diff --git a/src/scene/mod.rs b/src/scene/mod.rs
index 98f658bcf..3a3b95216 100644
--- a/src/scene/mod.rs
+++ b/src/scene/mod.rs
@@ -13686,3 +13686,134 @@ mod layout_cache_tests {
         assert!(after.iter().any(|(n, _, _)| n == "1:50"));
     }
 }
+
+#[cfg(test)]
+mod update_entities_batch_tests {
+    use super::Scene;
+    use codec::entities::{Block, BlockEnd, Line};
+    use codec::types::Vector3;
+    use codec::{EntityType, Handle};
+
+    /// Fresh scene holding two block sentinels (Block + BlockEnd) and one plain
+    /// line; returns their handles in that order.
+    fn scene_with_sentinels_and_line() -> (Scene, Handle, Handle, Handle) {
+        let mut scene = Scene::new();
+        let bh = scene
+            .document
+            .add_entity(EntityType::Block(Block::new("BATCH_TEST", Vector3::ZERO)))
+            .unwrap();
+        let be = scene
+            .document
+            .add_entity(EntityType::BlockEnd(BlockEnd::new()))
+            .unwrap();
+        let mut line = Line::new();
+        line.start = Vector3::new(0.0, 0.0, 1.0);
+        line.end = Vector3::new(3.0, 4.0, 2.0);
+        let lh = scene.document.add_entity(EntityType::Line(line)).unwrap();
+        (scene, bh, be, lh)
+    }
+
+    /// Stored entity as-is — the update shape the command arms actually pass.
+    fn snapshot(scene: &Scene, handle: Handle) -> EntityType {
+        scene.document.get_entity(handle).unwrap().clone()
+    }
+
+    /// The same line with an edited Z, still carrying its original handle.
+    fn edited_line(scene: &Scene, handle: Handle) -> EntityType {
+        let mut entity = snapshot(scene, handle);
+        if let EntityType::Line(line) = &mut entity {
+            line.start.z = 5.0;
+            line.end.z = 6.0;
+        }
+        entity
+    }
+
+    fn delta_count(scene: &Scene) -> usize {
+        scene.geometry_deltas.borrow().len()
+    }
+
+    #[test]
+    fn update_entities_batch_of_block_sentinels_collapses_to_one_full_delta() {
+        // Batched path: one coarse publish for the whole set.
+        let (mut batched, bh, be, _) = scene_with_sentinels_and_line();
+        let before = delta_count(&batched);
+        let updates = vec![snapshot(&batched, bh), snapshot(&batched, be)];
+        let moved = batched.update_entities(updates);
+        assert_eq!(moved, 2);
+        {
+            let ring = batched.geometry_deltas.borrow();
+            assert_eq!(
+                ring.len() - before,
+                1,
+                "batched sentinels must publish exactly one geometry delta"
+            );
+            assert!(
+                ring.back().unwrap().full,
+                "the batched delta must be full (bump_geometry)"
+            );
+        }
+
+        // Legacy path: each sentinel bumps on its own.
+        let (mut legacy, bh, be, _) = scene_with_sentinels_and_line();
+        let before = delta_count(&legacy);
+        let u1 = snapshot(&legacy, bh);
+        let u2 = snapshot(&legacy, be);
+        assert!(legacy.update_entity(u1));
+        assert!(legacy.update_entity(u2));
+        assert_eq!(
+            delta_count(&legacy) - before,
+            2,
+            "legacy per-entity updates must publish one delta each"
+        );
+
+        // Same final entity state either way.
+        for h in [bh, be] {
+            assert_eq!(
+                format!("{:?}", batched.document.get_entity(h)),
+                format!("{:?}", legacy.document.get_entity(h)),
+                "entity {h:?} differs between batched and legacy updates"
+            );
+        }
+    }
+
+    #[test]
+    fn update_entities_mixed_batch_matches_legacy_states_with_one_coarse_delta() {
+        // Batched: the sentinel's coarse (full) delta subsumes the line's
+        // would-be per-entity delta, and both slots are still written.
