microsoft/autogen · error · ValueError

Invalid arguments

Error message

Invalid arguments

What it means

The @message_handler entry function dispatches on how it was invoked: bare decorator usage (callable func), parenthesized usage with keyword options (func is None), or something else. If a non-callable, non-None value is passed (or a malformed combination), ValueError('Invalid arguments') is raised. This is a usage error in decorator application, not in handler logic.

Source

Thrown at python/packages/autogen-core/src/autogen_core/_routed_agent.py:172

                else:
                    logger.warning(f"Return type {type(return_value)} not in return types {return_types}")

            return return_value

        wrapper_handler = cast(MessageHandler[AgentT, ReceivesT, ProducesT], wrapper)
        wrapper_handler.target_types = list(target_types)
        wrapper_handler.produces_types = list(return_types)
        wrapper_handler.is_message_handler = True
        wrapper_handler.router = match or (lambda _message, _ctx: True)

        return wrapper_handler

    if func is None and not callable(func):
        return decorator
    elif callable(func):
        return decorator(func)
    else:
        raise ValueError("Invalid arguments")


@overload
def event(
    func: Callable[[AgentT, ReceivesT, MessageContext], Coroutine[Any, Any, None]],
) -> MessageHandler[AgentT, ReceivesT, None]: ...


@overload
def event(
    func: None = None,
    *,
    match: None = ...,
    strict: bool = ...,
) -> Callable[
    [Callable[[AgentT, ReceivesT, MessageContext], Coroutine[Any, Any, None]]],
    MessageHandler[AgentT, ReceivesT, None],
]: ...

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Use the decorator correctly: @message_handler above the method, or @message_handler(strict=False, match=...) with keyword arguments
2. Never pass positional arguments other than the decorated function itself
3. If applying decorators dynamically, pass the function object: handler = message_handler(strict=True)(func)

Example fix

# before
@message_handler(True)
async def handle(self, message: Msg, ctx: MessageContext) -> None: ...

# after
@message_handler(strict=True)
async def handle(self, message: Msg, ctx: MessageContext) -> None: ...

When it happens

Trigger: Calling message_handler(some_object) where some_object is not callable, e.g. message_handler(True), message_handler('handler'), or accidentally passing the result of calling the handler; also reachable via exotic double-application like @message_handler()(func) patterns with wrong arity.

Common situations: Typo such as @message_handler(strict) passing the boolean as a positional argument instead of strict=True; programmatically applying decorators via functools.partial or loops and passing the wrong value; copy-paste from a parenthesized example dropping the '='.

Related errors

• Message type not found
• Return type not found
• Message type not found. Please provide a type hint for the m
• Return type not found. Please use `None` as the type hint of
• SubscriptionInstantiationContext cannot be instantiated. It

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/09011383cd5ab652. Report an issue: GitHub.