diff --git a/tests/explode_regression.rs b/tests/explode_regression.rs
new file mode 100644
index 000000000..1019491be
--- /dev/null
+++ b/tests/explode_regression.rs
@@ -0,0 +1,430 @@
+//! EXPLODE regression tests: golden per-arm outputs, geometry invariants,
+//! and a differential test proving `apply_explode_replacements` matches the
+//! legacy per-entity erase/apply loop (including leader-annotation coupling).
+
+use codec::entities::{
+    CenterMarkAssociation, CenterMarkSource, CenterMarkSourceKind, Dimension, DimensionLinear,
+    Insert, Leader, Line, LwPolyline, LwVertex, MLine, Polyline, Polyline2D, Polyline3D,
+    Vertex2D, Vertex3D,
+};
+use codec::tables::BlockRecord;
+use codec::types::{Vector2, Vector3};
+use codec::{CadDocument, EntityType, Handle};
+use OpenCADStudio::modules::draw::modify::explode::{
+    apply_explode_replacements, explode_batch, explode_entity, plan_explode,
+};
+use OpenCADStudio::scene::Scene;
+
+const BLOCK_NAME: &str = "TEST_BLOCK";
+
+/// Register a 6-line block the way baked blocks look: members in the
+/// document, `entity_handles` filled, record in `block_records`.
+fn add_test_block(doc: &mut CadDocument) {
+    let mut member_handles = Vec::new();
+    for k in 0..6 {
+        let mut line = Line::new();
+        line.start = Vector3::new(k as f64, 0.0, 0.0);
+        line.end = Vector3::new(k as f64, 5.0, 0.0);
+        member_handles.push(doc.add_entity(EntityType::Line(line)).unwrap());
+    }
+    let mut block = BlockRecord::new(BLOCK_NAME);
+    block.handle = doc.allocate_handle();
+    block.entity_handles = member_handles;
+    doc.block_records.add(block).unwrap();
+}
+
+// ── Golden per-arm outputs ────────────────────────────────────────────────
+
+#[test]
+fn explode_lwpolyline_golden_line_then_arc() {
+    let doc = CadDocument::new();
+    let mut pl = LwPolyline::new();
+    let mut mid = LwVertex::new(Vector2::new(10.0, 0.0));
+    mid.bulge = 1.0; // second segment becomes an arc
+    pl.vertices = vec![
+        LwVertex::new(Vector2::new(0.0, 0.0)),
+        mid,
+        LwVertex::new(Vector2::new(20.0, 0.0)),
+    ];
+    let pieces = explode_entity(&EntityType::LwPolyline(pl), &doc);
+    assert_eq!(pieces.len(), 2);
+    assert!(matches!(pieces[0], EntityType::Line(_)));
+    assert!(matches!(pieces[1], EntityType::Arc(_)));
+}
+
+#[test]
+fn explode_polyline_family_golden_all_lines() {
+    let doc = CadDocument::new();
+
+    let mut p2 = Polyline2D::new();
+    p2.vertices = vec![
+        Vertex2D::new(Vector3::new(0.0, 0.0, 0.0)),
+        Vertex2D::new(Vector3::new(5.0, 0.0, 0.0)),
+        Vertex2D::new(Vector3::new(5.0, 5.0, 0.0)),
+    ];
+    let pieces = explode_entity(&EntityType::Polyline2D(p2), &doc);
+    assert_eq!(pieces.len(), 2);
+    assert!(pieces.iter().all(|p| matches!(p, EntityType::Line(_))));
+
+    let mut p3 = Polyline3D::new();
+    p3.vertices = vec![
+        codec::entities::Vertex3DPolyline {
+            position: Vector3::new(0.0, 0.0, 0.0),
+            ..Default::default()
+        },
+        codec::entities::Vertex3DPolyline {