+        let (mut batched, bh, _, lh) = scene_with_sentinels_and_line();
+        let before = delta_count(&batched);
+        let updates = vec![snapshot(&batched, bh), edited_line(&batched, lh)];
+        let moved = batched.update_entities(updates);
+        assert_eq!(moved, 2);
+        {
+            let ring = batched.geometry_deltas.borrow();
+            assert_eq!(
+                ring.len() - before,
+                1,
+                "mixed batch with a sentinel must publish one coarse delta"
+            );
+            assert!(ring.back().unwrap().full, "coarse delta must be full");
+        }
+
+        // Legacy: full delta for the sentinel + per-entity delta for the line.
+        let (mut legacy, bh, _, lh) = scene_with_sentinels_and_line();
+        let before = delta_count(&legacy);
+        let u1 = snapshot(&legacy, bh);
+        let u2 = edited_line(&legacy, lh);
+        assert!(legacy.update_entity(u1));
+        assert!(legacy.update_entity(u2));
+        assert_eq!(
+            delta_count(&legacy) - before,
+            2,
+            "legacy publishes one delta per update"
+        );
+
+        for h in [bh, lh] {
+            assert_eq!(
+                format!("{:?}", batched.document.get_entity(h)),
+                format!("{:?}", legacy.document.get_entity(h)),
+                "entity {h:?} differs between batched and legacy updates"
+            );
+        }
+    }
+}
diff --git a/tests/flatten_regression.rs b/tests/flatten_regression.rs
new file mode 100644
index 000000000..1e8eccb20
--- /dev/null
+++ b/tests/flatten_regression.rs
@@ -0,0 +1,221 @@
+//! FLATTEN regression: golden projections, collection filters, plan/apply
+//! invariants, and a legacy-vs-batch differential.
+
+use codec::entities::{Line, Polyline2D, Polyline3D, Solid, Vertex2D, Vertex3DPolyline};
+use codec::types::Vector3;
+use codec::{EntityType, Handle};
+
+use OpenCADStudio::modules::draw::modify::flatten::{
+    apply_flatten_updates, collect_flatten_handles, flatten_entity_z, plan_flatten,
+};
+use OpenCADStudio::scene::Scene;
+
+fn line_with_z(z0: f64, z1: f64) -> EntityType {
+    let mut line = Line::new();
+    line.start = Vector3::new(0.0, 0.0, z0);
+    line.end = Vector3::new(3.0, 4.0, z1);
+    line.thickness = 2.0;
+    EntityType::Line(line)
+}
+
+#[test]
+fn golden_line_projects_and_resets_thickness_normal() {
+    let flat = flatten_entity_z(&line_with_z(3.0, 5.0)).expect("moves");
+    let EntityType::Line(l) = flat else { panic!("variant") };
+    assert_eq!((l.start.z, l.end.z), (0.0, 0.0));
+    assert_eq!((l.start.x, l.start.y), (0.0, 0.0));
+    assert_eq!((l.end.x, l.end.y), (3.0, 4.0));
+    assert_eq!(l.thickness, 0.0);
+    assert_eq!(l.normal, Vector3::UNIT_Z);
+}
+
+#[test]
+fn golden_tilted_circle_becomes_ellipse() {
+    let mut c = codec::entities::Circle::from_center_radius(Vector3::new(1.0, 2.0, 4.0), 5.0);
+    c.normal = Vector3::new(0.0, 0.6, 0.8);
+    let flat = flatten_entity_z(&EntityType::Circle(c)).expect("moves");
+    assert!(matches!(flat, EntityType::Ellipse(_)), "tilted circle → ellipse");
+}
+
+#[test]
+fn golden_polyline3d_vertices_and_elevation_zeroed() {
+    let mut pl = Polyline3D::new();
+    pl.elevation = 5.0;
