Maybe the challenge was that it only looks at the first tag(?)
I think so. Another one could be getCommentsByTagsText. You could use "" for the search parameter.
Example: https://sds0.steemworld.org/content_search_api/getCommentsByTagsText/steem,book/%22%22
When checkin the returned data, I noticed that some comments have included the tags of the parent post in the metadata (the app ‘upvu’ is mostly entered in the metadata). This is why these are also returned in the tag search. This would actually also be a good suggestion for the Condenser.
someone uses that tag in a top-level post then I want to get notified of the post and all replies below the post
You probably won't be able to get them reliably directly. But if you check all comments live, it is possible. In SDS, the comments also contain the root permlink (I don't know whether this is also the case with the Steem API). This would then require an additional query for each comment.
The Steem API has the root author and permlink, too. That's how I was thinking to get it. It would be nice if I could find a way to avoid that second query, but (without secondary storage), I don't think there's a reliable way. As you noted.
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