Mercor Interview Questions
Reconstructed from 22 verified candidate reports across 8 questions. Oct 2025 – Oct 2026.
How the Mercorinterview runs, round by round, and how to prepare for it — built from candidate reports, not from job descriptions or one founder’s memory. The catalog gets a refresh pass every month.
AceOffer is not affiliated with, endorsed by, or sponsored by Mercor. Questions are reconstructed from interview experiences candidates shared, not from Mercor materials.
Key facts
- •8 distinct Mercor interview questions indexed
- •22 candidate reports across the catalog
- •Round mix across candidate reports: phone screen 53%, technical deep-dive 33%, system design 10%
- •5 of the 8 questions were last reported in Aug–Oct 2026
- •Reports span Oct 2025 – Oct 2026
- •Refreshed monthly · last updated October 2026
Browse Mercor interviews by topic
The Mercor loop, from candidate reports
Mercor's loop opens with a ~30-minute technical phone screen that is entirely verbal: about four rapid-fire questions (sorting theory, a probability puzzle, code reviews of a threaded cache and a CSV writer, an idempotent payment call), answered as many as you can in the time, and two candidates report failing after answering three. It moves to an in-person onsite in San Francisco that runs several hours and mixes a multi-part coding round, a system-design round, an algorithm round, and a 90-minute candidate-search build (75 minutes writing it with LLM tools and running it against an evaluation endpoint, then 15 minutes of discussion); for MLE roles the onsite adds a one-hour build where you implement and train a small autoregressive language model (nanoGPT) on Shakespeare, then a transformer-fundamentals review. The algorithm round can be entirely spoken (one report: find an exchange rate across currency pairs). Applied AI candidates get a different screen: debug an async refund function, then rapid-fire questions on idempotency, RAG versus fine-tuning, context limits, statefulness and prompt injection. The signature onsite coding problem is a 3-part referral network (async, screen-recorded, no LLM for the first two parts; the third done live), and the signature system design is a distributed job scheduler / work orchestration (candidates are pointed at the HelloInterview job-scheduler design). One report stresses that interviewers weight communication fit heavily (tailor depth to the interviewer's domain), and single reports describe an interviewer no-show and contradictory instructions about whether LLMs are allowed in the coding round.
What does Mercor ask in each interview round?
Mercor interviews span 4 distinct round types, broken down below. Counts reflect distinct questions per round, not number of times asked.
60 minute design rounds. Interviewers push hard on the specific dimension their team cares about (storage at scale, real-time fan-out, multi-tenancy).
60–75 minute live coding rounds. Multiple sub-problems progressing in difficulty. Test harness usually provided.
The Mercor question list
8 questions, grouped by round, with how often candidates reported each one and when it was last seen. Every question is free to read, no account needed.
Browse the Mercor questions →- •On the phone screen: keep every answer short. It is about five questions in 30 minutes, and a candidate who reached only three did not pass. The candidate who passed covered idempotency-key reuse and record-before-charge on payments, and duplicate computation plus lock scope on the cache review
- •On the job-scheduler system design: walk the HelloInterview framework fluently and make distributed correctness first-class — leader election, CAS/idempotency to prevent duplicate runs, and a DAG dependency-counter — rather than presenting a single-node design
- •On the faulty-comparator algorithm round: lead with the key insight that errors are deterministic (so re-calling the same pair never helps), then separately optimize comparison count vs wall-clock time (parallelize independent comparisons when each call is slow)
- •On the 3-part referral network: use cycle-safe traversals, clarify the flow-centrality definition before coding it, and write your own quick tests (the round provides none)
- •Tailor communication to the interviewer — reports say Mercor weights communication fit heavily and advise not going deep on domains the interviewer doesn't share
- •When a round allows a coding agent (some onsite/mini-project rounds do), use it to move fast but be able to explain every line — candidates who pasted AI output they couldn't defend got caught
- •Generating code with an LLM without understanding it — candidates explicitly reported being caught out this way on the coding rounds
- •Designing a single-node job scheduler that ignores leader election, exactly-once/idempotency, or DAG dependency management
- •Going deep on ML-specific content when the interviewer expected general systems knowledge (a reported failing pattern on the SD round)
- •Not clarifying under-specified prompts — the comparator's determinism, the referral flow-centrality definition, and mkdir-style ambiguities are left open on purpose
- •Being thrown by the chaotic logistics (interviewer no-shows, conflicting LLM-usage rules) instead of confirming the ground rules with the coordinator up front
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