fix(langchain): bind strict=True only for openai + grok

libs/langchain_v1/langchain/agents/factory.py

@@ -13,7 +13,6 @@
     get_type_hints,
 )
 
-from langchain_core.language_models.chat_models import BaseChatModel
 from langchain_core.messages import AIMessage, AnyMessage, SystemMessage, ToolMessage
 from langchain_core.tools import BaseTool
 from langgraph._internal._runnable import RunnableCallable
@@ -42,13 +41,15 @@
     ResponseFormat,
     StructuredOutputValidationError,
     ToolStrategy,
+    _supports_provider_strategy,
 )
 from langchain.chat_models import init_chat_model
 from langchain.tools.tool_node import ToolCallWithContext, _ToolNode
 
 if TYPE_CHECKING:
     from collections.abc import Awaitable, Callable, Sequence
 
+    from langchain_core.language_models.chat_models import BaseChatModel
     from langchain_core.runnables import Runnable
     from langgraph.cache.base import BaseCache
     from langgraph.graph.state import CompiledStateGraph
@@ -347,29 +348,6 @@ def _get_can_jump_to(middleware: AgentMiddleware[Any, Any], hook_name: str) -> l
     return []
 
 
-def _supports_provider_strategy(model: str | BaseChatModel) -> bool:
-    """Check if a model supports provider-specific structured output.
-
-    Args:
-        model: Model name string or `BaseChatModel` instance.
-
-    Returns:
-        `True` if the model supports provider-specific structured output, `False` otherwise.
-    """
-    model_name: str | None = None
-    if isinstance(model, str):
-        model_name = model
-    elif isinstance(model, BaseChatModel):
-        model_name = getattr(model, "model_name", None)
-
-    return (
-        "grok" in model_name.lower()
-        or any(part in model_name for part in ["gpt-5", "gpt-4.1", "gpt-oss", "o3-pro", "o3-mini"])
-        if model_name
-        else False
-    )
-
-
 def _handle_structured_output_error(
     exception: Exception,
     response_format: ResponseFormat,
@@ -932,16 +910,34 @@ def _get_bound_model(request: ModelRequest) -> tuple[Runnable, ResponseFormat |
 
         # Determine effective response format (auto-detect if needed)
         effective_response_format: ResponseFormat | None
+        model_name: str = cast(
+            "str",
+            (
+                request.model
+                if isinstance(request.model, str)
+                else getattr(request.model, "model_name", "")
+            ),
+        )
         if isinstance(request.response_format, AutoStrategy):
             # User provided raw schema via AutoStrategy - auto-detect best strategy based on model
-            if _supports_provider_strategy(request.model):
+            if _supports_provider_strategy(model_name):
                 # Model supports provider strategy - use it
                 effective_response_format = ProviderStrategy(schema=request.response_format.schema)
             else:
                 # Model doesn't support provider strategy - use ToolStrategy
                 effective_response_format = ToolStrategy(schema=request.response_format.schema)
+        elif isinstance(request.response_format, ProviderStrategy):
+            if not _supports_provider_strategy(model_name):
+                msg = (
+                    f"Cannot use ProviderStrategy with {model_name}. "
+                    "Supported models: OpenAI (gpt-5, gpt-4.1, gpt-oss, o3-pro, o3-mini), "
+                    "X.AI (Grok). "
+                    "Consider using a raw schema (which auto-selects the best strategy) or "
+                    "explicitly use `ToolStrategy` for unsupported providers."
+                )
+                raise ValueError(msg)
+            effective_response_format = request.response_format
         else:
-            # User explicitly specified a strategy - preserve it
             effective_response_format = request.response_format
 
         # Build final tools list including structured output tools
@@ -957,12 +953,9 @@ def _get_bound_model(request: ModelRequest) -> tuple[Runnable, ResponseFormat |
         if isinstance(effective_response_format, ProviderStrategy):
             # Use provider-specific structured output
             kwargs = effective_response_format.to_model_kwargs()
-            return (
-                request.model.bind_tools(
-                    final_tools, strict=True, **kwargs, **request.model_settings
-                ),
-                effective_response_format,
-            )
+            return request.model.bind_tools(
+                final_tools, **kwargs, **request.model_settings
+            ), effective_response_format
 
         if isinstance(effective_response_format, ToolStrategy):
             # Current implementation requires that tools used for structured output

libs/langchain_v1/langchain/agents/structured_output.py

@@ -31,6 +31,23 @@
 SchemaKind = Literal["pydantic", "dataclass", "typeddict", "json_schema"]
 
