diff --git a/ROADMAP.md b/ROADMAP.md
index 563be147..196acf0c 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -304,5 +304,11 @@ headline counts in-scope tabs only and lists the deferred ones separately.
 
 ## Kernel
 
+- Shell of a stepped, joined planar body no longer creates horizontal subdivisions across
+  continuous walls. Fresh axis-aligned Cut walls are stitched using their exact construction
+  support and shared topology, preserving real steps and intentional boundaries. The local
+  reported case returns from 86 to 49 faces with unchanged material and bounds; regressions
+  cover stepped stock, STEP round trips and saved projections from removed pieces.
+
 truck (Apache-2.0) behind our own `kernel` crate, with our own blend operation and boolean
 retry/validation; see `plan/adr/0001-geometry-kernel.md` (local) — summary in README.
diff --git a/crates/doc/src/config_shell_tests.rs b/crates/doc/src/config_shell_tests.rs
index c640a06d..5ecb5b81 100644
--- a/crates/doc/src/config_shell_tests.rs
+++ b/crates/doc/src/config_shell_tests.rs
@@ -457,3 +457,124 @@ fn saved_curved_strip_projection_is_lost_after_exact_wall_coalescence() {
         }
     }
 }
+
+#[test]
+fn saved_stepped_tab_cut_piece_projection_is_lost_without_expanding_material() {
+    use solvecraft_kernel::{BoolOp, FacePaint, Paint};
+    use solvecraft_sketch::{LinkKind, LinkSource, Sketch};
+    let mut doc = Document::new("public stepped tab case");
+    doc.set_param("length", "40", Some("mm"), None).unwrap();
+    doc.set_param("wall", "2", Some("mm"), None).unwrap();
+    for (name, kind) in [
+        ("Case", json!({"type":"box","corner":[0,0,0],"length":"length","width":"30","height":"20"})),
+        ("Tab", json!({"type":"box","corner":["length",10,12],"length":"8","width":"10","height":"8"})),
+    ] {
+        let id = doc.add_feature(serde_json::from_value(kind).unwrap(), Some(name)).unwrap();
+        doc.feature_mut(id).unwrap().body_names = vec![name.into()];
+    }
+    doc.add_feature(
+        FeatureKind::Combine { target: "Case".into(), tools: vec!["Tab".into()], operation: crate::Operation::Join, keep_tools: false },
+        None,
+    )
+    .unwrap();
+    let tool = doc
+        .add_feature(
+            serde_json::from_value(
+                json!({"type":"box","corner":["wall","wall","wall"],"length":"length-2*wall","width":"30-2*wall","height":"22-wall"}),
+            )
+            .unwrap(),
+            Some("Pocket tool"),
+        )
+        .unwrap();
+    doc.feature_mut(tool).unwrap().body_names = vec!["Pocket tool".into()];
+    let cut = doc
+        .add_feature(
+            FeatureKind::Combine { target: "Case".into(), tools: vec!["Pocket tool".into()], operation: crate::Operation::Cut, keep_tools: false },
+            Some("Pocket"),
+        )
+        .unwrap();
+    let mut model = Model::new();
+    model.evaluate(&doc);
+    assert!(model.results.iter().all(|r| r.error.is_none()), "{:?}", model.results);
+    let before = model.state_before(cut);
+    // A real per-face paint control preserves the former slab construction;
+    // no historical names or geometry are fabricated.
