No subscription. Unlock a problem once with credits and everything inside it is free: runs at any traffic, settings, changes, components, hints, Submit and the results. 6 problems are free to start.
| Action | Credits | Why it costs |
|---|---|---|
| Unlock an Easy problem | 10 | Once. It stays open on your account, with everything inside it free. |
| Unlock a Medium problem | 15 | Once. |
| Unlock a Hard problem | 25 | Once. |
| Unlock a solved design (Instagram, Netflix...) | 10 | Once: the working design and its design doc. |
| Unlock a component lesson | 2–3 | Once: 2 credits for traffic, compute, data and messaging components, 3 for AI, agent and security ones. The core components’ lessons are free. |
| Stress test | 5 | About ten runs of rising traffic plus an attack drill. |
| AI answer | 3 | A question answered by Claude about your run. |
| AI review | 20 | A full review of a finished run, every number checked against it. |
If an AI answer or review fails, its credits are released and nothing is charged. On the blank canvas, where you design your own system, components cost 3–12 credits each (every price), a copilot change 1, and runs above 5,000 req/s 1 per 5,000 req/s.
One Medium problem, from opening it to an accepted solve, with the prices as they stand today:
| Unlock it | 15 | Once; reopen it any time. |
|---|---|---|
| Run it, change settings, add components, rerun and compare | 0 | Free inside a problem you opened, at any traffic. |
| Hints, “what broke?”, Submit and the results | 0 | Including what you missed and how much you could save. |
| One question to the AI | 3 | Free-form questions go to Claude; a saved identical answer is free. |
| The problem | 18 | The 100 welcome credits cover about 5 problems like this, after the free ones. |
Two currencies, kept apart on purpose: credits are what you spend to practise; the cloud cost shown on every run is an estimate of what the design would cost to host, from illustrative list prices, and is never charged.
Change the numbers to match how you practise.
409 credits a month
About 19 credits per problem, or about $12.78 at the $5 pack price.
The 100 welcome credits cover about 5 problems like this after the free ones; then it is about 3 $5 packs of 160 credits a month.
| Pack | Price (outside India) | Per 1,000 credits | AI answers it pays for |
|---|---|---|---|
| 160 credits | $5 | $31.25 | about 53 |
| 400 credits | $10 | $25 | about 133 |
| 1,100 credits (+10%) | $25 | $22.73 | about 366 |
| 2,400 credits (+20%) | $50 | $20.83 | about 800 |
The AI coach is paid with credits like everything else: an answer is 3 credits and a full review 20. Hints, “what broke?”, the rule-based fixes and saved answers are free.
Payments go through Razorpay: cards, UPI and net banking in India, cards elsewhere. Unused purchased credits are refundable within 14 days: see refunds.
Choose how much to recharge. The amount steps through what the packs can add up to, and the calculator picks the packs that give the most credits.
Amounts step through what the packs can add up to: $5, $10, $15, $20, $25 and so on. Bigger packs carry a bonus, so the calculator always picks the combination with the most credits.
1,100 credits for $25
100 of them are bonus credits. That is $22.73 per 1,000 credits, and it unlocks about 73 Medium problems (everything inside each is free), or 183 components on a blank canvas, or 366 AI answers, or 110 solved case studies.
Teaching a distributed systems course or training a team? Ask about bulk credits through the contact form.