langchain-ai/langgraph · error · ValueError

Entrypoint function must have at least one parameter

Error message

Entrypoint function must have at least one parameter

What it means

Error "Entrypoint function must have at least one parameter" thrown in langchain-ai/langgraph.

Source

Thrown at libs/langgraph/langgraph/func/__init__.py:538

            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
        )

        def _pluck_return_value(value: Any) -> Any:
            """Extract the return_ value the entrypoint.final object or passthrough."""
            return value.value if isinstance(value, entrypoint.final) else value

        def _pluck_save_value(value: Any) -> Any:
            """Get save value from the entrypoint.final object or passthrough."""
            return value.save if isinstance(value, entrypoint.final) else value

        output_type, save_type = Any, Any
        if sig.return_annotation is not inspect.Signature.empty:
            # User does not parameterize entrypoint.final properly

View on GitHub (pinned to 38031739e5)

When it happens

Trigger: Thrown at libs/langgraph/langgraph/func/__init__.py:538 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/6daedb96808ae3fa. Report an issue: GitHub.