diff --git a/src/google/adk/sessions/vertex_ai_session_service.py b/src/google/adk/sessions/vertex_ai_session_service.py
index d36406d3797..c4df307aa0a 100644
--- a/src/google/adk/sessions/vertex_ai_session_service.py
+++ b/src/google/adk/sessions/vertex_ai_session_service.py
@@ -390,8 +390,14 @@ async def get_user_state(
 
   @override
   async def append_event(self, session: Session, event: Event) -> Event:
-    # Update the in-memory session.
-    await super().append_event(session=session, event=event)
+    if not event.partial:
+      # Apply temp-scoped state to the in-memory session and strip it from
+      # the event before the remote append succeeds. Normal state and the
+      # event itself are only applied to the session once the remote append
+      # succeeds, so a failed append leaves the session unchanged and a
+      # retry does not re-apply state or duplicate the event.
+      self._apply_temp_state(session, event)
+      event = self._trim_temp_delta_state(event)
 
     _validate_session_id(session.id)
     reasoning_engine_id = self._get_reasoning_engine_id(session.app_name)
@@ -496,6 +502,10 @@ async def _do_append(cfg: dict[str, Any]) -> None:
         if 'raw_event' in config:
           del config['raw_event']
         await _do_append(config)
+
+    if not event.partial:
+      self._update_session_state(session, event)
+      session.events.append(event)
     return event
 
   def _get_reasoning_engine_id(self, app_name: str) -> str:
diff --git a/tests/unittests/sessions/test_vertex_ai_session_service.py b/tests/unittests/sessions/test_vertex_ai_session_service.py
index b589d8c3685..9b847961dd7 100644
--- a/tests/unittests/sessions/test_vertex_ai_session_service.py
+++ b/tests/unittests/sessions/test_vertex_ai_session_service.py
@@ -11,6 +11,8 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
 import copy
 import datetime
 import re
@@ -1165,6 +1167,62 @@ async def test_append_event():
   assert retrieved_session.events[1] == event_to_append
 
 
+@pytest.mark.asyncio
+async def test_append_event_does_not_mutate_session_on_remote_failure() -> None:
+  """Regression test for #6998.
+
+  A failed remote append must leave normal state and the event list
+  untouched (temp state remains, since it is invocation-local), and a
+  successful retry must apply the delta and append the event exactly once.
+  """
+  append = mock.AsyncMock(side_effect=[RuntimeError('network failure'), None])
+  client = types.SimpleNamespace(
+      agent_engines=types.SimpleNamespace(
+          sessions=types.SimpleNamespace(
+              events=types.SimpleNamespace(append=append),
+          )
+      )
+  )
+
+  @asynccontextmanager
+  async def fake_client() -> AsyncIterator[types.SimpleNamespace]:
+    yield client
+
+  session_service = mock_vertex_ai_session_service()
+  session = Session(
+      id='1',
+      app_name='123',
+      user_id='user',
+      state={'existing': 'value'},
+  )
+  event = Event(
+      invocation_id='invocation',
+      author='model',
+      actions=EventActions(
+          state_delta={
+              'normal': 'persisted',
+              'temp:scratch': 'ephemeral',
+          }
+      ),
+  )
+
+  with mock.patch.object(session_service, '_get_api_client', fake_client):
+    with pytest.raises(RuntimeError):
+      await session_service.append_event(session, event)
+
+    assert session.state == {'existing': 'value', 'temp:scratch': 'ephemeral'}
+    assert len(session.events) == 0
+
+    await session_service.append_event(session, event)
+
+    assert session.state == {
+        'existing': 'value',
+        'temp:scratch': 'ephemeral',
+        'normal': 'persisted',
+    }
+    assert len(session.events) == 1
+
+
 @pytest.mark.asyncio
 @pytest.mark.usefixtures('mock_get_api_client')
 async def test_append_event_strips_unsupported_part_metadata(