System design interview assistant with real-time answers
System design rounds are open-ended by design: "How would you build a rate limiter?" has no single right answer, and the interviewer is judging how you structure it. LastRound AI listens to the question and gives you a structured answer to talk through, grounded in your resume, your domain and the job description.
Before the day, the same account has animated concept explainers for the building blocks interviewers ask about, and AI mock interviews to rehearse out loud.
Summarized question: How would you design a rate limiter for a public API?
I'd start with the requirements: a limit per API key, a clear 429 when a client is over, and no single point of failure.
Design: a token bucket per key, kept in Redis so every gateway node shares the same count.
Trade-off: a fixed window is simpler but lets bursts through at the window edge; the token bucket smooths that.
61interview languages
~1 sto the first words of an answer
Hiddenon screen share and recordings
1 creditper minute, free plan included
How it works
Four steps, all on the keyboard
- 1
Set your domain and experience
In Interview Setup, pick your category and domain from 25 categories and 500+ domains, and add your resume and the job description. Answers draw on them.
- 2
Start a Live interview session
The app captures your computer's audio, so the interviewer's question is transcribed as they speak. Nothing joins the call.
- 3
Check the Summarized question line
Before answering, the copilot shows what it understood. If it misheard, type the question into Ask instead.
- 4
Answer and follow up
Press ⌘/Ctrl + Enter. Follow-up questions build on the same session, so answers stay consistent as the design evolves.
What you get
Built for this round
Structure you can talk through
Answers are written in the first person so you can walk the interviewer through requirements, components and trade-offs in your own words.
Grounded in your background
Your resume, domain, job description and the hands-on experience you list shape the answer, so it sounds like your system, not a textbook.
Consistent across follow-ups
Session memory keeps later answers in line with the design you already described, even deep into a long round.
Whiteboard problems on screen
If the prompt is written in a shared doc or whiteboard, capture it with ⌘/Ctrl + H, or use Coding round with the System Design (Code) round type.
Learn the building blocks
238 animated concept explainers cover topics like load balancing, HTTPS and Kubernetes, free on every plan.
Rehearse out loud
AI mock interviews let you practise a design question with a scored report before the real round.
Watch the full walkthrough
The desktop app from setup to a streamed answer.
Before the interview
Add real numbers
The Include metrics switch uses figures you enter yourself, such as latencies or traffic you have handled. Answers never invent your numbers.
Test the audio
Interview Setup → Audio check confirms the app can hear the meeting in about 20 seconds (desktop 4.0.15 and later).
Learn the shortcuts
Answer with ⌘/Ctrl + Enter, capture the screen with ⌘/Ctrl + H and hide or show the overlay with ⌘/Ctrl + B. The shortcuts work while your meeting, browser or editor has focus.
Where to be careful
- Anyone who can see your physical screen sees the overlay. Stealth hides it from the video stream, not from the room.
- A transcript is never word-perfect. Read the Summarized question line before you answer from it.
System Design Interview Assistant: FAQ
Can the assistant draw the architecture diagram for me?
No. Answers are text you can talk through: the requirements, the components and how they connect, and the trade-offs. You draw the diagram yourself.
Does it work when the design question is written on a whiteboard tool?
Yes. Capture the prompt with ⌘/Ctrl + H and press Answer. In a Coding round you can also pick the System Design (Code) round type.
How do I prepare for system design interviews with LastRound AI?
Use the concept explainers for the building blocks, then run an AI mock interview on a design question to practise explaining it out loud and get a scored report.
How much does it cost?
Sessions bill 1 credit per minute, rounded up, from Start to End. Every account gets 15 free credits a month, so you can try it on a practice round before paying. Paid plans add more credits each month.
Other interview types
- Coding interviewsSolutions, hints and edge cases from a screenshot of the problem
- Behavioral interviewsSTAR answers built from your own stories and numbers
- Phone screen interviewsHelp on recruiter screens, on your laptop or your phone
- Online assessmentsMultiple choice, code output and puzzles from a screenshot
- HireVue and one-way videoPrompts read from audio or screen during recorded interviews
Learn more
Try it on a practice round first
Download the app, run a practice session with your free credits, and see the answers for yourself before the real one.