I have the same question and feel it is still not answered here.
My business requirement is: show top 5 threads that are relevant to a given search string.
That is, either "root message" or whatever other message in the thread contains the search string.
The top 5 threads shown should be unique (no duplication).
Also, for each thread shown I need to show some metadata (number of messages in the thread, last post date and user, etc).
So far the closest API method I've found is like:
/search/messages?q=my+search+string&collapse_discussion=true
but it returns messages separately (even if they are in the same thread) and the collapse_discussion parameters seems not working (no change in the result).