langchain-ai/langgraph · error · TypeError

A future is required for source argument

Error message

A future is required for source argument

What it means

Error "A future is required for source argument" thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/_internal/_future.py:92

        exception = source.exception()
        if exception is not None:
            dest.set_exception(_convert_future_exc(exception))
        else:
            result = source.result()
            dest.set_result(result)


def _chain_future(source: AnyFuture, destination: AnyFuture) -> None:
    """Chain two futures so that when one completes, so does the other.

    The result (or exception) of source will be copied to destination.
    If destination is cancelled, source gets cancelled too.
    Compatible with both asyncio.Future and concurrent.futures.Future.
    """
    if not asyncio.isfuture(source) and not isinstance(
        source, concurrent.futures.Future
    ):
        raise TypeError("A future is required for source argument")
    if not asyncio.isfuture(destination) and not isinstance(
        destination, concurrent.futures.Future
    ):
        raise TypeError("A future is required for destination argument")
    source_loop = _get_loop(source) if asyncio.isfuture(source) else None
    dest_loop = _get_loop(destination) if asyncio.isfuture(destination) else None

    def _set_state(future: AnyFuture, other: AnyFuture) -> None:
        if asyncio.isfuture(future):
            _copy_future_state(other, future)
        else:
            _set_concurrent_future_state(future, other)

    def _call_check_cancel(destination: AnyFuture) -> None:
        if destination.cancelled():
            if source_loop is None or source_loop is dest_loop:
                source.cancel()
            else:

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/_internal/_future.py:92 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/dc42ab76d5556bbd. Report an issue: GitHub.