fix(vertex-ai): default missing native content roles

litellm/llms/vertex_ai/google_genai/transformation.py

@@ -82,10 +82,18 @@ class VertexAIGoogleGenAIConfig(GoogleGenAIConfig):
         if generate_content_config_dict:
             self._normalize_response_schema(generate_content_config_dict, model)
 
-        # Build the request in Google GenAI format that Vertex AI expects
+        vertex_contents = (
+            [
+                {**content, "role": content.get("role", "user")} if isinstance(content, dict) else content
+                for content in contents
+            ]
+            if isinstance(contents, list)
+            else contents
+        )
+
         result = {
             "model": model,
-            "contents": contents,
+            "contents": vertex_contents,
         }
 
         # Add tools if provided

tests/test_litellm/llms/vertex_ai/google_genai/test_transformation.py

@@ -0,0 +1,24 @@
+from litellm.llms.vertex_ai.google_genai.transformation import (
+    VertexAIGoogleGenAIConfig,
+)
+
+
+def test_transform_generate_content_request_defaults_omitted_role():
+    config = VertexAIGoogleGenAIConfig()
+    contents = [
+        {"parts": [{"text": "Hello"}]},
+        {"role": "model", "parts": [{"text": "Hi"}]},
+    ]
+
+    result = config.transform_generate_content_request(
+        model="gemini-2.5-flash",
+        contents=contents,
+        tools=None,
+        generate_content_config_dict={},
+    )
+
+    assert result["contents"] == [
+        {"role": "user", "parts": [{"text": "Hello"}]},
+        {"role": "model", "parts": [{"text": "Hi"}]},
+    ]
+    assert "role" not in contents[0]