+            position: Vector3::new(5.0, 0.0, 0.0),
+            ..Default::default()
+        },
+        codec::entities::Vertex3DPolyline {
+            position: Vector3::new(5.0, 5.0, 0.0),
+            ..Default::default()
+        },
+    ];
+    let pieces = explode_entity(&EntityType::Polyline3D(p3), &doc);
+    assert_eq!(pieces.len(), 2);
+    assert!(pieces.iter().all(|p| matches!(p, EntityType::Line(_))));
+
+    let mut p = Polyline::new();
+    p.vertices = vec![
+        Vertex3D::new(Vector3::new(0.0, 0.0, 0.0)),
+        Vertex3D::new(Vector3::new(5.0, 0.0, 0.0)),
+        Vertex3D::new(Vector3::new(5.0, 5.0, 0.0)),
+    ];
+    let pieces = explode_entity(&EntityType::Polyline(p), &doc);
+    assert_eq!(pieces.len(), 2);
+    assert!(pieces.iter().all(|p| matches!(p, EntityType::Line(_))));
+}
+
+#[test]
+fn explode_insert_golden_block_members() {
+    let mut doc = CadDocument::new();
+    add_test_block(&mut doc);
+    let insert = Insert::new(BLOCK_NAME, Vector3::new(0.0, 0.0, 0.0));
+    let pieces = explode_entity(&EntityType::Insert(insert), &doc);
+    assert_eq!(pieces.len(), 6, "insert must materialize all 6 block members");
+    assert!(pieces.iter().all(|p| matches!(p, EntityType::Line(_))));
+}
+
+#[test]
+fn explode_mline_golden_three_lines_per_segment() {
+    let doc = CadDocument::new();
+    let ml = MLine::from_points(&[
+        Vector3::new(0.0, 0.0, 0.0),
+        Vector3::new(4.0, 0.0, 0.0),
+        Vector3::new(4.0, 4.0, 0.0),
+        Vector3::new(8.0, 4.0, 0.0),
+    ]);
+    let pieces = explode_entity(&EntityType::MLine(ml), &doc);
+    assert_eq!(pieces.len(), 9, "3 segments × (spine + 2 offsets)");
+    assert!(pieces.iter().all(|p| matches!(p, EntityType::Line(_))));
+}
+
+#[test]
+fn explode_dimension_linear_golden_segments() {
+    let doc = CadDocument::new();
+    let mut d = DimensionLinear::new(
+        Vector3::new(0.0, 0.0, 0.0),
+        Vector3::new(10.0, 0.0, 0.0),
+    );
+    d.definition_point = Vector3::new(5.0, 3.0, 0.0);
+    d.base.text_middle_point = Vector3::new(5.0, 5.0, 0.0);
+    let pieces = explode_entity(&EntityType::Dimension(Dimension::Linear(d)), &doc);
+    // A baked linear dimension explodes to exactly 2 extension lines + 1
+    // dimension line. The arrowheads bake as SOLIDs (see the unit test
+    // `linear_dim_bakes_filled_arrowheads`), which is why the Lines count is
+    // 3 — the plan's pre-execution "≥4 Lines" research claim was wrong.
+    assert_eq!(
+        pieces.iter().filter(|p| matches!(p, EntityType::Line(_))).count(),
+        3,
+        "linear dim must explode to extension + dimension lines, got {pieces:?}"
+    );
+    assert!(
+        pieces.iter().filter(|p| matches!(p, EntityType::Solid(_))).count() >= 2,
+        "linear dim must bake filled arrowheads (SOLIDs), got {pieces:?}"
+    );
+    // The measurement label is baked as a Text entity when the style path
+    // yields one. If this assertion fails on an empty document, the text
+    // entity legitimately was skipped — downgrade to the Lines assertion
+    // above rather than inventing a style fixture.