+    let old_stock = before
+        .body("Case")
+        .unwrap()
+        .body
+        .clone()
+        .with_paint(Some(Paint { opacity: 1., faces: vec![FacePaint { face: 0, color: [1., 0., 0.], opacity: 1. }] }));
+    let old_cut = solvecraft_kernel::boolean(&old_stock, &before.body("Pocket tool").unwrap().body, BoolOp::Cut).unwrap().unwrap();
+    let mut old_body = ModelBody::new("Case".into(), old_cut, cut);
+    old_body.names = crate::naming::NameCell::new(Some(std::sync::Arc::new(crate::naming::NamingOrigin {
+        feature: doc.feature(cut).unwrap().clone(),
+        before,
+        depth: 1,
+        mats: vec![],
+    })));
+    let at = Vec3::new(0., 15., 1.);
+    let former = crate::naming::face_name_at_body(&old_body, at).unwrap();
+    assert_ne!(former, crate::naming::strip_piece(&former), "actual former bounded wall patch has a full piece name");
+    let index = crate::naming::face_on_body(&old_body, &former, at).unwrap().0;
+    let mut old_state = crate::ModelState::default();
+    old_state.bodies.push(old_body);
+    let plane = solvecraft_geom::Plane::YZ;
+    let resolved = crate::project::resolve_face_indices(&old_state, &plane, LinkKind::Project, "Case", &[index]).unwrap();
+    let current_state = model.state();
+    let current = current_state.body("Case").unwrap();
+    let stem = crate::naming::strip_piece(&former);
+    let current_names = crate::naming::face_names(current);
+    let merged = current_names.iter().position(|name| name == stem).unwrap();
+    assert!(current.body.face_was_coalesced(merged), "new exact generated-wall construction supplies the loss-only witness");
+    assert!(!current_names.contains(&former));
+    let mut sketch = Sketch::new();
+    sketch.add_link(LinkKind::Project, LinkSource::Face { body: "Case".into(), at }, &resolved.geom).unwrap();
+    sketch.links[0].face_name = Some(former);
+    let original = sketch.clone();
+    let captured = original.points.iter().filter(|p| p.link.is_some()).map(|p| p.pos).collect::<Vec<_>>();
+    assert_eq!(captured.len(), 4);
+    assert!(captured.iter().all(|p| (p.x == 10. || p.x == 20.) && (p.y == 0. || p.y == 20.)));
+    let sk = doc.add_feature(FeatureKind::Sketch { plane: PlaneRef::Custom { plane }, sketch }, Some("Saved wall patch")).unwrap();
+    let material = doc
+        .add_feature(
+            FeatureKind::Extrude {
+                sketch: sk,
+                profiles: crate::ProfileSel::All,
+                extent: serde_json::from_value(json!({"distance":"1"})).unwrap(),
+                operation: crate::Operation::NewBody,
+                targets: vec![],
+            },
+            Some("Saved band stock"),
+        )
+        .unwrap();
+    for (length, wall) in [(40., 2.), (50., 3.)] {
+        doc.set_param("length", &length.to_string(), Some("mm"), None).unwrap();
+        doc.set_param("wall", &wall.to_string(), Some("mm"), None).unwrap();
+        model.evaluate(&doc);
+        let reopened = crate::format::read(&doc.to_json()).unwrap();
+        let mut cold = Model::new();
+        cold.evaluate(&reopened);
+        for current in [&model, &cold] {
+            let result = current.results.iter().find(|r| r.id == sk).unwrap();
+            assert!(result.error.is_none(), "{result:?}");
+            assert!(result.warning.as_deref().is_some_and(|w| w.contains("no longer exists; last geometry retained")), "{result:?}");
+            let state = current.state();
+            let saved = state.sketch(sk).unwrap();
+            assert!(saved.sketch.links[0].lost);
+            assert_eq!(saved.sketch.points, original.points);
+            assert_eq!(saved.sketch.curves, original.curves);
+            let stock = state.bodies.iter().find(|b| b.feature == material).unwrap();
+            assert!(
+                (solvecraft_kernel::measure(&stock.body).unwrap().volume - 200.).abs() < 1e-7,
+                "saved10x20 patch must not become30x20 wall; actualvolume={}, bounds={:?}, originalpoints={:?}",
+                solvecraft_kernel::measure(&stock.body).unwrap().volume,
+                stock.mesh().bounds(),
+                original.points
+            );
+            let case = state.body("Case").unwrap();
+            let expected = length * 30. * 20. + 8. * 10. * 8. - (length - 2. * wall) * (30. - 2. * wall) * (20. - wall);
+            assert!((solvecraft_kernel::measure(&case.body).unwrap().volume - expected).abs() < 1e-6);
+        }
+    }
+}
diff --git a/crates/kernel/src/body.rs b/crates/kernel/src/body.rs
index 6e671c97..4c989db2 100644
--- a/crates/kernel/src/body.rs
+++ b/crates/kernel/src/body.rs
@@ -33,6 +33,8 @@ pub struct Body {
     pub(crate) splits: Option<std::sync::Arc<Vec<Vec<Vec3>>>>,
     /// Exact construction/import source identities, in this body's face order.
     pub(crate) face_sources: Option<std::sync::Arc<Vec<Option<FaceSource>>>>,
+    /// Sticky transient veto: authored face boundaries must survive later rebuilds.
