API Developer Interview Questions & Answers

API developer interviews focus on the practical craft of building APIs — endpoint design, request/response modelling, authentication, error handling, and documentation. Where an <a href='/interview/api-engineer'>API engineer interview</a> tests platform-level API architecture (gateways, rate limiting, developer experience at scale), an API developer interview tests whether you can design and implement clean, well-documented APIs that consumers actually want to use.

Below you'll find 40-plus questions organised by the categories that appear most often in API developer loops — from REST conventions through testing and documentation. Use them to practise with our AI interviewer or as a self-study checklist.

What the API Developer Interview Process Looks Like

API developer loops test both design thinking and implementation skill. Expect at least one round where you design an API from a requirements spec.

REST Design, Resources & Endpoints

The core of API developer interviews. Interviewers test whether you understand REST conventions deeply enough to design intuitive, consistent APIs — not just build endpoints that return data.

• Design a REST API for a task management application. Walk through your resources, endpoints, HTTP methods, and response shapes.
• What makes a REST API 'RESTful'? Explain the Richardson Maturity Model levels and which level most production APIs actually reach.

  Sample Answer Guidance: The Richardson Maturity Model has four levels: Level 0 (plain HTTP/RPC), Level 1 (resources), Level 2 (HTTP verbs + status codes), and Level 3 (HATEOAS). Most production APIs reach Level 2 because Level 3 (hypermedia links for state transitions) adds complexity that many clients can't use; the pragmatic sweet spot is proper HTTP semantics with clear resource modelling.
• How do you design URL structures for nested resources? When do you use /users/123/orders versus /orders?user_id=123?
• Explain idempotency in REST APIs. Which HTTP methods are idempotent, and how do you make POST requests idempotent?

  Sample Answer Guidance: Idempotent operations produce the same result regardless of how many times they're called; GET, PUT, and DELETE are idempotent by definition. For POST, you can add an idempotency key header that clients provide (like a UUID) and store alongside the request — if the same key arrives again, return the cached response instead of reprocessing.
• How do you handle partial updates — PATCH versus PUT? What are the trade-offs of each approach?
• Design a search/filter API for a product catalogue with multiple optional filters, sorting, and pagination. What query parameter conventions do you follow?
• How do you design API responses for related resources — inline expansion, separate endpoints, or links? Compare the approaches.

  Sample Answer Guidance: Inline expansion (nested objects) reduces round-trips but increases response size and creates tight coupling; separate endpoints scale better but require multiple calls; links (HAL/JSON-LD) are flexible but require clients to parse and follow relations. I'd use inline expansion for one or two deeply-related objects, separate endpoints for optional or large collections, and links for hypermedia-driven clients.

Versioning, Documentation & Testing

Maintaining APIs over time is where most developers struggle. Interviewers test whether you can evolve an API without breaking consumers — this is the skill that separates API developers who've owned a production API from those who've only built greenfield projects. If you're also preparing for backend roles, our backend developer guide covers database and server-side topics that complement API development.

• Compare API versioning strategies: URL path (/v1/), header, query parameter, and content negotiation. Which do you prefer and why?
• How do you write an OpenAPI/Swagger specification? What sections are most important, and how do you keep the spec in sync with the implementation?
• How do you test an API? Walk through your approach to unit testing, integration testing, and contract testing for API endpoints.
• What is contract testing? How would you use Pact or a similar tool to prevent breaking changes between your API and its consumers?

  Sample Answer Guidance: Contract testing verifies that your API and its consumers agree on request/response formats without full integration tests. With Pact, each consumer writes expectations (I call GET /users and expect {id, name}), Pact generates a contract file, and your API runs against that contract to ensure it still satisfies all consumers. This catches breaking changes early — if I remove the name field, Pact fails before I deploy.
• How do you handle backward compatibility when adding new fields, removing fields, or changing response shapes?

  Sample Answer Guidance: Adding optional fields is safe — old clients ignore them. Removing fields breaks consumers relying on them — instead deprecate with a sunset header and migration period. Changing types or renaming fields requires either versioning or a parallel field (old_email alongside email). I'd use feature flags for conditional changes and communicate removal dates in release notes at least 6 months in advance.
• Design an API changelog and deprecation process. How do you communicate upcoming changes to consumers?

  Sample Answer Guidance: I'd document all changes (new fields, removed endpoints, behaviour changes) in a CHANGELOG.md versioned with releases. For deprecations, add a Deprecation header and link to migration docs, announce in email/Slack 3–6 months before removal, and mark deprecated endpoints in OpenAPI with the deprecated flag. For major versions, publish a migration guide with code examples showing how to update consumer calls.

What Interviewers Evaluate in API Developer Candidates