Brex Software Engineer Interview Questions

1.Tell me about a bug or system issue you discovered that technically belonged to another team. What did you do, and how far did you take it?

2.Walk me through a design doc or written proposal you wrote that actually changed how your team built something. What was the decision, and how did writing it down move things forward?

3.Tell me about a time you pushed back on a shortcut — a quick fix, a schema change, a skipped migration — because you knew it would cause problems later. How did you make your case, and what happened?

4.Tell me about a time you had to share bad news — a timeline slip, a production incident you caused, a design flaw you found late — with stakeholders who weren't expecting it. How did you approach it and what came of it?

5.Describe a time you were handed a vague problem — no clear owner, no clear requirements — and had to figure out from scratch what needed to be done. How did you structure it, and how did you get it to closure?

6.Tell me about a technical proposal you wrote that faced real pushback — where someone read your doc and disagreed with your approach. How did you handle the debate, and did you end up changing your mind?

7.You're building a Kotlin service that processes expense submissions. How would you model a money amount — say, $10.50 USD — to avoid rounding errors, and why does it matter here?

8.In a React + GraphQL frontend like Brex's spend dashboard, how do you decide what state lives in the GraphQL cache versus local React state? Give me a concrete example from a spend management context.

9.We publish a 'transaction.authorized' event on Kafka whenever a Brex card swipe is approved. Now a new team wants to consume that event to update an expense report in near-real-time. What could go wrong and how would you design the consumer to be resilient?

10.Brex's card authorization service needs to evaluate a spend policy — like 'no transactions over $500 at restaurants' — in under 100ms. How would you design the policy evaluation layer to hit that latency target at scale?

11.Walk me through how you'd design the double-entry ledger system for Brex's Business Account — specifically, how you'd model a wire transfer that debits the sender and credits the recipient atomically, with an audit trail that's immutable.

12.Brex's expense microservices communicate over gRPC. We're seeing a class of production incidents where a spike in one downstream service causes cascading timeouts across the entire request graph. How would you systematically address this at the infrastructure and code level?

13.Design a rate-limiting system for Brex's card transaction API — one that prevents a single compromised merchant from hammering our authorization endpoint while still letting legitimate high-volume businesses transact freely.

14.Brex needs to send real-time spending alerts — things like 'your card was just used at a restaurant for $240' — to business owners via push, email, and SMS. How would you design that notification fanout?

15.Design the reconciliation service that matches Brex card transactions against bank settlement files — basically, how do we detect when what our card network settles doesn't match what we recorded at authorization?

16.Brex Spend Management lets finance admins define policies like 'employees can only submit receipts in USD and EUR, capped at $200 per transaction, with manager approval over $100.' How would you design the policy engine that evaluates these rules at submission time?

17.Design the real-time fraud scoring service that sits in Brex's card authorization path — it needs to score every transaction before we approve or decline it, and it can't add more than 20ms to the auth latency.

18.Brex needs to support multi-currency expense reporting for Brex Empower — an employee in London pays £180 for a client dinner, submits it, and the company's finance team in San Francisco needs to see it in USD on their dashboard and book it at the right FX rate. Design the currency conversion and storage layer for this.

19.Tell me about a time you had to learn a new codebase or language quickly to get something done. How did you approach it, and what did you actually ship?

20.Think about a time you disagreed with a teammate's approach to something — a data model, an API contract, a testing strategy. How did you handle the disagreement day-to-day, not just in a meeting?

21.Tell me about a time you noticed your team's process was creating toil — repeated manual steps, flaky tests, slow deploys — and you did something about it. What did you change and how did you measure whether it worked?

22.Tell me about a time you had to get buy-in from engineers on another team for a change that affected their service — maybe an API contract change or a shared schema migration. How did you get them aligned?

23.Walk me through a time you were the most knowledgeable person on the team about a critical system — and then you had to hand that knowledge off when you moved teams or the system changed ownership. What did you do and what got lost?

24.Tell me about the most important 'no' you've said to a product request. What were you protecting — correctness, reliability, compliance — and how did you communicate it to a PM who really wanted the feature?

25.Estimate how many Brex card transactions happen in a single day, and how much storage that generates in our transactions database.

26.The weekly active users on the Brex spend dashboard drop 8% week-over-week. No deploys happened, no incidents were filed. How do you figure out what's going on?

27.Estimate the annual cost to Brex of storing receipt images that employees upload with their expense submissions — assume Brex has about 300,000 active expense submitters.

28.A Brex enterprise customer's finance admin calls in saying their monthly spend report is showing totals that are $12,000 higher than what their accounting system shows. Both systems pull from Brex data. How do you debug this?

29.We're considering adding a 'smart budget suggestions' feature to Brex Spend Management — it would analyze a team's past spending and recommend a quarterly budget. Estimate the infrastructure cost of running this for all Brex business customers, and flag the top two risks you'd surface in a design doc before building it.

30.Brex wants to offer real-time spend velocity alerts to enterprise finance admins — like 'your engineering team just hit 80% of their monthly SaaS budget with 12 days left.' Estimate the throughput and latency requirements this places on our data pipeline, and describe the two hardest engineering problems in making it actually real-time.