Did my Microsoft senior SWE loop last spring targeting L63. Passed. Sharing the system design portion specifically because there's a lot of outdated info floating around.
First, the format. At L63 (which roughly maps to senior IC, their version of L5 at other companies), you get one dedicated system design round and the interviewers are looking for something specific: can you drive the conversation, or do you wait to be led.
They gave me a classic: design a notification system at scale. I asked a few scoping questions, drew out the high-level architecture, then dove into the fanout problem. That part landed well. Where I stumbled briefly was when they asked me to talk through failure modes in the message queue layer. I hadn't fully thought through dead-letter queues and retry logic upfront, and they pushed on it. Not hostile, but direct.
What they weight at senior/L63: Breadth first, then depth where you choose. Don't go deep on the wrong thing for 20 minutes. They want you to make explicit tradeoffs. Say out loud why you picked Kafka over a simpler queue. "More scalable" is not an answer. Ambiguity handling. Senior signals start with how you handle an underspecified problem, not how fast you get to a solution. They do NOT have the same obsession with back-of-envelope math that Google does. Rough estimates are fine, exact numbers are not expected unless central to the problem.
The interviewer was a principal and asked follow-ups that probed my understanding of consistency vs. availability. We ended up in a pretty good discussion about eventual consistency in distributed caches. That felt like the real signal moment.
One thing I didn't expect: they asked me to walk through how I'd test this system. Not deep, maybe 5 minutes, but it's part of the L63 rubric apparently.
Total design round was 55 minutes with about 5 minutes of intro/questions at the end. I had time to cover most of it without feeling rushed.
Happy to answer specifics if you're prepping.