InMemorySessionService keeps empty user maps after sessions are deleted #7722

🔴 Required Information

Describe the Bug:

InMemorySessionService.delete_session removes the session itself but leaves the empty per-user dictionary in service.sessions. Once all users are empty, the app dictionary is left behind too.

Repeated create/delete cycles with new user IDs therefore keep growing the internal structure even though there are no live sessions.

Steps to Reproduce:

1. Create an InMemorySessionService.
2. Create and delete one session for each of 10,000 distinct users.
3. Inspect service.sessions.

Expected Behavior:

Deleting a user's last session should remove that user's empty session dictionary. If it was the app's last user, the empty app dictionary should be removed as well.

Observed Behavior:

All 10,000 empty user dictionaries remain reachable:

apps: 1
retained users: 10000
live sessions: 0

Environment Details:

• ADK Library Version: current main at da35ca77d71f7d74c8b7d7550f6e1b591a6690e0
• Desktop OS: Ubuntu 24.04.4 LTS
• Python Version: 3.14.4

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A

🟡 Optional Information

Regression:

Unknown.

Logs:

I also added a focused test that expected the empty app entry to be removed after deleting its only session. It fails on current main with:

AssertionError: assert 'my_app' not in {'my_app': {'user': {}}}

Screenshots / Video:

N/A

Additional Context:

The cleanup can stay local to _delete_session_impl: remove the session, then remove the user entry if its dictionary is empty, followed by the app entry if that dictionary is empty. User- and app-scoped state are stored separately, so this would only prune the session index.

Minimal Reproduction Code:

import asyncio

from google.adk.sessions.in_memory_session_service import InMemorySessionService


async def main():
  service = InMemorySessionService()

  for i in range(10_000):
    user_id = f'user-{i}'
    session = await service.create_session(app_name='app', user_id=user_id)
    await service.delete_session(
        app_name='app', user_id=user_id, session_id=session.id
    )

  print('apps:', len(service.sessions))
  print('retained users:', len(service.sessions['app']))
  print(
      'live sessions:',
      sum(len(sessions) for sessions in service.sessions['app'].values()),
  )


asyncio.run(main())

How often has this issue occurred?:

• Always (100%)