I’ve read where at least some agents accept queries on proposal for an unfinished manuscript when the author’s previous books are printed in the traditional print-run process. I did not see this addressed on your website and would appreciate knowing if this is something your agency ever considers.
Typically, if an author has been previously published from one of the bigger houses it is quite possible to sell the next work on proposal, without a full manuscript. In those cases the author will not need to finish the book before querying an agent. A proposal should be enough.
Now, that being said, of course there are exceptions to the “rule.” This will depend on the agent and agency. Everyone is different. It will also depend on the genre you were published under and the genre you are now writing in. For example, if you were previously published in category romance and now want to write women’s fiction, it is quite possible you’ll need to finish the manuscript before seeking publication.
To answer your question specifically, when querying BookEnds, if you are previously published, a proposal should be enough.