Interview Questions of API Testing

Introduction – Why Interview Questions of API Testing Matter

In modern applications, APIs are the backbone connecting web apps, mobile apps, databases, and third-party systems. While UI changes frequently, APIs remain stable and handle core business logic. Because of this, interviewers strongly focus on interview questions of API testing to evaluate a candidate’s real backend testing skills.

Whether you are a fresher, manual tester, automation tester, or experienced QA, interviewers expect you to:

• Understand API testing fundamentals
  
• Know REST concepts, HTTP methods, and status codes
  
• Validate business logic and data
  
• Use tools like Postman or SoapUI
  
• Have basic knowledge of API automation using Java or Python
  
• Explain real-time project scenarios clearly
  

This article is a complete, SEO-optimised, interview-focused guide, written in simple technical language and suitable for all experience levels.

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What Is API Testing? (Clear & Simple Explanation)

API testing is the process of testing Application Programming Interfaces directly to verify that:

• APIs accept valid requests
  
• APIs return correct responses
  
• Business rules are implemented correctly
  
• Errors are handled properly
  
• Security and performance requirements are met
  

API testing is done without UI, making it:

• Faster than UI testing
  
• More stable and reliable
  
• Ideal for early defect detection
  

Simple Example

For a User Login API:

• Valid credentials → 200 OK
  
• Invalid password → 401 Unauthorized
  
• Missing username → 400 Bad Request
  

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REST vs SOAP vs GraphQL (Interview Perspective)

Feature	REST	SOAP	GraphQL
Data Format	JSON / XML	XML only	JSON
Performance	Fast	Slower	Optimized
Popularity	Very High	Enterprise projects	Growing
Tools	Postman, Rest Assured	SoapUI	Special libraries
Interview Focus	High	Medium	Low

👉 Most interview questions of API testing focus mainly on REST APIs.

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Interview Questions of API Testing with Answers (100+)

Section 1: API & REST Fundamentals (Q1–Q20)

1. What is API testing?
   API testing validates backend services by testing requests, responses, and business logic.
   
2. Why is API testing important?
   It verifies core functionality without relying on UI.
   
3. What is a REST API?
   An API that follows REST principles and uses HTTP methods.
   
4. What does REST stand for?
   Representational State Transfer.
   
5. What are REST principles?
   Statelessness, client-server separation, cacheability, uniform interface.
   
6. What is an endpoint?
   A URL that represents an API resource.
   
7. What is a resource in REST?
   An object or entity exposed via API.
   
8. What is request payload?
   Data sent to the API.
   
9. What is response payload?
   Data returned by the API.
   
10. What is statelessness?
    Each request is independent and does not store session data.
    
11. What is idempotency?
    Same request produces the same result every time.
    
12. Difference between PUT and PATCH?
    PUT updates the full resource; PATCH updates partial fields.
    
13. What is authentication?
    Verifying user identity.
    
14. What is authorization?
    Verifying access permissions.
    
15. Common authentication methods?
    Bearer Token, API Key, OAuth, Basic Auth.
    
16. What is JWT?
    JSON Web Token used for stateless authentication.
    
17. What is JSON?
    

{

  “id”: 101,

  “name”: “Rahul”

}

18. What is XML?
    

<user>

  <id>101</id>

  <name>Rahul</name>

</user>

19. What is positive testing?
    Testing APIs with valid input.
    
20. What is negative testing?
    Testing APIs with invalid input.
    

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HTTP Methods – Core Interview Topic

Method	Purpose
GET	Retrieve data
POST	Create new data
PUT	Update full resource
PATCH	Update partial resource
DELETE	Delete resource

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HTTP Status Codes – Must-Know for Interviews

Code	Meaning	Example
200	OK	Successful GET
201	Created	Successful POST
204	No Content	Successful DELETE
400	Bad Request	Invalid input
401	Unauthorized	Invalid token
403	Forbidden	Access denied
404	Not Found	Resource missing
409	Conflict	Duplicate data
422	Validation error	Business rule failure
500	Server Error	Backend issue

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Section 2: API Validation, Tools & Automation (Q21–Q50)

