fix(langchain): allow Gemini 3 models to use ProviderStrategy with tools (#34464)

libs/langchain_v1/langchain/agents/factory.py

@@ -495,9 +495,14 @@ def _supports_provider_strategy(
         if (
             model_profile is not None
             and model_profile.get("structured_output")
-            # We make an exception for Gemini models, which currently do not support
-            # simultaneous tool use with structured output
-            and not (tools and isinstance(model_name, str) and "gemini" in model_name.lower())
+            # We make an exception for Gemini < 3-series models, which currently do not support
+            # simultaneous tool use with structured output; 3-series can.
+            and not (
+                tools
+                and isinstance(model_name, str)
+                and "gemini" in model_name.lower()
+                and "gemini-3" not in model_name.lower()
+            )
         ):
             return True
 

libs/langchain_v1/tests/unit_tests/agents/test_response_format.py

@@ -16,6 +16,7 @@ from pydantic import BaseModel, Field
 from typing_extensions import TypedDict
 
 from langchain.agents import create_agent
+from langchain.agents.factory import _supports_provider_strategy
 from langchain.agents.middleware.types import (
     AgentMiddleware,
     ModelCallResult,
@@ -897,3 +898,44 @@ def test_union_of_types() -> None:
 
     assert response["structured_response"] == EXPECTED_WEATHER_PYDANTIC
     assert len(response["messages"]) == 5
+
+
+class TestSupportsProviderStrategy:
+    """Unit tests for `_supports_provider_strategy`."""
+
+    @staticmethod
+    def _make_structured_model(model_name: str):
+        class GeminiTestChatModel(GenericFakeChatModel):
+            model_name: str
+
+        return GeminiTestChatModel(
+            messages=iter(
+                [
+                    AIMessage(content="test-response"),
+                ]
+            ),
+            profile={"structured_output": True},
+            model_name=model_name,
+        )
+
+    def test_blocks_gemini_v2_with_tools(self) -> None:
+        """Gemini 2 series models cannot use provider strategy with tools."""
+        model = self._make_structured_model("gemini-2.5-flash")
+        assert not _supports_provider_strategy(model, tools=[get_weather])
+
+    def test_allows_gemini_v3_with_tools(self) -> None:
+        """Gemini 3 series models support structured output alongside tools."""
+        model = self._make_structured_model("gemini-3-pro-preview")
+        assert _supports_provider_strategy(model, tools=[get_weather])
+
+    @pytest.mark.parametrize(
+        "alias",
+        [
+            "gemini-flash-latest",
+            "gemini-flash-lite-latest",
+        ],
+    )
+    def test_blocks_gemini_latest_aliases(self, alias: str) -> None:
+        """Latest aliases stay blocked until they point to Gemini 3."""
+        model = self._make_structured_model(alias)
+        assert not _supports_provider_strategy(model, tools=[get_weather])