30+ API Testing Interview Questions and Answers

Ask any seasoned QA professional and they’ll tell you: testing today is about far more than clicking through a user interface. With modern software architectures becoming increasingly API-first, the ability to test APIs has evolved from a "good-to-have" into a non-negotiable core skill.

More and more interviews are focusing on API Automation testing—not just on theoretical knowledge, but on real-world experience, automation skills, and the ability to troubleshoot across complex service integrations. If you're aiming for a role in QA, SDET, or Test Automation Engineering, you need to be ready to talk in depth about:

• Authentication challenges

• Schema validation

• CI/CD integration

• Real-world test scenarios

• Security and performance testing

This blog doesn’t just skim the surface. It dives deep into 30+ curated API testing interview questions, crafted from real interviews and years of hands-on QA experience. Each question includes practical, professional answers to help you stand out in technical discussions and interviews.

Let’s begin.

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Section 1: API Testing Basics

1. What is API Testing, and why is it important?

Answer: API testing validates the functionality, reliability, performance, and security of application programming interfaces. It's critical because it helps detect bugs at an early stage, especially in the service layer, which is more stable than the UI.

2. What are the types of APIs, and how do you test them?

Answer: Common types are REST, SOAP, and GraphQL APIs. REST uses HTTP methods; SOAP uses XML and follows strict standards; GraphQL allows querying only required data. Testing involves verifying endpoints, responses, status codes, schema, authentication, and edge cases.

3. Explain the structure of an HTTP request.

Answer: An HTTP request consists of:

• Method (GET, POST, etc.)

• URL/Endpoint

• Headers (e.g., Content-Type)

• Body (for POST, PUT)

• Authentication token (if required)

4. What are common HTTP response status codes?

Answer:

• 200: OK

• 201: Created

• 204: No Content

• 400: Bad Request

• 401: Unauthorized

• 403: Forbidden

• 404: Not Found

• 500: Internal Server Error

5. Difference between PUT, POST, and PATCH?

Answer:

• POST: Create a new resource

• PUT: Update/replace an entire resource

• PATCH: Update part of a resource

6. What are the key validations in API testing?

Answer:

• Response status codes

• Response body data (fields, values)

• Schema validation

• Response time

• Headers

• Error message consistency

7. What is contract testing in APIs?

Answer: Contract testing ensures that the interaction between services follows an agreed contract (e.g., OpenAPI spec). It helps detect breaking changes early.

8. What is idempotency in API testing?

Answer: An idempotent API produces the same result even after multiple identical requests. E.g., DELETE and PUT methods should be idempotent.

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Section 2: Tooling & Automation Questions

9. Which tools have you used for API testing?

Answer: Postman (manual and automated collections), Rest Assured (Java-based automation), Karate, Swagger, Insomnia, JMeter (for performance), and Newman (CLI for Postman).

10. How do you use Postman for API testing?

Answer: By creating collections, writing tests in JavaScript, using environments and variables, chaining requests, and executing via Collection Runner.

11. What is the use of Newman?

Answer: Newman is Postman’s CLI companion. It helps run Postman collections in CI/CD pipelines and generates detailed reports.

12. How do you validate JSON response in Postman?

Answer: Using tests tab with JavaScript:

pm.test("Status code is 200", function () {
    pm.response.to.have.status(200);
});
pm.test("Contains userId", function () {
    var jsonData = pm.response.json();
    pm.expect(jsonData).to.have.property('userId');
});

13. How do you handle dynamic data in API tests?

Answer: By using variables in Postman or extracting data from one request and using it in the next (chaining). In automation, use UUIDs, timestamps, or dynamic payloads.

14. How do you perform data-driven testing?

Answer: In Postman using CSV/JSON files with Collection Runner. In Rest Assured using data providers (like TestNG), or reading from external files like Excel or JSON.

15. How do you integrate API tests into CI/CD?

Answer: Using tools like Jenkins/GitLab/GitHub Actions. For Postman, Newman scripts are added to pipeline YAML; for code-based, Maven/Gradle commands are added.

16. What is schema validation and how do you perform it?

Answer: Schema validation ensures the response structure adheres to expected JSON schema. Tools: Postman (tv4, Ajv), Rest Assured with JSON schema validator.

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Section 3: Scenario-Based and Practical Questions

17. How would you test an undocumented API?

Answer:

• Use network tools (browser DevTools, Charles Proxy)

• Observe request/response behavior

• Use Swagger if partially available

• Collaborate with developers

• Trial and error with Postman

18. How do you validate chained API workflows?

Answer: Store data (e.g., token, userID) from the response of one API and use it in subsequent requests. Use global/environment variables or scripting.

19. How do you test API pagination?

Answer:

• Verify page size, page number parameters

• Check consistency across pages

• Validate total record counts and boundaries

20. How do you handle authentication in tests?

Answer:

• API keys in headers

• Bearer tokens (OAuth/JWT)

• Basic Auth

• Automate token generation if expired frequently

21. How do you test file upload and download APIs?

Answer:

• For upload: Use multipart/form-data

• For download: Validate content-disposition, file format, and data integrity

22. What is rate limiting and how to test it?

Answer: APIs restrict the number of requests per time. Use tools/scripts to send rapid requests and verify rate limit headers and 429 responses.

23. How do you mock APIs?

Answer: Using tools like Mockoon, WireMock, Postman Mock Server. Helpful when backend isn’t ready or for simulating edge conditions.

24. What challenges have you faced in API testing?

Answer:

• Unstable endpoints

• Frequent contract changes

• Auth token expiration

• Poor documentation

• Test data dependency

25. How do you test APIs in a microservices architecture?

Answer:

• Validate individual services

• Perform contract testing

• Use service virtualization

• Test message queues if involved

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Section 4: Security and Performance Testing

26. How do you test API security?

Answer:

• Token tampering

• Role-based access

• Injection attacks (SQL/command)

• TLS/SSL validation

• OWASP API Top 10 awareness

27. What tools do you use for API performance testing?

Answer: JMeter, k6, Apache Benchmark, Artillery. Measure TPS, latency, 95th percentile, and error rate.

28. How do you handle flaky API tests?

Answer:

• Add retries

• Improve wait conditions

• Mock unstable dependencies

• Add better assertions

29. What is the role of contract testing tools like Pact?

Answer: Pact verifies that a consumer and provider are aligned on API contracts. Prevents integration failures due to miscommunication.

30. Explain how GraphQL API testing differs from REST.

Answer: GraphQL allows flexible queries. Testing involves:

• Query structure validation

• Response shape matching the query

• Error handling for invalid fields

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End Note;

API testing isn’t just sending requests and getting responses—it’s about understanding the entire communication lifecycle. Mastering these questions prepares you not just to pass interviews but to lead robust test strategies.

If you’re preparing for a QA or SDET interview and want hands-on experience beyond theory, Testrig Technologies is here to help!

📩 Contact to leading API Testing Company to boost your career readiness.