Went through the full Slack SWE loop for a Staff role in February. Five rounds total: recruiter screen, technical screen, system design, two behavioral sessions back-to-back, then a brief hiring manager sync at the end.
The technical screen was LeetCode medium-ish. Graph traversal with a twist. Not brutal, but they moved fast. The system design round is where it got interesting. I got: design Slack's notification delivery system. They wanted to talk about push vs. pull, delivery guarantees for mobile vs. desktop, handling offline users, and the fan-out problem when you're in 500 channels. I drew a lot of boxes and was upfront about what I'd defer vs. tackle first. They seemed to like the prioritization reasoning more than me having the "right" architecture.
The behavioral rounds were the most Slack-specific part. Lots of questions about working across teams, decisions you made without full authority, and written communication. One interviewer literally asked me "how do you know a proposal has landed?" and I think they were half-evaluating my answer and half-watching how I constructed the explanation itself.
Overall the interviewers were sharp and prepared. Feedback loop was about 10 days after final round. I got the offer. Negotiated up from the initial number without drama.