fix(gemini): set responseLogprobs when top_logprobs is the only logprob param

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py

@@ -1690,6 +1690,7 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
                 optional_params["responseLogprobs"] = value
             elif param == "top_logprobs":
                 optional_params["logprobs"] = value
+                optional_params["responseLogprobs"] = True
             elif (param == "tools" or param == "functions") and isinstance(value, list) and value:
                 # Pass optional_params so _map_function can add toolConfig if needed
                 mapped_tools = with_integer_enums_that_fit(

tests/integration/translation/responses/params/test_responses_params_gemini_logprobs.py

@@ -14,7 +14,9 @@ GEMINI_3_5_FLASH_TOP_LOGPROBS_TEST_CASE: Final = replace(
     scenario="top_logprobs",
     litellm_request={**GEMINI_3_5_FLASH_TEST_CASE.litellm_request, "top_logprobs": 3},
     expected_provider_request=with_field(
-        GEMINI_3_5_FLASH_TEST_CASE.expected_provider_request, ("generationConfig", "logprobs"), 3
+        with_field(GEMINI_3_5_FLASH_TEST_CASE.expected_provider_request, ("generationConfig", "logprobs"), 3),
+        ("generationConfig", "responseLogprobs"),
+        True,
     ),
 )
 
@@ -22,5 +24,7 @@ CASES: Final = (GEMINI_3_5_FLASH_TOP_LOGPROBS_TEST_CASE,)
 
 
 @pytest.mark.parametrize("case", CASES, ids=lambda case: case.id)
-def test_responses_params_gemini_logprobs(case: TranslationTestCase, gateway: Gateway, provider: SharedProvider) -> None:
+def test_responses_params_gemini_logprobs(
+    case: TranslationTestCase, gateway: Gateway, provider: SharedProvider
+) -> None:
     assert_translation(case, gateway, provider)

tests/unit/llms/gemini/chat/test_transformation.py

@@ -1,12 +1,14 @@
+from typing import Final
+
 from litellm.llms.gemini.chat.transformation import GoogleAIStudioGeminiConfig
 
 
 def test_top_logprobs_maps_to_generation_config_logprobs():
-    config = GoogleAIStudioGeminiConfig()
+    config: Final = GoogleAIStudioGeminiConfig()
 
     assert "top_logprobs" in config.get_supported_openai_params("gemini-3.5-flash")
 
-    optional_params = config.map_openai_params(
+    optional_params: Final = config.map_openai_params(
         non_default_params={"logprobs": True, "top_logprobs": 3},
         optional_params={},
         model="gemini-3.5-flash",
@@ -15,3 +17,15 @@ def test_top_logprobs_maps_to_generation_config_logprobs():
 
     assert optional_params["responseLogprobs"] is True
     assert optional_params["logprobs"] == 3
+
+
+def test_top_logprobs_alone_enables_response_logprobs():
+    optional_params: Final = GoogleAIStudioGeminiConfig().map_openai_params(
+        non_default_params={"top_logprobs": 3},
+        optional_params={},
+        model="gemini-3.5-flash",
+        drop_params=False,
+    )
+
+    assert optional_params["responseLogprobs"] is True
+    assert optional_params["logprobs"] == 3