run-llama/llama_index · error · ValueError

fn or async_fn must be provided.

Error message

fn or async_fn must be provided.

What it means

Error "fn or async_fn must be provided." thrown in run-llama/llama_index.

Source

Thrown at llama-index-core/llama_index/core/tools/function_tool.py:90

    """
    Function Tool.

    A tool that takes in a function, optionally handles workflow context,
    and allows the use of callbacks. The callback can return a new ToolOutput
    to override the default one or a string that will be used as the final content.
    """

    def __init__(
        self,
        fn: Optional[Callable[..., Any]] = None,
        metadata: Optional[ToolMetadata] = None,
        async_fn: Optional[AsyncCallable] = None,
        callback: Optional[Callable[..., Any]] = None,
        async_callback: Optional[Callable[..., Any]] = None,
        partial_params: Optional[Dict[str, Any]] = None,
    ) -> None:
        if fn is None and async_fn is None:
            raise ValueError("fn or async_fn must be provided.")

        # Handle function (sync and async)
        self._real_fn = fn or async_fn
        if async_fn is not None:
            self._async_fn = async_fn
            self._fn = fn or async_to_sync(async_fn)
        else:
            assert fn is not None
            if inspect.iscoroutinefunction(fn):
                self._async_fn = fn
                self._fn = async_to_sync(fn)
            else:
                self._fn = fn
                self._async_fn = sync_to_async(fn)

        # Determine if the function requires context by inspecting its signature
        fn_to_inspect = fn or async_fn
        assert fn_to_inspect is not None

View on GitHub (pinned to afd0fef371)

Solutions

1. Provide fn (or async_fn) when constructing the FunctionTool.
2. Use FunctionTool.from_defaults(fn=my_function).

When it happens

Trigger: Thrown at llama-index-core/llama_index/core/tools/function_tool.py:90 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of run-llama/llama_index@afd0fef371 (2026-08-15). Data as JSON: /api/errors/25013059df89b19d. Report an issue: GitHub.