The fillet broke

Fillets are the most common step to fail, because rounding an edge is geometrically demanding. The good news is that the message tells you what went wrong, and the fix is usually small.

Symptom

• A fillet (or chamfer) step in the Recipe is red.
• The agent reported an error such as "The fillet radius (6 mm) is larger than the 4 mm wall".
• The step succeeded, but "matched 0 edges", so nothing was rounded.
• A fillet looks rounded in one place but is missing from another.
• The whole build stopped with a kernel error.

Likely causes

Fix it

1. Read the message

Click the red step. The message names the cause in plain language, and usually the numbers involved. If a radius is too big for a wall, reduce it, or thicken the wall.

2. Try a smaller radius

Shrink the radius until the step turns green, then grow it back towards what you need. As a rule, keep a fillet radius smaller than about half the thinnest adjacent wall.

3. Split the job

Instead of one fillet on 40 edges, do it in groups: first the long outer edges, then the vertical edges, then the small ones. A failure then points to one group.

Fillet the top outer edges at 2 mm, then the vertical outer edges at 3 mm, then the bottom edges at 0.5 mm. Do them as three separate steps.

4. Check the selection

Ask what the step is selecting:

What edges does the fillet step select right now, and how many matched? If it is zero, make the selection more robust.

By default, fillets and chamfers exclude tangent-continuous edges, which avoids the most common crash. If you really need those edges, round them as part of the original sketch instead (rounded rectangle corner radius), or use a different feature.

5. Change the order

Round before you cut small holes or add pockets near the edge, or after shelling. In general:

1. main shape,
2. big cuts and pockets,
3. shell (if any),
4. fillets and chamfers,
5. small holes and cosmetic details.

Move the step in the Recipe and see if it recovers.

6. Use a chamfer instead

A chamfer is geometrically simpler and often succeeds where a fillet fails. For printing, a 45 degree chamfer is often better than a fillet on a bottom edge, since it prints without supports.

7. Let the agent repair it

The fillet step is red. Diagnose why and propose the smallest change that makes it work.

You get a ghost preview of the fix. Read it, then accept.

How to avoid it

• Make the radius a formula that cannot exceed the wall: min(fillet_radius, wall * 0.8).
• Add a design intent rule: "Fillet radius is always less than half the wall thickness".
• Round in sketches when you can: a rounded rectangle sketch creates smooth corners with no fillet step at all.
• Select by meaning (outer vertical edges) rather than by clicking each one, so the step still works after changes.
• Fillet late, after the geometry is settled.

Try it

On the Enclosure template, deliberately break one:

Set the fillet radius on the outer edges to 8 mm, and the wall thickness to 2 mm.

When the step turns red, ask the agent to explain and repair it. Notice how it reduces the radius or adds material, and which one it chose.