diff --git a/src/app/block_editor_tests.rs b/src/app/block_editor_tests.rs
index e3c955bb8..feb4f7462 100644
--- a/src/app/block_editor_tests.rs
+++ b/src/app/block_editor_tests.rs
@@ -54,6 +54,11 @@ fn main_depth(app: &OpenCADStudio) -> usize {
         .unwrap_or(tab.history.undo_stack.len())
 }
 
+fn feed(app: &mut OpenCADStudio, text: &str) {
+    let _ = app.update(Message::CommandInput(text.to_string()));
+    let _ = app.update(Message::CommandSubmit);
+}
+
 #[test]
 fn local_undo_preserves_imported_native_networks_when_constraints_are_unchanged() {
     use codec::objects::{
@@ -922,6 +927,182 @@ fn close_all_resumes_after_discarding_a_dirty_block() {
     assert!(app.pending_tab_closes.is_empty());
 }
 
+#[test]
+fn explode_inside_session_edits_the_definition_and_marks_dirty() {
+    let (mut app, reference) = fixture();
+    let i = app.active_tab;
+    app.tabs[i].scene.define_block_raw(
+        "CHILD",
+        Vector3::ZERO,
+        vec![EntityType::Line(Line::from_points(
+            Vector3::ZERO,
+            Vector3::new(3.0, 0.0, 0.0),
+        ))],
+    );
+    begin(&mut app, reference);
+    let _ = app.dispatch_command("-INSERT");
+    feed(&mut app, "CHILD");
+    feed(&mut app, "0,5");
+    feed(&mut app, "1");
+    feed(&mut app, "1");
+    feed(&mut app, "0");
+    app.finish_pending_history(i);
+    let before = members(&app);
+    assert_eq!(before.len(), 2, "the nested INSERT joins the definition");
+    let insert_handle = *before.last().unwrap();
+    assert!(
+        matches!(
+            app.tabs[i].scene.document.get_entity(insert_handle),
+            Some(EntityType::Insert(_))
+        ),
+        "expected the member just added to be the INSERT"
+    );
+    let depth = main_depth(&app);
+    app.tabs[i].scene.select_entity(insert_handle, true);
+    let _ = app.dispatch_command("EXPLODE");
+    assert!(
+        app.block_edit_dirty(i),
+        "the explode marks the session dirty"
+    );
+    assert_eq!(main_depth(&app), depth, "the main stack is untouched");
+    let _ = app.dispatch_command("BSAVE");
+    let saved = members(&app);
+    assert!(
+        saved
+            .iter()
+            .all(|h| app.tabs[i].scene.document.get_entity(*h).is_some()),
+        "the saved definition carries no dangling member handle: {saved:?}"
+    );
+    assert_eq!(saved.len(), 2, "the INSERT is replaced by its piece");
+    assert!(
+        !saved.contains(&insert_handle),
+        "the exploded INSERT leaves the saved definition"
+    );
+    assert!(
+        saved.iter().any(|h| {
+            matches!(
+                app.tabs[i].scene.document.get_entity(*h),
+                Some(EntityType::Line(_))
+            )
+        }),
+        "the child line lands as a member of the edited block"
+    );
+    let _ = app.dispatch_command("BEDIT_DISCARD");
+}
+
+#[test]
+fn xclip_inside_session_stays_in_the_definition() {
+    let (mut app, reference) = fixture();
+    let i = app.active_tab;
+    app.tabs[i].scene.define_block_raw(
+        "CHILD",
+        Vector3::ZERO,
+        vec![EntityType::Line(Line::from_points(
+            Vector3::ZERO,
+            Vector3::new(3.0, 0.0, 0.0),
+        ))],
+    );
+    begin(&mut app, reference);
+    let _ = app.dispatch_command("-INSERT");
+    feed(&mut app, "CHILD");
+    feed(&mut app, "0,5");
+    feed(&mut app, "1");
+    feed(&mut app, "1");
+    feed(&mut app, "0");
+    app.finish_pending_history(i);
+    let before = members(&app);
+    let insert_handle = *before.last().unwrap();
+    let depth = main_depth(&app);
+    app.tabs[i].scene.select_entity(insert_handle, true);
+    let _ = app.dispatch_command("XCLIP");
+    feed(&mut app, "N");
+    feed(&mut app, "R");
+    feed(&mut app, "0,0");
+    feed(&mut app, "10,10");
+    app.finish_pending_history(i);
+    assert!(
+        crate::scene::pick::xclip::filter_handle(&app.tabs[i].scene.document, insert_handle)
+            .is_some(),
+        "the clip filter attaches to the in-block reference"
+    );
+    assert!(
+        members(&app).contains(&insert_handle),
