API Testing Interview Questions Answers

Introduction – Why API Testing Is Important in Interviews

In modern software systems, APIs act as the core communication layer between frontend, backend, mobile apps, and third-party services. Because of this, most QA, Automation, and SDET interviews now strongly focus on api testing interview questions answers.

Interviewers use API testing questions to evaluate whether a candidate:

• Understands backend logic beyond UI
  
• Can validate data, business rules, and integrations
  
• Knows REST concepts, HTTP methods, and status codes
  
• Has hands-on experience with tools like Postman, SoapUI, Rest Assured, Python
  
• Can think through real-time API failure scenarios
  

This article is a complete, interview-ready guide to api testing interview questions answers, suitable for freshers, mid-level testers, and experienced professionals.

---

What Is API Testing? (Clear & Simple)

API testing is the process of validating Application Programming Interfaces to ensure they:

• Accept correct requests
  
• Enforce business rules
  
• Return accurate responses and status codes
  
• Handle errors, security, and edge cases properly
  

Simple Example

For a login API:

• Valid credentials → 200 OK + token
  
• Invalid credentials → 401 Unauthorized
  
• Missing fields → 400 Bad Request
  

API testing focuses on data and logic, not UI.

---

REST vs SOAP vs GraphQL (Interview Comparison)

Feature	REST	SOAP	GraphQL
Protocol	HTTP	XML-based	HTTP
Payload	JSON / XML	XML only	JSON
Performance	Fast	Slower	Optimized
Flexibility	High	Low	Very High
Usage	Most modern apps	Banking / legacy	Modern APIs

👉 Most api testing interview questions answers focus on REST APIs.

---

API Testing Interview Questions Answers (90+ Q&A)

Section 1: API Basics (Q1–Q20)

1. What is an API?

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

---

2. What is API testing?

API testing validates API requests, responses, status codes, headers, and business logic.

---

3. Why is API testing important?

Because APIs drive multiple applications, a single API bug can break the entire system.

---

4. Difference between API testing and UI testing?

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

---

5. What types of APIs have you tested?

Mostly REST APIs; some exposure to SOAP APIs.

---

6. What are HTTP methods?

GET, POST, PUT, PATCH, DELETE.

---

7. Difference between PUT and PATCH?

• PUT → Full update
  
• PATCH → Partial update
  

---

8. What is an endpoint?

A specific URL representing an API resource, e.g., /users/10.

---

9. What is request payload?

Data sent to the API in the request body.

---

10. What is response body?

Data returned by the API.

---

11. What is stateless API?

Each request contains all required information independently.

---

12. What is idempotency?

Multiple identical requests give the same result.

---

13. What is API versioning?

Managing API changes using versions like /v1, /v2.

---

14. What is authentication?

Verifying identity using token, API key, or credentials.

---

15. What is authorization?

Verifying access rights to resources.

---

16. What authentication types have you tested?

Bearer token, Basic Auth, API key.

---

17. What is JWT?

JSON Web Token used for secure authentication.

---

18. What is API schema?

Defines structure and data types of request/response.

---

19. What is API chaining?

Using response of one API as input for another API.

---

20. What is negative API testing?

Testing API behavior with invalid input.

---

HTTP Status Codes – Must-Know for Interviews

Code	Meaning	Usage
200	OK	Successful request
201	Created	Resource created
204	No Content	Success without body
400	Bad Request	Invalid input
401	Unauthorized	Invalid token
403	Forbidden	No access
404	Not Found	Invalid endpoint
409	Conflict	Duplicate data
422	Unprocessable	Business rule failure
500	Server Error	Backend issue

---

Section 2: API Validation & Testing Types (Q21–Q45)

21. What validations do you perform in API testing?

• Status code
  
• Response body
  
• Headers
  
• Schema
  
• Response time
  

---

22. Is status code validation enough?

No. Response data and business rules must be validated.

---

23. What is positive API testing?

Testing API with valid input.

---

24. What is negative API testing?

Testing API with invalid or unexpected input.

---

25. What is boundary value testing in APIs?

Testing minimum and maximum input values.

