Went through the Block (formerly Square) senior SWE loop in March 2026. Got an offer for an L5 role in the Square Seller org. Wanted to write up the system design portion because it was a bit different from what I've seen at other fintech shops.
The system design round was 60 minutes, one interviewer. Not a panel. They gave me a prompt about 5 minutes in and let me drive. My prompt was something like: design a payment retry system that handles partial failures at scale. Pretty on-brand for Block given what they actually build.
What they cared about most, from what I could tell: Idempotency. This came up immediately when I talked about retries. They pushed hard on how you handle duplicate payment attempts if a client retries the same request. I had a good answer (idempotency keys stored in a distributed KV store with TTL) and the interviewer visibly relaxed. Consistency vs. availability tradeoffs. Classic fintech stuff. They want to know you understand why payment systems lean CP not AP on Brewer's theorem. Queue design. I proposed SQS with dead-letter queues. They asked about exactly-once delivery guarantees and how to handle poison pill messages. Database choices. I used Postgres for the ledger side. They seemed fine with that. Didn't push me toward any Block-specific tech stack.
The interviewer was engaged and asked good clarifying questions rather than gotcha stuff. I felt like they were genuinely curious whether I'd worked on real distributed systems, not just memorized Grokking the System Design Interview.
One thing I didn't expect: they asked me to scope the problem before jumping into design. Like explicitly: 'before you start drawing boxes, tell me what constraints matter.' That framing helped.
Total time from recruiter reach-out to verbal offer was about 5 weeks. The system design is the round I'd say is most differentiated from FAANG loops, mostly because of the payments-domain flavor on the prompts.