Just wrapped up the Block SWE loop (L4 role, Cash App org). Writing this because I couldn't find good info on their current online assessment format when I was prepping.
The OA came through HackerRank. Two problems, 90 minutes. From what I saw:
Problem 1 was a medium-difficulty graph or tree problem. Mine was a variant of finding connected components in a graph where edges had weights. Not hard if you know BFS/DFS, but the constraints were tight enough that a naive O(n^2) would TLE. I used union-find and it passed.
Problem 2 was harder. String manipulation with some DP. I recognized it as a pattern close to edit distance but with different cost rules. I got a working brute force first and then tried to optimize. Partially passed the hidden test cases. Not full marks.
Important: they do NOT give you a lot of runtime feedback on the hidden tests. You submit and mostly have to trust your analysis.
After the OA comes a recruiter screen, then a technical phone screen (also coding, 45 min, one problem, LeetCode medium), and then the onsite. So there's actually two coding checkpoints before you even get to onsite.
The phone screen problem I got was array manipulation, classic sliding window. The interviewer was a senior SWE, not an automated tool. They asked one or two brief follow-up questions after I solved it: 'what's the space complexity' and 'could you do it with less memory.' Pretty standard.
Overall difficulty of coding at Block: I'd put it at Leetcode medium-hard for senior roles. Not the hard-core Google style where you need to invent an algorithm on the spot. But they'll catch you if you can only do easy problems.