fix: ensure Gemini calls include a user message (fixes #9733)

When a Gemini model supports system messages, messages with the
system role get filtered out from the list of messages, so never
appended to the content (and passed as system_instructions instead).

This causes a 400 error, as both generateContent and streamGenerateContent
require contents (the systemInstruction object is optional).

This is a quick fix that does not check whether there was a system msg
or if the model supports it. It simply makes sure that if the list
of messages passed to  _gemini_convert_messages_with_history()
yields no contents, that there is at least a single (empty space)
user message set to prevent the call from failing.

The potential side effect of this is that we will not fail
a call lacking any message at all, but if that needs to be checked
(and AFAIU the OpenAI spec messages are required) it should be done
elsewhere (e.g. validate_and_fix_openai_messages() @ litellm/utils.py).

PS: Applied `make format` (`poetry run black [file changed]`) only to
files changed by me. The `make lint` command identified a bunch of
files requiring formatting that I am not modifying as I did not touch
them.

litellm/llms/vertex_ai/gemini/transformation.py

@@ -298,6 +298,19 @@ def _gemini_convert_messages_with_history(  # noqa: PLR0915
                 )
         if len(tool_call_responses) > 0:
             contents.append(ContentType(parts=tool_call_responses))
+
+        if len(contents) == 0:
+            verbose_logger.warning(
+                """
+                No contents in messages. Contents are required. See
+                https://cloud.google.com/vertex-ai/docs/reference/rest/v1/projects.locations.publishers.models/generateContent#request-body.
+                If the original request did not comply to OpenAI API requirements it should have failed by now,
+                but LiteLLM does not check for missing messages.
+                Setting an empty content to prevent an 400 error.
+                Relevant Issue - https://github.com/BerriAI/litellm/issues/9733
+                """
+            )
+            contents.append(ContentType(role="user", parts=[PartType(text=" ")]))
         return contents
     except Exception as e:
         raise e
@@ -403,19 +416,21 @@ def sync_transform_request_body(
     context_caching_endpoints = ContextCachingEndpoints()
 
     if gemini_api_key is not None:
-        messages, optional_params, cached_content = (
-            context_caching_endpoints.check_and_create_cache(
-                messages=messages,
-                optional_params=optional_params,
-                api_key=gemini_api_key,
-                api_base=api_base,
-                model=model,
-                client=client,
-                timeout=timeout,
-                extra_headers=extra_headers,
-                cached_content=optional_params.pop("cached_content", None),
-                logging_obj=logging_obj,
-            )
+        (
+            messages,
+            optional_params,
+            cached_content,
+        ) = context_caching_endpoints.check_and_create_cache(
+            messages=messages,
+            optional_params=optional_params,
+            api_key=gemini_api_key,
+            api_base=api_base,
+            model=model,
+            client=client,
+            timeout=timeout,
+            extra_headers=extra_headers,
+            cached_content=optional_params.pop("cached_content", None),
+            logging_obj=logging_obj,
         )
     else:  # [TODO] implement context caching for gemini as well
         cached_content = optional_params.pop("cached_content", None)

tests/llm_translation/test_prompt_factory.py

@@ -7,7 +7,7 @@ import pytest
 
 sys.path.insert(0, os.path.abspath("../.."))
 
-from typing import Union
+from typing import Union, List
 
 # from litellm.litellm_core_utils.prompt_templates.factory import prompt_factory
 import litellm
@@ -28,6 +28,7 @@ from litellm.litellm_core_utils.prompt_templates.common_utils import (
 from litellm.llms.vertex_ai.gemini.transformation import (
     _gemini_convert_messages_with_history,
 )
+from litellm.types.llms.openai import AllMessageValues
 from unittest.mock import AsyncMock, MagicMock, patch
 
 
@@ -472,6 +473,20 @@ def test_vertex_only_image_user_message():
         )
 
 
+def test_no_messages_yields_user_text():
+    """
+    Test that contents are not empty and have text when called without messages
+    This is to support blha blah
+    """
+    messages: List[AllMessageValues] = []
+
+    contents = _gemini_convert_messages_with_history(messages=messages)
+
+    expected_output = [{"role": "user", "parts": [{"text": " "}]}]
+
+    assert contents == expected_output
+
+
 def test_convert_url():
     convert_url_to_base64("https://picsum.photos/id/237/200/300")
 
@@ -630,7 +645,6 @@ def test_azure_tool_call_invoke_helper():
 def test_ensure_alternating_roles(
     messages, expected_messages, user_continue_message, assistant_continue_message
 ):
-
     messages = get_completion_messages(
         messages=messages,
         assistant_continue_message=assistant_continue_message,
@@ -651,7 +665,7 @@ def test_alternating_roles_e2e():
     http_handler = HTTPHandler()
 
     with patch.object(http_handler, "post", new=MagicMock()) as mock_post:
-        try: 
+        try:
             response = litellm.completion(
                 **{
                     "model": "databricks/databricks-meta-llama-3-1-70b-instruct",
@@ -663,7 +677,10 @@ def test_alternating_roles_e2e():
                         },
                         {"role": "user", "content": "What is Databricks?"},
                         {"role": "user", "content": "What is Azure?"},
-                        {"role": "assistant", "content": "I don't know anyything, do you?"},
+                        {
+                            "role": "assistant",
+                            "content": "I don't know anyything, do you?",
+                        },
                         {"role": "assistant", "content": "I can't repeat sentences."},
                     ],
                     "user_continue_message": {
@@ -712,7 +729,7 @@ def test_alternating_roles_e2e():
                         "role": "user",
                         "content": "Ok",
                     },
-                ]
+                ],
             }
         )