Affirm Staff Software Engineer Interview Questions

1.Tell me about a system you built or inherited where the business logic was so opaque that even your own team struggled to explain what it did. How did you fix it?

2.Walk me through a production incident where the root cause turned out to live in a completely different system than where the symptoms appeared. How did you trace it?

3.Tell me about a time you designed a feature where the obvious implementation had a non-obvious risk — something like a fraud vector, a compliance gap, or a data consistency issue. How did you catch it and what did you change?

4.Tell me about a time you pushed back on a technical decision made by someone more senior than you. What was the decision, how did you make your case, and how did it end?

5.Describe a time you had to communicate a complex system's guarantees — or lack of guarantees — to a product or business stakeholder who needed to make a decision based on it. What did you say, and what was the outcome?

6.Tell me about the last time a bug or incident turned out to require you to understand a domain completely outside your team's ownership — like credit risk logic, payment network behavior, or regulatory state — to fully diagnose. How did you build that understanding under pressure?

7.You're building a new installment loan repayment schedule generator in Python. What data structures and invariants would you use to ensure the payment amounts, due dates, and outstanding principal always stay consistent?

8.We publish loan lifecycle events — created, funded, payment received, charged off — over Kafka. A downstream team says their consumer is occasionally processing events out of order. Walk me through how you'd diagnose and fix it.

9.Affirm's underwriting service calls a feature store to get real-time ML features during checkout. Latency SLAs are tight — around 50ms end-to-end. Design the caching and fallback strategy for that feature fetch.

10.The Affirm Card needs to decide in under 200ms at the point of swipe whether to route a purchase as a BNPL installment or a standard debit from the user's Money Account. Walk me through how you'd design that decisioning service.

11.We're seeing double-charged payments appear in production — a small but non-zero percentage of repayment transactions are posting twice against users' bank accounts. ACH is involved. How do you approach finding and fixing this, and how do you make users whole?

12.Affirm's checkout latency is dominated by a synchronous chain of gRPC calls: fraud check, identity verification, credit decisioning, and offer generation — each calling the next. Product wants to expand this chain with two more services. How would you redesign it?

13.Design a payment retry system for Affirm's ACH repayments. When a repayment fails due to insufficient funds, how do you decide when and how often to retry, and how do you communicate that to the user?

14.Affirm's merchant dashboard lets partners see real-time transaction volume, approval rates, and GMV. Design the data pipeline that powers it, assuming some merchants do tens of thousands of transactions per day.

15.Design the system that generates and delivers Affirm's end-of-year tax documents — things like 1099-INTs for the Affirm Money Account — to millions of users by the IRS deadline. Walk me through the pipeline from data collection to delivery.

16.Design Affirm's virtual card provisioning system for the Affirm Card — specifically the flow that creates a single-use virtual card number scoped to a specific BNPL installment loan and binds it to an Apple Pay or Google Pay wallet. What are the consistency and security constraints you need to enforce?

17.Design Affirm's loan offer caching layer for Adaptive Checkout. We need to serve pre-computed BNPL offers to millions of shoppers at merchant checkout pages in under 100ms, but offers must reflect current underwriting policy and can't be stale by more than a few minutes. How do you build it?

18.Design Affirm's cross-product ledger reconciliation system. We need to detect and resolve discrepancies between the loan servicing ledger, the Money Account ledger, and our settlement records with card networks and ACH — daily, at scale, before we report to capital markets partners. Walk me through the architecture.

19.Tell me about a time you mentored or grew a more junior engineer into taking on a significantly larger scope. What did you do, and how did you know it was working?

20.Describe a time you identified that your team's on-call rotation was unsustainable and drove a fix. What was broken, and what did you change?

21.Tell me about a time you had to align two teams with genuinely competing priorities on a shared technical dependency. How did you get to a decision both could live with?

22.Tell me about a time you decided to slow down or stop a project your team was already executing because you recognized a risk that wasn't originally scoped. How did you make that call and sell it?

23.You've just joined a cross-functional initiative as the Staff Engineer, and you realize the existing technical approach will create significant data consistency problems down the line — but the team has months of momentum behind it. How do you handle that?

24.Tell me about a time you had to define the technical strategy for a domain your team owned, and a more senior engineer or architect strongly disagreed with your direction. How did that play out?

25.Estimate how many BNPL loans Affirm originates in a single day during peak shopping season — say, the week of Black Friday. Walk me through your assumptions.

26.Affirm's loan approval rate drops 8 percentage points overnight on a major electronics merchant's checkout page. No deploys happened, no incidents fired. How do you figure out what's going on?

27.Estimate the total ACH transaction volume Affirm processes in a year just from scheduled loan repayments — not originations, just repayments. What are the key variables, and what does that number imply for how you'd design the repayment pipeline?

28.Affirm wants to expand the Affirm Card to a new country. Walk me through how you'd estimate the engineering effort required, and what the highest-risk unknowns are.

29.Affirm's 'no late fees' model means we absorb the cost of delinquent loans differently than traditional lenders. Estimate what a 1% increase in the charge-off rate across Affirm's loan book would cost the company annually, and what that implies for how you'd prioritize a delinquency early-warning system.

30.Affirm is considering offering a 'Pay in Full' option at checkout alongside the standard installment plans — essentially letting merchants offer Affirm-processed debit payments with no interest and no installments. Estimate what percentage of Affirm's checkout volume would shift to Pay in Full if offered, and what the second-order effects on Affirm's revenue model and infrastructure would be.