fix(evaluation): skip invocations without user events in convert_events_to_eval_invocations

Sessions can contain invocation_ids whose events are all authored by agents
or tools (e.g. internal/background turns with no corresponding user message).
Previously, convert_events_to_eval_invocations left user_content as an empty
Content(parts=[]) for such invocations, and earlier versions used an empty
string, which caused a Pydantic ValidationError because Invocation.user_content
requires a genai_types.Content object.

Invocations without a user-authored event are not meaningful for evaluation,
so skip them instead of constructing an Invocation with a placeholder
user_content. A debug log line is emitted for each skipped invocation to aid
troubleshooting.

Fixes #3760

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Koushik-Salammagari

src/google/adk/evaluation/evaluation_generator.py

@@ -281,7 +281,7 @@ def convert_events_to_eval_invocations(
     for invocation_id, events in events_by_invocation_id.items():
       final_response = None
       final_event = None
-      user_content = Content(parts=[])
+      user_content = None
       invocation_timestamp = 0
       app_details = None
       if (
@@ -312,6 +312,18 @@ def convert_events_to_eval_invocations(
               events_to_add.append(event)
               break
 
+      if user_content is None:
+        # Skip invocations that have no user-authored event.  Such invocations
+        # arise from internal/system-driven turns (e.g. background agent tasks)
+        # and are not meaningful for evaluation purposes.  Including them would
+        # also cause a Pydantic ValidationError because Invocation.user_content
+        # requires a Content object.
+        logger.debug(
+            'Skipping invocation %s: no user-authored event found.',
+            invocation_id,
+        )
+        continue
+
       invocation_events = [
           InvocationEvent(author=e.author, content=e.content)
           for e in events_to_add

tests/unittests/evaluation/test_evaluation_generator.py

@@ -226,6 +226,52 @@ def test_convert_multi_agent_final_responses(
     assert intermediate_events[0].author == "agent1"
     assert intermediate_events[0].content.parts[0].text == "First response"
 
+  def test_invocation_without_user_event_is_skipped(self):
+    """Invocations with no user-authored event must be skipped.
+
+    Regression test for https://github.com/google/adk-python/issues/3760.
+    When a session contains an invocation_id whose events are all authored by
+    agents or tools (no 'user' event), convert_events_to_eval_invocations used
+    to leave user_content as a bare string, causing a Pydantic ValidationError
+    from Invocation.user_content which requires genai_types.Content.
+    The fix skips such invocations because they represent internal/system-driven
+    turns that are not meaningful for evaluation.
+    """
+    events = [
+        _build_event("agent", [types.Part(text="agent-only event")], "inv1"),
+    ]
+
+    # Must not raise a Pydantic ValidationError.
+    invocations = EvaluationGenerator.convert_events_to_eval_invocations(events)
+
+    assert invocations == [], (
+        "Invocations without a user event should be skipped."
+    )
+
+  def test_mixed_invocations_skips_only_agent_only_ones(self):
+    """Only agent-only invocations are skipped; normal invocations are kept.
+
+    Regression test for https://github.com/google/adk-python/issues/3760.
+    """
+    events = [
+        # inv1: normal user+agent turn — should be kept.
+        _build_event("user", [types.Part(text="Hello")], "inv1"),
+        _build_event("agent", [types.Part(text="Hi there!")], "inv1"),
+        # inv2: agent-only turn (e.g. background/system task) — should be skipped.
+        _build_event("agent", [types.Part(text="Internal work")], "inv2"),
+        # inv3: normal user+agent turn — should be kept.
+        _build_event("user", [types.Part(text="Follow-up")], "inv3"),
+        _build_event("agent", [types.Part(text="Sure!")], "inv3"),
+    ]
+
+    invocations = EvaluationGenerator.convert_events_to_eval_invocations(events)
+
+    assert len(invocations) == 2
+    assert invocations[0].invocation_id == "inv1"
+    assert invocations[0].user_content.parts[0].text == "Hello"
+    assert invocations[1].invocation_id == "inv3"
+    assert invocations[1].user_content.parts[0].text == "Follow-up"
+
 
 class TestGetAppDetailsByInvocationId:
   """Test cases for EvaluationGenerator._get_app_details_by_invocation_id method."""