langchain-ai/langgraph · error · ValueError

At least one of func or afunc must be provided.

Error message

At least one of func or afunc must be provided.

What it means

Constructor guard for the internal RunnableCallable: neither a sync func nor an async afunc was supplied, so the callable would have nothing to invoke. This fires when code constructs the wrapper with no function at all — a programming error in how the runnable is instantiated, fixed by passing at least one callable.

Solutions

1. Provide a sync function, an async function, or both when constructing the runnable.
2. Check upstream factory code that may pass None callables.

When it happens

Trigger: Thrown at libs/langgraph/langgraph/_internal/_runnable.py:336 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/caafc61536a92870. Report an issue: GitHub.

Appendix: source

Thrown at libs/langgraph/langgraph/_internal/_runnable.py:336

                    if func.__name__ != "<lambda>":
                        self.name = func.__name__
                except AttributeError:
                    pass
            elif afunc:
                try:
                    self.name = afunc.__name__
                except AttributeError:
                    pass
        self.func = func
        self.afunc = afunc
        self.tags = tags
        self.kwargs = kwargs
        self.trace = trace
        self.recurse = recurse
        self.explode_args = explode_args
        # check signature
        if func is None and afunc is None:
            raise ValueError("At least one of func or afunc must be provided.")

        self.func_accepts: dict[str, tuple[str, Any]] = {}
        params = inspect.signature(cast(Callable, func or afunc)).parameters

        for kw, typ, runtime_key, default in KWARGS_CONFIG_KEYS:
            p = params.get(kw)

            if p is None or p.kind not in VALID_KINDS:
                # If parameter is not found or is not a valid kind, skip
                continue

            if typ != (ANY_TYPE,) and p.annotation not in typ:
                # A specific type is required, but the function annotation does
                # not match the expected type.

                # If this is a config parameter with incorrect typing, emit a warning
                # because we used to support any type but are moving towards more correct typing
                if kw == "config" and p.annotation != inspect.Parameter.empty:

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