InMemorySessionService retains appended Event references, allowing later mutations to change saved history and state #7701

🔴 Required Information

Describe the Bug:
With the default deep-copy configuration, InMemorySessionService.append_event() retains the caller's mutable Event and nested state-delta values. Mutating that event after the append returns changes the stored conversation history and session state without another append. A subsequent get_session() returns those changed values.

Steps to Reproduce:

1. Check out main at 48d18a6aea65ed399bf036f29e383219ec167b3b (ADK 2.12.0) with runtime dependencies installed.
2. Save the code below as repro.py and run PYTHONPATH=src python repro.py.
3. Compare the in-memory result with the SQLite control. No model, credentials, or network are used.

Expected Behavior:
In the default deep-copy mode, a completed append should preserve the saved event and state. Changing the caller's retained event should leave the saved answer as original answer and the saved cart as {'items': ['coffee']}.

Observed Behavior:
The in-memory service reloads changed after append and {'items': ['coffee', 'tea']}. The SQLite control reloads the original answer and cart.

Environment Details:

• ADK Library Version: source 2.12.0 at the commit above.
• Desktop OS: macOS 27.0.1, arm64.
• Python Version: 3.13.16; dependencies installed from cache offline.

Model Information:

• LiteLLM: No.
• Model: N/A; session-service calls only.

🟡 Optional Information

Regression: Unknown; no earlier release was tested.

Logs:

InMemorySessionService: text='changed after append', cart={'items': ['coffee', 'tea']}
SqliteSessionService: text='original answer', cart={'items': ['coffee']}

Screenshots / Video: N/A.

Additional Context:
This is the append boundary, separate from #7676's initial create_session(state=...) snapshot and #7105's copies of scoped state returned to callers. It also reproduces on #7676's head 110262ad33f2c82b6eaa47faca9d6931e0566782. #7454 concerns duplicate delivery to stale callers; this reproduction appends one event once. No fix is included.

Minimal Reproduction Code:

import asyncio

from google.adk.events import Event
from google.adk.events.event_actions import EventActions
from google.adk.features import FeatureName, override_feature_enabled
from google.adk.sessions import InMemorySessionService
from google.adk.sessions.sqlite_session_service import SqliteSessionService
from google.genai import types


async def check(service):
    try:
        session = await service.create_session(app_name="app", user_id="user")
        event = Event(
            author="assistant",
            content=types.Content(
                role="model", parts=[types.Part(text="original answer")]
            ),
            actions=EventActions(state_delta={"cart": {"items": ["coffee"]}}),
        )
        await service.append_event(session, event)
        event.content.parts[0].text = "changed after append"
        event.actions.state_delta["cart"]["items"].append("tea")
        stored = await service.get_session(
            app_name="app", user_id="user", session_id=session.id
        )
        text = stored.events[0].content.parts[0].text
        print(f"{type(service).__name__}: text={text!r}, cart={stored.state['cart']}")
    finally:
        if isinstance(service, SqliteSessionService):
            await service.close()


async def main():
    override_feature_enabled(FeatureName.IN_MEMORY_SESSION_SERVICE_LIGHT_COPY, False)
    await check(InMemorySessionService())
    await check(SqliteSessionService(":memory:"))


asyncio.run(main())

How often has this issue occurred?: Always with the reproduction above.