Telling the ending isn't always the problem, sometimes a book still works even when you know how it ends. Giving away the whole plot in the prompt is a different thing though, so which one was it?
Does it count as spoiling if the person doing the spoiling is me, because I read the last page first every single time? My friends think I'm a monster for it, but I've never once had a book ruined by knowing who lives or who ends up with who. The journey is what gets me, the little moments where you can tell the author really cared about these people. Now if someone hands me a prompt that lays out every single turn of the plot, that's a whole different beast. There's nothing left to discover at that point, you're just checking boxes off a list. So for me, ending spoilers are fine, but full plot dumps can take a hike.