+    pub(crate) regularization_protected: bool,
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -360,6 +362,7 @@ impl Body {
             && self.paint == other.paint
             && self.splits == other.splits
             && self.face_sources == other.face_sources
+            && self.regularization_protected == other.regularization_protected
     }
 
     pub(crate) fn new(solid: Solid) -> Result<Body> {
@@ -398,9 +401,15 @@ impl Body {
             paint: None,
             splits: None,
             face_sources: None,
+            regularization_protected: false,
         })
     }
 
+    pub(crate) fn inherit_regularization_protection(mut self, input: &Body) -> Self {
+        self.regularization_protected |= input.regularization_protected;
+        self
+    }
+
     pub fn face_sources(&self) -> Option<&[Option<FaceSource>]> {
         self.face_sources.as_deref().map(Vec::as_slice)
     }
@@ -628,6 +637,7 @@ impl Body {
     /// The same body with the lines its faces were split along on purpose.
     pub fn with_split_lines(mut self, s: Option<std::sync::Arc<Vec<Vec<Vec3>>>>) -> Body {
         self.splits = s.filter(|l| !l.is_empty());
+        self.regularization_protected |= self.splits.is_some();
         self
     }
 
@@ -652,6 +662,7 @@ impl Body {
             })
             .filter(|p| p.opacity < 1.0 || !p.faces.is_empty())
             .map(std::sync::Arc::new);
+        self.regularization_protected |= self.paint().is_some_and(|paint| !paint.faces.is_empty());
         self
     }
 
@@ -1063,7 +1074,7 @@ impl Body {
                             })
                             .collect()
                     });
-                    Ok(result.with_face_sources(sources.map(std::sync::Arc::new)))
+                    Ok(result.with_face_sources(sources.map(std::sync::Arc::new)).inherit_regularization_protection(self))
                 })
                 .collect()
         })
@@ -1077,3 +1088,7 @@ mod lump_tests;
 #[cfg(test)]
 #[path = "refine_cache_tests.rs"]
 mod refine_cache_tests;
+
+#[cfg(test)]
+#[path = "regularization_protection_tests.rs"]
+mod regularization_protection_tests;
diff --git a/crates/kernel/src/meshbody.rs b/crates/kernel/src/meshbody.rs
index 9c2a28c6..7663ed64 100644
--- a/crates/kernel/src/meshbody.rs
+++ b/crates/kernel/src/meshbody.rs
@@ -109,6 +109,7 @@ pub fn mesh_body(positions: &[Vec3], triangles: &[[u32; 3]]) -> Result<Body> {
         paint: None,
         splits: None,
         face_sources: None,
+        regularization_protected: false,
     })
 }
 
@@ -201,6 +202,7 @@ impl Body {
             paint: self.paint.clone(),
             splits: self.splits.clone(),
             face_sources: None,
+            regularization_protected: self.regularization_protected,
         })
     }
 }
diff --git a/crates/kernel/src/ops.rs b/crates/kernel/src/ops.rs
index 8b62dc90..8a0763b4 100644
--- a/crates/kernel/src/ops.rs
+++ b/crates/kernel/src/ops.rs
@@ -221,6 +221,7 @@ pub fn boolean(a: &Body, b: &Body, op: BoolOp) -> Result<Option<Body>> {
             )),
         };
     }
+    r = r.map(|result| result.map(|body| body.inherit_regularization_protection(a).inherit_regularization_protection(b)));
     if matches!(op, BoolOp::Cut | BoolOp::Union) {
         r = r.and_then(|result| result.map(|body| body.with_preserved_boolean_planar_sources(a, b, op == BoolOp::Cut)).transpose());
     }
@@ -538,7 +539,11 @@ pub(crate) fn join_pieces(pieces: Vec<Body>) -> Result<Option<Body>> {
         _ => {
             let shells: Vec<mt::Shell> = pieces.iter().flat_map(|p| p.solid.boundaries().clone()).collect();
             let solid = guard("pieces", || Solid::try_new(shells).map_err(|e| KernelError::Failed(e.to_string())))?;
-            Body::new(solid).map(Some)
+            let protected = pieces.iter().any(|piece| piece.regularization_protected);
+            Body::new(solid).map(|mut body| {
+                body.regularization_protected = protected;
+                Some(body)
+            })
         }
     }
 }
@@ -558,8 +563,9 @@ pub fn transform_matrix(body: &Body, m: [[f64; 4]; 4]) -> Result<Body> {
         );
         // An affine placement retains face order and appearance metadata.
