Spring-ai Function Calling fails for registered functions without arguments (at least for OpenAI)

Bug description OpenAI and Azure OpenAI doesn't accept function definitions without "properties" parameter.

This example with a Request object without parameter produces the following error as the "properties" parameter in the request body is missing.

@Description("Fetches ingredients")
@Service("myFunction")
public class MyFunction implements Function<MyFunction.Request, MyFunction.Response> {
    public MyFunction() { }
    @Override
    public Response apply(Request request) {
        return new Response(List.of("Bacon"));
    }
    public record Request() {}
    public record Response(List<String> ingredients) {}
}

 com.azure.core.exception.HttpResponseException: Status code 400, "{
  "error": {
    "message": "Invalid schema for function 'fetchIngredientsAvailableAtHome': In context=(), object schema missing properties.",
    "type": "invalid_request_error",
    "param": "tools[0].function.parameters",
    "code": "invalid_function_parameters"
  }
}"]

tools:
- type: FUNCTION
   function: {
    "name": "myFunction",
    "description": "Fetches ingredients",
    "parameters": {
      "$schema": "..."
      "type": "object"
    }
  }

This example works as expected as it adds a "properties" parameter with empty value to the request body.

private Recipe fetchRecipeWithFunctionCallingFor(List<String> ingredients) {
        return chatClient.prompt()
                .user(...)
                .functions(FunctionCallback.builder()
                        .description("Fetches ingredients")
                        .function("myFunction", () -> List.of("Bacon"))
                        .build())
                .call()
                .entity(Recipe.class);
}

tools:
- type: FUNCTION
   function: {
    "name": "myFunction",
    "description": "Fetches ingredients",
    "parameters": {
      "$schema": "..."
      "type": "object"
      "properties": {},
    }
  }

Additionally, I don't see a way to define a function without arguments via the MethodInvokingSpec method(String methodName, Class<?>... argumentTypes); in FunctionCallback.Builder

Environment Spring AI M3/M4, Java 21

Steps to reproduce See examples

Expected behavior It should be supported to use functions without arguments in Function Calling

Comment From: tzolov

@timosalm since the Record is empty shouldn't you define your function like: Function<Void, MyFunction.Response> or even better Supplier<MyFunction.Response> instead?

Comment From: tzolov

Additionally, I don't see a way to define a function without arguments via the MethodInvokingSpec method(String methodName, Class<?>... argumentTypes); in FunctionCallback.Builder

The argumentTypes is java varargs so you can decide to not provide any arguments (e.g. without arguments) like this: .method("methodName")