Brex Staff Software Engineer Interview Questions

1.Tell me about a system or process outside your team's direct ownership that was clearly broken. What did you do about it, and how far did you take it?

2.Walk me through a design doc or written proposal you authored that actually changed the direction of a project or team. What was the decision before your doc, and what was it after?

3.Describe a time you pushed back on a 'quick fix' that would have shipped faster but created long-term technical debt. How did you make the case, and did it cost you anything?

4.Tell me about a time you had information — about a project failure, a technical risk, or a team problem — that was uncomfortable to share broadly. How did you decide what to say, to whom, and when?

5.Tell me about a time you inherited or stumbled into an engineering problem that had no clear owner, no existing plan, and significant business risk if left unresolved. How did you structure the path forward from nothing?

6.Tell me about a design doc you wrote where the process of writing it fundamentally changed your own technical recommendation — not just polished the argument, but reversed or significantly revised what you thought the right answer was.

7.We use Kafka heavily for event-driven workflows — like card transaction events flowing through our spend management pipeline. If a downstream consumer starts lagging and consumer group offsets fall hours behind, how do you triage and recover without losing data or double-processing events?

8.In our Elixir services, we rely heavily on pattern matching and immutable data pipelines. Say you're reviewing a PR where a junior engineer has solved a complex multi-step transaction validation flow using deep nested conditionals and mutable state — it works, but it's hard to follow. How do you approach that review, and what would you suggest instead?

9.We're rolling out a new spend policy enforcement feature for Brex Empower enterprise customers — rules that evaluate in real-time during card authorization. The policy rules are complex, customer-configurable, and must not add more than 20ms to auth latency. How would you design the evaluation engine?

10.Our expense reporting service exposes a GraphQL API consumed by our React frontend and mobile apps. We're seeing N+1 query patterns causing some expense list endpoints to hammer PostgreSQL — some requests are firing 200+ individual queries. How do you fix this at the API and data layer?

11.You're designing the ledger system for Brex Business Accounts — tracking cash balances, interest accruals, and transfers for thousands of companies. Walk me through how you'd model the data and guarantee consistency when multiple events can affect the same account balance concurrently.

12.We're migrating a critical authorization microservice from a monolithic PostgreSQL-backed model to a distributed architecture with DynamoDB for low-latency reads and Kafka for async downstream propagation. The service currently handles 10,000 authorization requests per second with sub-50ms P99. How do you execute this migration without a downtime window or a latency regression?

13.Design a webhook delivery system for Brex Spend Management that notifies customers' ERP systems when transactions are approved, declined, or categorized. Assume thousands of enterprise customers each with different endpoint reliability profiles.

14.Walk me through how you'd design the receipt matching pipeline for Brex Spend Management — automatically linking uploaded receipts to the correct card transaction for thousands of concurrent users.

15.Design the multi-entity spend consolidation system for Brex Empower — a large enterprise has 40 subsidiaries each with their own Brex accounts, cards, and budgets, and their CFO needs a single real-time view of global spend across all entities.

16.Design the reimbursement payout system for Brex Spend Management — employees submit out-of-pocket expenses, managers approve them, and Brex needs to initiate ACH transfers to employees' personal bank accounts. What does the end-to-end system look like?

17.Design the global card transaction search system for Brex — finance admins at large enterprises need to search, filter, and export millions of transactions across all their employees' cards in near-real-time, including transactions from the last 60 seconds.

18.Design the fraud detection system for Brex Card — it needs to evaluate every card authorization in real-time, flag suspicious spend patterns across millions of cards, and minimize both false positives (blocking legitimate business spend) and false negatives (letting fraud through), all within the 50ms authorization budget.

19.Tell me about a time you had to bring two engineering teams to agreement on a shared technical standard — a data model, an API contract, an error handling convention. How did you get there?

20.Walk me through how you've onboarded a new engineer into a codebase or domain that was genuinely complex — not just unfamiliar, but hard. What did you build or write to make it work?

21.Tell me about a time you had to replan a project mid-execution because the technical reality didn't match the original scope. How did you communicate that to stakeholders, and what did you protect versus cut?

22.Describe a situation where you spotted a gap between how your team believed a system was behaving and how it was actually behaving in production. What did you do, and how did you change the team's mental model?

23.Tell me about a time you had to advocate for a significant re-architecture or platform investment that had no immediate product payoff — how did you frame the case, and who did you have to convince?

24.Tell me about the most difficult personnel situation you've navigated as a Staff engineer or tech lead — not a management problem, but one where your technical credibility put you in an uncomfortable position with a peer or senior colleague.

25.Estimate the total annual ACH transfer volume Brex processes for employee reimbursements. Walk me through your assumptions.

26.Card approval rates dropped 3% last Tuesday with no deploys and no incidents filed. How do you figure out what happened?

27.Estimate what it costs Brex per year in compute if every card authorization runs a real-time spend policy evaluation that takes 8ms of CPU time at our current authorization volume. Now tell me how you'd drive that cost down.

28.We're considering adding AI-powered expense categorization to Brex Spend Management — it would auto-tag every transaction with an accounting category. How would you estimate whether the accuracy is good enough to ship to enterprise customers?

29.Brex issues corporate cards to thousands of startups. Estimate how much interchange revenue Brex earns annually, then tell me what the top three levers are to grow it — and what the engineering implications of each lever are.

30.We need to migrate Brex's core ledger — which tracks balances across cards, Business Accounts, and reimbursements — from a single-region PostgreSQL deployment to a multi-region active-active setup. Current write volume is around 50,000 ledger entries per second at peak. How do you approach this without introducing balance inconsistencies or a customer-visible outage?