Duffel Software Engineer interview questions & guide 2026

Our interview process is designed to evaluate your practical engineering skills and your ability to navigate trade-offs in real-world scenarios. While the specific questions may vary, the following categories represent the core areas we focus on during our assessment.

Technical Coding and API Integration

These questions assess your ability to handle data, work with JSON, and build functional solutions for real-world integration challenges.

• Given an API response with non-homogenous records and varying schemas, how would you sort and normalize this data?
• How would you structure a client-side or server-side service to aggregate data from multiple third-party endpoints?

Preparation at Duffel should focus on demonstrating how you think, not just your ability to provide a "correct" answer. We value clarity, technical depth, and the ability to articulate the "why" behind your engineering choices.

Technical Proficiency – We expect you to be comfortable with practical coding tasks. Focus on writing clean, maintainable code and being able to explain your logic under pressure.

System Design Thinking – You should be ready to discuss architecture not as a theoretical exercise, but as a series of trade-offs. Be prepared to defend your choices regarding latency, consistency, and scalability.

Problem-Solving Approach – We evaluate how you break down complex, ambiguous problems. When faced with a difficult scenario, communicate your thought process early and often to your interviewer.

Culture and Values Alignment – We are a team-oriented company. We look for candidates who are humble, collaborative, and genuinely interested in the problem space we are solving.

The Duffel interview process is designed to be rigorous yet transparent. We aim to provide a clear view of our engineering culture by focusing on practical, relevant tasks rather than abstract brain teasers. You can expect a series of stages that touch on coding, system design, and behavioral alignment, typically culminating in a conversation with our leadership team.

Our philosophy is to treat candidates as future colleagues. We strive to provide clear communication regarding expectations and feedback throughout the process. Because we value your time, we aim to keep the experience focused and meaningful, ensuring that each round provides both you and our team with the information needed to make an informed decision.

The timeline above illustrates the standard progression from initial screening to final-round interviews. Candidates should use this as a roadmap to manage their preparation energy, ensuring they are equally ready for both the deep-dive technical sessions and the behavioral discussions. Note that while this is our standard, slight variations may occur depending on the specific team or seniority of the role.

Practical Engineering

We evaluate your ability to translate requirements into working software. This is tested through pair programming or coding exercises where you are expected to write clean, production-ready code.

Be ready to go over:

• Handling API data and error states.
• Writing tests for your code.

As a Software Engineer at Duffel, you will be responsible for building and maintaining the core services that power our platform. This involves writing high-quality code, participating in regular code reviews, and collaborating closely with product managers and designers to define technical requirements.

You will often work on integrating complex third-party travel APIs, which requires a keen eye for data consistency and error handling. Beyond individual coding tasks, you will participate in architectural discussions, helping to shape the long-term technical roadmap of our systems. Collaboration is key; you will frequently work with cross-functional partners to ensure that the technical solutions you build align with the business goals and provide tangible value to our developer community.

We look for engineers who are curious, pragmatic, and eager to solve real-world problems. While we don't require specific experience in the travel industry, a strong foundation in backend engineering is essential.

• Must-have skills: Proficient in at least one modern programming language, strong understanding of RESTful API design, and experience working with databases.
• Nice-to-have skills: Experience with distributed systems, familiarity with cloud infrastructure (e.g., AWS, GCP), and experience with modern frontend frameworks if the role touches the client-side.
• Experience level: We value depth of experience and a track record of taking projects from conception to deployment.
• Soft skills: Clear communication, a collaborative team-first attitude, and the ability to explain complex technical concepts to non-technical stakeholders.

Q: How long does the interview process typically take? The process usually spans several weeks, depending on your availability and our scheduling. We aim to be as efficient as possible while ensuring you have enough time to meet the team and understand our culture.

Q: What is the best way to prepare for the technical rounds? Focus on building small, functional projects that involve API integrations and data processing. Being comfortable with your chosen language's standard library and common data structures is more important than memorizing algorithms.

Q: Is the culture at Duffel collaborative? Yes, collaboration is a cornerstone of our engineering team. We prioritize open communication, blameless post-mortems, and a supportive environment where everyone can contribute their ideas.

Q: What if I don't have experience with the specific tech stack at Duffel? We value engineering fundamentals over specific language expertise. If you have a strong grasp of software design principles and a track record of learning new technologies, we encourage you to apply.

• Prioritize clarity: When solving problems, communicate your assumptions and constraints out loud. This helps the interviewer understand your reasoning, which is often more important than the final code.
• Be prepared to discuss trade-offs: Whenever you propose a solution, be ready to explain why it is the right choice and what the potential downsides might be.
• Review your past work: Be ready to deep-dive into the technical challenges you’ve faced in previous roles. Use the STAR method (Situation, Task, Action, Result) to structure your answers.

Joining Duffel as a Software Engineer offers a unique opportunity to tackle significant technical challenges in a fast-moving, innovative environment. By focusing on your core engineering fundamentals, demonstrating a thoughtful approach to system design, and showcasing your collaborative spirit, you will be well-positioned to succeed in our process.

We encourage you to explore additional interview insights, practice questions, and preparation resources on Dataford to further refine your approach. Remember that our interview process is designed to find great engineers who are excited about what we are building. Stay confident, be yourself, and good luck with your preparation.

The compensation data provided reflects market benchmarks for this role, including base salary and potential equity components. Candidates should interpret these ranges as a starting point for discussion, as final offers are tailored based on individual experience, technical depth, and specific team requirements.