Type · Motivation
What interests you about working at Duffel, specifically within the travel tech industry and our mission to simplify travel for businesses?
How to Pass the Duffel Software Engineer Interview in 2026
Growth · Software Engineer Interview Guide
Sign up to see ATSHeadquartered in United KingdomInterview language: English
The Duffel DNA (TL;DR)
Duffel's core focus on simplifying travel APIs means interviews probe for your ability to tackle complex domains like "Flights Build" and "Search Book Ancillaries Order". They assess your capacity for structured problem deconstruction and delivering robust, scalable solutions, often through detailed discussions of technical decisions.
The Duffel Interview Loop
Your onsite loop will typically consist of 5 rounds.
- 1
Round 1
Recruiter ScreenMotivation, role fit, logistics. - 2
Round 2
Coding ScreenLeetCode-medium algorithmic problems under time pressure. - 3
Round 3
System DesignDistributed systems, trade-offs at scale, architecture under constraints. - 4
Round 4
Onsite CodingLeetCode-hard problems, reasoning about defects, code clarity, edge cases. - 5
Round 5
Behavioral / LeadershipPast evidence of ownership, influence, resolving conflict.
The Danger Zone: Top Reasons Candidates Fail
Based on our database of Duffel interview outcomes, avoid these common traps:
- Focusing only on theoretical knowledge without practical examples.
- Not handling layovers correctly (e.g., negative durations).
- Not handling edge cases like impossible connections or invalid input data.
- Failing to implement proper monitoring and alerting for GDS issues.
Test Yourself: Real Duffel Questions
Three real prompts pulled from our database.
Type · Data Structures
Implement a data structure that can efficiently store and retrieve flight routes between multiple cities, supporting queries for all direct flights from a given city and all flights arriving at a given city.
Type · Algorithmic
Given a list of flight bookings (each with a departure time, arrival time, and origin/destination airport codes), find the earliest possible connection time for a passenger needing to travel between two specified cities, considering layover times.
+ many more questions, signals, and worked examples
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Duffel Interview Question Bank
A sample from our database, grouped by round. Sign up to see the full set.
9 of 13 questions shown
1
Recruiter Screen
3- 1
Type · Motivation
What interests you about working at Duffel, specifically within the travel tech industry and our mission to simplify travel for businesses? - 2
Type · Role Fit
Describe your experience with building and maintaining APIs, particularly in a context where reliability and performance are critical. - + 1 more questions in this round (sign up to unlock)
2
Coding Screen
3- 3
Type · Algorithmic
Given a list of flight bookings (each with a departure time, arrival time, and origin/destination airport codes), find the earliest possible connection time for a passenger needing to travel between two specified cities, considering layover times. - 4
Type · Data Structures
Implement a data structure that can efficiently store and retrieve flight routes between multiple cities, supporting queries for all direct flights from a given city and all flights arriving at a given city. - + 1 more questions in this round (sign up to unlock)
3
System Design
3- 5
Type · API Design
Design the API for Duffel's flight search service. Consider how users will specify search criteria (dates, destinations, number of passengers, cabin class) and how results will be returned, including pagination and filtering. - 6
Type · Scalability
How would you design a system to handle a sudden surge in flight searches during a major travel sale event (e.g., Black Friday)? Discuss potential bottlenecks and solutions. - + 1 more questions in this round (sign up to unlock)
4
Onsite Coding
1- 7
Type · Edge Cases
Write a function that takes a list of flight segments and returns the total travel time, accounting for time zone differences and potential layovers. Consider edge cases like flights crossing the international date line or very short/long layovers.
5
Behavioral / Leadership
3- 8
Type · Conflict Resolution
At Duffel, we often have to choose between supporting a legacy GDS protocol that is brittle but essential for inventory, versus building a custom abstraction layer that improves developer experience but adds maintenance overhead. Tell me about a time you had to advocate for a specific technical trade-off in a high-stakes integration. How did you navigate the conflicting priorities between short-term delivery and long-term system maintainability? - 9
Type · Ownership
Describe a situation where you took ownership of a challenging technical problem or project that fell outside your immediate responsibilities. What steps did you take, and what was the impact? - + 1 more questions in this round (sign up to unlock)
Unlock all 13 Duffel questions, free
No credit card. Every question with its framework, the grading signals interviewers score against, and a worked answer for each.
Interview tracks at Duffel
How Duffel's DNA translates across functions. Pick your role.
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Practice Duffel interviews end-to-end
Duffel Mock Interview
Run a live mock interview with our AI interviewer using Duffel-style prompts. Get scored on structure, signal, and answer length - exactly how the real loop grades you.
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STAR Stories for Duffel Behavioral Rounds
Build a Story Bank of your past wins, mapped to the leadership signals Duffel interviewers grade on. Reuse them across every behavioral round.
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Duffel Interview Prep Hub
The frameworks behind every Duffel round: CIRCLES for product sense, hypothesis-driven debugging for analytical, STAR for behavioral. Learn each one in 10 minutes.
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Interview Frameworks
CIRCLES, STAR, AARRR, RICE, MECE. The exact frameworks that make Duffel interviewers nod instead of frown. Step-by-step playbooks with the moves and the pitfalls.
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Sample answers
What a strong answer to these Duffel interview questions shows.
What interests you about working at Duffel, specifically within the travel tech industry and our mission to simplify travel for businesses?
A strong answer shows: Genuine interest in travel tech; Understanding of Duffel's value proposition.
Implement a data structure that can efficiently store and retrieve flight routes between multiple cities, supporting queries for all direct flights from a given city and all flights arriving at a given city.
A strong answer shows: Hash map usage; Adjacency list implementation; Understanding of time/space complexity trade-offs.
Frequently asked questions
How long does the Duffel interview process take?
Most candidates spend between 4 and 8 weeks from recruiter screen to offer. The onsite loop itself runs in a single day or is split across two half-days, with debrief and offer typically within 5 business days after.
How should I prepare specifically for Duffel?
Focus on three things: (1) the company DNA shown above - what they actually grade for, (2) the rounds in your loop, especially the round most candidates underestimate, and (3) drilling on the question types in this guide using a structured framework like CIRCLES or STAR.
Does this apply to engineering or design roles at Duffel?
The DNA stays the same - what changes is the round mix. SWE candidates face coding screens instead of Product Sense; designers face portfolio reviews and design exercises. The "what they value" and behavioral signals carry across all functions.
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