at ±30 mm. If a later change needs to be reusable, ask for a supported parametric definition with named values. Arbitrary edits do not automatically create a constraint system or a persistent dependency graph.

Close the loop with evidence.

• Geometry: inspect the intended objects, their bounds, and available closure diagnostics.
• Appearance: review front, side, and isometric views. A hole can disappear behind a face in a single view.
• Editability: save FORM JSON and confirm the source values you need are retained.
• Export: open the exported file in its destination tool and check scale and representation.

If the result is wrong, identify the object and the mismatch before requesting a fix. Save or undo before a major revision. For surface work, explain whether you need an open NURBS patch or a solid with thickness; those are different modeling tasks.

When choosing an assistant, compare repeated attempts and retained files. The FORM benchmark provides one published task and its measurement method, rather than a guarantee that an assistant will solve every CAD brief.