+        "the clipped reference stays a member of the definition"
+    );
+    assert!(app.block_edit_dirty(i), "the clip marks the session dirty");
+    assert_eq!(main_depth(&app), depth, "the main stack is untouched");
+    let _ = app.dispatch_command("BEDIT_DISCARD");
+}
+
+#[test]
+fn attdef_inside_session_lands_in_the_definition_and_undo_stays_local() {
+    let (mut app, reference) = fixture();
+    let i = app.active_tab;
+    let depth = main_depth(&app);
+    begin(&mut app, reference);
+    assert_eq!(members(&app).len(), 1);
+    let _ = app.dispatch_command("-ATTDEF");
+    feed(&mut app, ""); // modes: done
+    feed(&mut app, "TAG1");
+    feed(&mut app, "prompt text");
+    feed(&mut app, ""); // default value
+    feed(&mut app, ""); // single line
+    feed(&mut app, "0,5");
+    feed(&mut app, "2");
+    feed(&mut app, "0");
+    app.finish_pending_history(i);
+    let after = members(&app);
+    assert_eq!(after.len(), 2, "the ATTDEF joins the definition");
+    assert!(
+        matches!(
+            app.tabs[i]
+                .scene
+                .document
+                .get_entity(*after.last().unwrap()),
+            Some(EntityType::AttributeDefinition(_))
+        ),
+        "the new member is the ATTDEF"
+    );
+    assert!(
+        app.block_edit_dirty(i),
+        "the ATTDEF marks the session dirty"
+    );
+    assert_eq!(main_depth(&app), depth, "the main stack is untouched");
+    app.undo_steps(1);
+    assert_eq!(members(&app).len(), 1, "session undo removes the ATTDEF");
+    assert_eq!(main_depth(&app), depth, "the main stack is still untouched");
+    let _ = app.dispatch_command("BEDIT_DISCARD");
+}
+
+#[test]
+fn save_after_session_undo_does_not_write_a_dangling_member() {
+    let (mut app, reference) = fixture();
+    begin(&mut app, reference);
+    circle(&mut app, 1.0);
+    assert_eq!(members(&app).len(), 2);
+    app.undo_steps(1);
+    assert_eq!(members(&app).len(), 1);
+    let depth = main_depth(&app);
+    let _ = app.dispatch_command("BSAVE");
+    assert!(app.tabs[app.active_tab].active_block_edit.is_some());
+    let saved = members(&app);
+    assert!(
+        saved.iter().all(|h| app.tabs[app.active_tab]
+            .scene
+            .document
+            .get_entity(*h)
+            .is_some()),
+        "save after a session undo must not write a member handle whose entity is gone: {saved:?}"
+    );
+    assert_eq!(
+        main_depth(&app),
+        depth,
+        "a clean save commits no new drawing step"
+    );
+    let _ = app.dispatch_command("BEDIT_DISCARD");
+    assert_eq!(members(&app).len(), 1);
+}
+
 #[test]
 fn locked_bedit_no_selection_opens_edit_block_definition_dialog() {
     let mut app = OpenCADStudio::new_for_test();
diff --git a/src/app/control/entities.rs b/src/app/control/entities.rs
index 2e9f9131b..dd701dd0f 100644
--- a/src/app/control/entities.rs
+++ b/src/app/control/entities.rs
@@ -65,6 +65,12 @@ impl OpenCADStudio {
     /// layers they name, and commit everything as one undoable step. All definitions are validated before the first
     /// entity is added, so the batch is all-or-nothing.
     pub(super) fn control_entities_create(&mut self, req: &Value) -> Result<Task<Message>, Value> {
+        if self.tabs[self.active_tab].scene.block_edit_block.is_some() {
+            return Err(failure(
+                "block_editor_open",
+                "finish the open block editor before adding entities to a block",
+            ));
+        }
         let list = req["entities"]
             .as_array()
             .filter(|v| !v.is_empty())
diff --git a/src/app/update/block_ops_tests.rs b/src/app/update/block_ops_tests.rs
index 05a3c25a7..c8b070667 100644
--- a/src/app/update/block_ops_tests.rs
+++ b/src/app/update/block_ops_tests.rs
@@ -1227,6 +1227,47 @@ fn locked_mcp_block_delete_with_force_still_refuses_an_open_bedit_session() {
     let _ = app.dispatch_command("BEDIT_DISCARD");
 }
 
+/// Control/MCP `entities_create` must not route new entities into the open
+/// block while pushing a main-stack entry (same policy as the plugin guard).
