API Software Testing Interview Questions

Introduction – Why API Testing Is Important in Interviews

In modern software development, APIs act as the core communication layer between applications—web, mobile, microservices, and third-party systems. Because of this, interviewers heavily focus on api software testing interview questions to assess whether candidates truly understand how backend systems work beyond the UI.

API testing skills show that a tester can:

• Validate business logic early
  
• Detect defects before UI is built
  
• Test integrations and microservices
  
• Reduce cost and time in the SDLC
  

For freshers, interviewers test API fundamentals.
For experienced professionals, they focus on real-time scenarios, negative testing, security, and tool usage.

This article is a complete, interview-ready guide with theory, examples, code snippets, and scenario-based REST API testing questions.

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What Is API Testing? (Clear & Simple)

API testing is the process of validating Application Programming Interfaces by sending requests and verifying responses for correctness, reliability, performance, and security—without using the user interface.

In simple words:

API testing ensures that 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/enterprise	Modern frontend apps

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80+ API Software Testing Interview 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 UI.
   
4. What are the common API types?
   REST, SOAP, 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 self-contained.
   
10. What is idempotency?
    Multiple identical requests return the same result.
    

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REST API Interview Questions

11. Which HTTP methods are most 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 a resource; PATCH updates part of it.
    
14. What is JSON?
    Lightweight data-interchange format.
    

{

  “id”: 101,

  “name”: “Srushti”,

  “role”: “QA Engineer”

}

15. What is a query parameter?
    Parameter passed after ? in the URL.
    
16. What is a path parameter?
    Dynamic value inside the endpoint path.
    
17. What is pagination?
    Splitting large responses into pages.
    
18. What is API versioning?
    Managing API changes using /v1, /v2, 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 Software Testing Tools Questions

21. Which tools are commonly used for API testing?
    Postman, SoapUI, Rest Assured, curl.
    
22. What is Postman?
    Postman is a tool used for manual and automated API testing.
    
23. What is a Postman collection?
    A group of related API requests.
    
24. What are Postman environments?
    Variables for different environments like QA, staging, prod.
    
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 for API automation testing.
    
28. Can APIs be tested without UI?
    Yes, APIs are tested independently 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 file 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

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 should exist
  
• Message text 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 Testing Interview Questions (10+)

41. API returns 200 but wrong data – what do you do?
    Validate business rules and raise a defect.
    
42. How do you test authentication APIs?
    Valid, invalid, and expired tokens.
    
43. How do you test rate limiting?
    Send multiple requests and expect 429.
    
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.
    
46. How do you test file upload APIs?
    Validate file size, format, and response.
    
47. How do you test backward compatibility?
    Test older API versions.
    
48. How do you debug API failures?
    Check headers, payload, and response body.
    
49. How do you test API performance manually?
    Measure response time.
    
50. How do you test concurrency?
    Send parallel requests and verify data integrity.
    

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How Interviewers Evaluate Your Answers

Interviewers look for:

• Strong API fundamentals
  
• Understanding of HTTP methods & status codes
  
• Ability to explain real-time testing scenarios
  
• Awareness of manual + automation tools
  
• Clear and logical communication
  

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API Software Testing Cheat Sheet (Quick Revision)

• Validate status code + response body
  
• Test positive & negative scenarios
  
• Verify headers and payload
  
• Check business logic
  
• Use Postman efficiently
  
• Log bugs with request & response
  

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FAQs – API Software Testing Interview Questions

Q1. Is API testing mandatory for testers today?
Yes, API testing is a core skill.

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

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

Q4. Which APIs are more common in interviews—REST or SOAP?
REST APIs are far more common.