Mock system design interview at Slack

A live 45-minute round on a question reported in Slack interviews, run by an AI interviewer that asks one thing at a time, probes what you say, looks at your whiteboard, and weighs what that round is reported to weigh. It ends with a debrief: a score, what was strong, what was missing.

your question

Design Slack

Real-time team messaging: channels, DMs, presence, history and search for millions of concurrent users.

You will not see the requirements, the numbers or the design before you start. That is the point.

Walk through it first

What the Slack round is reported to look like

45–60 minutes. Prompts are reported to sit close to Slack: messaging, notifications or presence, or integrations that act on commands typed into a message and call outside services.

What it weighs

Clarifying the product and technical requirements before designing
Back-and-forth with the interviewer, not a finished design presented in one go
Tradeoffs explained with reasons, for a product someone has to ship and support
On messaging prompts: the data model, real-time delivery and fan-out to channels

The common mistake

Presenting a finished design without talking it through. A Slack engineering leader has said the back-and-forth matters as much as the solution, so ask about the requirements and check in with the interviewer as the design takes shape.

From public interview guides and candidate reports, and Slack’s own interview guidance where it publishes any. Not endorsed by Slack. The interviewer is an AI running in that style; it is practice, not a prediction.

Start the round

Timed · 45 minutesInterview conditions. The AI interviewer owns the clock; the AI Mentor is off.With the AI Mentor · untimedNo clock, and an AI Mentor beside you to ask when you are stuck.

The interviewer’s opening question is free. Answering it is part of Pro. See an example round and its debrief →

The 3 questions this round draws from

Each is reported in Slack interviews. Pick one to sit, or read its reference design and walk through a strong answer first.

Design SlackReal-time team messaging: channels, DMs, presence, history and search for millions of concurrent users.
drawn
Design a Webhook Delivery SystemDeliver 3 billion signed webhooks a day to a million customer endpoints, retrying for three days, without one slow endpoint delaying anyone else.
Design DiscordCommunity chat and voice: servers with a million members, years of history, roles and presence, for 20 M concurrent users.

Other companies: Anthropic · Google · Meta · Amazon · OpenAI · Microsoft · Uber · DoorDash