diff --git a/changes/1698.fix.md b/changes/1698.fix.md
new file mode 100644
index 0000000000..788035b714
--- /dev/null
+++ b/changes/1698.fix.md
@@ -0,0 +1 @@
+Allow `Undefined` value of `ModifyGroupInput.user_update_mode` field to enable client-py updates group.
diff --git a/src/ai/backend/manager/models/group.py b/src/ai/backend/manager/models/group.py
index b3134a05fe..d97180369e 100644
--- a/src/ai/backend/manager/models/group.py
+++ b/src/ai/backend/manager/models/group.py
@@ -20,6 +20,7 @@
 import graphene
 import sqlalchemy as sa
 from graphene.types.datetime import DateTime as GQLDateTime
+from graphql import Undefined
 from sqlalchemy.engine.row import Row
 from sqlalchemy.ext.asyncio import AsyncConnection as SAConnection
 from sqlalchemy.orm import relationship
@@ -486,7 +487,7 @@ async def mutate(
 
         if "name" in data and _rx_slug.search(data["name"]) is None:
             raise ValueError("invalid name format. slug format required.")
-        if props.user_update_mode not in (None, "add", "remove"):
+        if props.user_update_mode not in (None, Undefined, "add", "remove"):
             raise ValueError("invalid user_update_mode")
         if not props.user_uuids:
             props.user_update_mode = None