         let sources = body.transformed_sources(m)?;
-        Body::new(placed_topology(&body.solid, mat)?)
-            .map(|placed| placed.with_color(body.color()).with_paint(body.paint().cloned()).with_face_sources(sources))
+        Body::new(placed_topology(&body.solid, mat)?).map(|placed| {
+            placed.with_color(body.color()).with_paint(body.paint().cloned()).with_face_sources(sources).inherit_regularization_protection(body)
+        })
     })
 }
 
diff --git a/crates/kernel/src/planar_regularize.rs b/crates/kernel/src/planar_regularize.rs
index 593284c8..7dda1687 100644
--- a/crates/kernel/src/planar_regularize.rs
+++ b/crates/kernel/src/planar_regularize.rs
@@ -117,6 +117,36 @@ fn merged_face(faces: &[&mt::Face], members: &[usize], plane: Plane, eligible: &
 
 impl Body {
     pub(crate) fn regularize_certified_planar_faces(self) -> Result<Self> {
+        self.regularize_planar_faces(false)
+    }
+
+    /// Only newly produced Cut walls call this path. The temporary planes carry
+    /// construction support, never persistent ancestor/source-role authority.
+    pub(crate) fn regularize_generated_planar_faces(self) -> Result<Self> {
+        let body = self.regularize_planar_faces(true)?;
+        let sources = body.face_sources().map(|sources| {
+            sources
+                .iter()
+                .map(|source| {
+                    let mut source = source.clone();
+                    if let Some(super::FaceSource::ExtrudeSide { plane, coalesced, .. }) = &mut source {
+                        if plane.is_some() && !*coalesced {
+                            source = None;
+                        } else {
+                            *plane = None;
+                        }
+                    }
+                    source
+                })
+                .collect()
+        });
+        Ok(body.with_face_sources(sources.map(Arc::new)))
+    }
+
+    fn regularize_planar_faces(self, generated: bool) -> Result<Self> {
+        if self.regularization_protected {
+            return Ok(self);
+        }
         let Some(sources) = self.face_sources() else { return Ok(self) };
         if sources.len() > 16_384 || self.paint().is_some_and(|p| !p.faces.is_empty()) {
             return Ok(self);
@@ -148,7 +178,16 @@ impl Body {
                 .zip(sources)
                 .map(|(f, s)| {
                     s.as_ref()
-                        .and_then(super::planar_sources::certified_plane)
+                        .and_then(|source| {
+                            if generated {
+                                match source {
+                                    super::FaceSource::ExtrudeSide { segment: solvecraft_geom::Seg2::Line { .. }, plane: Some(p), .. } => Some(*p),
+                                    _ => None,
+                                }
+                            } else {
+                                super::planar_sources::certified_plane(source)
+                            }
+                        })
                         .filter(|p| super::planar_sources::actual_plane(f).is_some_and(|actual| super::planar_sources::same_support(p, &actual)))
                 })
                 .collect();
@@ -179,7 +218,7 @@ impl Body {
                 };
                 let Some(left) = identity.checked_sub(cost) else { return Ok(None) };
                 identity = left;
-                if sa != sb {
+                if !generated && sa != sb {
                     continue;
                 }
                 let Some(cost) = split_points.checked_mul(6).and_then(|n| n.checked_add(1)) else { return Ok(None) };
@@ -235,7 +274,14 @@ impl Body {
                         let Some(left) = metadata.checked_sub(cost) else { return Ok(None) };
                         metadata = left;
                     }
-                    output_sources.push(source.cloned());
+                    let mut source = source.cloned();
+                    if generated
+                        && replacements.contains_key(&leader)
+                        && let Some(super::FaceSource::ExtrudeSide { coalesced, .. }) = &mut source
+                    {
+                        *coalesced = true;
+                    }
+                    output_sources.push(source);
                     offset += 1;
                 }
                 shells.push(mt::Shell::from(output));
diff --git a/crates/kernel/src/planar_sources.rs b/crates/kernel/src/planar_sources.rs
index 2fb19bed..7faa92eb 100644
--- a/crates/kernel/src/planar_sources.rs
+++ b/crates/kernel/src/planar_sources.rs
@@ -114,13 +114,15 @@ impl Body {
     /// Retain exact shell roles, and construction roles for Cut, only on unchanged
     /// supports without a competing coplanar tool. No imported or fitted role is inferred.
