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Whatnot interview notes, page 3 of 4

Mid-sized

Most of these Whatnot write-ups come from Software Engineers (25), then Machine Learning Engineers (3) and Mobile Engineers (3). The most reported rounds are the technical phone screen (20), the system design round (7), the product sense round (3) and the hiring manager screen (1).

32 interview notes · updated Sep 2026

Whatnot·Software Engineer·Onsite - System Design / Architecture

Jun 2026
System design round at Whatnot for a software engineer role, focused entirely on building a direct messaging system for an e-commerce marketplace. Pretty deep dive, lots of follow-ups, and the multi-device consistency angle caught me more off guard than I expected.
  • Design a direct messaging system for an e-commerce marketplace where buyers and sellers can chat one-on-one, with real-time delivery when online and SMS/email fallback when offline.
  • Is the messaging strictly one-to-one, or should the data model account for group threads from the start?
  • What delivery semantics would you target: at-least-once with client deduplication, or exactly-once? Is any message loss acceptable?
  • Walk through how a client recovers a missed message after disconnecting mid-delivery, with no loss and no duplicates.
  • Two devices from the same user read the same conversation simultaneously and both try to advance the read cursor. How do you keep that state consistent?
  • How do you guarantee strict per-conversation message ordering when the store is sharded and multiple app servers might assign sequence numbers for the same conversation concurrently?
  • How would you extend this design to support group conversations without rewriting the core delivery path?
  • How do you make offline SMS and email notifications idempotent and debounced so a user offline for an hour with 20 messages doesn't receive 20 separate texts?

“This is the core question and it ate up most of the session.”

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Whatnot·Machine Learning Engineer·Onsite - Product Sense / Strategy

Jun 2026
Whatnot ML engineer interview with a product-minded twist. The whole exercise was basically: use the live-shopping app, find what's broken, and tell us how you'd fix it with data. More product sense than ML, which threw me a bit.
  • You're evaluating a live-shopping mobile app as a product-minded engineer. What's your read on the experience, and what would you change?
  • How would you identify which product categories are underperforming on the platform?
  • The team suspects a bug is affecting category performance or user experience. Walk through how you'd debug it.

“This felt open-ended in a way that was either a gift or a trap depending on how you handle ambiguity.”

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Whatnot·Mobile Engineer·Onsite - Product Sense / Strategy

Jun 2026
Product sense round for a Mobile Engineer role at Whatnot. The interviewer asked how you'd improve their app, then kept drilling into implementation details, success metrics, A/B testing, and downstream effects on other users. More of a conversation than a one-and-done answer situation.
  • How would you improve the Whatnot app?

“This sounds open-ended but the follow-ups are where it gets real.”

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Whatnot·Software Engineer·Technical Phone Screen

Jun 2026
Technical phone screen for a software engineer role at Whatnot. One coding problem, pretty algorithmic, centered on text filtering with a Trie. They also wanted a complexity discussion at the end which I wasn't fully prepared for.
  • Given an array of messages (each a dict mapping a username to text) and a list of unsafe multi-word phrases, filter out any message whose text contains one of those phrases as a contiguous word sequence. Tokenize correctly so punctuation doesn't break matches. Implement using a Trie over phrase tokens, then discuss time and space complexity.

“This was a meaty one.”

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Whatnot·Software Engineer·Technical Phone Screen

Jun 2026
Did a technical phone screen for a software engineer role at Whatnot. Pretty short session, just one coding problem and that was about it.
  • Given a sorted array of integers, return a new array of the squares of each number, also in sorted order.

“Classic two-pointer problem once you see it, but I fumbled around for a bit thinking about just squaring everything and sorting after.”

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Whatnot·Software Engineer·Technical Phone Screen

Jul 2026
Whatnot software engineer interview with a two-part coding problem around building a content safety filter for a chat platform. The problem started straightforward but the follow-up pushed into trie territory pretty fast.
  • Given a list of chat messages and a list of banned words, return only the messages whose text contains none of the banned words. Tokenization is whitespace-only, punctuation stays attached to tokens, and matching is case-insensitive.
  • Extend the filter to support banned phrases (multi-word sequences). A phrase matches only if its words appear as consecutive tokens in the message, in order, case-insensitively. The follow-up asks you to do this efficiently using a single left-to-right pass over each message's tokens rather than checking every phrase independently at each position.

“The punctuation rule is what trips people up.”

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Whatnot·Mobile Engineer·Technical Phone Screen

Jul 2026
Whatnot mobile interview had me build a small app with two list views that had to stay in sync. The time pressure was real and caught me off guard more than the actual problem did.
  • Build a small app with two list views that both fetch and update data, where changes made in one view are reflected in the other in real time.

“The sync part is what trips you up if you're not thinking about it from the start.”

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Whatnot·Frontend Engineer·Onsite - Behavioral / Leadership

Jul 2026
Behavioral round at Whatnot for a frontend role that went deeper into product and data territory than I expected. The whole thing was basically one long project deep-dive centered on a conversion-rate metric, and they really wanted to get into the weeds on experimentation and trade-offs.
  • Walk me through a project where you worked to improve a business conversion-rate metric. What was the goal, what hypotheses did you form, what experiments did you run, and what happened?

“This was basically the entire interview.”

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Whatnot·Software Engineer·Onsite - System Design / Architecture

Jul 2026
One system design question for a software engineer role at Whatnot. Short and to the point, nothing else to report.
  • Design a platform like LeetCode from scratch.

“Classic system design prompt but there's actually a lot hiding under the surface.”

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Whatnot·Software Engineer·Technical Phone Screen

Jul 2026
Interviewed for a software engineer role at WhatNot and came out of it feeling genuinely rattled, less because of the problems themselves and more because of how the interviewer handled the feedback mid-interview. Two coding questions each with follow-ups in 40 minutes, and the back half turned into a pretty uncomfortable experience.
  • Given a string, remove all runs of consecutive duplicate characters. Follow-up: what if a run can contain any number of matching characters, not just pairs?
  • Merge two sorted arrays and return a sorted array of their squared values. Follow-up: handle negative numbers in the input arrays.

“The base version was fine, stack-based, explained complexity, tested it.”

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