feat: support pre-populated session events in SessionInput for eval c…

…ases Add an optional 'events' field to SessionInput that accepts a list of Event objects. After create_session, the eval framework appends these events to the session before starting inference. This seeds the session with conversation history so that eval turns run with realistic prior context without being scored. Closes #4896
google · brucearctor · Mar 20, 2026 · Mar 20, 2026 · Mar 22, 2026 · Mar 26, 2026
commit 600308ec5dbfef89a73d7f81f0dfd515288c4efd
diff --git a/src/google/adk/evaluation/eval_case.py b/src/google/adk/evaluation/eval_case.py
@@ -23,6 +23,7 @@
 from pydantic import model_validator
 from typing_extensions import TypeAlias
 
+from ..events.event import Event
 from .app_details import AppDetails
 from .common import EvalBaseModel
 from .conversation_scenarios import ConversationScenario
@@ -124,6 +125,14 @@ class SessionInput(EvalBaseModel):
   state: SessionState = Field(default_factory=dict)
   """The state of the session."""
 
+  events: Optional[list[Event]] = None
+  """Optional list of events to seed the session with prior conversation history.
+
+  When provided, these events are appended to the session after creation but
+  before inference begins. This enables eval cases to test agent behavior that
+  depends on prior context without scoring the context turns.
+  """
+
 
 StaticConversation: TypeAlias = list[Invocation]
 """A conversation where the user's queries for each invocation are already specified."""

diff --git a/src/google/adk/evaluation/evaluation_generator.py b/src/google/adk/evaluation/evaluation_generator.py
@@ -212,13 +212,19 @@ async def _generate_inferences_from_root_agent(
     user_id = initial_session.user_id if initial_session else "test_user_id"
     session_id = session_id if session_id else str(uuid.uuid4())
 
-    _ = await session_service.create_session(
+    session = await session_service.create_session(
         app_name=app_name,
         user_id=user_id,
         state=initial_session.state if initial_session else {},
         session_id=session_id,
     )
 
+    # Seed session with prior conversation history so that eval turns
+    # run with realistic prior context without being scored.
+    if initial_session and initial_session.events:
+      for event in initial_session.events:
+        await session_service.append_event(session=session, event=event)
+
     if not artifact_service:
       artifact_service = InMemoryArtifactService()
 

diff --git a/tests/unittests/evaluation/test_evaluation_generator.py b/tests/unittests/evaluation/test_evaluation_generator.py
@@ -457,3 +457,112 @@ async def mock_generate_inferences_side_effect(
     mock_generate_inferences.assert_called_once()
     called_with_content = mock_generate_inferences.call_args.args[3]
     assert called_with_content.parts[0].text == "message 1"
+
+  @pytest.mark.asyncio
+  async def test_seed_events_are_appended_to_session(self, mocker, mock_runner):
+    """Tests that seed events from SessionInput.events are appended after create_session."""
+    from google.adk.evaluation.eval_case import SessionInput
+
+    mock_agent = mocker.MagicMock()
+    mock_user_sim = mocker.MagicMock(spec=UserSimulator)
+    mock_user_sim.get_next_user_message = mocker.AsyncMock(
+        return_value=NextUserMessage(
+            status=UserSimulatorStatus.STOP_SIGNAL_DETECTED
+        )
+    )
+
+    # Create a real InMemorySessionService so we can verify events are appended
+    mock_session_service = mocker.MagicMock()
+    mock_session = mocker.MagicMock()
+    mock_session_service.create_session = mocker.AsyncMock(
+        return_value=mock_session
+    )
+    mock_session_service.append_event = mocker.AsyncMock()
+
+    mocker.patch(
+        "google.adk.evaluation.evaluation_generator.EvaluationGenerator._get_app_details_by_invocation_id"
+    )
+    mocker.patch(
+        "google.adk.evaluation.evaluation_generator.EvaluationGenerator.convert_events_to_eval_invocations"
+    )
+
+    seed_event_1 = Event(
+        content=types.Content(
+            parts=[types.Part(text="How do I pay my water bill?")],
+            role="user",
+        ),
+        author="user",
+        invocation_id="seed-0",
+    )
+    seed_event_2 = Event(
+        content=types.Content(
+            parts=[types.Part(text="You can pay by calling ABC.")],
+            role="model",
+        ),
+        author="root_agent",
+        invocation_id="seed-0",
+    )
+
+    initial_session = SessionInput(
+        app_name="test_app",
+        user_id="test_user",
+        events=[seed_event_1, seed_event_2],
+    )
+
+    await EvaluationGenerator._generate_inferences_from_root_agent(
+        root_agent=mock_agent,
+        user_simulator=mock_user_sim,
+        initial_session=initial_session,
+        session_service=mock_session_service,
+    )
+
+    # Verify append_event was called for each seed event
+    assert mock_session_service.append_event.call_count == 2
+    mock_session_service.append_event.assert_any_call(
+        session=mock_session, event=seed_event_1
+    )
+    mock_session_service.append_event.assert_any_call(
+        session=mock_session, event=seed_event_2
+    )
+
+  @pytest.mark.asyncio
+  async def test_no_seed_events_when_events_is_none(self, mocker, mock_runner):
+    """Tests that append_event is not called when SessionInput.events is None."""
+    from google.adk.evaluation.eval_case import SessionInput
+
+    mock_agent = mocker.MagicMock()
+    mock_user_sim = mocker.MagicMock(spec=UserSimulator)
+    mock_user_sim.get_next_user_message = mocker.AsyncMock(
+        return_value=NextUserMessage(
+            status=UserSimulatorStatus.STOP_SIGNAL_DETECTED
+        )
+    )
+
+    mock_session_service = mocker.MagicMock()
+    mock_session = mocker.MagicMock()
+    mock_session_service.create_session = mocker.AsyncMock(
+        return_value=mock_session
+    )
+    mock_session_service.append_event = mocker.AsyncMock()
+
+    mocker.patch(
+        "google.adk.evaluation.evaluation_generator.EvaluationGenerator._get_app_details_by_invocation_id"
+    )
+    mocker.patch(
+        "google.adk.evaluation.evaluation_generator.EvaluationGenerator.convert_events_to_eval_invocations"
+    )
+
+    initial_session = SessionInput(
+        app_name="test_app",
+        user_id="test_user",
+    )
+
+    await EvaluationGenerator._generate_inferences_from_root_agent(
+        root_agent=mock_agent,
+        user_simulator=mock_user_sim,
+        initial_session=initial_session,
+        session_service=mock_session_service,
+    )
+
+    # Verify append_event was NOT called
+    mock_session_service.append_event.assert_not_called()