     pub(crate) fn with_preserved_boolean_planar_sources(self, input: &Body, tool: &Body, constructed: bool) -> Result<Self> {
+        let self_ = self.inherit_regularization_protection(input).inherit_regularization_protection(tool);
         if !input.face_sources().is_some_and(|sources| sources.iter().flatten().any(|s| boolean_source(s, constructed))) {
-            return Ok(self);
+            return Ok(self_);
         }
-        self.map_planar_sources(input, None, Some(tool), &[], constructed)
+        self_.map_planar_sources(input, None, Some(tool), &[], constructed)
     }
 
     fn map_planar_sources(mut self, input: &Body, shell: Option<f64>, blocker: Option<&Body>, open: &[Vec3], constructed: bool) -> Result<Self> {
+        self.regularization_protected |= input.regularization_protected;
         let Some(sources) = input.face_sources() else { return Ok(self) };
         if input.solid.face_iter().count() > 16_384
             || self.solid.face_iter().count() > 16_384
diff --git a/crates/kernel/src/polyhedron.rs b/crates/kernel/src/polyhedron.rs
index 4c772f0f..13a5b7e6 100644
--- a/crates/kernel/src/polyhedron.rs
+++ b/crates/kernel/src/polyhedron.rs
@@ -342,7 +342,15 @@ pub fn round_all_edges(b: &Body, r: f64) -> Result<Body> {
 /// by the thickness (open faces outward, past the body) keeping the body's topology, which
 /// gives the cavity; the shell is the body minus the cavity.
 fn shell_planar(b: &Body, open: &[Vec3], thickness: f64) -> Result<Body> {
-    let healed = Body::new(crate::heal::heal_keep(b.deep_copy(), b.size(), b.split_keep()))?;
+    // Face paint and authored splits are construction boundaries. Retain their
+    // eligibility veto through the temporary body used for the cavity Cut.
+    // Paint indices stay in source order by avoiding healing in that case.
+    let solid = if b.paint().is_some_and(|paint| !paint.faces.is_empty()) {
+        b.deep_copy()
+    } else {
+        crate::heal::heal_keep(b.deep_copy(), b.size(), b.split_keep())
+    };
+    let healed = Body::new(solid)?.with_paint(b.paint().cloned()).with_split_lines(b.split_lines().cloned()).inherit_regularization_protection(b);
     let size = healed.size();
     let mesh = healed.tessellate((size * 1e-3).max(1e-3))?;
     // Faces to open: the face of the triangle nearest each point.