---

26. What is API regression testing?

Re-testing APIs after changes.

---

27. What is API smoke testing?

Basic health check of APIs.

---

28. What is API security testing?

Testing authentication, authorization, and data protection.

---

29. What is API performance testing?

Testing response time and throughput.

---

30. What is API rate limiting?

Restricting number of requests per user.

---

31. What is pagination testing?

Validating page-wise API responses.

---

32. What is filtering testing?

Testing query parameters for data filtering.

---

33. What is sorting testing?

Validating sorted API responses.

---

34. What is schema validation?

Ensuring response structure matches API contract.

---

35. What is API mocking?

Simulating API responses when backend is unavailable.

---

36. What is API rollback?

Reverting operation when a failure occurs.

---

37. What is API data consistency testing?

Ensuring same data across systems.

---

38. What is API concurrency testing?

Testing multiple requests at the same time.

---

39. What is API caching?

Storing responses temporarily for performance.

---

40. What is content-type validation?

Ensuring correct response format (JSON/XML).

---

41. What is header validation?

Validating headers like Authorization and Cache-Control.

---

42. What is response time SLA?

Maximum allowed API response time.

---

43. What is contract testing?

Validating agreement between API provider and consumer.

---

44. What is API monitoring?

Tracking API uptime and failures.

---

45. What is API throttling?

Limiting API traffic to protect backend.

---

Real-Time API Validation Example

Sample Request

POST /api/login

Content-Type: application/json

{

  “username”: “testuser”,

  “password”: “pass123”

}

Sample Response

{

  “token”: “abc.def.xyz”,

  “expires_in”: 3600,

  “userId”: 101

}

Validations

• Status code = 200
  
• Token is not null
  
• expires_in > 0
  
• userId is numeric
  

---

API Automation Snippets (Interview-Friendly)

Postman Test Script

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

  pm.response.to.have.status(200);

});

pm.test(“Token exists”, function () {

  const json = pm.response.json();

  pm.expect(json.token).to.not.be.undefined;

});

---

Rest Assured (Java)

given()

 .contentType(“application/json”)

 .body(payload)

.when()

 .post(“/login”)

.then()

 .statusCode(200)

 .body(“token”, notNullValue());

---

Python Requests

import requests

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

assert res.status_code == 200

assert “token” in res.json()

---

Scenario-Based API Testing Interview Questions Answers (15)

1. API returns 200 but incorrect data – what do you validate?
   
2. Login API works, profile API fails – possible reasons?
   
3. Token expired but API still accessible – what defect?
   
4. API works in Postman but fails in application – why?
   
5. API slow only in production – what could be the cause?
   
6. Duplicate records created – what validation missed?
   
7. Unauthorized user accesses secured API – issue type?
   
8. API crashes for special characters – what testing?
   
9. Same request returns different responses – why?
   
10. Payment deducted but order not created – what testing?
    
11. API returns null fields – how do you catch this?
    
12. API response schema changes suddenly – impact?
    
13. API fails only in CI pipeline – possible reasons?
    
14. API returns XML instead of JSON – what issue?
    
15. Partial data saved after failure – what testing missed?
    

---

How Interviewers Evaluate Your API Testing Answers

Interviewers look for:

• Clear understanding of API fundamentals
  
• Validation beyond just status codes
  
• Real-time examples from projects
  
• Logical debugging approach
  
• Tool knowledge (Postman + basic automation)
  

👉 Practical thinking > memorized definitions

---

API Testing Interview Cheatsheet

• Never trust only status code
  
• Always validate response body
  
• Test positive & negative scenarios
  
• Check headers and schema
  
• Understand authentication clearly
  
• Be ready with real examples
  

---

FAQs – API Testing Interview Questions Answers

Q1. Is API testing mandatory for QA roles?

Yes, most modern projects require API testing knowledge.

Q2. Is Postman enough for API testing interviews?

Yes for manual testing; automation knowledge is a plus.

Q3. Should freshers learn API testing?

Absolutely. It improves backend understanding.

Q4. REST or SOAP – which is more important?

REST is more commonly used today.

Q5. Biggest mistake candidates make?

Validating only the status code.