I got my first query letter out the door with a synopsis and the first three chapters. I realize that the first version of everything I send in this regard will likely be the least polished and/or worst but I’ve decided to start with one of the most experienced/high profile agencies I could find that had someone who wanted sci-fi. I expect many, many rejections - another would-be author on twitter mentioned that he’d racked up 80 rejections. And even if someone accepts it, I’ll then have a 2-3 year road to it seeing the light of day.
But it's a first step on the road.