In a Query, do you tell the agent you are querying others?

Most agents assume you are submitting to multiple agents, however, there are a few agents that want to be informed.
Go by what their guidlines say on their website. If the ask to be informed just include a small sentence at the end of your query stating that you are submitting to multiple agents. Short and sweet.

Well a query is different than submitting a manuscript. I know that may seem like an annoying technicality and why am I mentioning that, but it will matter in the future if an agent asks if you have submitted a manuscript to anyone and they want an exclusive. They will not be referring to queries.

So I hope you are submitting queries and not manuscripts. Agents hate unsolicited manuscripts and they will likely toss your email or package. Research each agent to see what they want. Some want a query letter plus 5 pages, 10 pages, 50 pages and many want only your query letter at first.

It is assumed that you will query many agents. Considering the fact that many authors on here say they sent 125 to 250 queries before they got their agent, then if you queried one at a time, you'd probably never get one. Agents get hundreds of submissions, choose a handful to request partials from and then may choose one or none to represent. So it's largely a numbers game.

I would not put that in my query.

Also, when I've gotten partial or full requests to see my manuscript, I didn't mention that other agents were looking at it unless they asked for an exclusive. I only had one out of 9 ask for an exclusive. It was already being looked at by another agent and she was okay with that.

What Alisha said. I've seen this addressed by blogging agents before, and the assumption is that you're sending out multiple queries. It's generally not until the partial/full stage that they start wondering who they might be competing with.