Agent A requests a partial. Likes it, but is not enthusiastic enough to offer representation. You go on to write another book. When sending the query about the new book off to Agent A would it be ok to mention that they prevously requested a partial on your other novel or would it be better to just leave that out completely?
I'd leave it off, UNLESS the agent specifically said "let me see something else you've got". Then you say "you asked to see other things, here's my new novel". Do NOT say "you rejected me" "in your rejection letter" or anything else that sounds like rejection, pass, or later gator.
It's not like a magic incantation where if you say the wrong words you conjure up the Snark, but it's a mind set. You don't mention efforts that didn't succeed; you focus on the work at hand. It's just presenting yourself as fresh, vibrant, and positive. There's time enough to have a good laugh that it took multiple queries for your agent to recognize your merit. The best time to mention it is the drinks celebration when she sells your book.