AI Mock System Design Interview

I lost a Meta interview at minute 15 once. I spent 12 minutes drawing boxes and arrows before asking a single requirements question. Here's what actually happened. Minutes 0–3: I clarified the problem. Good start. Minutes 3–15: I immediately jumped into: "We'll use this architecture, cache here, database there, here's the schema…" The interviewer was nodding. I was feeling confident. But I had already failed. Because I never asked: - What scale are we designing for? - What are the core use cases? - What's critical vs. nice to have? - What are the latency and reliability expectations? - Are there any business constraints or trade-offs? By minute 15, the interviewer had already decided: "This candidate doesn't gather requirements. They design systems without understanding business needs. They'll confidently build the wrong thing." The rest of the interview? I was already fighting uphill. That failure taught me something: you might not need more system design knowledge. You've been building systems after all. You need reps under time pressure with someone who'll call you out when you skip steps, probe your choices and provide actionable feedback afterwards. So I built this for myself first. It forces phase transitions whether you're ready or not. It tracks if you actually asked about scale, trade-offs, and requirements. It tells you exactly where you lost points and which phases you ran out of time in. No theory. No lectures. Just practice the interview format until asking the right questions becomes muscle memory.