langchain-ai/langgraph · error · ValueError

Validation node only works with tools that have a pydantic B

Error message

Validation node only works with tools that have a pydantic BaseModel args_schema. Got {schema.name} with args_schema: {schema.args_schema}.

What it means

Error "Validation node only works with tools that have a pydantic BaseModel args_schema. Got {schema.name} with args_schema: {schema.args_schema}." thrown in langchain-ai/langgraph.

Source

Thrown at libs/prebuilt/langgraph/prebuilt/tool_validator.py:151

            format_error: A function that takes an exception, a ToolCall, and a schema
                and returns a formatted error string. By default, it returns the
                exception repr and a message to respond after fixing validation errors.
            name: The name of the node.
            tags: A list of tags to add to the node.
        """
        super().__init__(self._func, None, name=name, tags=tags, trace=False)
        self._format_error = format_error or _default_format_error
        self.schemas_by_name: dict[str, type[BaseModel]] = {}
        for schema in schemas:
            if isinstance(schema, BaseTool):
                if schema.args_schema is None:
                    raise ValueError(
                        f"Tool {schema.name} does not have an args_schema defined."
                    )
                elif not isinstance(
                    schema.args_schema, type
                ) or not is_basemodel_subclass(schema.args_schema):
                    raise ValueError(
                        "Validation node only works with tools that have a pydantic BaseModel args_schema. "
                        f"Got {schema.name} with args_schema: {schema.args_schema}."
                    )
                self.schemas_by_name[schema.name] = schema.args_schema
            elif isinstance(schema, type) and issubclass(
                schema, (BaseModel, BaseModelV1)
            ):
                self.schemas_by_name[schema.__name__] = cast(type[BaseModel], schema)
            elif callable(schema):
                base_model = create_schema_from_function("Validation", schema)
                self.schemas_by_name[schema.__name__] = base_model
            else:
                raise ValueError(
                    f"Unsupported input to ValidationNode. Expected BaseModel, tool or function. Got: {type(schema)}."
                )

    def _get_message(
        self, input: list[AnyMessage] | dict[str, Any]

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/prebuilt/langgraph/prebuilt/tool_validator.py:151 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of langchain-ai/langgraph@38031739e5 (2026-08-26). Data as JSON: /api/errors/d8d15682663bf51a. Report an issue: GitHub.