Do you require a complete manuscript to be considered?
So, the short answer is yes, if you are seeking representation as a novelist, you need a finished manuscript to be considered with very few exceptions. (E.g. – mg, ya or adult non-fiction, in which case you’d need a complete proposal that includes three sample chapters.)
Before you query an agent, you should have a complete and fully revised manuscript that’s already been seen by your critique partners and beta readers. Perhaps you paid for a professional critique (or two…) for the manuscript and/or your query letter and synopsis.
But most of us have novels that we started and lost steam for half way through, and so for a number of reasons, an agent just can’t take on a writer in the hope that they will be able to complete and polish the manuscript up to publishable standards.