Software Engineer · Phone Screen · Coding — Stripe

Stripe — Software Engineer ✅ Passed

Level: Senior-Level

Round: Phone Screen · Type: Coding · Difficulty: 6/10 · Duration: 60 min · Interviewer: Unfriendly

Topics: Data Integrity, Fraud Detection, Risk Control Rules, User Behavior Analysis, Error Reporting, CSV Processing

Location: San Francisco Bay Area

Interview date: 2026-01-20

Summary

This phone screen involved a coding exercise focused on fraud detection. The exercise was divided into four parts, with each part building upon the previous one.

Details

During the 60-minute phone screen, I spent about 45 minutes on the coding exercise. The exercise had multiple parts, and I could only unlock the next part after completing the current one.

Part 1: Verify Transaction Data Integrity This part focused on establishing a foundation for fraud detection by verifying the integrity of the data. I had to read six fields from a CSV file and determine if they were all non-empty.

Part 2: High-Risk Rule Validation This part built upon the basic transaction information by adding two core risk control rules:

Amount rule: The amount must be within the business-defined normal range.
Payment method rule: The payment method cannot be in the blocked method list.

If any of these conditions were violated, the transaction was marked as SUSPICIOUS.

Part 3: User Behavior Matching This part required me to verify the degree to which a transaction matched the user's behavioral pattern. I needed to ensure that the transaction's behavioral characteristics (such as frequently used countries, time periods, and average transaction amount ranges) matched at least 50% of the user's historical behavior. This involved feature extraction, normalization, and calculating a match ratio. If the current transaction's match with user behavior was less than 50%, it was considered a behavior mismatch and marked as SUSPICIOUS.

Part 4: Smart Fraud Error Reporting The last part involved implementing smart error reporting, replacing the generic "SUSPICIOUS" label with specific error codes. The system was required to output a maximum of two error codes based on priority. If there were no errors, it should output "OK". The report output needed to maintain column alignment for readability.