fix(vertex_ai): keep Gemini 1.x off the responseJsonSchema channel

Neither the global setting nor the per request override can select a channel the
model has no field for, so asking for the JSON Schema channel on a Gemini 1.x
model now logs a warning and keeps the natively converted responseSchema.

litellm/llms/vertex_ai/common_utils.py

@@ -271,10 +271,16 @@ def supports_response_json_schema(model: str) -> bool:
     return bool(gemini_2_plus_pattern.search(model_lower))
 
 
+GEMINI_1_MODEL_PATTERN: Final = re.compile(r"gemini-1(?:\.|-)")
 VERTEX_AI_USE_RESPONSE_JSON_SCHEMA_PARAM: Final = "vertex_ai_use_response_json_schema"
 VERTEX_AI_VERBATIM_RESPONSE_SCHEMA_PARAM: Final = "litellm_param_vertex_ai_verbatim_response_schema"
 
 
+def _rejects_response_json_schema(model: str) -> bool:
+    """Gemini 1.x generateContent has no responseJsonSchema field, so no override can reach it"""
+    return bool(GEMINI_1_MODEL_PATTERN.search(model.lower()))
+
+
 def should_use_response_json_schema(model: str, request_override: bool | None = None) -> bool:
     """
     Resolve which structured output channel a json_schema response_format goes to.
@@ -283,13 +289,20 @@ def should_use_response_json_schema(model: str, request_override: bool | None =
     natively converted ``responseSchema`` (nullable unions flattened, constraints
     hoisted, ``propertyOrdering`` added). Precedence: per request
     ``vertex_ai_use_response_json_schema``, then
-    ``litellm.vertex_ai_use_response_json_schema``, then the model heuristic
+    ``litellm.vertex_ai_use_response_json_schema``, then the model heuristic. Neither
+    override can select a channel the model has no field for
     """
-    if request_override is not None:
-        return request_override
-    if litellm.vertex_ai_use_response_json_schema is not None:
-        return litellm.vertex_ai_use_response_json_schema
-    return supports_response_json_schema(model)
+    override: Final = request_override if request_override is not None else litellm.vertex_ai_use_response_json_schema
+    if override is None:
+        return supports_response_json_schema(model)
+    if override and _rejects_response_json_schema(model):
+        verbose_logger.warning(
+            "vertex_ai_use_response_json_schema=True ignored for model=%s: it has no responseJsonSchema field, "
+            "so the schema stays on responseSchema",
+            model,
+        )
+        return False
+    return override
 
 
 from typing import Literal

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

@@ -1,5 +1,7 @@
 
 import json
+from collections.abc import Mapping
+from typing import Optional
 
 import pytest
 
@@ -354,7 +356,14 @@ RESPONSE_SCHEMA_CHANNEL_CLIENT_SCHEMA = {
 }
 
 
-def _gemini_request_body(model: str, litellm_params: dict, **completion_kwargs) -> RequestBody:
+def _gemini_request_body(
+    model: str,
+    litellm_params: Mapping[str, bool],
+    request_override: Optional[bool] = None,
+) -> RequestBody:
+    override_kwargs: dict[str, bool] = (
+        {} if request_override is None else {"vertex_ai_use_response_json_schema": request_override}
+    )
     optional_params = litellm.utils.get_optional_params(
         model=model,
         custom_llm_provider="vertex_ai",
@@ -365,14 +374,14 @@ def _gemini_request_body(model: str, litellm_params: dict, **completion_kwargs)
                 "schema": json.loads(json.dumps(RESPONSE_SCHEMA_CHANNEL_CLIENT_SCHEMA)),
             },
         },
-        **completion_kwargs,
+        **override_kwargs,
     )
     return transformation._transform_request_body(
         messages=[{"role": "user", "content": "extract it"}],
         model=model,
         optional_params=optional_params,
         custom_llm_provider="vertex_ai",
-        litellm_params=litellm_params,
+        litellm_params=dict(litellm_params),
         cached_content=None,
     )
 
@@ -382,9 +391,7 @@ def test__transform_request_body_per_request_response_json_schema_opt_out():
     vertex_ai_use_response_json_schema=False on the request puts the schema on Vertex's native
     responseSchema channel, and the knob itself never reaches the provider body
     """
-    body = _gemini_request_body(
-        "gemini-2.5-flash", {}, vertex_ai_use_response_json_schema=False
-    )
+    body = _gemini_request_body("gemini-2.5-flash", {}, request_override=False)
 
     generation_config = body["generationConfig"]
     assert "response_json_schema" not in generation_config
@@ -398,9 +405,7 @@ def test__transform_request_body_per_request_response_json_schema_opt_out():
 
 def test__transform_request_body_deployment_response_json_schema_opt_out():
     """A deployment's litellm_params opts every request routed to it out of responseJsonSchema"""
-    body = _gemini_request_body(
-        "gemini-2.5-flash", {"vertex_ai_use_response_json_schema": False}
-    )
+    body = _gemini_request_body("gemini-2.5-flash", {"vertex_ai_use_response_json_schema": False})
 
     generation_config = body["generationConfig"]
     assert "response_json_schema" not in generation_config
@@ -414,9 +419,7 @@ def test__transform_request_body_per_request_opt_in_beats_global_opt_out(monkeyp
     """
     monkeypatch.setattr(litellm, "vertex_ai_use_response_json_schema", False)
 
-    body = _gemini_request_body(
-        "gemini-2.5-flash", {}, vertex_ai_use_response_json_schema=True
-    )
+    body = _gemini_request_body("gemini-2.5-flash", {}, request_override=True)
 
     generation_config = body["generationConfig"]
     assert "response_schema" not in generation_config

tests/test_litellm/llms/vertex_ai/test_vertex_ai_common_utils.py

@@ -200,7 +200,9 @@ CLIENT_SCHEMA = {
         (None, None, "gemini-2.5-flash", True),
         (None, None, "gemini-1.5-pro", False),
         (False, None, "gemini-2.5-flash", False),
-        (True, None, "gemini-1.5-pro", True),
+        (True, None, "gemini-flash-latest", True),
+        (True, None, "gemini-1.5-pro", False),
+        (None, True, "gemini-1.5-pro", False),
         (False, True, "gemini-2.5-flash", True),
         (True, False, "gemini-2.5-flash", False),
     ],
@@ -208,7 +210,10 @@ CLIENT_SCHEMA = {
 def test_should_use_response_json_schema_precedence(
     monkeypatch, global_setting, request_override, model, expected
 ):
-    """Per request override beats litellm.vertex_ai_use_response_json_schema, which beats the model heuristic"""
+    """
+    Per request override beats litellm.vertex_ai_use_response_json_schema, which beats the model
+    heuristic, and neither can put a schema on a channel Gemini 1.x has no field for
+    """
     monkeypatch.setattr(litellm, "vertex_ai_use_response_json_schema", global_setting)
 
     assert should_use_response_json_schema(model, request_override) is expected