How I Practice System Design with AI (URL Shortener Walkthrough)

I've been doing system design interviews for years - both as a candidate and as an interviewer. The hardest part to practice alone is not the knowledge. It's the process: starting from requirements, running the numbers, evaluating multiple options before committing, and making explicit trade-off arguments at the end. Reading about it helps. Actually working through it is different. A few months ago I built a structured multi-step AI plan that walks through a complete system design session end-to-end. I've been using it to practice against different systems.