langchain/tools/base.py/_make_tool
def tool(
    *args: Union[str, Callable],
    return_direct: bool = False,
    args_schema: Optional[Type[BaseModel]] = None,
    infer_schema: bool = True,
) -> Callable:
    """Make tools out of functions, can be used with or without arguments.

    Args:
        *args: The arguments to the tool.
        return_direct: Whether to return directly from the tool rather
            than continuing the agent loop.
        args_schema: optional argument schema for user to specify
        infer_schema: Whether to infer the schema of the arguments from
            the function's signature. This also makes the resultant tool
            accept a dictionary input to its `run()` function.

    Requires:
        - Function must be of type (str) -> str
        - Function must have a docstring

    Examples:
        .. code-block:: python

            @tool
            def search_api(query: str) -> str:
                # Searches the API for the query.
                return

            @tool("search", return_direct=True)
            def search_api(query: str) -> str:
                # Searches the API for the query.
                return
    """

    def _make_with_name(tool_name: str) -> Callable:
        def _make_tool(func: Callable) -> BaseTool:
            if infer_schema or args_schema is not None:
                return StructuredTool.from_function(
                    func,
                    name=tool_name,
                    return_direct=return_direct,
                    args_schema=args_schema,
                    infer_schema=infer_schema,
                )
            # If someone doesn't want a schema applied, we must treat it as
            # a simple string->string function
            assert func.__doc__ is not None, "Function must have a docstring"
            return Tool(
                name=tool_name,
                func=func,
                description=f"{tool_name} tool",
                return_direct=return_direct,
            )

        return _make_tool

    if len(args) == 1 and isinstance(args[0], str):
        # if the argument is a string, then we use the string as the tool name
        # Example usage: @tool("search", return_direct=True)
        return _make_with_name(args[0])
    elif len(args) == 1 and callable(args[0]):
        # if the argument is a function, then we use the function name as the tool name
        # Example usage: @tool
        return _make_with_name(args[0].__name__)(args[0])
    elif len(args) == 0:
        # if there are no arguments, then we use the function name as the tool name
        # Example usage: @tool(return_direct=True)
        def _partial(func: Callable[[str], str]) -> BaseTool:
            return _make_with_name(func.__name__)(func)

        return _partial
    else:
        raise ValueError("Too many arguments for tool decorator")