Replit System Design & Production Engineering interview questions

A Copilot/Cursor-style assistant lives or dies on the latency budget. The strong answer spends every millisecond of a 200ms end-to-end budget on purpose, separates the inline-ghost-text path from the chat path, and shows why prefix caching and speculative decoding are the only ways to hit it at 10k concurrent users.

Every queue, webhook, and retry loop you'll ever deploy at a customer hides this distinction. Interviewers use it to separate people who've shipped from people who've read about shipping.

A staple at OpenAI and Anthropic in both coding and design rounds. Most candidates can name the algorithms; few can say which one lets 2x your limit through, and that's the part that gets scored.

Averages lie, and interviewers know most candidates quote them anyway. The fastest way to sound production-grade in any latency discussion is to reason about percentiles correctly, including the one trap almost everyone falls into.

The signature FDE debugging genre: a moving fault, an opaque client, and a customer who's sure it's your fault. The winning method splits the problem at the boundary, and knows the four classic culprits hiding on their side.

A Fermi estimate with a paycheck attached. The interviewers' favorite filter: candidates who jump to a dollar figure miss the two numbers that actually break deployments, peak QPS and tokens-per-minute limits.

One malformed message can silently wedge an entire pipeline, and it happens at every customer eventually. The DLQ answer interviewers want covers what most candidates forget: what happens *after* the message lands there.

This isn't a deployment question, it's a judgment question wearing one. Interviewers are scoring whether you have a decision framework, a safety checklist, and the spine to say no to a customer politely.

Standard observability tells you the request returned 200 in 800ms. It cannot tell you the answer was wrong. The strong answer names the new failure plane LLMs introduce, and takes a real position on the prompts-and-PII question.

A signature FDE triage question with a hidden rubric: interviewers score the *order* of your moves, not just the list. There's a reason 'what changed?' beats 'check the dashboards', and a reason mitigation beats diagnosis.

The bug only appears under load, the code 'hasn't changed,' and the customer just got charged twice. There's one root-cause pattern behind almost every version of this incident, and a fix with a subtle race most candidates miss.

The Palantir decomposition round has no right answer, it has a right METHOD, and it's the most-failed interview in the FDE loop. Here is the four-move rubric interviewers actually score: clarify, model, spine, iterate.

The decomposition prompt we use to teach the skill that actually decides these interviews: graceful iteration under perturbation. Your score isn't your design; it's the delta when the design gets punched.

The round is conversational discovery plus live Excalidraw diagramming, and it's graded on sequence: candidates who draw before asking fail. The discovery battery, the voice-pipeline boxes worth drawing, and the latency budget that anchors the whole design.

Under 500ms from end of speech to first audio out, across three models and a network. The trick is that you never wait for any stage to finish: you stream and pipeline everything.

Vercel's signature triage question. 3s TTFB is almost never 'the server is slow', it's a rendering-strategy problem with four classic causes, and the rubric rewards proving each fix with a measurement, not vibes.