 
+def _supports_provider_strategy(model_name: str) -> bool:
+    """Check if a model supports provider-specific structured output.
+
+    Args:
+        model_name: Model name string.
+
+    Returns:
+        `True` if the model supports provider-specific structured output, `False` otherwise.
+    """
+    return (
+        "grok" in model_name.lower()
+        or any(part in model_name for part in ["gpt-5", "gpt-4.1", "gpt-oss", "o3-pro", "o3-mini"])
+        if model_name
+        else False
+    )
+
+
 class StructuredOutputError(Exception):
     """Base class for structured output errors."""
 
@@ -238,7 +255,56 @@ def _iter_variants(schema: Any) -> Iterable[Any]:
 
 @dataclass(init=False)
 class ProviderStrategy(Generic[SchemaT]):
-    """Use the model provider's native structured output method."""
+    """Use the model provider's native structured output method.
+
+    `ProviderStrategy` uses provider-specific structured output APIs that enforce
+    JSON schema validation at the model level. This provides stronger guarantees
+    than tool-based approaches but is only supported by certain providers.
+
+    Supported Providers:
+        - **OpenAI**: All models that support structured outputs (requires `strict=True`)
+        - **X.AI (Grok)**: All models that support structured outputs (requires `strict=True`)
+
+    Important:
+        When using `ProviderStrategy`, the agent will validate at runtime that the
+        model provider is supported. If you're using an unsupported provider, consider:
+
+        - Using a **raw schema** (recommended): Automatically selects the best strategy
+          based on model capabilities
+        - Using **`ToolStrategy`**: Explicitly use tool-based structured output for any
+          provider
+
+    Example:
+        ```python
+        from langchain.agents import create_agent
+        from langchain.agents.structured_output import ProviderStrategy
+        from pydantic import BaseModel
+
+
+        class WeatherResponse(BaseModel):
+            temperature: float
+            condition: str
+
+
+        # Explicitly use provider strategy (only for OpenAI/Grok)
+        agent = create_agent(
+            model="openai:gpt-4", tools=[], response_format=ProviderStrategy(WeatherResponse)
+        )
+
+        # Or use raw schema for automatic strategy selection (recommended)
+        # This will auto-select ProviderStrategy for OpenAI/Grok, ToolStrategy for others
+        agent = create_agent(
+            model="openai:gpt-4",
+            tools=[],
+            response_format=WeatherResponse,  # Auto-selects best strategy
+        )
+        ```
+
+    Note:
+        `ProviderStrategy` can be used with middleware that changes the model at runtime.
+        Validation occurs after the model is resolved, allowing dynamic model selection
+        while ensuring provider compatibility.
+    """
 
     schema: type[SchemaT]
     """Schema for native mode."""
@@ -255,17 +321,32 @@ def __init__(
         self.schema_spec = _SchemaSpec(schema)
 
     def to_model_kwargs(self) -> dict[str, Any]:
-        """Convert to kwargs to bind to a model to force structured output."""
-        # OpenAI:
-        # - see https://platform.openai.com/docs/guides/structured-outputs
+        """Convert to kwargs to bind to a model to force structured output.
+
+        Args:
+            model: The model instance to check provider for conditional `strict` param.
+
+        Returns:
+            Model kwargs with `response_format` and optionally `strict`.
+        """
+        # Provider-specific structured output:
+        # - OpenAI: https://platform.openai.com/docs/guides/structured-outputs
+        #   - Uses strict=True for schema validation
+        # - X.AI (Grok): https://docs.x.ai/docs/guides/structured-outputs
+        #   - Uses strict=True for schema validation (required)
         response_format = {
             "type": "json_schema",
             "json_schema": {
                 "name": self.schema_spec.name,
                 "schema": self.schema_spec.json_schema,
             },
         }
-        return {"response_format": response_format}
+
+        # Set strict=True for OpenAI and X.AI (Grok) models
+        # Both providers require strict=True for structured output
+        kwargs: dict[str, Any] = {"response_format": response_format, "strict": True}
+
+        return kwargs
 
