[Feature]: Support Gemini requests with only system prompt (#14010)

* _default_user_message_when_system_message_passed

* test_system_prompt_only_adds_blank_user_message

* test_system_message_with_no_user_message

litellm/llms/vertex_ai/gemini/transformation.py

@@ -35,6 +35,7 @@ from litellm.types.llms.openai import (
     ChatCompletionFileObject,
     ChatCompletionImageObject,
     ChatCompletionTextObject,
+    ChatCompletionUserMessage,
 )
 from litellm.types.llms.vertex_ai import *
 from litellm.types.llms.vertex_ai import (
@@ -475,6 +476,13 @@ async def async_transform_request_body(
         optional_params=optional_params,
     )
 
+def _default_user_message_when_system_message_passed() -> ChatCompletionUserMessage:
+    """
+    Returns a default user message when a "system" message is passed in gemini fails.
+
+    This adds a blank user message to the messages list, to ensure that gemini doesn't fail the request.
+    """
+    return ChatCompletionUserMessage(content=".", role="user")
 
 def _transform_system_message(
     supports_system_message: bool, messages: List[AllMessageValues]
@@ -510,6 +518,13 @@ def _transform_system_message(
                 messages.pop(idx)
 
     if len(system_content_blocks) > 0:
+        #########################################################
+        # If no messages are passed in, add a blank user message
+        # Relevant Issue - https://github.com/BerriAI/litellm/issues/13769
+        #########################################################
+        if len(messages) == 0:
+            messages.append(_default_user_message_when_system_message_passed())
+        #########################################################
         return SystemInstructions(parts=system_content_blocks), messages
 
     return None, messages

tests/llm_translation/base_llm_unit_tests.py

@@ -119,6 +119,28 @@ class BaseLLMChatTest(ABC):
             pytest.skip("Model is overloaded")
 
         assert response.choices[0].message.content is not None
+    
+    def test_system_message_with_no_user_message(self):
+        """
+        Test that the system message is translated correctly for non-OpenAI providers.
+        """
+        base_completion_call_args = self.get_base_completion_call_args()
+        messages = [
+            {
+                "role": "system",
+                "content": "Be a good bot!",
+            },
+        ]
+        try:
+            response = self.completion_function(
+                **base_completion_call_args,
+                messages=messages,
+            )
+            assert response is not None
+        except litellm.InternalServerError:
+            pytest.skip("Model is overloaded")
+
+        assert response.choices[0].message.content is not None
 
     def test_content_list_handling(self):
         """Check if content list is supported by LLM API"""

tests/test_litellm/llms/vertex_ai/test_vertex.py

@@ -1469,3 +1469,37 @@ def test_vertex_parallel_tool_calls_false_single_tool():
         parallel_tool_calls=False,
     )
     assert "tools" in optional_params
+
+
+from litellm.llms.vertex_ai.gemini.transformation import _transform_request_body
+
+
+def test_system_prompt_only_adds_blank_user_message():
+    """
+    Test that the system prompt only adds a blank user message when a system message is passed in.
+
+    Relevant Issue - https://github.com/BerriAI/litellm/issues/13769
+    """
+    SYSTEM_INSTRUCTION = "System instructions for the model"
+    data = _transform_request_body(
+        messages=[{"role": "system", "content": SYSTEM_INSTRUCTION}],
+        model="gemini-2.5-flash",
+        optional_params={},
+        custom_llm_provider="vertex_ai",
+        litellm_params={},
+        cached_content=None,
+    )
+    print("Final data: ", data)
+
+    # validate that a blank user message is added when a system message is passed in
+    assert len(data["contents"]) == 1
+    first_content = data["contents"][0]
+    assert first_content["role"] == "user"
+    assert len(first_content["parts"]) == 1
+
+
+    #########################################################
+    # system message was passed in
+    #########################################################
+    assert len(data["system_instruction"]) == 1
+    assert data["system_instruction"]["parts"][0]["text"] == SYSTEM_INSTRUCTION