API Testing in Manual Testing Interview Questions

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Introduction – Why API Testing Is Important in Interviews

In today’s software systems, the backend APIs power everything—from mobile apps and web portals to integrations with third-party services. That’s why interviewers increasingly ask api testing in manual testing interview questions to check whether candidates understand how applications really work behind the UI.

For manual testers, API knowledge proves that you can:

  • Validate business logic without UI dependency
  • Catch defects earlier in the SDLC
  • Test integrations and data flow
  • Communicate better with developers

In interviews, freshers are usually tested on API fundamentals, HTTP methods, and Postman usage, while experienced candidates are evaluated on real-time scenarios, negative testing, authentication, and debugging skills.

This article is a complete interview guide with theory, examples, status codes, JSON/XML samples, and scenario-based questions, written in a simple and interview-focused style.

What Is API Testing? (Clear & Simple)

API testing is the process of testing Application Programming Interfaces by sending requests directly to the backend and validating responses for correct data, status codes, performance, and security, without using the UI.

In one line:

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

REST vs SOAP vs GraphQL

FeatureRESTSOAPGraphQL
TypeArchitectural styleProtocolQuery language
Data formatJSONXMLJSON
PerformanceFast & lightweightSlowerOptimized
FlexibilityHighLowVery high
UsageMost modern appsLegacy systemsModern frontend apps

70+ API Testing in Manual Testing Interview Questions & Answers

Basic API Testing Questions (Freshers)

  1. What is API testing in manual testing?
     Manual API testing is validating APIs by manually sending requests and checking responses using tools like Postman.
  2. Why is API testing important for manual testers?
     It validates backend logic early and reduces dependency on UI testing.
  3. What is an API?
     An API allows two software systems to communicate with each other.
  4. What tools are commonly used for manual API testing?
     Postman, SoapUI, curl.
  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/PATCH requests.
  7. What is a response payload?
     Data returned by the server after processing the request.
  8. What are HTTP headers?
     Metadata such as Content-Type, Authorization, and Accept.
  9. What is Content-Type?
     It specifies the format of the request/response (e.g., application/json).
  10. What is statelessness in REST?
     Each request is independent and does not store client state.

REST API Interview Questions

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

{

  “id”: 101,

  “name”: “Srushti”,

  “role”: “QA Engineer”

}

  1. What is a query parameter?
     A parameter passed after ? in the URL.
  2. What is a path parameter?
     A dynamic value inside the endpoint path.
  3. What is pagination?
     Splitting large responses into multiple pages.
  4. What is API versioning?
     Managing changes using /v1, /v2, or headers.
  5. What is idempotency?
     Multiple identical requests produce the same result.
  6. What is caching in REST APIs?
     Storing responses to reduce server load.

Manual API Testing Using Postman

  1. What is Postman?
     Postman is a tool used for manual and automated API testing.
  2. What is a Postman collection?
     A group of related API requests.
  3. What is a Postman environment?
     A set of variables for different environments like QA, staging, and production.
  4. What are environment variables?
     Dynamic values used across requests.
  5. How do you validate API responses manually in Postman?
     By checking status code, response body, and headers.
  6. How do you test headers in Postman?
     Validate Content-Type, Authorization, and custom headers.
  7. What is API chaining in manual testing?
     Using response data from one API in another request.
  8. How do you handle authentication APIs manually?
     By passing tokens, API keys, or basic authentication.
  9. What is Newman (basic awareness)?
     A command-line tool to run Postman collections.
  10. How do you test negative scenarios manually?
     Send invalid payloads, missing headers, or wrong endpoints.

SOAP & XML Interview Questions

  1. What is SOAP?
     A protocol for exchanging XML-based messages.
  2. What is WSDL?
     An XML document describing SOAP services.
  3. How do you test SOAP APIs manually?
     Send XML requests and validate XML responses.

<response>

  <status>SUCCESS</status>

</response>

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

HTTP Status Codes – Must Know

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

Real-Time API Validation Example (Manual Testing)

Request

POST /api/users

Payload

{

  “email”: “test@example.com”,

  “password”: “Test@123”

}

Response

{

  “id”: 501,

  “message”: “User created successfully”

}

Manual Validations

  • Status code = 201
  • id should not be null
  • Message text is correct

Automation Awareness (Interview Advantage)

Even for manual testing roles, interviewers like basic awareness of automation.

Postman Test Script (Basic)

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

SoapUI (Awareness)

  • JSONPath assertions
  • XPath assertions

Rest Assured (Awareness)

given().when().get(“/users/101”).then().statusCode(200);

Python Requests (Awareness)

import requests

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

Scenario-Based API Testing in Manual Testing Interview Questions (10+)

  1. API returns 200 but wrong data—what do you do?
     Validate business logic and log a defect.
  2. How do you test authentication manually?
     Test valid, invalid, and expired tokens.
  3. How do you test rate limiting manually?
     Send multiple rapid requests and expect 429.
  4. How do you test missing headers?
     Remove required headers and validate error response.
  5. How do you test invalid payload fields?
     Use wrong data types or remove mandatory fields.
  6. How do you test dependent APIs manually?
     Chain requests using response values.
  7. How do you test file upload APIs manually?
     Validate file size, format, and response message.
  8. How do you test backward compatibility?
     Test older API versions.
  9. How do you debug API failures?
     Check request, headers, payload, and response.
  10. How do you test API performance manually?
     Measure response time using Postman.

How Interviewers Evaluate Your Answer

Interviewers evaluate:

  • Understanding of API fundamentals
  • Ability to explain manual testing steps
  • Knowledge of HTTP methods and status codes
  • Real-time problem-solving skills
  • Clear and structured explanations

API Testing Cheat Sheet (Quick Revision)

  • Validate status code + response body
  • Test positive and negative scenarios
  • Verify headers and payload
  • Check business rules
  • Use Postman efficiently
  • Log defects with request/response details

FAQs – API Testing in Manual Testing Interview Questions

Q1. Is API testing mandatory for manual testers?
 Yes, it is a highly valued skill today.

Q2. Is Postman enough for manual API testing?
 Yes, Postman is widely used and sufficient.

Q3. Do freshers need automation knowledge?
 Not mandatory, but basic awareness helps.

Q4. Are REST APIs more important than SOAP?
 Yes, REST APIs are more common in interviews.

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