fix: accumulate list values when merging parallel tool call state_delta

Parallel tool calls writing list values to the same state_delta key
were silently dropped because deep_merge_dicts only recursed into
dict values; lists hit the overwrite branch.

Closes #5190

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 905229706

Author: GWeale

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

@@ -1134,10 +1134,12 @@ def __build_response_event(
 
 
 def deep_merge_dicts(d1: dict, d2: dict) -> dict:
-  """Recursively merges d2 into d1."""
+  """Recursively merges d2 into d1; concatenates list values at the same key."""
   for key, value in d2.items():
     if key in d1 and isinstance(d1[key], dict) and isinstance(value, dict):
       d1[key] = deep_merge_dicts(d1[key], value)
+    elif key in d1 and isinstance(d1[key], list) and isinstance(value, list):
+      d1[key] = d1[key] + value
     else:
       d1[key] = value
   return d1

tests/unittests/flows/llm_flows/test_functions_simple.py

@@ -20,6 +20,7 @@
 from google.adk.events.event import Event
 from google.adk.events.event_actions import EventActions
 from google.adk.events.ui_widget import UiWidget
+from google.adk.flows.llm_flows.functions import deep_merge_dicts
 from google.adk.flows.llm_flows.functions import find_matching_function_call
 from google.adk.flows.llm_flows.functions import handle_function_calls_async
 from google.adk.flows.llm_flows.functions import handle_function_calls_live
@@ -1236,3 +1237,62 @@ async def mock_run(*args, **kwargs):
   # Verify the image was converted to a blob
   assert len(response_part.parts) == 1
   assert response_part.parts[0].inline_data is not None
+
+
+def test_deep_merge_dicts_concatenates_lists_at_same_key():
+  d1 = {'items': ['a', 'b']}
+  d2 = {'items': ['c', 'd']}
+  assert deep_merge_dicts(d1, d2) == {'items': ['a', 'b', 'c', 'd']}
+
+
+def test_deep_merge_dicts_concatenates_nested_lists():
+  d1 = {'state_delta': {'items': ['a']}}
+  d2 = {'state_delta': {'items': ['b']}}
+  assert deep_merge_dicts(d1, d2) == {'state_delta': {'items': ['a', 'b']}}
+
+
+def test_deep_merge_dicts_overwrites_when_types_differ():
+  d1 = {'x': ['a']}
+  d2 = {'x': 'b'}
+  assert deep_merge_dicts(d1, d2) == {'x': 'b'}
+
+  d1 = {'x': 'a'}
+  d2 = {'x': ['b']}
+  assert deep_merge_dicts(d1, d2) == {'x': ['b']}
+
+
+def test_deep_merge_dicts_overwrites_scalars():
+  d1 = {'x': 1, 'y': 'old'}
+  d2 = {'x': 2, 'y': 'new'}
+  assert deep_merge_dicts(d1, d2) == {'x': 2, 'y': 'new'}
+
+
+def test_merge_parallel_function_response_events_concatenates_state_delta_lists():
+  """Regression test for https://github.com/google/adk-python/issues/5190."""
+  function_response1 = types.FunctionResponse(
+      id='func_1', name='append_item', response={'result': 'ok'}
+  )
+  function_response2 = types.FunctionResponse(
+      id='func_2', name='append_item', response={'result': 'ok'}
+  )
+
+  event1 = Event(
+      invocation_id='inv_1',
+      author='test_agent',
+      content=types.Content(
+          role='user', parts=[types.Part(function_response=function_response1)]
+      ),
+      actions=EventActions(state_delta={'items': ['a']}),
+  )
+  event2 = Event(
+      invocation_id='inv_1',
+      author='test_agent',
+      content=types.Content(
+          role='user', parts=[types.Part(function_response=function_response2)]
+      ),
+      actions=EventActions(state_delta={'items': ['b']}),
+  )
+
+  merged_event = merge_parallel_function_response_events([event1, event2])
+
+  assert merged_event.actions.state_delta == {'items': ['a', 'b']}