langchain-ai/langgraph · error · NotImplementedError

When specifying __root__ no other fields should be provided.

Error message

When specifying __root__ no other fields should be provided. Got {field_definitions}

What it means

Error "When specifying __root__ no other fields should be provided. Got {field_definitions}" thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/_internal/_pydantic.py:211

    Args:
        model_name: The name of the model.
        module_name: The name of the module where the model is defined.
            This is used by Pydantic to resolve any forward references.
        field_definitions: The field definitions for the model.
        root: Type for a root model (RootModel)

    Returns:
        Type[BaseModel]: The created model.
    """
    field_definitions = field_definitions or {}

    if root:
        if field_definitions:
            msg = (
                "When specifying __root__ no other "
                f"fields should be provided. Got {field_definitions}"
            )
            raise NotImplementedError(msg)

        if isinstance(root, tuple):
            kwargs = {"type_": root[0], "default_": root[1]}
        else:
            kwargs = {"type_": root}

        try:
            named_root_model = _create_root_model_cached(model_name, **kwargs)
        except TypeError:
            # something in the arguments into _create_root_model_cached is not hashable
            named_root_model = _create_root_model(
                model_name,
                **kwargs,
            )
        return named_root_model

    # No root, just field definitions
    names = set(field_definitions.keys())

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/_internal/_pydantic.py:211 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/055fbd98659436dc. Report an issue: GitHub.