AI Mock Interview Practice for Software Engineers
A calibrated mock interview plan for SWE loops at big tech, unicorns, and Series B startups — with the exact question shapes, a worked system-design answer, and the leveling rubric your interviewer is likely using.
The loop you're prepping for
Most software engineers loops share the same skeleton. Rehearse each round on its own — a single "general" mock trains you for none of them.
| Round | Length | What they score |
|---|---|---|
| Recruiter screen | 20–30 min | Motivation, level fit, comp expectations, one behavioural. |
| Coding (phone) | 45 min | Clarifying questions, brute-force → optimal, edge cases, complexity. |
| System design | 60 min | Requirements gathering, capacity math, data model, tradeoffs, failure modes. |
| Behavioural / bar-raiser | 45–60 min | Ownership, ambiguity, disagreement, learning from failure — STAR stories. |
| Domain deep dive | 45 min | Depth on a project on your resume — architecture, decisions, tradeoffs. |
Real questions to practice — by round
Coding warm-ups (talk out loud)
- Find the k-th largest element in a stream of integers.
- Merge overlapping intervals — then extend to a streaming version.
- Design a data structure with insert / delete / getRandom in O(1).
- LRU cache — implement with a rubric of 'works, then optimal'.
System design
- Design a URL shortener that survives 50k RPS.
- Design a rate limiter shared across 20 services.
- Design the newsfeed for a photo-sharing app.
- Design a distributed job scheduler with at-least-once semantics.
Behavioural (bar-raiser)
- Tell me about a technical decision you made that turned out wrong.
- Describe a time you disagreed with a more senior engineer.
- Walk me through a project you shipped under an aggressive deadline.
- Tell me about the hardest bug you've debugged.
Worked example
Question
Design a rate limiter that serves 50k RPS across 20 microservices.
Strong sample answer
First I'd clarify requirements: 50k RPS aggregate or per service, per-user or per-API-key limits, and whether we fail open or closed under Redis failure. I'll assume per (api_key, route), 10k unique keys, and fail-closed with a stricter local fallback. For the algorithm I'd pick token bucket over fixed window — smoother, allows short bursts, and fits well in Redis. Each key holds two integers: tokens_left and last_refill_ts. I'd implement the refill + decrement atomically in a Lua script so we don't race on multi-step transactions. Storage: Redis Cluster, sharded by hash(api_key). At 50k RPS with a 100μs Lua script that's about 5ms of Redis CPU per second per shard — 6 shards gives comfortable headroom. TTL on the bucket key = 2× the window so cold keys don't leak memory. Failure modes: if Redis is down, each service falls back to an in-process bucket at 25% of the true limit — fail-closed but not hard block. If the Lua script fails, we log and allow (better a false negative than a customer outage during a code bug we own). Tradeoffs I'd flag to the interviewer: token bucket doesn't play well with hard SLA fairness — for that we'd want a leaky bucket queue. And the per-service local fallback drifts under partition; we accept up to 4× the true rate for the ~10s of a Redis failover.
The rubric interviewers use
Problem clarification
Did you turn a vague prompt into a spec before typing? Naming assumptions is scored, not penalised.
Design + tradeoffs
You picked an approach and named what you gave up. Silent choices lose points.
Coding fluency
Compiled in your head. Edge cases named without prompting. Complexity stated out loud.
Communication
Interviewer never had to ask 'what are you doing right now?'. Signposting > cleverness.
Tips that actually move your score
- Always spend the first 5 minutes on requirements — even if you 'know' the question. Interviewers score the clarification.
- In system design, do the back-of-envelope math out loud (RPS × payload = bandwidth). It signals seniority faster than any diagram.
- For behavioural, prepare one story that ends 'and here's what I'd do differently now'. That single line lifts bar-raiser scores.
- Practice on a webcam with a shared editor — coding while talking is a separate skill from either one alone.
Frequently asked questions
How long should I prep for a FAANG-level SWE loop?
6–10 weeks part-time is typical for someone with 3+ years of experience: 4 weeks of coding drills, 3 weeks of system design, and 2 weeks of full-loop mocks.
Can AI mock interviews replace LeetCode?
No — pattern practice on LeetCode builds the muscle; AI mocks build the talking-through-it muscle. You need both. Do LeetCode alone in silence, then re-do 30% of them out loud on camera.
What's the fastest way to improve at system design?
Redo the same 5 designs (URL shortener, rate limiter, newsfeed, chat, scheduler) three times each. Each rep, add one dimension — sharding, then failover, then multi-region. Depth beats breadth.
Also read: STAR method interview questions & examples · Mock interview practice hub.
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