How/when to tell a new agent you fired your last agent

In my opinion, you should tell them once they've requested your full ms, and include a list of all the places it's been to at that time.

Also in my opinion: don't mention a whisper of it in your query. Even if you think you're being honest, you're just giving the prospective agent another data point on the 'too much trouble to request, let alone rep' scale.

This one I know from recent and personal experience. Only be honest when they're already wowed with the mms.

I have a different experience. I also fired my previous agent, and in my queries I wrote, "I was previously represented by Agent Name, with whom I have amicably parted ways." I did not mention whether tit was me or my former agent who terminated the relationship, but it didn't seem to hurt my request rate at all. I ended up with multiple offers, and all of the offering agents asked for details during the call, at which point I was honest about the reasons I had for firing the previous agent. It was important for me to find a new agent who saw eye-to-eye with me about the things my last agent and I disagreed on, so I think honesty (but of course being polite... I really DO think the world of my former agent but even if you don't, still be nice!) was the best option.

Hikarinotsubasa, when you queried new agents, was it for the same manuscript the prior agent had repped? I'm wondering if that affects the interest level at the query stage or not.

I lost my first agent when he left the business, and I mentioned it in my new query. (The manuscript had supposedly been shopped to four places.) I seemed to get a good response and full request rate from that approach. I wrote something similar to Hikarinotsubasa in my query because I was worried an agent might be mad if I withheld that info. Sounds like that's not necessarily the case?

I didn't bring it up till I reached the phone-call stage with agents (and yes, then I was open about why I'd terminated my relationship with agent #1.) Since agent #1 had sold books for me, I figured it was sort of public record that I'd had previous representation, and so saved the space in my query letter for actual stuff about the book being queried. Honestly, I don't think there's a wrong or right way, so long as you disclose to an interested agent whether or not a manuscript has been shopped before signing with him or her.

I have done it both ways: queried a manuscript that had been represented and not submitted, and one that my former agent had not even read. I ended up finding new representation for the new one. I personally haven't queried one that has been on submission, but I'd definitely mention that too. It'll ultimately be the agent's call whether they have a decent chance of selling a book that has been subbed to 4 or 14 or 40 publishers, and if I were an agent I'd want to have that information when deciding whether to offer rep.

To make things a little clearer, I love my ex agent, I'm just writing things she doesn't feel comfortable representing. Amicable split. But I did notice a clear difference in agent reaction this year, when I mentioned that I'd parted with that previous agent...and when I didn't mention the relationship in the query. I'm no longer querying that mms, pending a major revision. None of the five mms I'm working on now are suitable for or require agent rep. I might pick up the agent quest next summer with a new project.

I'd make my past clear if an interested agent asks, but I'm never again going to put it in a query.