diff --git a/crates/kernel/src/prism.rs b/crates/kernel/src/prism.rs
index 6c06877c..9a33493e 100644
--- a/crates/kernel/src/prism.rs
+++ b/crates/kernel/src/prism.rs
@@ -848,6 +848,16 @@ fn sew_connected_sections(plane: &Plane, d: Vec3, levels: &[f64], slabs: &[Vec<R
     }
     let mut faces: Vec<FaceSpec> = Vec::new();
     let mut sources = Vec::new();
+    let frame_is_orthonormal = plane.origin.is_finite()
+        && plane.x.is_finite()
+        && plane.y.is_finite()
+        && d.is_finite()
+        && (plane.x.len2() - 1.).abs() <= 64. * f64::EPSILON
+        && (plane.y.len2() - 1.).abs() <= 64. * f64::EPSILON
+        && (d.len2() - 1.).abs() <= 64. * f64::EPSILON
+        && plane.x.dot(plane.y).abs() <= 64. * f64::EPSILON
+        && plane.x.dot(d).abs() <= 64. * f64::EPSILON
+        && plane.y.dot(d).abs() <= 64. * f64::EPSILON;
     for (first, end, sg) in runs {
         let (z0, z1) = (levels[first], levels[end]);
         let (a, b) = (sg.start(), sg.end());
@@ -868,7 +878,28 @@ fn sew_connected_sections(plane: &Plane, d: Vec3, levels: &[f64], slabs: &[Vec<R
             _ => SurfSpec::Plane,
         };
         faces.push(FaceSpec { loops: vec![lp3], surface, probe: Some((at(sg.point_at(0.5), (z0 + z1) / 2.0), outward)) });
-        sources.push((end > first + 1).then_some(crate::FaceSource::ExtrudeSide { segment: sg, plane: None, coalesced: true }));
+        // Axis-aligned generated lines have a common oriented support frame
+        // independent of axial stations or along-line endpoint interpolation.
+        let generated_plane = if coalesce
+            && frame_is_orthonormal
+            && let Seg2::Line { a, b } = sg
+        {
+            let (origin, x) = if a.y == b.y && a.x != b.x {
+                (at(Vec2::new(0., a.y), 0.), plane.x * (b.x - a.x).signum())
+            } else if a.x == b.x && a.y != b.y {
+                (at(Vec2::new(a.x, 0.), 0.), plane.y * (b.y - a.y).signum())
+            } else {
+                (Vec3::ZERO, Vec3::ZERO)
+            };
+            (origin.is_finite() && x.len2() > 0.).then_some(Plane { origin, x, y: d })
+        } else {
+            None
+        };
+        sources.push((generated_plane.is_some() || end > first + 1).then_some(crate::FaceSource::ExtrudeSide {
+            segment: sg,
+            plane: generated_plane,
+            coalesced: end > first + 1,
+        }));
     }
     for (z, up, rs) in &caps {
         for r in rs {
@@ -885,7 +916,11 @@ fn sew_connected_sections(plane: &Plane, d: Vec3, levels: &[f64], slabs: &[Vec<R
             sources.push(None);
         }
     }
-    Some(crate::sew::sew_touching_with_sources(&faces, &sources))
+    Some(crate::sew::sew_touching_with_sources(&faces, &sources).and_then(
+        |body| {
+            if coalesce { body.regularize_generated_planar_faces() } else { Ok(body) }
+        },
+    ))
 }
 
 fn simple_prism_boolean(a: &Body, b: &Body, op: BoolOp) -> Option<Result<Option<Body>>> {
@@ -1109,6 +1144,8 @@ fn stack_boolean(a: &Body, b: &Body, op: BoolOp) -> Option<Result<Option<Body>>>
         &slabs,
         tol,
         op == BoolOp::Cut
+            && !a.regularization_protected
+            && !b.regularization_protected
             && a.split_keep().is_empty()
             && b.split_keep().is_empty()
             && a.paint().is_none_or(|p| p.faces.is_empty())
@@ -1216,6 +1253,62 @@ mod planar_tests {
         assert_eq!(crate::step_export(&[&tool], "tool").unwrap(), before_tool);
         assert_eq!(crate::step_export(&[&roof], "roof").unwrap(), before_roof);
     }
+    fn stepped_tab_stock() -> Body {
+        let stock = crate::box_solid(Vec3::new(-20., -15., 0.), Vec3::new(20., 15., 24.)).unwrap();
+        let tab = crate::box_solid(Vec3::new(-12., -25., 8.), Vec3::new(-4., -15., 16.)).unwrap();