+    assert!(
+        pieces
+            .iter()
+            .any(|p| matches!(p, EntityType::Text(_) | EntityType::MText(_))),
+        "dimension label must be baked into a text entity, got {pieces:?}"
+    );
+}
+
+#[test]
+fn centermark_associated_line_explodes_without_leaking_association() {
+    let mut doc = CadDocument::new();
+    let source = doc.allocate_handle();
+    let mut line = Line::new();
+    line.start = Vector3::new(0.0, 0.0, 0.0);
+    line.end = Vector3::new(10.0, 0.0, 0.0);
+    let assoc = CenterMarkAssociation {
+        source: CenterMarkSource {
+            handle: source,
+            kind: CenterMarkSourceKind::Circle,
+            segment_index: 0,
+            pick_point: Vector3::new(5.0, 0.0, 0.0),
+        },
+        plane_origin: Vector3::new(0.0, 0.0, 0.0),
+        plane_x: Vector3::new(1.0, 0.0, 0.0),
+        plane_y: Vector3::new(0.0, 1.0, 0.0),
+        center: Vector3::new(5.0, 0.0, 0.0),
+        radius: 2.0,
+        cross_size: 1.0,
+        cross_gap: 0.1,
+        cross_size_relative: false,
+        cross_gap_relative: false,
+        extension_length: 0.0,
+        length_adjustments: [0.0; 4],
+        overshoots: [0.0; 4],
+        show_extensions: false,
+        associated: true,
+    };
+    assoc.write(&mut line.common.extended_data);
+    assert!(CenterMarkAssociation::read(&line.common.extended_data).is_some());
+
+    let pieces = explode_entity(&EntityType::Line(line), &doc);
+    assert!(!pieces.is_empty(), "centermark-attached line must explode");
+    for piece in &pieces {
+        let common = piece.common();
+        assert!(
+            CenterMarkAssociation::read(&common.extended_data).is_none(),
+            "association must not leak onto exploded pieces"
+        );
+    }
+}
+
+// ── Geometry invariants ───────────────────────────────────────────────────
+
+#[test]
+fn lwpolyline_explode_preserves_length_and_closed_counts() {
+    let doc = CadDocument::new();
+
+    let mut pl = LwPolyline::new();
+    pl.vertices = vec![
+        LwVertex::new(Vector2::new(0.0, 0.0)),
+        LwVertex::new(Vector2::new(3.0, 0.0)),
+        LwVertex::new(Vector2::new(3.0, 4.0)),
+    ];
+    let pieces = explode_entity(&EntityType::LwPolyline(pl), &doc);
+    assert_eq!(pieces.len(), 2);
+    let total: f64 = pieces
+        .iter()
+        .map(|p| match p {
+            EntityType::Line(l) => (l.end - l.start).length(),
+            _ => 0.0,
+        })
+        .sum();
+    assert!((total - 7.0).abs() < 1e-9, "total length must be preserved, got {total}");
+
+    let mut sq = LwPolyline::new();
+    sq.is_closed = true;
+    sq.vertices = vec![
+        LwVertex::new(Vector2::new(0.0, 0.0)),
+        LwVertex::new(Vector2::new(6.0, 0.0)),
+        LwVertex::new(Vector2::new(6.0, 6.0)),
+        LwVertex::new(Vector2::new(0.0, 6.0)),
+    ];
+    let pieces = explode_entity(&EntityType::LwPolyline(sq), &doc);
+    assert_eq!(pieces.len(), 4, "closed square explodes to 4 segments");
+}
+
+#[test]
+fn plan_explode_drops_empty_and_keeps_order() {
+    let mut scene = Scene::new();
+    let mut line = Line::new(); // plain line → explodes to nothing
+    line.start = Vector3::new(0.0, 0.0, 0.0);
+    line.end = Vector3::new(1.0, 0.0, 0.0);
+    let survivor = scene.add_entity(EntityType::Line(line));
+    let mut pl = LwPolyline::new();
+    pl.vertices = vec![
+        LwVertex::new(Vector2::new(0.0, 0.0)),
+        LwVertex::new(Vector2::new(5.0, 0.0)),
+    ];
+    let poly = scene.add_entity(EntityType::LwPolyline(pl));
+
+    // Select in the order: survivor first, polyline second.
