Stop studying system design. Start practicing it.

A recorded system design interview between an interviewer and a strong candidate, played one turn at a time. You press Next, the candidate answers, and the whiteboard fills in as they talk: requirements, numbers, the API, then a design that gets fixed one bottleneck at a time, then the deep dives. Skip to any stage or straight to a key moment, and ask the AI Mentor about anything along the way.

A chat beside the interview, never part of it. Ask it anything about what you have seen so far (why this database, what a term means, whether your own idea would work) and it answers with the interview and the whiteboard in front of it. Each turn also comes with suggested questions whose answers are written in advance; those are free for everyone. Live questions are part of Pro.

You are the candidate. An AI interviewer runs the round the way a real one does: it states the problem, asks one thing at a time, probes what you said and what you drew on the whiteboard, and ends with a debrief (a score out of 100, what was strong, what was missing, and a note on each stage with a link to the same stage of the walkthrough).

Your choice at the start. Timed is interview conditions: a 45-minute clock the interviewer owns, and the AI Mentor is off. With the AI Mentor there is no clock and the AI Mentor sits beside you; it gives hints rather than answers unless you ask it outright, and the debrief notes that you had the AI Mentor.

You can see what you would be buying first. Every walkthrough is free with a Google sign-in, so you can watch a strong candidate answer any question in full; the AI Mentor’s suggested answers are free; and anyone can start a mock interview and hear the AI interviewer open the round. Answering it, and live questions to the AI Mentor, are Pro. A first purchase can be refunded within 7 days if you have started fewer than 3 mock interviews.

Every walkthrough, with no account, up to its deep dives, including the AI Mentor’s suggested questions. Design a URL Shortener is free in full. Sign in with Google and every other walkthrough is free in full too. On a mock interview, anyone can hear the interviewer open the round. The study guides, the interactive concepts, the reference designs and which companies ask each question are free too. Pro adds the AI mock interviews and live questions to the AI Mentor.

Reading gives you the final answer. A walkthrough shows you how a strong candidate gets there out loud (the order, the pacing, the numbers, and the design being fixed one problem at a time) and lets you stop and ask why. The mock then makes you produce it yourself, under pressure, with someone following up on the weak part.

You can, and with a good prompt it makes a usable interviewer. Three things get in the way: it sees your diagram only when you upload a picture of it, any side question either derails the interview or goes to a second chat that knows nothing about it, and it has no reference answer to grade you against. Here the interviewer reads your whiteboard as you draw, the AI Mentor sits beside the interview with the whole conversation in front of it, and every question has a written reference design and a strong candidate’s walkthrough behind it.

AI mock interviews on every question, timed or with the AI Mentor, each ending in a scored debrief; live questions to the AI Mentor, including comparing your answers with the candidate’s; and the company-style mock interviews (Google, Meta, Amazon, OpenAI and others). Every question added later is included, and a daily fair-use limit on AI messages keeps it fast for everyone. All three plans include the same thing; they differ only in how you pay.

Monthly and annual plans renew until you cancel. Cancel from the billing portal at any time and you keep access until the end of the period you paid for. Lifetime is a single payment with no renewal.

Yes. Open the billing portal from the pricing page and switch plans; Stripe prorates the unused part of the current period.

Yes, on a first purchase. Email us within 7 days of it, having started fewer than 3 mock interviews, and it is refunded in full. Renewals are not refundable, so cancel before the renewal date if you are done. The Terms have the details.

Not for the free walkthrough, the guides, or to hear the AI interviewer open a round. Signing in with Google opens every walkthrough in full, and is needed to buy Pro.

Public interview guides and candidate reports. They indicate where a question has been reported, not a verified dataset, and a company name never implies endorsement.

Senior and staff loops at large tech companies, where the interview is 45 to 60 minutes, expects numbers, and spends most of its time on deep dives. Mid-level candidates use it too; the walkthroughs explain every term as they go, and the AI Mentor will explain anything they do not.