diff --git a/backend/app/core/step_sync.py b/backend/app/core/step_sync.py
index 5b92e5ca..cd90325f 100644
--- a/backend/app/core/step_sync.py
+++ b/backend/app/core/step_sync.py
@@ -170,15 +170,20 @@ async def sync_steps_from_tree(
     # Soft-delete previously synced steps that no longer exist in the tree
     current_node_ids = [s["source_node_id"] for s in extracted]
     if current_node_ids:
+        # Build NOT IN using explicit named placeholders — asyncpg does not
+        # auto-cast a Python list to a PostgreSQL array in text() queries.
+        placeholders = ", ".join(f":id_{i}" for i in range(len(current_node_ids)))
+        params = {f"id_{i}": nid for i, nid in enumerate(current_node_ids)}
+        params.update({"tree_id": str(tree_id), "now": now})
         await db.execute(
-            text("""
+            text(f"""
                 UPDATE step_library
                 SET is_active = false, updated_at = :now
                 WHERE source_tree_id = :tree_id
                   AND is_flow_synced = true
-                  AND source_node_id != ALL(:node_ids)
+                  AND source_node_id NOT IN ({placeholders})
             """),
-            {"tree_id": str(tree_id), "node_ids": current_node_ids, "now": now}
+            params
         )
     else:
         await db.execute(
@@ -194,7 +199,11 @@ async def sync_steps_from_tree(
 
 
 async def deactivate_synced_steps_for_tree(db: AsyncSession, tree_id: UUID) -> None:
-    """Soft-delete all synced library entries for a tree (on tree delete/deactivate)."""
+    """Soft-delete all synced library entries for a tree (on tree delete/deactivate).
+
+    Must be called BEFORE deleting the tree row — after deletion the FK ondelete='SET NULL'
+    will null source_tree_id, making the WHERE clause match nothing.
+    """
     await db.execute(
         text("""
             UPDATE step_library