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Stripe·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Stripe coding round for a software engineer role, building on a previous multi-currency payment problem with a new wrinkle around cross-currency balance conversions and layered fees. The core logic wasn't too bad but getting the fee math right under pressure was a different story.

Questions Asked (1)

Q1

You have a payment system where a user's balance may be in a different currency than the payment. Given a fixed exchange rate table and a conversion fee on top of the existing payment method fee, determine whether the balance covers the full cost after conversion. If it does, update the balances accordingly.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

This was a follow-up to an earlier multi-currency problem so I thought I had a head start.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then outline a step-by-step algorithm that handles currency conversion, fee calculation, and balance updates. Emphasize precision in monetary calculations and discuss trade-offs between simplicity and robustness.

Pro tip: Always use integer arithmetic for monetary values (e.g., cents) to avoid floating-point errors, and explicitly state rounding rules for fees and conversions. Mention idempotency and transactional integrity to show production readiness.

1. Clarify Requirements and Assumptions

Ask about the exchange rate table format, fee structure (percentage or fixed), rounding rules, and whether multiple currencies or concurrent updates are involved. Confirm that the balance is in a single currency and that the payment amount is fixed.

2. Define the Conversion and Fee Calculation

Outline how to convert the payment amount to the balance currency using the exchange rate, then apply the conversion fee and the existing payment method fee. Specify the order of operations and rounding at each step.

3. Check Sufficiency and Update Balances

Compare the total cost (converted amount + fees) against the balance. If sufficient, deduct the total cost from the balance and update any other relevant balances (e.g., merchant's account) atomically.

4. Handle Edge Cases and Errors

Discuss scenarios like missing exchange rates, zero or negative amounts, insufficient balance, and rounding discrepancies. Explain how to handle failures gracefully and ensure idempotency.

5. Discuss Trade-offs and Scalability

Talk about trade-offs between using floating-point vs. integer arithmetic, caching exchange rates, and handling concurrent transactions. Mention potential optimizations and how the design would scale.

Key Points to Mention

  • Use integer arithmetic (e.g., cents) to avoid floating-point precision issues.
  • Clearly define rounding rules for conversion and fees (e.g., round half up, round down).
  • Ensure atomicity and idempotency when updating balances to prevent race conditions.
  • Consider the order of applying fees and conversion (e.g., convert then fee, or fee then convert).
  • Handle missing or stale exchange rates gracefully (e.g., fallback, error).
  • Discuss how to extend the solution to support multiple currencies and dynamic exchange rates.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.