The Top 20 Tricky API Testing Interview Questions to Watch Out For

API testing is a crucial component of the software quality assurance process, with the aim to ensure APIs function optimally, reliably, and securely.

If you're prepping for an API testing interview, brace yourself for questions that not only test your technical knowledge but also your problem-solving skills. Below are 20 tricky questions that you might encounter, designed to stretch your understanding and gauge your real-world expertise.

1. What distinguishes API testing from unit testing, and why is it important?
2. Can you explain the difference between SOAP and REST APIs and where you would use each?
3. How do you validate responses from API calls?
4. What HTTP methods are typically used in RESTful APIs, and what do they mean?
5. Could you discuss the most critical aspects to consider when performing API security testing?
6. How would you handle API testing for a system that is not yet fully developed?
7. Explain what is meant by ‘rate limiting’ in API testing and why it's important.
8. Describe an approach to test an API that has rate limiting implemented.
9. What are the common status codes returned by APIs, and what do they signify?
10. How do you test an API that uses OAuth for authentication?
11. Can you illustrate a time you automated API testing? What tools did you use, and what challenges did you face?
12. Discuss how you would validate JSON or XML responses in API testing.
13. How would you conduct performance testing on APIs?
14. What are API mockups, and how do you use them in testing?
15. Explain the term 'API Chaining' and how you would test it.
16. What is an API endpoint, and how do you verify it?
17. How do you identify and handle dependencies when testing APIs?
18. What’s your strategy for testing APIs that are not documented?
19. Explain the concept of 'statelessness' in REST APIs and its implications for testing.
20. What is a 'breaking change' in an API, and how would you detect it during testing?

Approach these questions as opportunities to display not just your technical knowledge but also your critical thinking and ability to apply concepts in practical scenarios. Your interviewer is looking for clarity of thought, depth of understanding, and a knack for detail—qualities that are indispensable for a proficient API tester.

Remember, the trickiest questions often have no single right answer; they are a launchpad for discussion that reveals your experience level, thought process, and problem-solving abilities. So, take a deep breath and tackle each question with confidence and poise.