fix: skipping state_delta overwrite on function_response-only events

Co-authored-by: Xuan Yang <xygoogle@google.com>
PiperOrigin-RevId: 911411431

Author: xuanyang15

src/google/adk/agents/llm_agent.py

@@ -845,6 +845,17 @@ def __maybe_save_output_to_state(self, event: Event):
     # Handle text responses
     if event.is_final_response() and event.content and event.content.parts:
 
+      # Skip if no text parts at all to avoid overwriting state_delta values
+      # already set (e.g. after_tool_callback with skip_summarization
+      # on function_response-only events).
+      has_text_part = any(
+          part.text is not None and not part.thought
+          for part in event.content.parts
+      )
+
+      if not has_text_part:
+        return
+
       result = ''.join(
           part.text
           for part in event.content.parts
@@ -857,12 +868,6 @@ def __maybe_save_output_to_state(self, event: Event):
         if not result.strip():
           return
         result = validate_schema(self.output_schema, result)
-      elif not result:
-        # No text parts found and no output_schema. Skip to avoid
-        # overwriting state_delta values already set by callbacks
-        # (e.g. after_tool_callback with skip_summarization on
-        # function_response-only events).
-        return
       event.actions.state_delta[self.output_key] = result
 
   @model_validator(mode='after')

tests/unittests/agents/test_llm_agent_output_save.py

@@ -279,7 +279,9 @@ def test_maybe_save_output_to_state_handles_empty_final_chunk_with_schema(
 
   def test_maybe_save_output_to_state_skips_function_response_only_event(self):
     """Test that state_delta set by callback is not overwritten when event
-    only has function_response parts and no text."""
+
+    only has function_response parts and no text.
+    """
     agent = LlmAgent(name="test_agent", output_key="result")
 
     # Simulate a function_response-only event (no text parts)
@@ -307,3 +309,25 @@ def test_maybe_save_output_to_state_skips_function_response_only_event(self):
 
     # The callback-set value should be preserved, not overwritten with ""
     assert event.actions.state_delta["result"] == [1, 2, 3]
+
+  def test_maybe_save_output_to_state_saves_empty_string_when_text_is_empty(
+      self,
+  ):
+    """Test that output is saved as empty string when part.text is explicitly empty."""
+    agent = LlmAgent(name="test_agent", output_key="result")
+
+    # Explicitly construct a part with empty string text
+    parts = [types.Part(text="")]
+    content = types.Content(role="model", parts=parts)
+    event = Event(
+        invocation_id="test_invocation",
+        author="test_agent",
+        content=content,
+        actions=EventActions(),
+    )
+
+    agent._LlmAgent__maybe_save_output_to_state(event)
+
+    # Assert key exists and value is empty string
+    assert "result" in event.actions.state_delta
+    assert not event.actions.state_delta["result"]