diff --git a/src/google/adk/flows/llm_flows/functions.py b/src/google/adk/flows/llm_flows/functions.py
index 724bcd3f0e..672b997011 100644
--- a/src/google/adk/flows/llm_flows/functions.py
+++ b/src/google/adk/flows/llm_flows/functions.py
@@ -1151,10 +1151,17 @@ def __build_response_event(
 
 
 def deep_merge_dicts(d1: dict, d2: dict) -> dict:
-  """Recursively merges d2 into d1."""
+  """Recursively merges d2 into d1.
+
+  For dict values, merges recursively. For list values, concatenates instead of
+  overwriting so that parallel tool calls don't silently drop list entries
+  (e.g. state_delta lists from concurrent function responses).
+  """
   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
diff --git a/tests/unittests/flows/llm_flows/test_functions_parallel.py b/tests/unittests/flows/llm_flows/test_functions_parallel.py
index f192f2e95c..79f1dcf1db 100644
--- a/tests/unittests/flows/llm_flows/test_functions_parallel.py
+++ b/tests/unittests/flows/llm_flows/test_functions_parallel.py
@@ -13,7 +13,10 @@
 # limitations under the License.
 
 from google.adk.agents.llm_agent import Agent
+from google.adk.events.event import Event
 from google.adk.events.event_actions import EventActions
+from google.adk.flows.llm_flows.functions import deep_merge_dicts
+from google.adk.flows.llm_flows.functions import merge_parallel_function_response_events
 from google.adk.tools.tool_context import ToolContext
 from google.genai import types
 import pytest
@@ -105,3 +108,83 @@ async def transfer_to_agent(
       },
       transfer_to_agent='test_sub_agent',
   )
+
+
+def test_deep_merge_dicts_concatenates_lists():
+  """Test that deep_merge_dicts concatenates list values instead of overwriting."""
+  d1 = {'state_delta': {'items': ['a']}}
+  d2 = {'state_delta': {'items': ['b']}}
+  result = deep_merge_dicts(d1, d2)
+  assert result['state_delta']['items'] == ['a', 'b']
+
+
+def test_deep_merge_dicts_overwrites_non_list_non_dict():
+  """Test that deep_merge_dicts still overwrites scalar values."""
+  d1 = {'key': 'old'}
+  d2 = {'key': 'new'}
+  result = deep_merge_dicts(d1, d2)
+  assert result['key'] == 'new'
+
+
+def test_deep_merge_dicts_merges_nested_dicts():
+  """Test that deep_merge_dicts recursively merges nested dicts."""
+  d1 = {'a': {'b': 1, 'c': 2}}
+  d2 = {'a': {'b': 3, 'd': 4}}
+  result = deep_merge_dicts(d1, d2)
+  assert result == {'a': {'b': 3, 'c': 2, 'd': 4}}
+
+
+def test_deep_merge_dicts_handles_mixed_list_and_non_list():
+  """Test that deep_merge_dicts overwrites when types differ (list vs non-list)."""
+  d1 = {'key': 'not_a_list'}
+  d2 = {'key': ['a', 'b']}
+  result = deep_merge_dicts(d1, d2)
+  assert result['key'] == ['a', 'b']
+
+  d1 = {'key': ['a', 'b']}
+  d2 = {'key': 'not_a_list'}
+  result = deep_merge_dicts(d1, d2)
+  assert result['key'] == 'not_a_list'
+
+
+def test_merge_parallel_function_response_events_merges_state_delta_lists():
+  """Test that parallel events with list state_delta values are concatenated, not overwritten."""
+  invocation_id = 'base_invocation_123'
+
+  event1 = Event(
+      invocation_id=invocation_id,
+      author='tool',
+      content=types.Content(
+          role='user',
+          parts=[
+              types.Part(
+                  function_response=types.FunctionResponse(
+                      name='func_1',
+                      response={'result': 'ok'},
+                  )
+              )
+          ],
+      ),
+      actions=EventActions(state_delta={'items': ['a']}),
+  )
+
+  event2 = Event(
+      invocation_id=invocation_id,
+      author='tool',
+      content=types.Content(
+          role='user',
+          parts=[
+              types.Part(
+                  function_response=types.FunctionResponse(
+                      name='func_2',
+                      response={'result': 'ok'},
+                  )
+              )
+          ],
+      ),
+      actions=EventActions(state_delta={'items': ['b']}),
+  )
+
+  merged_event = merge_parallel_function_response_events([event1, event2])
+
+  assert merged_event.actions.state_delta == {'items': ['a', 'b']}