+        crate::boolean(&stock, &tab, BoolOp::Union).unwrap().unwrap()
+    }
+
+    fn coplanar_joins(body: &Body) -> usize {
+        let faces: Vec<_> = body.solid.face_iter().collect();
+        let mut adjacency = std::collections::HashMap::<_, Vec<usize>>::new();
+        for (i, face) in faces.iter().enumerate() {
+            for edge in face.edge_iter() {
+                adjacency.entry(edge.id()).or_default().push(i);
+            }
+        }
+        adjacency
+            .values()
+            .filter(|faces| {
+                let [a, b] = faces.as_slice() else { return false };
+                let (Some(pa), Some(pb)) = (body.planar_face(*a), body.planar_face(*b)) else { return false };
+                let (na, nb) = (pa.normal(), pb.normal());
+                na.dot(nb) > 1. - 1e-12 && (pb.origin - pa.origin).dot(na).abs() < 1e-10
+            })
+            .count()
+    }
+
+    #[test]
+    fn shell_of_joined_stepped_tabs_has_no_artificial_planar_stations() {
+        let stock = stepped_tab_stock();
+        let snapshot = crate::step_export(&[&stock], "unchanged stock").unwrap();
+        assert_eq!(coplanar_joins(&stock), 0);
+        let shell = crate::shell(&stock, &[Vec3::new(0., 0., 24.)], 1.).unwrap();
+        assert_eq!(coplanar_joins(&shell), 0, "Shell must not add internal edges across continuous planar walls");
+        assert!(shell.validity().is_empty());
+        let mesh = shell.tessellate(1e-5).unwrap();
+        let expected = 40. * 30. * 24. + 8. * 10. * 8. - (38. * 28. * 23. + 6. * 10. * 6.);
+        assert!((mesh.measure().volume - expected).abs() < 1e-6);
+        for (point, material) in [
+            (Vec3::new(0., 0., 0.5), true),
+            (Vec3::new(0., 0., 10.), false),
+            (Vec3::new(0., -14.5, 10.), true),
+            (Vec3::new(19.5, 0., 10.), true),
+            (Vec3::new(-8., -24.5, 12.), true),
+            (Vec3::new(-8., -20., 12.), false),
+            (Vec3::new(0., -18., 9.), false),
+        ] {
+            assert_eq!(mesh.contains(point), material, "wrong material at {point:?}");
+        }
+        let step = crate::step_export(&[&shell], "stepped tab shell").unwrap();
+        let reopened = crate::step_import(&step).unwrap();
+        let body = &reopened.bodies[0].body;
+        assert_eq!(body.face_count(), shell.face_count());
+        assert_eq!(coplanar_joins(body), 0);
+        assert!((body.tessellate(1e-5).unwrap().measure().volume - expected).abs() < 1e-6);
+        assert_eq!(crate::step_export(&[&stock], "unchanged stock").unwrap(), snapshot);
+    }
+
     #[test]
     fn contiguous_analytic_walls_remove_faces_but_retain_vertical_stations() {
         for curved in [false, true] {
diff --git a/crates/kernel/src/regularization_protection_tests.rs b/crates/kernel/src/regularization_protection_tests.rs
new file mode 100644
index 00000000..05c3534a
--- /dev/null
+++ b/crates/kernel/src/regularization_protection_tests.rs
@@ -0,0 +1,65 @@
+use super::*;
+use solvecraft_geom::Plane;
+
+fn stamped_stock() -> Body {
+    crate::box_solid(Vec3::new(-20., -15., 0.), Vec3::new(20., 15., 24.)).unwrap().with_box_certificate(7).unwrap()
+}
+
+fn cup(stock: &Body) -> Body {
+    crate::shell(stock, &[Vec3::new(0., 0., 24.)], 1.).unwrap().with_shell_certificate(9).unwrap()
+}
+
+#[test]
+fn stamped_shell_keeps_authored_painted_and_split_boundaries() {
+    let stock = stamped_stock();
+    let ordinary = cup(&stock);
+    assert!(!ordinary.regularization_protected);
+    let painted = stock.clone().with_paint(Some(Paint { opacity: 1., faces: vec![FacePaint { face: 0, color: [1., 0., 0.], opacity: 1. }] }));
+    let side = stock
+        .solid
+        .face_iter()
+        .enumerate()
+        .find(|(i, _)| stock.planar_face(*i).is_some_and(|p| p.normal().dot(Vec3::X) > 0.999 && p.origin.x == 20.))