+    pl.vertices.push(Vertex3DPolyline::new(Vector3::new(1.0, 2.0, 7.5)));
+    let flat = flatten_entity_z(&EntityType::Polyline3D(pl)).expect("moves");
+    let EntityType::Polyline3D(pl) = flat else { panic!("variant") };
+    assert_eq!(pl.elevation, 0.0);
+    assert!(pl.vertices.iter().all(|v| v.position.z == 0.0 && v.position.x == 1.0));
+}
+
+#[test]
+fn golden_polyline2d_elevation_zeroed() {
+    let mut pl = Polyline2D::new();
+    pl.elevation = 9.0;
+    pl.vertices.push(Vertex2D::new(Vector3::new(3.0, 4.0, 0.0)));
+    let flat = flatten_entity_z(&EntityType::Polyline2D(pl)).expect("moves");
+    let EntityType::Polyline2D(pl) = flat else { panic!("variant") };
+    assert_eq!(pl.elevation, 0.0);
+    assert_eq!(pl.vertices[0].location.z, 0.0);
+}
+
+#[test]
+fn golden_text_insertion_and_normal_zeroed() {
+    let mut t = codec::entities::Text::new();
+    t.insertion_point = Vector3::new(1.0, 2.0, 3.0);
+    t.thickness = 1.0;
+    let flat = flatten_entity_z(&EntityType::Text(t)).expect("moves");
+    let EntityType::Text(t) = flat else { panic!("variant") };
+    assert_eq!(t.insertion_point.z, 0.0);
+    assert_eq!(t.thickness, 0.0);
+    assert_eq!(t.normal, Vector3::UNIT_Z);
+}
+
+#[test]
+fn golden_solid_corners_zeroed() {
+    let s = Solid::new(
+        Vector3::new(0.0, 0.0, 1.0),
+        Vector3::new(1.0, 0.0, 2.0),
+        Vector3::new(1.0, 1.0, 3.0),
+        Vector3::new(0.0, 1.0, 4.0),
+    );
+    let flat = flatten_entity_z(&EntityType::Solid(s)).expect("moves");
+    let EntityType::Solid(s) = flat else { panic!("variant") };
+    assert_eq!(
+        [s.first_corner.z, s.second_corner.z, s.third_corner.z, s.fourth_corner.z],
+        [0.0; 4]
+    );
+}
+
+#[test]
+fn already_flat_and_unsupported_return_none() {
+    let mut flat_line = Line::new(); // zero thickness + z-normal by default
+    flat_line.start = Vector3::new(0.0, 0.0, 0.0);
+    flat_line.end = Vector3::new(3.0, 4.0, 0.0);
+    assert!(flatten_entity_z(&EntityType::Line(flat_line)).is_none());
+    let ray = codec::entities::Ray::new(Vector3::new(0.0, 0.0, 5.0), Vector3::new(0.0, 0.0, 1.0));
+    assert!(flatten_entity_z(&EntityType::Ray(ray)).is_none());
+}
+
+fn scene_with_three_lines() -> (Scene, [Handle; 3]) {
+    let mut scene = Scene::new();
+    let a = scene.document.add_entity(line_with_z(1.0, 2.0)).unwrap();
+    let b = scene.document.add_entity(line_with_z(1.0, 2.0)).unwrap();
+    let c = scene.document.add_entity(line_with_z(1.0, 2.0)).unwrap();
+    (scene, [a, b, c])
+}
+
+#[test]
+fn collect_excludes_locked_layer_and_respects_selection() {
+    let (mut scene, [a, b, c]) = scene_with_three_lines();
+    // Lock b's layer.
+    let mut layer = codec::tables::Layer::new("LOCKED");
+    layer.flags.locked = true;
+    scene.document.layers.add(layer).unwrap();
+    scene.document.get_entity_mut(b).unwrap().common_mut().layer = "LOCKED".into();
+
+    // Empty selection → all active/unlocked candidates (b excluded).
+    let candidates = collect_flatten_handles(&scene);
+    assert!(candidates.contains(&a) && candidates.contains(&c));
+    assert!(!candidates.contains(&b));
+
+    // Non-empty selection → only the selected handles (minus locked).
