get-posts-stateless - how to get 1st level comments?

There is a 'FetchSubComments' flag to set (which does not seem to work, maybe it is dependent on other flags?).

I see no top level comments option for this api call. I need to get all of 1st level comments for each post without having to call the 'get-single-post' api. is this possible?

Start by tracing the get-posts-stateless API and the FetchSubComments flag, then compare its behavior with the get-single-post API. Determine whether first-level comments can be returned per post without additional calls; done means the requested comments are available through get-posts-stateless and the flag behavior is clear.

Written by the indexing model from the issue text.