+        .unwrap()
+        .0;
+    let split = crate::split_faces(&stock, &[side], &crate::SplitTool::Plane(Plane { origin: Vec3::new(0., 0., 8.), ..Plane::XY })).unwrap();
+    assert!(split.split_lines().is_some());
+    for protected in [painted, split] {
+        let result = cup(&protected);
+        assert!(result.regularization_protected);
+        assert!(result.face_count() > ordinary.face_count(), "authored boundaries must survive both Shell regularizers");
+        assert!(result.validity().is_empty());
+        let expected = 40. * 30. * 24. - 38. * 28. * 23.;
+        let mesh = result.tessellate(1e-5).unwrap();
+        assert!((mesh.measure().volume - expected).abs() < 1e-6);
+        assert!(mesh.contains(Vec3::new(19.5, 0., 12.)));
+        assert!(!mesh.contains(Vec3::new(0., 0., 12.)));
+        let count = result.face_count();
+        assert_eq!(result.clone().regularize_certified_planar_faces().unwrap().face_count(), count);
+        assert_eq!(result.clone().regularize_generated_planar_faces().unwrap().face_count(), count);
+        let placed = crate::transform(&result, Vec3::new(30., -40., 7.), Vec3::ZERO, Vec3::Z, 0.3).unwrap();
+        assert!(placed.regularization_protected);
+        for lump in placed.lumps().unwrap() {
+            assert!(lump.regularization_protected);
+        }
+        let cleared = result.clone().with_paint(None).with_split_lines(None);
+        assert!(cleared.regularization_protected, "cleared appearance must not erase inherited boundary protection");
+        assert_eq!(cleared.regularize_certified_planar_faces().unwrap().face_count(), count);
+    }
+}
+
+#[test]
+fn protection_is_identity_and_survives_tool_operand_and_compounds() {
+    let a = stamped_stock();
+    let protected = a.clone().with_paint(Some(Paint { opacity: 1., faces: vec![FacePaint { face: 0, color: [0., 1., 0.], opacity: 1. }] }));
+    let cleared = protected.with_paint(None);
+    assert!(!a.same(&cleared));
+    let tool = crate::box_solid(Vec3::new(-5., -5., 23.), Vec3::new(5., 5., 30.)).unwrap().inherit_regularization_protection(&cleared);
+    let cut = crate::boolean(&a, &tool, crate::BoolOp::Cut).unwrap().unwrap();
+    assert!(cut.regularization_protected);
+    let remote = crate::box_solid(Vec3::new(100., 0., 0.), Vec3::new(110., 10., 10.)).unwrap();
+    let joined = crate::ops::join_pieces(vec![cleared, remote]).unwrap().unwrap();
+    assert!(joined.regularization_protected);
+    assert_eq!(joined.lumps().unwrap().len(), 2);
+    assert!(joined.lumps().unwrap().iter().all(|body| body.regularization_protected));
+}
diff --git a/crates/kernel/src/splitface.rs b/crates/kernel/src/splitface.rs
index 8cfaf654..10f29cab 100644
--- a/crates/kernel/src/splitface.rs
+++ b/crates/kernel/src/splitface.rs
@@ -886,6 +886,7 @@ pub(crate) fn split_any(body: &Body, faces: &[usize], tool: &SplitTool) -> Resul
                 paint: None,
                 splits: body.splits.clone(),
                 face_sources: None,
+                regularization_protected: true,
             },
             imp,
         ))
diff --git a/crates/kernel/src/surfaces.rs b/crates/kernel/src/surfaces.rs
index 83f08f58..95dd7880 100644
--- a/crates/kernel/src/surfaces.rs
+++ b/crates/kernel/src/surfaces.rs
@@ -30,6 +30,7 @@ fn surface_body(faces: Vec<mt::Face>) -> Body {
         paint: None,
         splits: None,
         face_sources: None,
+        regularization_protected: false,
     }
 }
 
@@ -47,6 +48,7 @@ fn body_of(faces: Vec<mt::Face>) -> Result<Body> {
             paint: None,
             splits: None,
             face_sources: None,
+            regularization_protected: false,
         })
     }
 }