Just finished my Square SWE loop (backend, mid-senior level). Sharing because I couldn't find much recent info when I was prepping.
Rounds: Recruiter screen, 20 min, pretty standard. Asked about comp expectations early, which I appreciated. Technical phone screen with an engineer. One medium-ish graph problem. Not brutal, but they asked a lot of follow-up questions about edge cases and scalability. Be ready to talk through why you'd handle failures a certain way, not just what you'd do. Virtual onsite: 4 panels over about 4.5 hours. Two coding, one system design, one behavioral.
System design was the hardest part for me. The prompt was loosely "design a payment retry system" which is... extremely on-brand for Square. They wanted to hear about idempotency explicitly. If you don't say that word at some point I think they write it down.
Behavioral was focused on conflict and ambiguity. Specifically how I handled disagreements with product about scope. Have a real story, not a sanitized one.
Coding rounds were Leetcode medium but the interviewers asked me to optimize mid-solution which threw me the first round. Got the offer. Timeline was about 5 weeks total from first reach-out to offer call.