fix: Add a fallback user message to LiteLLM requests if the last user message is empty

Related to #3255
Close #2560

PiperOrigin-RevId: 825143315

src/google/adk/models/lite_llm.py

@@ -155,6 +155,53 @@ def _safe_json_serialize(obj) -> str:
     return str(obj)
 
 
+def _part_has_payload(part: types.Part) -> bool:
+  """Checks whether a Part contains usable payload for the model."""
+  if part.text:
+    return True
+  if part.inline_data and part.inline_data.data:
+    return True
+  if part.file_data and (part.file_data.file_uri or part.file_data.data):
+    return True
+  return False
+
+
+def _append_fallback_user_content_if_missing(
+    llm_request: LlmRequest,
+) -> None:
+  """Ensures there is a user message with content for LiteLLM backends.
+
+  Args:
+    llm_request: The request that may need a fallback user message.
+  """
+  for content in reversed(llm_request.contents):
+    if content.role == "user":
+      parts = content.parts or []
+      if any(_part_has_payload(part) for part in parts):
+        return
+      if not parts:
+        content.parts = []
+      content.parts.append(
+          types.Part.from_text(
+              text="Handle the requests as specified in the System Instruction."
+          )
+      )
+      return
+  llm_request.contents.append(
+      types.Content(
+          role="user",
+          parts=[
+              types.Part.from_text(
+                  text=(
+                      "Handle the requests as specified in the System"
+                      " Instruction."
+                  )
+              ),
+          ],
+      )
+  )
+
+
 def _content_to_message_param(
     content: types.Content,
 ) -> Union[Message, list[Message]]:
@@ -818,6 +865,7 @@ class LiteLlm(BaseLlm):
     """
 
     self._maybe_append_user_content(llm_request)
+    _append_fallback_user_content_if_missing(llm_request)
     logger.debug(_build_request_log(llm_request))
 
     messages, tools, response_format, generation_params = (

tests/unittests/models/test_litellm.py

@@ -548,6 +548,41 @@ async def test_generate_content_async(mock_acompletion, lite_llm_instance):
   )
 
 
+@pytest.mark.asyncio
+async def test_generate_content_async_adds_fallback_user_message(
+    mock_acompletion, lite_llm_instance
+):
+  llm_request = LlmRequest(
+      contents=[
+          types.Content(
+              role="user",
+              parts=[],
+          )
+      ]
+  )
+
+  async for _ in lite_llm_instance.generate_content_async(llm_request):
+    pass
+
+  mock_acompletion.assert_called_once()
+
+  _, kwargs = mock_acompletion.call_args
+  user_messages = [
+      message for message in kwargs["messages"] if message["role"] == "user"
+  ]
+  assert any(
+      message.get("content")
+      == "Handle the requests as specified in the System Instruction."
+      for message in user_messages
+  )
+  assert (
+      sum(1 for content in llm_request.contents if content.role == "user") == 1
+  )
+  assert llm_request.contents[-1].parts[0].text == (
+      "Handle the requests as specified in the System Instruction."
+  )
+
+
 litellm_append_user_content_test_cases = [
     pytest.param(
         LlmRequest(