API Software Testing Interviews Questions

Introduction – Why API Testing Is Important in Interviews

In today’s application architecture, APIs act as the core communication layer between frontend, backend, mobile apps, microservices, and third-party systems. Because of this, interviewers place strong emphasis on api software testing interviews questions to assess whether a candidate understands how systems really work behind the UI.

API testing interviews are designed to check:

• Backend and business-logic validation skills
  
• Understanding of REST/SOAP concepts
  
• Ability to test integrations and microservices
  
• Real-time problem-solving and debugging skills
  

For freshers, interviewers focus on API basics, HTTP methods, and Postman usage.
For experienced candidates, they expect scenario-based explanations, negative testing, security awareness, and automation basics.

This article is a complete interview preparation guide with theory, practical examples, API samples, status codes, and scenario-based REST API testing questions.

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What Is API Testing? (Clear & Simple)

API testing is the process of testing Application Programming Interfaces by sending requests and validating responses for correctness, reliability, performance, and security, without involving the UI.

In simple words:

API testing ensures the backend logic of an application works correctly.

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REST vs SOAP vs GraphQL

Feature	REST	SOAP	GraphQL
Type	Architectural style	Protocol	Query language
Data format	JSON	XML	JSON
Performance	Fast & lightweight	Slower	Highly optimized
Flexibility	High	Low	Very high
Usage	Most modern apps	Legacy systems	Modern frontend-driven apps

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80+ API Software Testing Interviews Questions & Answers

Basic API Testing Questions (Freshers)

1. What is an API?
   An API allows two software systems to communicate with each other.
   
2. What is API software testing?
   Testing APIs to validate request handling, response data, and business logic.
   
3. Why is API testing important?
   It validates core functionality early and independently of the UI.
   
4. What are common API types?
   REST, SOAP, and GraphQL.
   
5. What is an endpoint?
   A URL where an API receives requests.
   
6. What is a request payload?
   Data sent to the server in POST/PUT requests.
   
7. What is a response payload?
   Data returned by the server.
   
8. What are HTTP headers?
   Metadata such as Content-Type and Authorization.
   
9. What is statelessness in REST?
   Each request is independent and contains all required data.
   
10. What is idempotency?
    Multiple identical requests produce the same result.
    

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REST API Interview Questions

11. Which HTTP methods are commonly used?
    GET, POST, PUT, PATCH, DELETE.
    
12. Difference between GET and POST?
    GET retrieves data; POST creates data.
    
13. Difference between PUT and PATCH?
    PUT replaces the entire resource; PATCH updates part of it.
    
14. What is JSON?
    A lightweight data-interchange format.
    

{

  “id”: 101,

  “name”: “Srushti”,

  “role”: “QA Engineer”

}

15. What is a query parameter?
    Parameters passed after ? in a URL.
    
16. What is a path parameter?
    Dynamic values inside the endpoint path.
    
17. What is pagination?
    Dividing large responses into pages.
    
18. What is API versioning?
    Managing changes using /v1, /v2, or headers.
    
19. What is caching in REST APIs?
    Storing responses to reduce server load.
    
20. What is HATEOAS?
    REST principle where responses include navigation links.
    

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API Testing Tools Interview Questions

21. Which tools are commonly used for API testing?
    Postman, SoapUI, Rest Assured, curl.
    
22. What is Postman?
    Postman is a popular tool for manual and automated API testing.
    
23. What is a Postman collection?
    A group of related API requests.
    
24. What are Postman environments?
    Variable sets for QA, staging, and production.
    
25. What is API chaining?
    Using response data from one API in another request.
    
26. What is SoapUI used for?
    Testing REST and SOAP APIs with assertions.
    
27. What is Rest Assured?
    A Java library used for API automation testing.
    
28. Can APIs be tested without UI?
    Yes, API testing is independent of UI.
    

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SOAP & XML Interview Questions

29. What is SOAP?
    A protocol for exchanging XML-based messages.
    
30. What is WSDL?
    An XML document that describes SOAP services.
    
31. How do you test SOAP APIs?
    Send XML requests and validate XML responses.
    

<response>

  <status>SUCCESS</status>

</response>

32. How do you validate XML responses?
    By checking nodes, values, and schema.
    

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HTTP Status Codes – Must Know for Interviews

Status Code	Meaning
200	OK
201	Created
204	No Content
400	Bad Request
401	Unauthorized
403	Forbidden
404	Not Found
409	Conflict
500	Internal Server Error

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Real-Time API Validation Example

Request

POST /api/users

Payload

{

  “email”: “test@example.com”,

  “password”: “Test@123”

}

Response

{

  “id”: 501,

  “message”: “User created successfully”

}

Validations

• Status code = 201
  
• id exists
  
• Success message is correct
  

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Automation Awareness (Interview Advantage)

Postman Test Script

pm.response.to.have.status(201);

pm.expect(pm.response.json().id).to.exist;

Rest Assured (Java)

given()

.when()

.get(“/users/101”)

.then()

.statusCode(200);

Python (Requests)

import requests

assert requests.get(url).status_code == 200

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Scenario-Based API Software Testing Interview Questions (10+)

41. API returns 200 but incorrect data. What do you do?
    Validate business logic and raise a functional defect.
    
42. How do you test authentication APIs?
    Test valid, invalid, and expired tokens.
    
43. How do you test rate limiting?
    Send multiple requests and expect a 429 response.
    
44. How do you test negative scenarios?
    Invalid payloads, missing headers, wrong data types.
    
45. How do you test dependent APIs?
    Chain requests and validate output data.
    
46. How do you test file upload APIs?
    Validate file size, format, and response message.
    
47. How do you test backward compatibility?
    Test older API versions with new backend.
    
48. How do you debug API failures?
    Check headers, payload, logs, and response body.
    
49. How do you test API performance manually?
    Measure response time and compare with SLA.
    
50. How do you test concurrency issues?
    Send parallel requests and verify data consistency.
    

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How Interviewers Evaluate Your Answer

Interviewers look for:

• Strong API fundamentals
  
• Clear understanding of HTTP methods and status codes
  
• Ability to explain real-time testing scenarios
  
• Awareness of manual and automation tools
  
• Logical and structured explanations
  

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API Software Testing Cheat Sheet (Quick Revision)

• Validate status code + response body
  
• Cover positive and negative test cases
  
• Verify headers and payload
  
• Check business logic
  
• Use Postman efficiently
  
• Log defects with request & response details
  

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FAQs – API Software Testing Interviews Questions

Q1. Are API software testing interviews questions hard?
No, strong fundamentals and practice are enough.

Q2. Do freshers need automation knowledge?
Not mandatory, but basic awareness helps.

Q3. Is Postman enough for API testing interviews?
Yes, Postman is widely used for manual API testing.

Q4. Which API style is most common in interviews?
REST APIs are the most common.