From ae902052adca1d055dbdeb94e2487321d267ceb2 Mon Sep 17 00:00:00 2001
From: Alexander Kuzmik <alexander.kuzmik99@gmail.com>
Date: Wed, 28 Jan 2026 16:55:08 +0100
Subject: [PATCH 1/3] Make _part_has_payload respect function response

---
 src/google/adk/models/lite_llm.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/google/adk/models/lite_llm.py b/src/google/adk/models/lite_llm.py
index 79182d7b0a..651e7d9818 100644
--- a/src/google/adk/models/lite_llm.py
+++ b/src/google/adk/models/lite_llm.py
@@ -470,6 +470,8 @@ def _part_has_payload(part: types.Part) -> bool:
     return True
   if part.file_data and (part.file_data.file_uri or part.file_data.data):
     return True
+  if part.function_response:
+    return True
   return False
 
 

From 04657c1d1c63ffe566f409135c3d3774875475e0 Mon Sep 17 00:00:00 2001
From: Alexander Kuzmik <alexander.kuzmik99@gmail.com>
Date: Wed, 28 Jan 2026 17:42:05 +0100
Subject: [PATCH 2/3] Add new test

---
 tests/unittests/models/test_litellm.py | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index 2ebbc5dfe8..b4bf69d949 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -1035,6 +1035,23 @@ async def test_generate_content_async_adds_fallback_user_message(
         4,
         id="user content is not the last message scenario",
     ),
+    pytest.param(
+        LlmRequest(
+            contents=[
+                types.Content(
+                    role="user",
+                    parts=[
+                        types.Part.from_function_response(
+                            name="test_function",
+                            response={"result": "test_result"},
+                        )
+                    ],
+                ),
+            ]
+        ),
+        1,
+        id="user content with function_response has payload",
+    ),
 ]
 
 

From 0fe6e39a49198718d1a69c9785c23024a87eb55e Mon Sep 17 00:00:00 2001
From: Alexander Kuzmik <alexander.kuzmik99@gmail.com>
Date: Wed, 28 Jan 2026 17:57:26 +0100
Subject: [PATCH 3/3] update test

---
 tests/unittests/models/test_litellm.py | 58 ++++++++++++++++++--------
 1 file changed, 41 insertions(+), 17 deletions(-)

diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index b4bf69d949..8c07ee7061 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -990,6 +990,47 @@ async def test_generate_content_async_adds_fallback_user_message(
   )
 
 
+@pytest.mark.asyncio
+async def test_generate_content_async_no_fallback_for_function_response(
+    mock_acompletion, lite_llm_instance
+):
+  """Tests that no fallback message is added for a user message with a function response."""
+  llm_request = LlmRequest(
+      contents=[
+          types.Content(
+              role="user",
+              parts=[
+                  types.Part.from_function_response(
+                      name="test_function",
+                      response={"result": "test_result"},
+                  )
+              ],
+          )
+      ]
+  )
+
+  # Run generate_content_async which calls _append_fallback_user_content_if_missing
+  async for _ in lite_llm_instance.generate_content_async(llm_request):
+    pass
+
+  # Verify that the fallback message was NOT added to the llm_request
+  assert len(llm_request.contents) == 1
+  assert len(llm_request.contents[0].parts) == 1
+  assert llm_request.contents[0].parts[0].function_response is not None
+
+  # Verify that the message sent to litellm does not contain the fallback text
+  mock_acompletion.assert_called_once()
+  _, kwargs = mock_acompletion.call_args
+  user_messages = [
+      message for message in kwargs["messages"] if message["role"] == "user"
+  ]
+  assert not any(
+      message.get("content")
+      == "Handle the requests as specified in the System Instruction."
+      for message in user_messages
+  )
+
+
 litellm_append_user_content_test_cases = [
     pytest.param(
         LlmRequest(
@@ -1035,23 +1076,6 @@ async def test_generate_content_async_adds_fallback_user_message(
         4,
         id="user content is not the last message scenario",
     ),
-    pytest.param(
-        LlmRequest(
-            contents=[
-                types.Content(
-                    role="user",
-                    parts=[
-                        types.Part.from_function_response(
-                            name="test_function",
-                            response={"result": "test_result"},
-                        )
-                    ],
-                ),
-            ]
-        ),
-        1,
-        id="user content with function_response has payload",
-    ),
 ]