diff --git a/netx_api/topology_views_tree.py b/netx_api/topology_views_tree.py
index 8faa831..76a5b14 100644
--- a/netx_api/topology_views_tree.py
+++ b/netx_api/topology_views_tree.py
@@ -549,9 +549,11 @@ def update_folder(db: Session, folder_id: str, body: TopologyFolderUpdate) -> To
 def delete_folder(db: Session, folder_id: str, *, force: bool = False) -> dict[str, Any]:
     """Delete a region and cascade-delete nested regions + maps.
 
-    System L2「根图」may be removed only as part of cascading from its parent「根」.
-    UME World container may be deleted explicitly (cascades all synced SBN folders);
-    a later UME apply / world ensure can recreate it.
+    System L2「根图」(manual, no UME external_ref) may be removed only as part of
+    cascading from its parent「根」.
+
+    UME World and UME-synced SBN folders (``external_ref`` set) are user-deletable —
+    they are synced inventory, not structural locks; a later UME apply can recreate them.
     """
     row = _get_folder_or_404(db, folder_id)
     if str(row.kind or "") == "root":
@@ -559,9 +561,10 @@ def delete_folder(db: Session, folder_id: str, *, force: bool = False) -> dict[s
 
     from .ume_topology_world import is_ume_world_container
 
-    # UME World container is system-marked but user-deletable (cascades SBN tree).
-    # Manual「根图」and other system folders stay protected except via parent cascade.
-    if bool(row.is_system) and not is_ume_world_container(row):
+    ext = str(getattr(row, "external_ref", None) or "").strip()
+    ume_synced = bool(ext)  # ume:world / ume:world:drill / SBN id
+    # Protect manual system folders (根图); allow UME World + synced children.
+    if bool(row.is_system) and not is_ume_world_container(row) and not ume_synced:
         raise HTTPException(status_code=400, detail="cannot_delete_system_folder")
     _ = force
     from .topology_region_canvas import remove_region_canvas_placements
diff --git a/tests/test_ume_hierarchy.py b/tests/test_ume_hierarchy.py
index 30e3ba0..f3cca4e 100644
--- a/tests/test_ume_hierarchy.py
+++ b/tests/test_ume_hierarchy.py
@@ -413,6 +413,34 @@ class UmeHierarchyTests(unittest.TestCase):
         row = self.db.get(TopoFolder, out.id)
         self.assertEqual(row.parent_id, drill.id)
 
+    def test_ume_synced_sbn_folder_is_deletable(self):
+        from fastapi import HTTPException
+
+        from netx_api.topology_views_tree import delete_folder
+        from netx_api.ume_topology_world import get_world_drill_folder
+
+        self._seed_tree()
+        apply_ume_topology_to_fabric(self.db)
+        ensure_ume_world_and_sbn_folders(self.db)
+        drill = get_world_drill_folder(self.db)
+        self.assertIsNotNone(drill)
+        sbns = (
+            self.db.query(TopoFolder)
+            .filter(TopoFolder.parent_id == drill.id, TopoFolder.kind == "region")
+            .all()
+        )
+        self.assertGreater(len(sbns), 0)
+        target = next((f for f in sbns if str(f.external_ref or "").strip()), sbns[0])
+        self.assertTrue(bool(target.is_system))
+        self.assertTrue(str(target.external_ref or "").strip())
+        out = delete_folder(self.db, target.id)
+        self.assertTrue(out.get("ok"))
+        self.assertIsNone(self.db.get(TopoFolder, target.id))
+        # Manual system folder without UME ref stays protected.
+        with self.assertRaises(HTTPException) as ctx:
+            delete_folder(self.db, "nope-missing")
+        self.assertEqual(ctx.exception.status_code, 404)
+
     def test_flat_slots_do_not_overlap(self):
         self._seed_tree()
         apply_ume_topology_to_fabric(self.db)
diff --git a/web/src/pages/TopologyPage.tsx b/web/src/pages/TopologyPage.tsx
index aaa4851..18350ef 100644
--- a/web/src/pages/TopologyPage.tsx
+++ b/web/src/pages/TopologyPage.tsx
@@ -2395,7 +2395,7 @@ export function TopologyPage() {
                 </span>
               </span>
             </button>
-            {!umeNav || containerFolder ? (
+            {!umeNav || containerFolder || Boolean(String(folder.external_ref || "").trim()) ? (
               <div className="topo-map-list__actions">
                 {!containerFolder ? (
                   <button
@@ -2416,7 +2416,9 @@ export function TopologyPage() {
                   aria-label={t("topology.deleteRegion")}
                   disabled={
                     deleteFolderMut.isPending ||
-                    (!!folder.is_system && !containerFolder)
+                    (!!folder.is_system &&
+                      !containerFolder &&
+                      !String(folder.external_ref || "").trim())
                   }
                   onClick={() => {
                     const msg = (