fix(sessions): raise SessionNotFoundError when appending to an unknown Redis session

src/google/adk/integrations/redis/_redis_session_service.py

@@ -27,6 +27,7 @@
 from typing import Optional
 
 from ...errors.already_exists_error import AlreadyExistsError
+from ...errors.session_not_found_error import SessionNotFoundError
 from ...events.event import Event
 from ...platform import uuid as platform_uuid
 from ...sessions import _session_util
@@ -339,6 +340,12 @@ async def get_user_state(
   async def append_event(self, session: Session, event: Event) -> Event:
     """Appends an event to the session and synchronizes state in Redis."""
     client = self._get_redis()
+    key = self._session_key(session.app_name, session.user_id, session.id)
+    # Check before any write: the app/user state deltas below are stored under
+    # their own keys, and the session write would otherwise create a session
+    # that was never stored instead of failing like the other services.
+    if not event.partial and not await client.exists(key):
+      raise SessionNotFoundError(f"Session {session.id} not found.")
     event = await super().append_event(session, event)
     # Stamp the session with the event's own timestamp, matching
     # InMemorySessionService, SqliteSessionService and DatabaseSessionService.
@@ -388,8 +395,6 @@ async def append_event(self, session: Session, event: Event) -> Event:
         last_update_time=session.last_update_time,
     )
 
-    key = self._session_key(session.app_name, session.user_id, session.id)
-
     async def append_to_storage(pipe: Any) -> None:
       raw = await pipe.get(key)
       if raw:

tests/unittests/integrations/redis/_fake_redis.py

@@ -57,6 +57,9 @@ async def get(self, key: str) -> str | None:
       return None
     return self._store.get(key)
 
+  async def exists(self, *keys: str) -> int:
+    return sum(not self._is_expired(key) for key in keys)
+
   async def set(
       self,
       key: str,

tests/unittests/integrations/redis/test_redis_session_service.py

@@ -20,11 +20,13 @@
 from unittest import mock
 
 from google.adk.errors.already_exists_error import AlreadyExistsError
+from google.adk.errors.session_not_found_error import SessionNotFoundError
 from google.adk.events.event import Event
 from google.adk.events.event import EventActions
 from google.adk.integrations.redis._config import RedisSessionServiceConfig
 from google.adk.integrations.redis._redis_session_service import RedisSessionService
 from google.adk.sessions.base_session_service import GetSessionConfig
+from google.adk.sessions.session import Session
 from google.genai import types
 import pytest
 
@@ -806,3 +808,23 @@ async def test_cumulative_user_state_creation(session_service):
       user_id="u1",
   )
   assert user_state == {"theme": "dark", "lang": "en"}
+
+
+@pytest.mark.asyncio
+async def test_append_event_to_unknown_session_writes_nothing(
+    session_service, fake_redis
+):
+  """A rejected append leaves no session, app state or user state behind."""
+  session = Session(app_name="app1", user_id="u1", id="never_created")
+  event = Event(
+      author="agent",
+      actions=EventActions(
+          state_delta={"count": 1, "user:score": 100, "app:status": "active"}
+      ),
+  )
+
+  with pytest.raises(SessionNotFoundError):
+    await session_service.append_event(session, event)
+
+  assert fake_redis._store == {}
+  assert session.events == []

tests/unittests/sessions/_conformance.py

@@ -140,13 +140,6 @@ async def _make_per_agent_database(
     _Backend(
         'redis',
         _make_redis,
-        divergences={
-            'test_append_event_to_unknown_session_raises_session_not_found': (
-                'Redis writes the session key unconditionally on append, so'
-                ' appending to a session it has never stored creates one'
-                ' instead of raising.'
-            ),
-        },
     ),
     _Backend('per_agent_database', _make_per_agent_database),
 ]