langchain-ai/langgraph · error · ValueError

Unsupported input to ValidationNode. Expected BaseModel, too

Error message

Unsupported input to ValidationNode. Expected BaseModel, tool or function. Got: {type(schema)}.

What it means

Error "Unsupported input to ValidationNode. Expected BaseModel, tool or function. Got: {type(schema)}." thrown in langchain-ai/langgraph.

Source

Thrown at libs/prebuilt/langgraph/prebuilt/tool_validator.py:164

                        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]
    ) -> tuple[str, AIMessage]:
        """Extract the last AIMessage from the input."""
        if isinstance(input, list):
            output_type = "list"
            messages: list = input
        elif messages := input.get("messages", []):
            output_type = "dict"
        else:
            raise ValueError("No message found in input")
        message: AnyMessage = messages[-1]
        if not isinstance(message, AIMessage):
            raise ValueError("Last message is not an AIMessage")
        return output_type, message

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/prebuilt/langgraph/prebuilt/tool_validator.py:164 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/7c89d04c0282e013. Report an issue: GitHub.