To practise system design with AI, pick a real prompt, give yourself 45 to 60 minutes, and talk it through in the order an interviewer expects: requirements, rough numbers, a high-level design, the data model, then scaling and failure. The AI plays the interviewer, pushing back on your choices and scoring you afterwards. Devana runs this as a voice round with a whiteboard, scored on 8 metrics: 40% communication and 60% design.
Step by step: practising a system design round with AI
- Pick one prompt at your level. A URL shortener, a rate limiter, a news feed or a chat service are classics because they force the same decisions real systems do. Choose one you have not rehearsed.
- Set the clock. Most real rounds run 45 to 60 minutes. Roughly: 5 to 10 minutes on requirements, 5 on estimates, 15 to 20 on the high-level design, 15 going deep and the last 5 on trade-offs. The complete guide to system design mock interviews breaks the hour down further.
- Scope out loud. Ask about users, features, read and write patterns, latency and consistency. Write the answers where you can see them.
- Do the numbers. Rough requests per second, storage per year, bandwidth. Precision does not matter; showing which part of the system the numbers make hard does.
- Draw and narrate. Put the main components on the board and say what each one does as you add it. Going quiet while you draw is the easiest way to lose an interviewer.
- Go deep where you are pushed. Data model, partitioning, caching and invalidation, queues, consistency. Let the follow-up questions choose where.
- Close, then review. Summarise the design, name its biggest weakness and what you would build next. Then read the scorecard and pick one thing to change in the next session.
A general AI chatbot or a mock interview tool?
You can run a decent system design drill with a general AI assistant: paste a prompt like the one below and answer in its voice mode. It is free and flexible. Its limits are that it tends to agree with you, it cannot see what you draw, and nothing keeps time.
A prompt to start from
You are interviewing me for a senior backend role. Give me one system design problem and let me drive. Answer my clarifying questions briefly. Push back on every major choice with one "why" and one "what happens when this fails" question. Do not suggest components unless I am stuck for over a minute. After 45 minutes, score me on requirements, estimates, high-level design, data model, scaling, reliability and trade-offs, and name my weakest area.
A purpose-built tool adds what a chatbot lacks: a voice interviewer that keeps the pace, a timer, a whiteboard, and the same rubric every time, so this week's score means the same as last week's.
How Devana runs a system design round
Josh plays a senior software architect. He presents the system in two or three sentences and then lets you drive, as most real interviewers do.
- A whiteboard the interviewer reads. You draw boxes, arrows and text. Josh is told in words what is on the board: the components you have named, how they connect and any notes. Label every box.
- Follow-up questions. Josh probes your choices: why that database, where the cache goes and how it stays correct, what happens when a service goes down, and what if the traffic doubles.
- Real round lengths. Up to 60 minutes on the Sprint Pass and Pro, and up to 20 minutes on the Free plan.
- 357 system design questions in company sets, so you can practise the designs a company tends to ask about, such as checkout and order placement for Amazon.
- Designs from your own resume. A resume-based round can give you a problem close to systems you have built.
How the round is scored: 4 + 4 metrics, 40/60
The score is out of 100. Forty percent comes from how you communicated and collaborated, sixty percent from the design itself. Each half has four metrics:
| Metric | Half | Points | What earns them |
|---|---|---|---|
| Requirements scoping | Communication (40%) | 30 | Functional and non-functional requirements, traffic scale and latency targets, settled up front |
| Driving the discussion | Communication (40%) | 25 | Structuring the session and leading it, rather than waiting to be asked |
| Trade-offs | Communication (40%) | 25 | Weighing options out loud: SQL or NoSQL, sync or async, throughput or latency |
| Verbal clarity | Communication (40%) | 20 | Clear explanations, including of what is on the board |
| High-level architecture | Design (60%) | 30 | A sensible breakdown into APIs, load balancers, services, caches and queues |
| Data modelling and storage | Design (60%) | 25 | Database choice, schema, indexes, partitioning and sharding |
| Scalability | Design (60%) | 25 | Scaling techniques, and finding the bottlenecks and single points of failure |
| Reliability | Design (60%) | 20 | Replication, failover, rate limiting, circuit breakers and recovery |
From the grader's rubric. Each half's metrics add up to 100; the round score is 40% of the first half plus 60% of the second. No code is graded in this round.
For every question the report lists at least four things that raised your score and four that lowered it, each tied to a moment in the session, such as jumping to components before asking about read and write volumes, or a primary database with no replication plan. That is the point of a fixed rubric: you can see whether the habit you worked on actually moved.
Mistakes the scorecard catches most
- Designing before scoping. Requirements scoping is the single largest communication metric, and skipping it also costs you on the design half, because you are solving an unstated problem.
- No numbers. Without an estimate, "add a cache" is a guess. With one, it is a decision.
- One of everything. A single database, a single queue, no failover: each is a single point of failure the interviewer will ask about.
- A cache with no invalidation plan. The follow-up is always "how does it stay correct?"
- Waiting to be led. Driving the discussion is a quarter of the communication score. Say what you will cover next, then cover it.
Interface design comes up inside most designs, so API design interview questions is worth reading alongside this, and backend engineer interview prep covers the concurrency and idempotency questions senior rounds add.
When to use a human instead
AI practice is best for the volume of reps that makes the structure automatic. For a senior or staff-level design, book at least one session with an experienced engineer before your onsite: interviewing.io's sessions with engineers start at $179, and it also has a free AI interviewer for system design. If you want structured study material rather than a mock, Hello Interview sells guided system design practice (Premium is $47 for a month or $78 for a year as of October 2026); its human mock interviews ended on May 31, 2026.
Frequently asked questions
Can AI really run a system design mock interview?
It can run the part most people never practise: talking through a design under time pressure while someone asks why. An AI interviewer presents the problem, pushes back on your choices and scores the session. It does not replace a senior engineer's judgment on a staff-level design, so book a human for a final check if you can.
How long should a system design practice session be?
Practise at the length of the real round, usually 45 to 60 minutes. On Devana's paid plans a system design round runs up to 60 minutes; on the Free plan it is up to 20 minutes, which is enough to practise requirements, estimates and a high-level design.
Does the Devana interviewer see my whiteboard?
It reads it. The round has a whiteboard with boxes, arrows and text, and the interviewer is told in words what is on it: the components you have named, how they connect and any notes. Label every box, because an unlabelled shape tells it very little.
What does the system design score measure?
Eight things. Forty percent of the score is communication and collaboration: requirements scoping, driving the discussion, trade-offs and verbal clarity. Sixty percent is the design itself: high-level architecture, data modelling and storage, scalability, and reliability. No code is graded in this round.
Is system design practice included on the free plan?
Yes. Every round type is on the Free plan, including system design, for 3 interviews a month of up to 20 minutes each.