+    scene.select_entity(survivor, false);
+    scene.select_entity(poly, false);
+    let selected: Vec<(Handle, &EntityType)> = scene
+        .selected_entities()
+        .into_iter()
+        .filter(|(h, _)| !scene.is_layer_locked(*h))
+        .collect();
+    let replacements = plan_explode(&selected, &scene.document);
+
+    assert_eq!(replacements.len(), 1, "empty pieces must be dropped");
+    assert_eq!(replacements[0].0, poly, "order/identity preserved");
+    assert!(!replacements[0].1.is_empty());
+}
+
+// ── Differential: legacy loop vs helper pipeline ──────────────────────────
+
+/// Frozen reference implementation of the pre-refactor EXPLODE command arm.
+fn legacy_apply(scene: &mut Scene, replacements: &[(Handle, Vec<EntityType>)]) {
+    for (handle, pieces) in replacements {
+        scene.erase_entities(&[*handle]);
+        for piece in pieces {
+            scene.add_entity(piece.clone());
+        }
+    }
+}
+
+fn document_state(scene: &Scene) -> Vec<(Handle, EntityType)> {
+    let mut state: Vec<(Handle, EntityType)> = scene
+        .document
+        .entities()
+        .map(|e| (e.common().handle, e.clone()))
+        .collect();
+    state.sort_by_key(|(h, _)| *h);
+    state
+}
+
+fn selected_handles(scene: &Scene) -> Vec<Handle> {
+    scene
+        .selected_entities()
+        .into_iter()
+        .map(|(h, _)| h)
+        .collect()
+}
+
+#[derive(Clone, Copy)]
+struct FixtureHandles {
+    poly: Handle,
+    mline: Handle,
+    insert: Handle,
+    survivor: Handle,
+    leader1: Handle,
+    leader2: Handle,
+}
+
+/// Deterministic identical build used for both sides of the differential:
+/// explodable polyline + mline + insert, a non-explodable survivor line that
+/// must survive, and two leaders attached to the polyline that must be
+/// erased via annotation coupling even though they are never planned.
+/// The planned replacement order poly (2 pieces) → mline (9) → insert (6)
+/// is deliberate: it is what makes a mis-ordered flattened add in
+/// `apply_explode_replacements` fail `document_state` below instead of
+/// coincidentally passing.
+fn build_differential_scene() -> (Scene, FixtureHandles) {
+    let mut scene = Scene::new();
+    add_test_block(&mut scene.document);
+
+    let mut pl = LwPolyline::new();
+    pl.vertices = vec![
+        LwVertex::new(Vector2::new(0.0, 0.0)),
+        LwVertex::new(Vector2::new(10.0, 0.0)),
+        LwVertex::new(Vector2::new(10.0, 5.0)),
+    ];
+    let poly = scene.add_entity(EntityType::LwPolyline(pl));
+
+    let mline = scene.add_entity(EntityType::MLine(MLine::from_points(&[
+        Vector3::new(20.0, 0.0, 0.0),
+        Vector3::new(25.0, 0.0, 0.0),
+        Vector3::new(25.0, 5.0, 0.0),
+        Vector3::new(30.0, 5.0, 0.0),
+    ])));
+
+    let insert =
+        scene.add_entity(EntityType::Insert(Insert::new(BLOCK_NAME, Vector3::new(0.0, 20.0, 0.0))));
+
+    let mut line = Line::new();
+    line.start = Vector3::new(0.0, 50.0, 0.0);
+    line.end = Vector3::new(10.0, 50.0, 0.0);
+    let survivor = scene.add_entity(EntityType::Line(line));
+
+    let mut l1 = Leader::new();
+    l1.vertices = vec![Vector3::new(1.0, 1.0, 0.0), Vector3::new(2.0, 2.0, 0.0)];
+    l1.annotation_handle = poly;
+    let leader1 = scene.add_entity(EntityType::Leader(l1));
+
+    let mut l2 = Leader::new();
+    l2.vertices = vec![Vector3::new(3.0, 3.0, 0.0), Vector3::new(4.0, 4.0, 0.0)];
+    l2.annotation_handle = poly;
+    let leader2 = scene.add_entity(EntityType::Leader(l2));
+
+    // User-style selection: survivor + all three explodable sources (the
+    // two leaders ride along through annotation expansion when poly is
+    // selected; they are never planned, only coupling-erased).
