Katkat
Back

Walk Through a System You Built: Key Technical Decisions and the Hardest Part

EliseAISoftware Engineer · Technical Screen
EasySoftware EngineerTechnical ScreenBehavioral & Leadership

Last updated

Describe a technical system you implemented at a previous company, the technical decisions you made along the way and the hardest part of the work. Tests clear ownership, trade-off reasoning between real alternatives, technical depth under probing and honest reflection on what was difficult.

In a 30-minute software engineer phone screen that is mostly behavioral and experience questions, the interviewer asks you to talk about something technical you implemented at a previous company, including the technical decisions you made, and then asks what the hardest part of it was.

Clarifying Questions

  • Should the example be the most complex project, or the one most relevant to this role?
  • Is a team project acceptable if you make clear which parts were yours?
  • How deep should the answer go: architecture and trade-offs, or down to code-level details?

Part 1 — What you implemented and the decisions you made

Describe a technical thing you implemented at a previous company. Explain what it did, why it was needed, and the technical decisions you made along the way.

What This Part Should Cover

  • The problem, its constraints (scale, latency, deadlines, existing systems) and the candidate's own role
  • A short architecture overview a listener can follow without a diagram
  • Two or three key decisions, with the alternatives considered and the trade-offs accepted
  • The outcome, measured where possible

Part 2 — The hardest part

What was the hardest part of that work?

What This Part Should Cover

  • A specific difficulty, technical or organizational, rather than general time pressure
  • What the candidate tried, including approaches that did not work
  • How it was resolved, and what the candidate learned or would do differently

Hints

Hint 1

Decisions, not a feature list: For each major decision, be ready to name the alternative you rejected and the constraint that ruled it out.

Hint 2

Hard for a specific reason: Pick the difficulty that actually cost the most effort or carried the most risk, and be ready to explain why it was hard and how you got through it.

Follow-up questions

  • If you rebuilt it today, which decision would you change, and why?
  • How did you know it was working in production, and what did you measure?
  • What would break first if the load grew tenfold?
  • Who disagreed with one of your decisions, and how was it settled?