langchain-ai/langgraph · error · NotImplementedError

Generators are not supported in the Functional API.

Error message

Generators are not supported in the Functional API.

What it means

Error "Generators are not supported in the Functional API." thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/func/__init__.py:527

        """Value to return. A value will always be returned even if it is `None`."""
        save: S
        """The value for the state for the next checkpoint.

        A value will always be saved even if it is `None`.
        """

    def __call__(self, func: Callable[..., Any]) -> Pregel:
        """Convert a function into a Pregel graph.

        Args:
            func: The function to convert. Support both sync and async functions.

        Returns:
            A Pregel graph.
        """
        # wrap generators in a function that writes to StreamWriter
        if inspect.isgeneratorfunction(func) or inspect.isasyncgenfunction(func):
            raise NotImplementedError(
                "Generators are not supported in the Functional API."
            )

        bound = get_runnable_for_entrypoint(func)
        stream_mode: StreamMode = "updates"

        # get input and output types
        sig = inspect.signature(func)
        first_parameter_name = next(iter(sig.parameters.keys()), None)
        if not first_parameter_name:
            raise ValueError("Entrypoint function must have at least one parameter")
        input_type = (
            sig.parameters[first_parameter_name].annotation
            if sig.parameters[first_parameter_name].annotation
            is not inspect.Signature.empty
            else Any
        )

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/func/__init__.py:527 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/981953c618e7187e. Report an issue: GitHub.