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Stripe·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026Remote

Summary

Stripe coding interview for a software engineer role, this one was a multi-part problem built around a fraud detection pipeline. The final part asked you to replace a generic label with specific error codes and then render everything as a formatted plain-text report. Pretty involved for a single session.

Questions Asked (2)

Q1

You have a multi-stage fraud detection pipeline that labels each transaction as VALID, INVALID, or SUSPICIOUS. Refactor it so that SUSPICIOUS is replaced by specific prioritized error codes (missing field, amount out of range, blocked payment method, behavior mismatch). Each transaction should output at most two error codes ordered by a configurable priority, or OK if clean. How do you design and implement this?

System DesignTechnical Trade-offsAlgorithms & Data Structures
Author's notes

This was the meat of the whole interview.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, then propose a modular design with a configurable priority list and a rule engine that evaluates each transaction against specific checks. Discuss how to integrate this into the existing pipeline, ensuring backward compatibility and testability, and cover trade-offs like performance and extensibility.

Pro tip: Emphasize configurability and observability: make the priority order easily changeable without code deployment, and log which rules triggered to aid debugging and auditing. This shows you think about production readiness and maintainability.

1. Clarify Requirements and Constraints

Ask about expected transaction volume, latency requirements, and whether the priority order should be dynamic or static. Confirm that each transaction should output at most two error codes and that 'OK' means no errors.

2. Design the Rule Engine and Priority Configuration

Propose a rule-based system where each error code corresponds to a specific validation rule. Use a configurable priority list (e.g., via config file or database) to determine the order of error codes.

3. Implement the Evaluation Logic

For each transaction, evaluate all rules, collect triggered error codes, sort them by the configured priority, and select the top two (or fewer). Return 'OK' if none triggered.

4. Integrate into the Pipeline and Ensure Testability

Refactor the existing pipeline to replace the SUSPICIOUS label with the new error codes. Write unit tests for each rule and integration tests for the overall behavior, including priority ordering.

5. Discuss Trade-offs and Extensibility

Address performance implications (e.g., evaluating all rules vs. short-circuiting), how to add new error codes, and how to handle changes in priority without redeploying.

Key Points to Mention

  • Configurable priority: Use an external configuration (e.g., JSON, YAML, or database) to define the order of error codes, allowing changes without code deployment.
  • Rule engine design: Each error code maps to a specific validation function; evaluate all rules and collect triggered codes.
  • Output format: Return at most two error codes sorted by priority, or 'OK' if no errors.
  • Backward compatibility: Ensure the refactor doesn't break existing consumers; consider a transition period or feature flags.
  • Observability: Log triggered rules and final output for debugging and auditing.
  • Performance: Consider caching rule evaluations or short-circuiting if latency is critical, but balance with completeness.

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

Q2

For the final output, produce a column-aligned plain-text report with transaction ID, status, and up to two error codes. How do you compute column widths and handle padding?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

Honestly the formatting part felt like a trap at first because it seems trivial but there are real edge cases.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements: what defines a column, how to handle missing error codes, and whether the output is for a fixed-width console or a file. Then explain that you compute each column's width as the maximum length of its content (including header) and pad each cell with spaces to that width, using left or right alignment as appropriate. Finally, discuss edge cases like truncation, Unicode, and dynamic data.

Pro tip: Mention that you would use a two-pass approach: first collect all rows and compute max widths, then format and output. This avoids buffering issues and ensures correct alignment even with streaming data.

1. Clarify requirements and constraints

Ask about the expected output format (fixed-width text, console, file), whether columns have fixed or dynamic widths, and how to handle missing or overly long values.

2. Determine column widths

For each column, compute the maximum length of the header and all cell values. This can be done in a single pass if all data is available, or in two passes if streaming.

3. Format and pad each cell

For each cell, convert to string, then pad with spaces to the computed width. Use left alignment for text and right alignment for numbers, or as specified.

4. Handle edge cases

Address missing values (e.g., empty string or placeholder), truncation with ellipsis if a value exceeds a maximum width, and Unicode characters that may have varying display widths.

5. Assemble and output

Join padded cells with a delimiter (e.g., space or pipe) and output rows. Optionally include a separator line between header and data.

Key Points to Mention

  • Two-pass approach: first compute max widths, then format rows.
  • Padding with spaces using string formatting methods (e.g., ljust, rjust, or format specifiers).
  • Handling missing error codes: use empty string or a placeholder like 'N/A'.
  • Truncation strategy for long values: truncate with ellipsis to maintain alignment.
  • Unicode and wide characters: consider using a library or wcwidth to compute display width.
  • Performance considerations: avoid repeated string concatenation; use a list and join.

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