API Testing Interview Questions

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Introduction – Why API Testing Interview Questions Matter

In modern applications, APIs are the backbone connecting web apps, mobile apps, databases, and third-party services. Even if the UI looks perfect, a single API failure can break the entire system.

That’s why interviewers focus heavily on api testing interview questions to check whether candidates:

  • Understand backend logic beyond UI
  • Can validate data, responses, and error handling
  • Know REST concepts, HTTP methods, and status codes
  • Can test APIs manually and automate them

Whether you are a fresher, manual tester, automation engineer, or SDET, strong preparation in api testing interview questions is essential.

What Is API Testing? (Simple Explanation)

API testing is the process of validating APIs to ensure they:

  • Return correct responses
  • Handle valid and invalid inputs properly
  • Are secure, fast, and reliable

Simple Example

If a login API is called with valid credentials:

  • Status code should be 200
  • Response should contain a token
  • Invalid credentials should return 401

API testing focuses on data and logic, not UI.

REST vs SOAP vs GraphQL (Interview Comparison)

FeatureRESTSOAPGraphQL
ProtocolHTTPXML-basedHTTP
PayloadJSON / XMLXML onlyJSON
FlexibilityHighLowVery High
PerformanceFastSlowerOptimized
UsageMost modern appsBanking/LegacyModern microservices

Interviewers often include this in REST API interview questions.

API Testing Interview Questions and Answers (100+ Q&A)

Basics (Q1–Q15)

1. What is an API?

An API allows two software systems to communicate with each other.

2. What is API testing?

Validating API requests, responses, status codes, and data correctness.

3. Why is API testing important?

Because APIs power multiple applications, a single API bug can impact many systems.

4. Difference between API testing and UI testing?

API testing validates backend logic; UI testing validates frontend behavior.

5. What are HTTP methods?

  • GET – Fetch data
  • POST – Create data
  • PUT – Update full data
  • PATCH – Update partial data
  • DELETE – Remove data

6. What is REST API?

An architectural style using HTTP methods to interact with resources.

7. What is endpoint?

A specific URL that accepts API requests.
Example: /api/users/1

8. What is request payload?

Data sent in the request body.

9. What is response body?

Data returned by the server.

10. What is API schema?

Defines structure, data types, and fields of the API response.

11. What is JSON?

A lightweight data-exchange format.

{

  “id”: 101,

  “name”: “Rahul”,

  “role”: “Tester”

}

12. What is XML?

Markup language used mainly in SOAP APIs.

13. What is idempotency?

Multiple identical requests give the same result.

14. What is stateless API?

Each request is independent and carries its own data.

15. What is API versioning?

Maintaining multiple API versions (v1, v2) without breaking clients.

HTTP Status Codes (Q16–Q25)

CodeMeaningUsage
200OKSuccessful request
201CreatedResource created
204No ContentSuccessful, no response
400Bad RequestInvalid input
401UnauthorizedAuthentication failure
403ForbiddenAccess denied
404Not FoundResource missing
409ConflictDuplicate data
500Server ErrorBackend failure

16. What is 200 status code?

Request processed successfully.

17. What is 201?

Resource created successfully.

18. What is 400?

Client sent invalid request.

19. What is 401?

Authentication failed.

20. What is 403?

User is authenticated but not authorized.

21. What is 404?

Requested resource does not exist.

22. What is 409?

Conflict like duplicate records.

23. What is 500?

Internal server error.

24. Can 200 be wrong?

Yes. Data may be incorrect even if status is 200.

25. What should you validate apart from status?

Response body, headers, schema, and business rules.

API Validation & Testing Types (Q26–Q40)

26. What is positive API testing?

Testing with valid inputs.

27. What is negative API testing?

Testing with invalid or unexpected inputs.

28. What is boundary testing?

Testing minimum and maximum values.

29. What is API security testing?

Validating authentication and authorization.

30. What is API performance testing?

Testing response time and throughput.

31. What is contract testing?

Validating agreement between consumer and provider.

32. What is schema validation?

Checking response structure and data types.

33. What is API rate limiting?

Restricting number of API requests per user.

34. What is pagination?

Splitting large data into pages.

35. What is filtering in APIs?

Fetching specific data using query params.

36. What is sorting?

Ordering API response data.

37. What is API caching?

Storing response temporarily to improve speed.

38. What is token-based authentication?

Using tokens like JWT for API access.

39. What is OAuth?

Authorization framework for secure access.

40. What is API mocking?

Simulating API responses when backend is unavailable.

API Validation Example

Sample Request

POST /login

Content-Type: application/json

{

  “username”: “testuser”,

  “password”: “pass123”

}

Sample Response

{

  “token”: “eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9”,

  “expires_in”: 3600

}

Assertions

  • Status code = 200
  • Token is not null
  • Token expiry > 0

Postman / Automation Snippets

Postman Test Script

pm.test(“Status code is 200”, function () {

    pm.response.to.have.status(200);

});

pm.test(“Token exists”, function () {

    var jsonData = pm.response.json();

    pm.expect(jsonData.token).to.not.be.null;

});

RestAssured (Java)

given()

 .contentType(“application/json”)

 .body(payload)

.when()

 .post(“/login”)

.then()

 .statusCode(200)

 .body(“token”, notNullValue());

Python (Requests)

import requests

response = requests.post(url, json=payload)

assert response.status_code == 200

assert “token” in response.json()

Scenario-Based API Testing Interview Questions (15)

  1. API returns 200 but wrong data – what do you do?
  2. Login works but profile API fails – why?
  3. Payment deducted but order not created – what testing applies?
  4. API slow only in production – reason?
  5. Duplicate records created – what status code expected?
  6. Unauthorized user accesses secured API – defect type?
  7. API works in Postman but not in UI – why?
  8. Token expired but API still accessible – issue?
  9. API fails under heavy load – what test?
  10. Missing fields in response – what validation?
  11. API returns null values – what check?
  12. API crashes on special characters – what testing?
  13. Same request gives different responses – why?
  14. API works for GET but fails for POST – reason?
  15. API returns XML instead of JSON – issue?

How Interviewers Evaluate Your API Answers

They check whether you:

  • Validate more than status codes
  • Understand backend logic
  • Think in real-time scenarios
  • Explain clearly with examples

Real-time thinking > memorized answers

API Testing Interview Cheatsheet

  • Always validate response body
  • Test positive & negative scenarios
  • Check headers and authentication
  • Validate business rules
  • Never trust only 200 status

FAQs – API Testing Interview Questions

Q1. Is API testing mandatory for QA roles?
 Yes, most projects require API testing.

Q2. Is Postman enough for interviews?
 Yes for manual API testing basics.

Q3. Should freshers learn API testing?
 Absolutely. It boosts backend understanding.

Q4. REST or SOAP – which is more important?
 REST is more commonly used today.

Q5. What is the biggest API testing mistake?
 Validating only status code.

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