Brex Senior Software Engineer Interview Questions

1.Tell me about a production incident or broken system you discovered that wasn't your team's responsibility. What did you do, and how far did you take it?

2.Describe a time you had to share news your team or stakeholders didn't want to hear — a missed deadline, a flawed technical decision, a scope you couldn't deliver. How did you handle it, and what was the fallout?

3.Walk me through a design doc or RFC you wrote that actually changed what your team built. What was the disagreement it resolved, and how did the written form matter?

4.Tell me about a time you inherited a broken or dysfunctional system — bad architecture, no tests, missing ownership — and had to decide how much to fix versus how much to work around. What was your call, and how did you make it?

5.Tell me about a time you pushed back on taking a shortcut — a quick fix, a fast migration path, a 'good enough' schema — when you knew the right solution would take significantly longer. How did you make the case, and did it hold up?

6.Walk me through a time your written proposal — a design doc, a 6-pager, an RFC — was rejected or significantly pushed back on. What was the core objection, how did you respond, and what did the final outcome look like?

7.In Elixir, what's the difference between using a GenServer and a plain module with pure functions? When would you actually reach for one over the other?

8.We use GraphQL on top of our microservices layer for the Brex dashboard. What are the main performance failure modes you'd watch for in a GraphQL API serving finance-team users, and how do you address them?

9.We're seeing duplicate expense records appearing in the database intermittently after a network retry storm hit our expense submission service. How would you design the idempotency layer to prevent this, and what would the data model look like?

10.Walk me through how you'd design the authorization flow for a Brex Card transaction — from the moment the merchant terminal sends an auth request to when we respond with approve or decline. What are the latency constraints and failure modes you'd design for?

11.We have a Kafka consumer in our expense pipeline that's falling behind — consumer lag is growing and it's been running for 6 hours. How do you triage it, and what are the architectural levers you have to recover?

12.Brex is expanding Empower to support multi-entity companies — a parent company with 20 subsidiaries, each with their own budgets, cards, and expense policies, but consolidated reporting at the top. Walk me through how you'd model the data layer and what the hardest consistency problems are.

13.Walk me through how you'd design the real-time spend limit enforcement system for Brex Card — specifically how you'd ensure a cardholder can't exceed their budget even when multiple transactions are flying in simultaneously.

14.Design a webhook delivery system for Brex's platform that notifies customers when key spend events occur — transactions posted, expenses approved, reimbursements paid. How do you guarantee delivery without overwhelming customer endpoints?

15.Design the expense reimbursement payout system for Brex — employees submit out-of-pocket expenses, and Brex needs to pay them back via ACH. Walk me through the end-to-end flow and where the hard problems are.

16.Design a global spend policy engine for Brex Empower — a system where enterprise admins can define rules like 'no first-class flights', 'meals capped at $75 per person', or 'require receipt over $25', and those rules are evaluated in real-time at transaction time across 50,000 employees worldwide.

17.Design the double-entry ledger system that underpins Brex Business Account — every credit, debit, transfer, fee, and yield credit needs to be recorded correctly, queryable in real time, and consistent even when the services writing to it are distributed.

18.Design Brex's multi-currency expense and reporting system for global Empower customers — a company with entities in the US, UK, Germany, and Singapore needs accurate expense reports in each local currency, consolidated P&L in USD, and correct FX rate application at time-of-transaction. What breaks at scale and how do you design around it?

19.Tell me about a time you had to get an engineer or team you had no authority over to change how they were building something. How did you make it happen?

20.Describe a time you mentored a more junior engineer through a problem that was genuinely hard — not just walking them through syntax, but helping them think through a real design or debugging challenge. What was your approach?

21.Tell me about a time you identified a gap in your team's technical direction — something nobody had formally decided — and took it on yourself to define and align the team around it. How did you do it and what happened?

22.Walk me through a time when two teams you depended on had conflicting priorities, and your project was caught in the middle. How did you unblock yourself?

23.Tell me about a time you changed how your team made technical decisions — the process itself, not just one specific decision. What was broken, what did you change, and how did you know it was better?

24.Describe a time you had to advocate for slowing down or stopping a project that had significant momentum — stakeholders were bought in, engineers were mid-build, and you believed it was the wrong path. What did you do and what happened to your standing after?

25.Estimate the total number of expense transactions Brex processes in a single business day. Walk me through your reasoning.

26.Brex offers rewards points on corporate card spend. If a bug caused reward points to be double-credited for 48 hours before anyone noticed, how would you estimate the financial exposure and what's your first move?

27.Card authorization approval rates dropped 2% last Tuesday between 2–4 PM. No deploys happened, no alerts fired. Walk me through how you'd diagnose it.

28.Estimate what a 10ms increase in average latency on Brex's card authorization API would cost the business annually. What are the inputs you'd need and how would you build the model?

29.Brex's bill pay product needs to support a company that processes 10,000 vendor invoices per month across 15 currencies. Estimate the FX conversion volume and model where the error accumulation risks are over a fiscal quarter.

30.Brex is deciding whether to build a native travel booking experience inside Empower or deepen the integration with a third-party travel management company. You're asked to structure the build-vs-integrate analysis. How do you frame it, and what's your recommendation framework?