I kept watching engineers freeze in system design interviews, so I built a practice tool

I've interviewed a lot of engineers for system design rounds over the years. The pattern that kept bugging me: candidates clearly understood the theory but fell apart when they had to actually build something from scratch. They'd watched all the YouTube videos. Memorized the classic walkthroughs. But give them a slightly different problem and they'd freeze. Same thing when friends asked me to help them prep. We'd jump on a call, I'd give them a prompt, and they'd stare at the screen not knowing where to start. The problem was never that they didn't know what a load balancer does — it's that they'd never practiced going from a blank page to a full architecture while tracking requirements and defending choices out loud. And when I wasn't around to run the mock? They had no way to get feedback. You can practice drawing diagrams all day, but without someone pushing back on your decisions and asking "what happens when this fails?" — you're just reinforcing your own blind spots. So I built S