diff --git a/backend/open_webui/migrations/versions/461111b60977_add_missing_primary_keys_to_legacy_.py b/backend/open_webui/migrations/versions/461111b60977_add_missing_primary_keys_to_legacy_.py
index c061e1febc..67b737c530 100644
--- a/backend/open_webui/migrations/versions/461111b60977_add_missing_primary_keys_to_legacy_.py
+++ b/backend/open_webui/migrations/versions/461111b60977_add_missing_primary_keys_to_legacy_.py
@@ -19,10 +19,12 @@ depends_on: Union[str, Sequence[str], None] = None
 # Tables bootstrapped by the old Peewee migration layer that may have
 # UNIQUE(id) but no PRIMARY KEY constraint.  Fresh Alembic installs
 # already have correct PKs from 7e5b5dc7342b_init.py.
-LEGACY_TABLES = [
-    'auth', 'chat', 'chatidtag', 'document', 'file',
-    'function', 'memory', 'model', 'prompt', 'tag', 'tool', 'user',
-]
+# 'tag' uses a composite PK since the same tag name can exist for multiple users.
+LEGACY_TABLES = {
+    'auth': ['id'], 'chat': ['id'], 'chatidtag': ['id'], 'document': ['id'],
+    'file': ['id'], 'function': ['id'], 'memory': ['id'], 'model': ['id'],
+    'prompt': ['id'], 'tag': ['id', 'user_id'], 'tool': ['id'], 'user': ['id'],
+}
 
 
 def upgrade() -> None:
@@ -30,30 +32,31 @@ def upgrade() -> None:
     inspector = sa.inspect(conn)
     existing_tables = set(inspector.get_table_names())
 
-    for table_name in LEGACY_TABLES:
+    for table_name, pk_columns in LEGACY_TABLES.items():
         if table_name not in existing_tables:
             continue
 
         pk = inspector.get_pk_constraint(table_name)
         pk_cols = pk.get('constrained_columns', [])
 
-        # Already has a proper PK on 'id' — nothing to do
-        if pk_cols == ['id']:
+        # Already has the correct PK — nothing to do
+        if sorted(pk_cols) == sorted(pk_columns):
             continue
 
-        # Check that an 'id' column actually exists
+        # Check that all PK columns exist
         columns = {c['name'] for c in inspector.get_columns(table_name)}
-        if 'id' not in columns:
+        if not all(c in columns for c in pk_columns):
             continue
 
-        print(f"Promoting UNIQUE(id) → PRIMARY KEY for '{table_name}'")
+        print(f"Promoting UNIQUE(id) -> PRIMARY KEY({', '.join(pk_columns)}) for '{table_name}'")
 
+        conn.execute(sa.text(f'DROP TABLE IF EXISTS _alembic_tmp_{table_name}'))
         with op.batch_alter_table(table_name) as batch_op:
             # Drop existing PK if any (e.g. on wrong column)
             if pk_cols and pk.get('name'):
                 batch_op.drop_constraint(pk['name'], type_='primary')
 
-            batch_op.create_primary_key(f'pk_{table_name}_id', ['id'])
+            batch_op.create_primary_key(f'pk_{table_name}', pk_columns)
 
 
 def downgrade() -> None: