As an agent I am not there to overhaul your manuscript in an extensive editing process. My job is to get it in the best shape I can for submission to editors, but substantial editing is not in my job description because agents don’t have time for it and not all agents are trained editors.
I edit my client’s work to improve the novel’s structure, flush out characters, procure consistency, ask the right questions to get the novel to where it needs to go–as well as copyedit and proofread–before we take it to acquiring editors.
Don’t submit to an agent thinking they’ll solve the problems in your manuscript, that’s not our job. If that’s what you are looking for you need a freelance editor, not an agent. You only have one chance to make a first impression so don’t spend it on a manuscript that hasn’t been carefully edited.
Agents do take on projects that need work, but that agent needs to be able to imagine the finished product down the line to know it can get there–and know that the writer has the chops.
Publishing is a business so editing takes time away from selling, but business is also about relationship building which is an important part of the agent-author partnership.
So give agents a reason to invest their time! They have to love it and want to go through this process with you.