chore: add a service_tier field mapping from openai to gemini

litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py

@@ -363,6 +363,17 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
         """
         return Tools(googleSearch={})
 
+    def _map_service_tier_param(self, value: str, optional_params: dict) -> None:
+        """
+        Map OpenAI service_tier (string) to Gemini serviceTier.
+        'auto' maps to 'priority'.
+        Other values are passed blindly.
+        """
+        if value == "auto":
+            optional_params["service_tier"] = "priority"
+        else:
+            optional_params["service_tier"] = value
+
     def _transform_computer_use_config(self, computer_use_config: dict) -> dict:
         """
         Transform Computer Use configuration to Gemini API format.
@@ -1123,7 +1134,7 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
                     optional_params, [_tools]
                 )
             elif param == "service_tier" and isinstance(value, str):
-                optional_params["service_tier"] = value
+                self._map_service_tier_param(value, optional_params)
             elif param == "include_server_side_tool_invocations" and value is True:
                 optional_params["include_server_side_tool_invocations"] = True
         if litellm.vertex_ai_safety_settings is not None:

tests/test_litellm/llms/vertex_ai/gemini/test_vertex_and_google_ai_studio_gemini.py

@@ -3674,18 +3674,33 @@ def test_vertex_ai_service_tier_in_map_openai_params():
     )
 
     v = VertexGeminiConfig()
+    
+    # Test pass-through
     optional_params = {}
     non_default_params = {"service_tier": "FLEX"}
 
     result = v.map_openai_params(
         non_default_params=non_default_params,
         optional_params=optional_params,
-        model="gemini-pro",
+        model="gemini-3-pro-preview",
         drop_params=True,
     )
 
     assert result["service_tier"] == "FLEX"
 
+    # Test auto -> priority
+    optional_params_auto = {}
+    non_default_params_auto = {"service_tier": "auto"}
+
+    result_auto = v.map_openai_params(
+        non_default_params=non_default_params_auto,
+        optional_params=optional_params_auto,
+        model="gemini-3-pro-preview",
+        drop_params=True,
+    )
+
+    assert result_auto["service_tier"] == "priority"
+
 
 def test_vertex_ai_usage_metadata_with_video_tokens_in_prompt():
     """Test promptTokensDetails with VIDEO modality for video inputs.