API tests are faster, more stable, and catch business logic bugs earlier than UI tests. Build a strong API test layer and use UI tests only for user journey validation.
Strong answers cover: testing response codes, headers, payloads, error handling, authentication/authorisation, rate limiting, data validation, contract testing, and performance at the API level. Best candidates explain why API testing is often more reliable and faster than UI testing for business logic validation, and discuss contract testing for microservices.
Important modern QA skill. Candidates who only test through the UI are missing a crucial layer. API testing catches bugs faster and more reliably. Ask about specific tools and assertion strategies.