QA API Testing Interview Questions

Introduction – Why API Testing Is Important in QA Interviews

In modern software projects, the QA role is no longer limited to UI testing. Applications today are built on microservices, backend integrations, and third-party systems, all connected through APIs.

Because of this shift, almost every QA interview now includes qa api testing interview questions to evaluate whether a tester can:

• Validate backend functionality without UI
  
• Understand REST APIs and HTTP methods
  
• Check data correctness, not just screens
  
• Use tools like Postman or SoapUI
  
• Think in real-time failure and edge-case scenarios
  

Whether you are a manual tester, API tester, or QA automation engineer, strong preparation in qa api testing interview questions significantly increases your chances of clearing interviews.

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What Is API Testing? (Clear & Simple)

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

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

Simple Example

For a login API:

• Valid username & password → 200 OK + token
  
• Invalid credentials → 401 Unauthorized
  
• Missing parameters → 400 Bad Request
  

API testing focuses on data and logic, not UI elements.

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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
Flexibility	High	Low	Very High
Usage in QA Projects	Very High	Legacy projects	Growing

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

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QA API Testing Interview Questions and Answers (90+)

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

1. What is an API?

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

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2. What is API testing?

API testing validates backend services by checking requests, responses, status codes, headers, and business logic.

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3. Why is API testing important for QA?

Because APIs drive multiple applications, one API defect can impact the entire system.

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4. Difference between API testing and UI testing?

API testing checks backend logic, while UI testing checks frontend behavior.

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5. What types of APIs have you tested?

Mostly REST APIs; some exposure to SOAP APIs.

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6. What are HTTP methods?

GET, POST, PUT, PATCH, DELETE.

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7. What is GET request?

Used to retrieve data from the server.

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8. What is POST request?

Used to create new data.

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9. Difference between PUT and PATCH?

• PUT → Full update
  
• PATCH → Partial update
  

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10. What is DELETE request?

Used to remove data.

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11. What is an endpoint?

A specific URL representing an API resource, e.g. /users/101.

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12. What is request payload?

Data sent to the API in the request body.

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13. What is response body?

Data returned by the API after processing.

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14. What is stateless API?

Each request is independent and contains all required information.

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15. What is idempotency?

Repeating the same request multiple times gives the same result.

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16. What is authentication?

Verifying identity using token, API key, or credentials.

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17. What is authorization?

Verifying user access permissions.

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18. What authentication types have you tested?

Bearer Token, Basic Auth, API Key.

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19. What is JWT?

JSON Web Token used for secure authentication.

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20. What is negative API testing?

Testing APIs with invalid or unexpected inputs.

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HTTP Status Codes – Must Know for QA API Testing

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	No access
404	Not Found	Invalid endpoint
409	Conflict	Duplicate record
422	Unprocessable	Business rule failure
500	Server Error	Backend failure

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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
  

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22. Is validating only status code enough?

No. Response data and business logic must be validated.

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23. What is positive API testing?

Testing APIs with valid input data.

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24. What is negative API testing?

Testing APIs with invalid or missing parameters.

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25. What is boundary value testing in APIs?

Testing minimum and maximum allowed values.

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26. What is API regression testing?

Re-testing APIs after code changes.

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27. What is API smoke testing?

Basic health check of APIs.

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28. What is API security testing?

Testing authentication and authorization.

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29. What is API performance testing?

Testing response time and throughput.

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30. What is API rate limiting?

Restricting number of requests per user.

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31. What is pagination testing?

Validating page-wise API responses.

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32. What is filtering testing?

Validating query parameters.

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33. What is sorting testing?

Validating order of response data.

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34. What is schema validation?

Ensuring response structure matches API contract.

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35. What is API mocking?

Simulating API responses when backend is unavailable.

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36. What is API rollback?

Reverting operations when a failure occurs.

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37. What is data consistency testing?

Ensuring same data across systems.

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38. What is API concurrency testing?

Testing multiple requests at the same time.

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39. What is API caching?

Storing responses temporarily to improve performance.

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40. What is content-type validation?

Ensuring response format is JSON/XML.

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41. What is header validation?

Validating headers like Authorization and Content-Type.

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42. What is response time SLA?

Maximum acceptable API response time.

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43. What is contract testing?

Validating agreement between API provider and consumer.

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44. What is API monitoring?

Tracking API uptime and failures.

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45. What is API throttling?

Limiting traffic to protect backend systems.

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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

}

QA Validations

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

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Postman & Automation Basics for QA

Postman Test Script

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

  pm.response.to.have.status(200);

});

pm.test(“Token exists”, function () {

  var json = pm.response.json();

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

});

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Rest Assured (Java – Basic)

given()

  .contentType(“application/json”)

  .body(payload)

.when()

  .post(“/login”)

.then()

  .statusCode(200);

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Python Requests (Basic)

import requests

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

assert response.status_code == 200

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Scenario-Based QA API Testing Interview Questions (15)

These are very common in QA interviews:

1. API returns 200 but wrong data – what do you validate?
   
2. Login API works, profile API fails – possible reasons?
   
3. Token expired but API still accessible – defect type?
   
4. API works in Postman but not in UI – why?
   
5. API slow only in production – possible causes?
   
6. Duplicate records created – what validation missed?
   
7. Unauthorized user accesses secured API – issue?
   
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 handle?
    
12. API schema changes suddenly – impact on QA?
    
13. API fails only in CI pipeline – possible reasons?
    
14. API returns XML instead of JSON – issue?
    
15. Partial data saved after failure – what testing missed?
    

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How Interviewers Evaluate QA API Testing Answers

Interviewers look for:

• Clear understanding of API fundamentals
  
• Validation beyond status code
  
• Real-time project examples
  
• Logical debugging approach
  
• Tool knowledge (Postman mandatory)
  

👉 Practical thinking > memorized answers

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QA API Testing Interview Cheatsheet

• Never trust only 200 status
  
• Always validate response body
  
• Test negative scenarios
  
• Check headers and schema
  
• Understand authentication clearly
  
• Practice real APIs in Postman
  

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FAQs – QA API Testing Interview Questions

Q1. Is API testing mandatory for QA roles?
Yes, most modern QA roles require API testing skills.

Q2. Is Postman enough for QA API interviews?
Yes for manual API testing; automation knowledge is a plus.

Q3. REST or SOAP – which is more important?
REST is more commonly used.

Q4. Biggest mistake QA candidates make?
Validating only status codes.

Q5. How should freshers prepare?
Practice APIs using Postman with real examples.