langchain-ai/langgraph · error · NotImplementedError

Remapping for fields starting with '_' or fields with a name

Error message

Remapping for fields starting with '_' or fields with a name matching a reserved name {_RESERVED_NAMES} is not supported if  the field is a pydantic Field instance. Got {key}.

What it means

Error "Remapping for fields starting with '_' or fields with a name matching a reserved name {_RESERVED_NAMES} is not supported if the field is a pydantic Field instance. Got {key}." thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/_internal/_pydantic.py:165

# "model_post_init", "model_rebuild", "model_validate", "model_validate_json",
# "model_validate_strings"
_RESERVED_NAMES = {key for key in dir(BaseModel) if not key.startswith("_")}


def _remap_field_definitions(field_definitions: dict[str, Any]) -> dict[str, Any]:
    """This remaps fields to avoid colliding with internal pydantic fields."""

    remapped = {}
    for key, value in field_definitions.items():
        if key.startswith("_") or key in _RESERVED_NAMES:
            # Let's add a prefix to avoid colliding with internal pydantic fields
            if isinstance(value, FieldInfo):
                msg = (
                    f"Remapping for fields starting with '_' or fields with a name "
                    f"matching a reserved name {_RESERVED_NAMES} is not supported if "
                    f" the field is a pydantic Field instance. Got {key}."
                )
                raise NotImplementedError(msg)
            type_, default_ = value
            remapped[f"private_{key}"] = (
                type_,
                Field(
                    default=default_,
                    alias=key,
                    serialization_alias=key,
                    title=key.lstrip("_").replace("_", " ").title(),
                ),
            )
        else:
            remapped[key] = value
    return remapped


def create_model(
    model_name: str,
    *,

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/_internal/_pydantic.py:165 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/4d97fc12076b6b1d. Report an issue: GitHub.