Okay so I just did the Citadel online assessment for the new grad SWE track and I have notes because I was desperately searching for this information beforehand and couldn't find much.
Format: Two coding problems, 90 minutes total, HackerRank platform. Timing is strict.
Difficulty: Both problems were what I'd call LeetCode hard in terms of the thinking required, but they weren't trick questions. One was a graph problem that needed BFS with some state tracking. The other involved dynamic programming on intervals. Both had financial flavor in the problem statement (portfolio optimization framing, order book matching framing) but you could solve them without knowing anything about markets.
The inputs were large, so brute force will TLE. You need to think about complexity before you start coding.
One thing I didn't expect: There was a short-answer section too. Three questions, roughly: explain your approach to a recent complex technical problem, describe a time you disagreed with a team decision, and something about handling ambiguous requirements. These are not throwaway. People I know who got moved forward said they spent real time on the short answers.
What I'd tell past-me: Practice LC hard graphs and DP, specifically on problems with non-obvious state definitions. Also actually write out your short answers before the test. I winged mine and I think it hurt me.
I'm still waiting to hear back. Will update this thread when I do.