+    scene.selected.insert(a);
+    scene.selected.insert(b);
+    let candidates = collect_flatten_handles(&scene);
+    assert_eq!(candidates, vec![a]);
+}
+
+#[test]
+fn collect_excludes_entities_outside_active_space() {
+    let (mut scene, [a, b, c]) = scene_with_three_lines();
+    // Enter block editor scoping on block b: entities whose owner block
+    // differs from the active space must be excluded.
+    scene.block_edit_block = Some(b);
+    scene.document.get_entity_mut(a).unwrap().common_mut().owner_handle = c; // wrong owner
+    scene.document.get_entity_mut(c).unwrap().common_mut().owner_handle = b; // active block
+
+    let candidates = collect_flatten_handles(&scene);
+    assert!(!candidates.contains(&a), "wrong-owner entity must be excluded");
+    assert!(candidates.contains(&c), "owner of the active block must be included");
+}
+
+#[test]
+fn plan_moves_only_movable_candidates() {
+    let (mut scene, [a, b, c]) = scene_with_three_lines();
+    let ray = scene
+        .document
+        .add_entity(EntityType::Ray(codec::entities::Ray::new(
+            Vector3::new(0.0, 0.0, 5.0),
+            Vector3::new(0.0, 0.0, 1.0),
+        )))
+        .unwrap();
+    let handles = collect_flatten_handles(&scene);
+    assert_eq!(handles.len(), 4);
+    let updates = plan_flatten(&scene, &handles);
+    assert_eq!(updates.len(), 3, "ray must not be planned");
+    assert!(updates.iter().all(|e| !matches!(e, EntityType::Ray(_))));
+
+    let moved = apply_flatten_updates(&mut scene, updates);
+    assert_eq!(moved, 3);
+    for h in [a, b, c] {
+        let EntityType::Line(l) = scene.document.get_entity(h).unwrap() else {
+            panic!("variant")
+        };
+        assert_eq!((l.start.z, l.end.z), (0.0, 0.0));
+    }
+    // Ray untouched.
+    assert!(matches!(scene.document.get_entity(ray).unwrap(), EntityType::Ray(_)));
+}
+
+#[test]
+fn differential_legacy_loop_vs_apply_flatten_updates() {
+    let build = || {
+        let mut scene = Scene::new();
+        let mut h = Vec::new();
+        for i in 0..50 {
+            let line = line_with_z(1.0 + i as f64, 2.0);
+            h.push(scene.document.add_entity(line).unwrap());
+            if i % 10 == 0 {
+                h.push(
+                    scene
+                        .document
+                        .add_entity(EntityType::Ray(codec::entities::Ray::new(
+                            Vector3::new(0.0, 0.0, 5.0),
+                            Vector3::new(0.0, 0.0, 1.0),
+                        )))
+                        .unwrap(),
+                );
+            }
+            if i % 7 == 0 {
+                h.push(scene.document.add_entity(line_with_z(0.0, 0.0)).unwrap());
+            }
+        }
+        let handles = collect_flatten_handles(&scene);
+        let updates = plan_flatten(&scene, &handles);
+        (scene, handles, updates)
+    };
+
+    // Reference: the legacy per-entity loop (what the command arm did).
+    let (mut legacy, handles, updates) = build();
+    let mut moved_legacy = 0usize;
+    for entity in updates {
+        if legacy.update_entity(entity) {
+            moved_legacy += 1;
+        }
+    }
+
+    // Candidate: the helper under test.
+    let (mut batched, handles2, updates2) = build();
+    let moved_batched = apply_flatten_updates(&mut batched, updates2);
+
+    assert_eq!(moved_legacy, moved_batched);
+    assert_eq!(handles, handles2);
+    for h in handles {
+        assert_eq!(
+            format!("{:?}", legacy.document.get_entity(h)),
+            format!("{:?}", batched.document.get_entity(h)),
+            "entity {h:?} differs between legacy loop and apply_flatten_updates"
+        );
+    }
+}