fix: ensure AgentTool text output when skip_summarization is True

When skip_summarization=True, AgentTool previously terminated the flow with a
FunctionResponse event that contained no text, leaving UIs stuck. Most UIs do
not render function responses, so the agent appeared to hang or return an empty
result.

This change appends a Text part to the response event when summarization is
skipped, so the tool's output is displayed. If the result is a string it is
used directly; otherwise it is safely serialized to JSON. Error responses are
left unchanged, preserving existing behavior for normal flows.

Added regression tests verifying text output is present for string,
JSON-string, and structured (output_schema) tool results.

Closes #3881

Co-authored-by: Haran Rajkumar <haranrk@google.com>
PiperOrigin-RevId: 938286418

Author: 2 people

src/google/adk/flows/llm_flows/functions.py

@@ -23,6 +23,7 @@
 import contextvars
 import copy
 import inspect
+import json
 import logging
 import threading
 from typing import Any
@@ -1181,6 +1182,9 @@ def __build_response_event(
     tool_context: ToolContext,
     invocation_context: InvocationContext,
 ) -> Event:
+  # Capture the raw result for display purposes before any normalization.
+  display_result = function_result
+
   # Specs requires the result to be a dict.
   if not isinstance(function_result, dict):
     function_result = {'result': function_result}
@@ -1198,6 +1202,25 @@ def __build_response_event(
       function_response_parts,
   )
 
+  # When summarization is skipped, ensure a displayable text part is added so
+  # the tool's output is not lost in UIs that don't render function responses.
+  # Control-flow tools (e.g. exit_loop) set skip_summarization but return no
+  # meaningful output; their None result is normalized to {'result': None}, so
+  # skip those to avoid emitting a noisy "null" text part.
+  has_displayable_result = display_result is not None and display_result != {
+      'result': None
+  }
+  if (
+      tool_context.actions.skip_summarization
+      and 'error' not in function_result
+      and has_displayable_result
+  ):
+    if isinstance(display_result, str):
+      result_text = display_result
+    else:
+      result_text = json.dumps(display_result, ensure_ascii=False, default=str)
+    content.parts.append(types.Part.from_text(text=result_text))
+
   function_response_event = Event(
       invocation_id=invocation_context.invocation_id,
       author=invocation_context.agent.name,

tests/unittests/tools/test_agent_tool.py

@@ -1531,3 +1531,98 @@ async def close(self):
   assert captured['new_message'] is not None
   text = captured['new_message'].parts[0].text
   assert text == expected_text
+
+
+@pytest.fixture
+def setup_skip_summarization_runner():
+  def _setup_runner(tool_agent_model_responses, tool_agent_output_schema=None):
+    tool_agent_model = testing_utils.MockModel.create(
+        responses=tool_agent_model_responses
+    )
+    tool_agent = Agent(
+        name='tool_agent',
+        model=tool_agent_model,
+        output_schema=tool_agent_output_schema,
+    )
+
+    agent_tool = AgentTool(agent=tool_agent, skip_summarization=True)
+
+    root_agent_model = testing_utils.MockModel.create(
+        responses=[
+            function_call_no_schema,
+            'final_summary_text_that_should_not_be_reached',
+        ]
+    )
+
+    root_agent = Agent(
+        name='root_agent',
+        model=root_agent_model,
+        tools=[agent_tool],
+    )
+    return testing_utils.InMemoryRunner(root_agent)
+
+  return _setup_runner
+
+
+def test_agent_tool_skip_summarization_has_text_output(
+    setup_skip_summarization_runner,
+):
+  """Tests that when skip_summarization is True, the final event contains text content."""
+  runner = setup_skip_summarization_runner(
+      tool_agent_model_responses=['tool_response_text']
+  )
+  events = runner.run('start')
+
+  final_events = [e for e in events if e.is_final_response()]
+  assert final_events
+  last_event = final_events[-1]
+  assert last_event.is_final_response()
+
+  assert any(p.function_response for p in last_event.content.parts)
+
+  assert [p.text for p in last_event.content.parts if p.text] == [
+      'tool_response_text'
+  ]
+
+
+def test_agent_tool_skip_summarization_preserves_json_string_output(
+    setup_skip_summarization_runner,
+):
+  """Tests that structured output string is preserved as text when skipping summarization."""
+  runner = setup_skip_summarization_runner(
+      tool_agent_model_responses=['{"field": "value"}']
+  )
+  events = runner.run('start')
+
+  final_events = [e for e in events if e.is_final_response()]
+  assert final_events
+  last_event = final_events[-1]
+  assert last_event.is_final_response()
+
+  text_parts = [p.text for p in last_event.content.parts if p.text]
+
+  # Check that the JSON string content is preserved exactly
+  assert text_parts == ['{"field": "value"}']
+
+
+def test_agent_tool_skip_summarization_handles_non_string_result(
+    setup_skip_summarization_runner,
+):
+  """Tests that non-string (dict) output is correctly serialized as JSON text."""
+
+  class CustomOutput(BaseModel):
+    value: int
+
+  runner = setup_skip_summarization_runner(
+      tool_agent_model_responses=['{"value": 123}'],
+      tool_agent_output_schema=CustomOutput,
+  )
+  events = runner.run('start')
+
+  final_events = [e for e in events if e.is_final_response()]
+  assert final_events
+  last_event = final_events[-1]
+
+  text_parts = [p.text for p in last_event.content.parts if p.text]
+
+  assert text_parts == ['{"value": 123}']