21. What validations are done in API testing?
    Status code, response body, headers, schema, response time.
    
22. Is validating status code enough?
    No, response data and business logic must be validated.
    
23. What is header validation?
    Validating headers like Authorization and Content-Type.
    
24. What is schema validation?
    Validating the response structure.
    
25. What is response time testing?
    Validating API performance.
    
26. What is API smoke testing?
    Basic API health check.
    
27. What is API regression testing?
    Re-testing APIs after changes.
    
28. What is API chaining?
    Using the response of one API in another.
    
29. What is API mocking?
    Simulating API responses when backend is unavailable.
    
30. What is rate limiting?
    Restricting the number of API requests.
    
31. What is concurrency testing?
    Testing multiple users simultaneously.
    
32. What is backend validation?
    Validating database updates after API calls.
    
33. What is API security testing?
    Testing authentication and authorization.
    
34. What is environment testing?
    Testing APIs in dev, QA, UAT, and prod.
    
35. What is API contract testing?
    Validating agreement between client and server.
    
36. What is data-driven API testing?
    Testing APIs with multiple datasets.
    
37. What is logging in API testing?
    Capturing request and response logs.
    
38. What is CI/CD integration?
    Running API tests in pipelines.
    
39. What tools are used for API testing?
    Postman, SoapUI, Rest Assured, Python requests.
    
40. What is Postman?
    A tool for manual API testing.
    
41. What is SoapUI?
    A tool for SOAP and REST API testing.
    
42. What is Rest Assured?
    A Java library for API automation.
    
43. What is Python requests library?
    Python library to send HTTP requests.
    
44. What is pagination testing?
    Validating page size and page number.
    
45. What is filtering testing?
    Validating query parameters.
    
46. What is sorting testing?
    Validating sorted API responses.
    
47. What is caching in APIs?
    Temporary storage of responses.
    
48. What is cache invalidation?
    Refreshing stale cache.
    
49. Why API testing before UI testing?
    To catch backend defects early.
    
50. What is assertion?
    Validation of expected results.
    

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Real-Time API Validation Example

Request

POST /api/login

{

  “username”: “testuser”,

  “password”: “pass123”

}

Response

{

  “token”: “abc123”,

  “expiresIn”: 3600

}

Validations

• Status code = 200
  
• Token is not null
  
• Expiry time is valid
  
• Token works for secured APIs
  

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API Automation Snippets (Interview-Level)

Postman Test Script

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

  pm.response.to.have.status(200);

});

Rest Assured (Java)

given()

.when()

.get(“/users/1”)

.then()

.statusCode(200);

Python Requests

import requests

res = requests.get(url)

assert res.status_code == 200

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Scenario-Based Interview Questions of API Testing (15)

1. API returns 200 but wrong data – how do you detect it?
   
2. Duplicate records created – how do you prevent?
   
3. Token expired but API still works – issue?
   
4. API returns 500 for invalid input – correct behavior?
   
5. Same request gives different responses – why?
   
6. API slow under load – what test is needed?
   
7. Partial data saved after failure – how test rollback?
   
8. API works in Postman but fails in application – reason?
   
9. Unauthorized user accesses data – defect?
   
10. Schema change breaks clients – how to detect?
    
11. Rate limiting not implemented – impact?
    
12. API fails only in production – possible causes?
    
13. Cache returns stale data – how identify?
    
14. Bulk API partially succeeds – how validate?
    
15. Backend updated but UI shows old data – where is the issue?
    

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How Interviewers Evaluate Your API Testing Answers

Interviewers usually check:

• Conceptual clarity of REST and API fundamentals
  
• Ability to explain real-time scenarios
  
• Validation beyond just status codes
  
• Tool awareness (Postman / automation basics)
  
• Logical, structured communication
  

👉 Clear explanations with examples score higher than memorised definitions.

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Interview Questions of API Testing – Quick Revision Cheatsheet

• Understand REST & HTTP methods
  
• Memorize key status codes
  
• Validate response data, not just status
  
• Practice Postman regularly
  
• Prepare real project examples
  

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FAQs – Interview Questions of API Testing

Q1. Is API testing mandatory for QA roles?
Yes, basic API testing knowledge is expected.

Q2. Is Postman enough for interviews?
Yes, for manual testing roles.

Q3. Do freshers need API automation knowledge?
Basic awareness is sufficient.

Q4. Biggest mistake candidates make?
Only checking HTTP status codes.

Q5. How to prepare quickly for API testing interviews?
Practice CRUD APIs and scenario-based questions.