@@ -155,7 +155,7 @@ Snippet: [`snippets/decimate_to_budget.py`](https://github.com/TMHSDigital/Blend
 
 ### 8. Generate collider
 
-Convex hull via `bmesh.ops.convex_hull`, then delete `geom_interior` and `geom_unused`. Copy `matrix_world` onto the collider. Hull a coarse cage when the render mesh would blow a per-piece face budget. Witness: [`examples/collision-hull-proxy/`](https://github.com/TMHSDigital/Blender-Developer-Tools/tree/main/examples/collision-hull-proxy).
+Convex hull via `bmesh.ops.convex_hull` over the points only: remove the source edges first (`bm.edges.remove`, which keeps the verts), hull, then delete verts left with no faces. Hulling a mesh that still has faces keeps every source face lying on the hull, so a cube collider comes out with 18 faces and 12 non-manifold edges instead of 12 closed triangles. Copy `matrix_world` onto the collider. Hull a coarse cage when the render mesh would blow a per-piece face budget. Witness: [`examples/collision-hull-proxy/`](https://github.com/TMHSDigital/Blender-Developer-Tools/tree/main/examples/collision-hull-proxy).
 
 Snippet: [`snippets/convex_hull_collider.py`](https://github.com/TMHSDigital/Blender-Developer-Tools/blob/main/snippets/convex_hull_collider.py).
 
 
@@ -1,7 +1,10 @@
 # Build a convex hull collision mesh via bmesh.ops.convex_hull.
-# Delete geom_interior and geom_unused from the result dict so leftover
-# input verts do not remain inside the hull. Copy matrix_world onto
-# the collider so it sits on the source object.
+# Strip the source edges/faces first: the op adds hull triangles but keeps
+# source faces lying on the hull (a cube came out V8 E18 F18 with 12
+# non-manifold edges). Then drop verts left inside the hull. The result is
+# a closed, all-triangle hull (cube: F12, 0 non-manifold edges). A flat
+# input has no volume and yields an open hull. Copy matrix_world onto the
+# collider so it sits on the source object.
 #
 # bmesh.new() must be paired with bm.free() in try/finally.
 #
@@ -18,13 +21,15 @@ def convex_hull_collider(obj, name=None):
     bm = bmesh.new()
     try:
         bm.from_mesh(obj.data)
-        result = bmesh.ops.convex_hull(bm, input=bm.verts)
-        interior = result.get("geom_interior") or []
-        unused = result.get("geom_unused") or []
-        if interior:
-            bmesh.ops.delete(bm, geom=interior, context="VERTS")
-        if unused:
-            bmesh.ops.delete(bm, geom=unused, context="VERTS")
+        # Hull the points, not the surface: convex_hull keeps any source
+        # edge or face whose verts lie on the hull, leaving duplicate,
+        # non-manifold faces. edges.remove() drops faces but keeps verts.
+        for edge in bm.edges[:]:
+            bm.edges.remove(edge)
+        bmesh.ops.convex_hull(bm, input=bm.verts[:])
+        loose = [v for v in bm.verts if not v.link_faces]
+        if loose:
+            bmesh.ops.delete(bm, geom=loose, context="VERTS")
         bm.to_mesh(mesh)
         mesh.update()
     finally:
 
@@ -68,6 +68,7 @@ as `export-preset-axis` number their own checks independently.
 | 4 | `--lod-budgets` missing, non-positive, or not strictly decreasing |
 | 5 | Import produced no mesh |
 | 6 | `outdir` is not a directory, or glTF export failed / wrote no file |
+| 7 | `--collider convex` produced a hull that is not closed (an edge without exactly two faces), e.g. a flat input |
 
 ## Expected environment
 
 
@@ -28,6 +28,7 @@
 #   docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.fbx
 
 import argparse
+import collections
 import os
 import sys
 
@@ -183,13 +184,15 @@ def convex_hull_collider(obj, name=None):
     bm = bmesh.new()
     try:
         bm.from_mesh(obj.data)
-        result = bmesh.ops.convex_hull(bm, input=bm.verts)
-        interior = result.get("geom_interior") or []
-        unused = result.get("geom_unused") or []
-        if interior:
-            bmesh.ops.delete(bm, geom=interior, context="VERTS")
-        if unused:
-            bmesh.ops.delete(bm, geom=unused, context="VERTS")
+        # Hull the points, not the surface: convex_hull keeps any source
+        # edge or face whose verts lie on the hull, leaving duplicate,
+        # non-manifold faces. edges.remove() drops faces but keeps verts.
+        for edge in bm.edges[:]:
+            bm.edges.remove(edge)
+        bmesh.ops.convex_hull(bm, input=bm.verts[:])
+        loose = [v for v in bm.verts if not v.link_faces]
+        if loose:
+            bmesh.ops.delete(bm, geom=loose, context="VERTS")
         bm.to_mesh(mesh)
         mesh.update()
     finally:
@@ -200,6 +203,14 @@ def convex_hull_collider(obj, name=None):
     return collider
 
 
+def is_closed_hull(mesh):
+    """True when every edge borders exactly two faces (a closed surface)."""
+    if not mesh.polygons:
+        return False
+    counts = collections.Counter(k for p in mesh.polygons for k in p.edge_keys)
+    return all(counts[e.key] == 2 for e in mesh.edges)
+
+
 def box_collider(obj, name=None):
     mesh_in = obj.data
     n = len(mesh_in.vertices)
@@ -374,6 +385,12 @@ def main():
     if args.collider == "convex":
         collider = convex_hull_collider(hero)
         print(f"collider={collider.name} verts={len(collider.data.vertices)}")
+        if not is_closed_hull(collider.data):
+            print(
+                f"ERROR: convex collider {collider.name} is not a closed hull",
+                file=sys.stderr,
+            )
+            return 7
     elif args.collider == "box":
         collider = box_collider(hero)
         print(f"collider={collider.name} verts={len(collider.data.vertices)}")