+#[test]
+fn locked_control_entities_create_refused_while_block_editor_open() {
+    let mut app = app();
+    let name = fixture_unreferenced(&mut app);
+    let insert = add_insert(&mut app, &name);
+    begin_bedit(&mut app, insert);
+    assert!(bedit_open(&app));
+    let depth = entity_count(&app);
+    let main_depth = app.tabs[app.active_tab]
+        .block_edit_history
+        .as_ref()
+        .map(|h| h.main.undo_stack.len())
+        .unwrap_or(app.tabs[app.active_tab].history.undo_stack.len());
+    let doc = app.automation_op(r#"{"protocol":1,"op":"state"}"#)["document_id"]
+        .as_u64()
+        .unwrap();
+    let response = app.automation_op(&format!(
+        r#"{{"protocol":1,"op":"entities_create","request_id":"p11s15","document_id":{doc},"entities":[{{"type":"Point","location":[5,5]}}]}}"#
+    ));
+    assert_eq!(response["ok"], false, "{response}");
+    assert_eq!(response["code"], "block_editor_open", "{response}");
+    assert_eq!(
+        response["error"], "finish the open block editor before adding entities to a block",
+        "{response}"
+    );
+    assert_eq!(
+        entity_count(&app),
+        depth,
+        "no entity may be created through the control path"
+    );
+    let main_depth_after = app.tabs[app.active_tab]
+        .block_edit_history
+        .as_ref()
+        .map(|h| h.main.undo_stack.len())
+        .unwrap_or(app.tabs[app.active_tab].history.undo_stack.len());
+    assert_eq!(main_depth_after, main_depth, "no undo entry may be pushed");
+    let _ = app.dispatch_command("BEDIT_DISCARD");
+}
+
 // ── PURGE ────────────────────────────────────────────────────────────────
 
 #[test]
diff --git a/src/scene/block_base_point_tests.rs b/src/scene/block_base_point_tests.rs
index b02a6e2cb..15ba6aefa 100644
--- a/src/scene/block_base_point_tests.rs
+++ b/src/scene/block_base_point_tests.rs
@@ -1255,3 +1255,71 @@ fn locked_redefine_block_raw_refuses_cycle() {
         "expected cycle error, got: {err}"
     );
 }
+
+#[test]
+fn locked_redefine_block_raw_refuses_open_block() {
+    let mut scene = Scene::new();
+    let open_name = "REDEF_OPEN";
+    let other_name = "REDEF_CLOSED";
+
+    scene.define_block_raw(
+        open_name,
+        Vector3::ZERO,
+        vec![EntityType::Line(Line::from_points(
+            Vector3::ZERO,
+            Vector3::new(1.0, 0.0, 0.0),
+        ))],
+    );
+    scene.define_block_raw(
+        other_name,
+        Vector3::ZERO,
+        vec![EntityType::Line(Line::from_points(
+            Vector3::ZERO,
+            Vector3::new(2.0, 0.0, 0.0),
+        ))],
+    );
+
+    let open_record = scene.document.block_records.get(open_name).unwrap().handle;
+    let members_before = scene
+        .document
+        .block_records
+        .get(open_name)
+        .unwrap()
+        .entity_handles
+        .clone();
+    scene.block_edit_block = Some(open_record);
+
+    let res = scene.redefine_block_raw(
+        open_name,
+        Vector3::ZERO,
+        vec![EntityType::Line(Line::from_points(
+            Vector3::ZERO,
+            Vector3::new(9.0, 9.0, 0.0),
+        ))],
+    );
+    let err = res.expect_err("redefining the open block must be refused");
+    assert!(
+        err.contains(open_name) && err.contains("Block Editor"),
+        "expected refusal naming the block and the editor, got: {err}"
+    );
+    assert_eq!(
+        scene
+            .document
+            .block_records
+            .get(open_name)
+            .unwrap()
+            .entity_handles,
+        members_before,
+        "the open definition must be untouched"
+    );
+
+    let res = scene.redefine_block_raw(
+        other_name,
+        Vector3::ZERO,
+        vec![EntityType::Line(Line::from_points(
+            Vector3::ZERO,
+            Vector3::new(8.0, 0.0, 0.0),
+        ))],
+    );
+    assert!(res.is_ok(), "a closed block must still redefine: {res:?}");
+}
diff --git a/src/scene/entity.rs b/src/scene/entity.rs
index a14eea8d4..4e0358c7c 100644
--- a/src/scene/entity.rs
+++ b/src/scene/entity.rs
@@ -1432,6 +1432,11 @@ impl Scene {
             self.define_block_raw(name, base_point, entities);
             return Ok(());
         };
+        if self.block_edit_block == Some(br_handle) {
+            return Err(format!(
+                "block \"{name}\" is open in the Block Editor; close it before redefining"
+            ));
+        }
         let owned: Vec<Handle> = self
             .document
             .entities()