API Testing Related Interview Questions

Introduction – Why API Testing Is Important in Interviews

Modern applications are API-driven. Web apps, mobile apps, microservices, third-party integrations—everything communicates through APIs. Because of this, interviewers across QA, Automation, and SDET roles ask api testing related interview questions to check whether a candidate truly understands how systems interact behind the UI.

Interviewers want to evaluate:

• Your understanding of backend communication
  
• Knowledge of REST and SOAP APIs
  
• Ability to validate data, logic, and errors
  
• Awareness of real-time production issues
  
• Familiarity with tools like Postman, SoapUI, Rest Assured, Python
  

This article is a complete, interview-focused guide with simple explanations, practical examples, JSON/XML samples, and scenario-based questions, suitable for freshers to experienced professionals.

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What Is API Testing? (Clear & Simple)

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

• Accept valid requests
  
• Return correct responses
  
• Follow business rules
  
• Handle errors and edge cases properly
  

Unlike UI testing, API testing:

• Works directly with requests and responses
  
• Is faster and more reliable
  
• Finds bugs earlier in the development cycle
  

Simple Example

For a User Login API:

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

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REST vs SOAP vs GraphQL (Interview Comparison)

Feature	REST	SOAP	GraphQL
Protocol	HTTP	XML-based	HTTP
Data Format	JSON / XML	XML only	JSON
Performance	Fast	Slower	Optimized
Complexity	Easy	Moderate	Moderate
Usage	Most modern apps	Banking/legacy	Modern apps

👉 Most api testing related interview questions focus on REST, but SOAP basics are still important for enterprise projects.

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API Testing Related Interview Questions & Answers (100+)

Section 1: API Fundamentals (Q1–Q20)

1. What is an API?
   An API allows two applications to communicate with each other.
   
2. What is API testing?
   Validating API requests, responses, status codes, headers, and business logic.
   
3. Why is API testing important?
   Because APIs connect systems, and a defect can affect multiple applications.
   
4. API testing vs UI testing?
   API testing checks backend logic; UI testing checks frontend behavior.
   
5. What types of APIs are commonly tested?
   REST and SOAP APIs.
   
6. What is REST?
   A lightweight architecture using HTTP methods.
   
7. What is SOAP?
   An XML-based protocol with strict contracts (WSDL).
   
8. What is an endpoint?
   A URL representing an API resource, e.g., /users/101.
   
9. What is request payload?
   Data sent to the API.
   
10. What is response payload?
    Data returned by the API.
    
11. What is statelessness?
    Each API request is independent.
    
12. What is idempotency?
    Same request repeated gives the same result.
    
13. What is authentication?
    Verifying user identity.
    
14. What is authorization?
    Verifying user permissions.
    
15. Common authentication types?
    Basic Auth, Bearer Token, API Key, OAuth.
    
16. What is JSON?
    

{ “id”: 101, “name”: “Anita” }

17. What is XML?
    

<user><id>101</id><name>Anita</name></user>

18. What is positive testing?
    Testing with valid input.
    
19. What is negative testing?
    Testing with invalid input.
    
20. What is API versioning?
    Managing changes using /v1, /v2.
    

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HTTP Methods – Interview Must-Know

Method	Purpose
GET	Fetch data
POST	Create data
PUT	Update full resource
PATCH	Update partial resource
DELETE	Remove data

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HTTP Status Codes – Very Important

Code	Meaning	Usage
200	OK	Successful request
201	Created	Resource created
204	No Content	Successful delete
400	Bad Request	Invalid input
401	Unauthorized	Invalid auth
403	Forbidden	No access
404	Not Found	Wrong endpoint
409	Conflict	Duplicate data
500	Server Error	Backend failure

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

21. What validations are done in API testing?
    Status code, response body, headers, schema, response time.
    
22. Is checking status code enough?
    No, business logic and data must be validated.
    
23. What is header validation?
    Validating headers like Authorization, Content-Type.
    
24. What is schema validation?
    Validating response structure.
    
25. What is response time testing?
    Checking how fast API responds.
    
26. What is smoke testing?
    Basic API health check.
    
27. What is regression testing?
    Re-testing APIs after changes.
    
28. What is API security testing?
    Testing authentication and authorization.
    
29. What is API performance testing?
    Testing speed, load, and scalability.
    
30. What is pagination testing?
    Testing page-wise responses.
    
31. What is filtering testing?
    Testing query parameters.
    
32. What is sorting testing?
    Testing ordered responses.
    
33. What is API chaining?
    Using response of one API in another.
    
34. What is API mocking?
    Simulating API responses.
    
35. What is rate limiting?
    Restricting number of API calls.
    
36. What is throttling?
    Controlling traffic to protect backend.
    
37. What is boundary value testing?
    Testing min and max values.
    
38. What is data consistency testing?
    Ensuring same data across systems.
    
39. What is API rollback testing?
    Ensuring no partial data on failure.
    
40. What is content-type validation?
    Ensuring JSON/XML format.
    
41. What is API documentation?
    Defines how API works.
    
42. What is Swagger/OpenAPI?
    API documentation tool.
    
43. What is API contract testing?
    Validating client-server agreement.
    
44. What is concurrency testing?
    Testing multiple users simultaneously.
    
45. What is API caching?
    Storing responses temporarily.
    

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Real-Time API Validation Example

Request

POST /api/login

{

  “username”: “testuser”,

  “password”: “pass123”

}

Response

{

  “token”: “xyz123”,

  “expiresIn”: 3600

}

Validations

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

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Postman / SoapUI / Automation Basics

Postman – Simple Test Script

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

  pm.response.to.have.status(200);

});

SoapUI – XPath Assertion

//token != ”

Rest Assured (Java – Basic Awareness)

given().when().get(“/users/1”).then().statusCode(200);

Python Requests (Basic Awareness)

import requests

res = requests.get(url)

assert res.status_code == 200

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Scenario-Based API Testing Related Interview Questions (15)

1. API returns 200 but wrong data—what do you validate?
   
2. API works in Postman but fails in application—why?
   
3. Missing parameter returns 500—is it correct?
   
4. API allows access without token—what issue?
   
5. Duplicate records created—what testing missed?
   
6. API slow for large datasets—what test?
   
7. API returns null values—how validate?
   
8. Invalid input accepted—what defect?
   
9. API returns wrong status code—impact?
   
10. Same request gives different responses—why?
    
11. Unauthorized user accesses data—issue?
    
12. API fails only in production—possible reasons?
    
13. API schema changes—what breaks?
    
14. Partial data saved after failure—what testing needed?
    
15. API rate limiting not working—impact?
    

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How Interviewers Evaluate Your Answers

Interviewers look for:

• Clear understanding of core concepts
  
• Logical thinking over memorization
  
• Ability to explain real-time scenarios
  
• Awareness of negative and edge cases
  
• Confidence and clarity
  

👉 Explaining “why” matters more than naming tools.

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API Testing Interview Cheatsheet

• Understand REST basics
  
• Know HTTP methods & status codes
  
• Validate response data, not just status
  
• Practice Postman regularly
  
• Think about negative scenarios
  
• Keep explanations simple and clear
  

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FAQs – API Testing Related Interview Questions

Q1. Are API testing questions asked for freshers?
Yes, basic API knowledge is expected.

Q2. Is Postman enough for interviews?
Yes, Postman is sufficient for fundamentals.

Q3. Do I need automation knowledge?
Not mandatory, but awareness is helpful.

Q4. Biggest mistake candidates make?
Only validating status codes.

Q5. How to prepare quickly?
Practice simple REST APIs daily.