 
 @dataclass

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

@@ -11,6 +11,7 @@
 
 from langchain_core.callbacks import CallbackManagerForLLMRun
 from langchain_core.language_models import BaseChatModel, LanguageModelInput
+from langchain_core.language_models.base import LangSmithParams
 from langchain_core.messages import (
     AIMessage,
     BaseMessage,
@@ -29,6 +30,7 @@ class FakeToolCallingModel(BaseChatModel, Generic[StructuredResponseT]):
     structured_response: StructuredResponseT | None = None
     index: int = 0
     tool_style: Literal["openai", "anthropic"] = "openai"
+    model_name: str = "fake-model"
 
     def _generate(
         self,

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

@@ -619,7 +619,9 @@ def test_pydantic_model(self) -> None:
         ]
 
         model = FakeToolCallingModel[WeatherBaseModel](
-            tool_calls=tool_calls, structured_response=EXPECTED_WEATHER_PYDANTIC
+            tool_calls=tool_calls,
+            structured_response=EXPECTED_WEATHER_PYDANTIC,
+            model_name="gpt-4.1",
         )
 
         agent = create_agent(
@@ -637,7 +639,9 @@ def test_dataclass(self) -> None:
         ]
 
         model = FakeToolCallingModel[WeatherDataclass](
-            tool_calls=tool_calls, structured_response=EXPECTED_WEATHER_DATACLASS
+            tool_calls=tool_calls,
+            structured_response=EXPECTED_WEATHER_DATACLASS,
+            model_name="gpt-4.1",
         )
 
         agent = create_agent(
@@ -657,7 +661,7 @@ def test_typed_dict(self) -> None:
         ]
 
         model = FakeToolCallingModel[WeatherTypedDict](
-            tool_calls=tool_calls, structured_response=EXPECTED_WEATHER_DICT
+            tool_calls=tool_calls, structured_response=EXPECTED_WEATHER_DICT, model_name="gpt-4.1"
         )
 
         agent = create_agent(
@@ -675,7 +679,7 @@ def test_json_schema(self) -> None:
         ]
 
         model = FakeToolCallingModel[dict](
-            tool_calls=tool_calls, structured_response=EXPECTED_WEATHER_DICT
+            tool_calls=tool_calls, structured_response=EXPECTED_WEATHER_DICT, model_name="gpt-4.1"
         )
 
         agent = create_agent(
@@ -697,13 +701,13 @@ def test_middleware_model_swap_provider_to_tool_strategy(self) -> None:
         on the middleware-modified model (not the original), ensuring the correct strategy is
         selected based on the final model's capabilities.
         """
-        from unittest.mock import patch
         from langchain.agents.middleware.types import AgentMiddleware, ModelRequest
         from langchain_core.language_models.fake_chat_models import GenericFakeChatModel
 
-        # Custom model that we'll use to test whether the tool strategy is applied
-        # correctly at runtime.
+        # Custom model that we'll use to test whether the provider strategy is applied
+        # correctly at runtime. Use a model_name that supports provider strategy.
         class CustomModel(GenericFakeChatModel):
+            model_name: str = "gpt-4.1"
             tool_bindings: list[Any] = []
 
             def bind_tools(
@@ -736,14 +740,6 @@ def wrap_model_call(
                 request.model = model
                 return handler(request)
 
-        # Track which model is checked for provider strategy support
-        calls = []
-
-        def mock_supports_provider_strategy(model) -> bool:
-            """Track which model is checked and return True for ProviderStrategy."""
-            calls.append(model)
-            return True
-
         # Use raw Pydantic model (not wrapped in ToolStrategy or ProviderStrategy)
         # This should auto-detect strategy based on model capabilities
         agent = create_agent(
@@ -754,14 +750,7 @@ def mock_supports_provider_strategy(model) -> bool:
             middleware=[ModelSwappingMiddleware()],
         )
 
-        with patch(
-            "langchain.agents.factory._supports_provider_strategy",
-            side_effect=mock_supports_provider_strategy,
-        ):
-            response = agent.invoke({"messages": [HumanMessage("What's the weather?")]})
-
-        # Verify strategy resolution was deferred: check was called once during _get_bound_model
-        assert len(calls) == 1
+        response = agent.invoke({"messages": [HumanMessage("What's the weather?")]})
 
         # Verify successful parsing of JSON as structured output via ProviderStrategy
         assert response["structured_response"] == EXPECTED_WEATHER_PYDANTIC