diff --git a/backend/app/services/material_service.py b/backend/app/services/material_service.py
index d0f2002..0da33f2 100644
--- a/backend/app/services/material_service.py
+++ b/backend/app/services/material_service.py
@@ -160,27 +160,31 @@ async def update_material(db: AsyncSession, material: Material, payload: Materia
 
     v2.5/2.6: category is DERIVED (from the first linked supplier) — it is
     no longer part of the update payload.
+
+    NOTE: no ``db.begin()`` here — the router already starts the transaction
+    (it loads the row first), and a nested begin raises
+    "A transaction is already begun on this Session" (500). The request
+    session commits via the get_db dependency.
     """
-    async with db.begin():
-        if payload.name is not None:
-            duplicate = await db.scalar(
-                select(Material.id).where(
-                    Material.name == payload.name,
-                    Material.id != material.id,
-                )
+    if payload.name is not None:
+        duplicate = await db.scalar(
+            select(Material.id).where(
+                Material.name == payload.name,
+                Material.id != material.id,
             )
-            if duplicate is not None:
-                raise ConflictError("متریالی با این نام در کل کاتالوگ قبلاً ثبت شده است", field="name")
-        if payload.name is not None:
-            material.name = payload.name
-        if payload.topic is not None:
-            material.topic = payload.topic
-        if payload.canonical_unit_id is not None and payload.canonical_unit_id != material.canonical_unit_id:
-            if not await db.get(Unit, payload.canonical_unit_id):
-                raise NotFoundError("واحد مرجع یافت نشد", field="canonical_unit_id")
-            material.canonical_unit_id = payload.canonical_unit_id
-        if payload.status is not None:
-            material.status = payload.status
+        )
+        if duplicate is not None:
+            raise ConflictError("متریالی با این نام در کل کاتالوگ قبلاً ثبت شده است", field="name")
+    if payload.name is not None:
+        material.name = payload.name
+    if payload.topic is not None:
+        material.topic = payload.topic
+    if payload.canonical_unit_id is not None and payload.canonical_unit_id != material.canonical_unit_id:
+        if not await db.get(Unit, payload.canonical_unit_id):
+            raise NotFoundError("واحد مرجع یافت نشد", field="canonical_unit_id")
+        material.canonical_unit_id = payload.canonical_unit_id
+    if payload.status is not None:
+        material.status = payload.status
     return material