73% of Engineers Fail This 5-Minute System Design Question. Here's Exactly Why.

The question is "design a basic chat system." Not design Twitter. Not design a distributed database. A chat system. And 73% of software engineers are failing it in FAANG interviews. I've been thinking about why this keeps happening and the answer is genuinely useful regardless of whether you're actively interviewing or not — because it's really about how engineers think under pressure, not just what they know. What actually goes wrong The moment most candidates hear the prompt, they jump straight into implementation mode. Database schema. WebSocket setup. API endpoints. Moving fast, looking busy, heading straight off a cliff. The problem isn't that those decisions are wrong. It's that they're making them before establishing what they're actually building. "Design a chat system" is massively underspecified. The right architecture for 200 concurrent users is genuinely different from the right architecture for 10 million. A single WebSocket server works fine at small scale. At large scale