diff --git a/crates/kernel/src/modify_history_tests.rs b/crates/kernel/src/modify_history_tests.rs
index 04880f70b6..1c6165ff2d 100644
--- a/crates/kernel/src/modify_history_tests.rs
+++ b/crates/kernel/src/modify_history_tests.rs
@@ -537,6 +537,21 @@ mod smlib {
         assert!(e.to_string().contains("no room"), "{e}");
     }
 
+    /// A hole's wall is stored as two halves; a pick on the line between them opens the whole
+    /// wall (kernel-cases shell/4010). The cavity is the plate inset by 2.5 less the hole's
+    /// cylinder, open to the hole.
+    #[test]
+    fn shell_open_at_a_hole_wall_opens_the_whole_wall() {
+        let hole = Loop2::circle(Vec2::new(20.0, 15.0), 5.0);
+        let plate = extrude(&Plane::XY, &[Region2 { outer: square(0.0, 0.0, 40.0, 30.0), holes: vec![hole] }], 0.0, 12.5).unwrap().pop().unwrap();
+        let r = offset_backend::shell(&plate, &[Vec3::new(25.0, 15.0, 6.25)], 2.5).unwrap();
+        let pi = std::f64::consts::PI;
+        let want = (15000.0 - pi * 25.0 * 12.5) - (35.0 * 25.0 * 7.5 - pi * 25.0 * 7.5);
+        // The measure is a mesh volume (within 1e-5 of the true one on this round hole).
+        assert!((volume(&r) - want).abs() < want * 1e-5, "{} vs {want}", volume(&r));
+        verified(&r, &plate, reach(2.5 * 1.5));
+    }
+
     #[test]
     fn offset_moves_a_face() {
         let b = ell();
diff --git a/crates/kernel/src/offset_backend.rs b/crates/kernel/src/offset_backend.rs
index 091e0137dd..10ecbe7a88 100644
--- a/crates/kernel/src/offset_backend.rs
+++ b/crates/kernel/src/offset_backend.rs
@@ -73,14 +73,19 @@ fn run(name: &str, b: &Body, op: impl FnOnce(&Brep) -> std::result::Result<Brep,
     Ok((body, history))
 }
 
-/// [`crate::shell`] through SMLib: walls of `thickness` inside, open at the faces nearest to
-/// `open`.
+/// [`crate::shell`] through SMLib: walls of `thickness` inside, open at the faces under the
+/// points `open` (the face of the nearest triangle, as truck's shell picks them) and the faces
+/// on the same surface joined to them: a round wall stored in pieces (a hole's two halves) opens
+/// whole. SMLib is given one point inside each, never the pick itself, which may lie on the
+/// line between two pieces.
 pub(crate) fn shell(b: &Body, open: &[Vec3], thickness: f64) -> Result<Body> {
     if !(thickness.is_finite() && thickness > 1e-6) {
         return Err(KernelError::Invalid("shell thickness must be positive".into()));
     }
+    let picked = faces_under(b, open)?;
+    let opened = crate::offset::with_same_surface(b, &picked);
     let tol = pick_tolerance(b);
-    let pts = points(open);
+    let pts = points(&face_points(b, &opened)?);
     let (r, h) = run("shell", b, |s| s.shell(&pts, tol, -thickness))?;
     let h = h.and_then(|h| with_offsets(&r, b, h, thickness));
     Ok(r.with_checked_history(h, &record::counts(&[b])))
@@ -118,6 +123,30 @@ pub(crate) fn thicken(b: &Body, thickness: f64, symmetric: bool) -> Result<Body>
     Ok(r.with_checked_history(h, &record::counts(&[b])))
 }
 
+/// The face of `b` under each point: the face of the nearest triangle, within the picking reach.
+fn faces_under(b: &Body, at: &[Vec3]) -> Result<Vec<usize>> {
+    let reach = pick_tolerance(b);
+    let mesh = b.tessellate((b.size() * 1e-3).max(1e-3))?;
+    let mut out: Vec<usize> = Vec::new();
+    for p in at {
+        let near = mesh
+            .triangles
+            .iter()
+            .zip(&mesh.tri_face)
+            .filter_map(|(t, f)| mesh.tri(t).map(|[x, y, z]| (crate::boolean_guard::point_triangle_dist(*p, x, y, z), *f as usize)))
+            .min_by(|x, y| x.0.total_cmp(&y.0));
+        match near {
+            Some((d, f)) if d < reach => {
+                if !out.contains(&f) {
+                    out.push(f);
+                }
+            }
+            _ => return Err(KernelError::Invalid(format!("no face at {:?}", [p.x, p.y, p.z]))),
+        }
+    }
+    Ok(out)
+}
+
 /// One point inside each face `faces` of `b`: the centroid of its largest triangle.
 fn face_points(b: &Body, faces: &[usize]) -> Result<Vec<Vec3>> {
     let mesh = b.tessellate((b.size() * 1e-3).max(1e-3))?;
diff --git a/crates/kernel/src/polyhedron.rs b/crates/kernel/src/polyhedron.rs
index 5c2607ff00..f87648396a 100644
--- a/crates/kernel/src/polyhedron.rs
+++ b/crates/kernel/src/polyhedron.rs
@@ -879,11 +879,12 @@ pub fn offset_faces(b: &Body, at: &[Vec3], distance: f64) -> Result<Body> {
     let run = |body: &Body| -> Result<Body> {
         let healed = crate::boolean_record::healed(body)?.with_split_lines(body.splits.clone());
         let chosen = faces_at(&healed, at)?;
-        #[cfg(feature = "smlib")]
-        if crate::backend()? == crate::Backend::Smlib {
-            return crate::offset_backend::offset_faces(&healed, &chosen, distance).map(|r| record::after(r, &healed, &record::counts(&[body])));
-        }
-        match offset_planar(&healed, |fi, _| if chosen.contains(&fi) { distance } else { 0.0 }) {
+        let moved = match crate::backend()? {
+            #[cfg(feature = "smlib")]
+            crate::Backend::Smlib => crate::offset_backend::offset_faces(&healed, &chosen, distance),
+            _ => offset_planar(&healed, |fi, _| if chosen.contains(&fi) { distance } else { 0.0 }),
+        };
+        match moved {
             Ok(r) => Ok(record::after(r, &healed, &record::counts(&[body]))),
             // A piece of a split face (its neighbour stays): the piece swept along its normal
             // joins on (outward) or cuts in, with walls along the split lines.