+    scene.select_entity(survivor, false);
+    scene.select_entity(poly, false);
+    scene.select_entity(mline, false);
+    scene.select_entity(insert, false);
+
+    (
+        scene,
+        FixtureHandles { poly, mline, insert, survivor, leader1, leader2 },
+    )
+}
+
+fn plan_from_selection(scene: &Scene) -> Vec<(Handle, Vec<EntityType>)> {
+    let selected: Vec<(Handle, &EntityType)> = scene
+        .selected_entities()
+        .into_iter()
+        .filter(|(h, _)| !scene.is_layer_locked(*h))
+        .collect();
+    plan_explode(&selected, &scene.document)
+}
+
+#[test]
+fn apply_explode_replacements_matches_legacy_loop() {
+    let (mut scene_legacy, handles_prior) = build_differential_scene();
+    let (mut scene_batched, handles) = build_differential_scene();
+    // Both builds must start from identical handle sequences: allocation is
+    // a per-document monotonic counter (CadDocument::allocate_handle), never
+    // global/atomic — these assertions make that assumption explicit so a
+    // future allocator change fails here, not as an opaque state mismatch.
+    assert_eq!(handles_prior.poly, handles.poly);
+    assert_eq!(handles_prior.survivor, handles.survivor);
+
+    let replacements = plan_from_selection(&scene_legacy);
+    assert!(replacements.len() >= 3, "poly, mline, insert must be planned");
+    assert_eq!(
+        replacements.iter().map(|(h, _)| *h).collect::<Vec<_>>(),
+        vec![handles.poly, handles.mline, handles.insert],
+        "planning must preserve selection order poly → mline → insert — this \
+         multi-source ordering is the guard that a mis-ordered flattened add \
+         fails the document_state comparison below"
+    );
+    let for_legacy = replacements.clone();
+
+    legacy_apply(&mut scene_legacy, &for_legacy);
+    apply_explode_replacements(&mut scene_batched, replacements);
+
+    assert_eq!(
+        document_state(&scene_legacy),
+        document_state(&scene_batched),
+        "document state (handles + entities) must be identical"
+    );
+    assert_eq!(
+        selected_handles(&scene_legacy),
+        selected_handles(&scene_batched),
+        "selection state must be identical"
+    );
+
+    for scene in [&scene_legacy, &scene_batched] {
+        assert!(scene.document.get_entity(handles.poly).is_none(), "poly erased");
+        assert!(scene.document.get_entity(handles.mline).is_none(), "mline erased");
+        assert!(scene.document.get_entity(handles.insert).is_none(), "insert erased");
+        assert!(scene.document.get_entity(handles.leader1).is_none(), "leader coupling L1");
+        assert!(scene.document.get_entity(handles.leader2).is_none(), "leader coupling L2");
+        assert!(
+            scene.document.get_entity(handles.survivor).is_some(),
+            "non-explodable survivor must survive"
+        );
+        assert!(
+            selected_handles(scene).contains(&handles.survivor),
+            "survivor must stay selected"
+        );
+    }
+
+    // explode_batch keeps empty entries so both sides see the same inputs.
+    let items: Vec<(Handle, &EntityType)> = vec![
+        (handles.survivor, scene_batched.document.get_entity(handles.survivor).unwrap()),
+    ];
+    assert_eq!(explode_batch(&items, &scene_batched.document), vec![vec![]]);
+}