Good, better, best: eight prompts side by side
The same eight requests written three ways, with what the extra detail buys you each time. Copy, paste, and swap in your own numbers.
A good prompt is not longer, it is more informative. The pattern behind every "best" below is the same:
- What it is and what it is for.
- Real numbers, in millimetres, wherever size matters.
- Rules to keep ("always", "keep", "never").
- How it will be made (printer, material, process).
- What success looks like.
You do not need all five every time. For a quick edit, one sentence is perfect. Use the extra detail when a wrong guess would cost you a print.
1. A new object
Good
Make a phone stand.
Better
Make a phone stand that holds a phone 75 mm wide at about 65 degrees.
Best
Make a phone stand for a phone that is 75 mm wide and 9 mm thick with its case. Tilted back 65 degrees, with a gap at the bottom for a charging cable. One piece, printed in PETG on an Ender 3 without supports.
What changed: the best version gives the thickness (so the slot fits), the cable requirement (a feature the agent would otherwise miss), and the print constraints (so it avoids overhangs from the start).
2. A box for electronics
Good
Make a box for an ESP32.
Better
Make a box for an ESP32 DevKit V1 with a lid and a hole for the USB cable.
Best
Add an ESP32 DevKit V1 from the library and design a two-piece case around it: 1 mm clearance around the board, 2.4 mm walls, a cutout for the USB port sized for a plug, side vents, and a lid with a locating lip held by four M3 heat-set screws. Make the sizes follow the board through parameters.
What changed: a real library board, so the dimensions are exact; rules for the walls and clearance; and a request for parameters so you can adapt it.
3. Making something stronger
Good
Make this stronger.
Better
Make the walls thicker and add fillets.
Best
This bracket cracked at the inside corner where the arm meets the base. Add a 4 mm fillet there, and thicken the arm from 4 to 6 mm. Keep the mounting holes and overall length unchanged. I will print it with layers running along the arm.
What changed: it names the failure, so the agent fixes the cause instead of adding material everywhere. "Keep" protects what matters.
4. Rounding edges
Good
Round the edges.
Better
Round the outer edges with a 2 mm fillet.
Best
Select the outer vertical edges (these four). Round them with a 3 mm radius. Leave the top and bottom edges sharp so the part still sits flat on the bed.
What changed: the selection removes ambiguity, the radius is explicit, and the last sentence says what not to touch and why.
5. Holes and fasteners
Good
Add some holes for screws.
Better
Add four holes for M3 screws in the corners.
Best
Add four clearance holes for M3 screws in the corners of this face, 6 mm in from each edge. Countersink them so the heads sit flush. Always keep them symmetric about the center, even if I change the width.
What changed: "clearance holes for M3" tells the agent the intent (the hole size comes from the screw), the position is exact, the countersink says how the head sits, and "always keep them symmetric" becomes a lasting rule.
6. A lid that fits
Good
Add a lid.
Better
Add a lid that fits the box with a lip.
Best
Add a lid with a 1.5 mm locating lip that sits inside the box walls. Use a slip fit with 0.25 mm clearance per side, stored in a parameter called fit_clearance. Keep the lid the same width as the box, always. Print it flat, outer face down.
What changed: a numeric fit, a named parameter (so you can retune after a test print), a rule, and an orientation that influences the finish.
7. Making it printable
Good
Make it printable.
Better
Make it printable without supports.
Best
Check this for FDM on a Prusa MK4 in PETG. Fix any overhangs steeper than 45 degrees with chamfers rather than supports. Keep the outer dimensions and all hole positions. Tell me the best orientation and the printability score before and after.
What changed: it names the printer and material (which sets the rules), prefers a design fix over supports, protects what must stay, and asks for evidence.
8. Making it lighter
Good
Make it lighter.
Better
Reduce the weight by 20 percent.
Best
Create a branch called lighter. Reduce the weight by at least 20 percent using lightening holes in the web, not thinner walls. Keep the 8 mm bore, the 60 mm outer diameter and the mounting holes. Never go below 1.6 mm wall. Show me the weight change.
What changed: a method, a measurable target, protections, and a branch so the experiment is safe.
Habits to take with you
| Habit | Example |
|---|---|
| Give the size of anything that mates with something real | "a slot for a 9 mm thick phone" |
| Name the process | "FDM, PETG, no supports" |
| State what to keep | "keep the mounting holes" |
| Use "always" for rules | "always centered" |
| Say how success is measured | "weighs under 100 g" |
| Ask for assumptions | "list anything you assumed" |
Try it
Pick any prompt above, send the "Good" version, and reject the result. Then send the "Best" version. Compare the two ghost previews and the credits spent.
Did this help?
Keep reading
Prompt cookbook
Phrases the agent understands
A short vocabulary of words and phrases that map directly to something the agent does: keep, always, this, symmetric, fits an M3, centered, no supports and more.
Beginner5 min readPrompt cookbook
Prompts with images: what to say when you attach one
For each kind of image (sketch, drawing, part photo, device photo, markup, concept) here is what the agent does with it and the prompt that gets the best result.
Beginner4 min readTroubleshooting
The agent misunderstood me
The preview is not what you meant. Find out whether the cause was selection, missing numbers, a mixed-up request or the wrong mode, and recover without losing work.
Beginner5 min read