API Manual Testing Interview Questions -

Introduction – Why API Testing Is Important in Interviews

In today’s software landscape, applications rely heavily on APIs to connect web apps, mobile apps, databases, and third-party services. Because of this, interviewers frequently ask API manual testing interview questions to evaluate whether a candidate understands backend validation, request–response flow, and real-time defect detection—without depending on automation.

For freshers, interviewers usually focus on:

HTTP methods

Status codes

JSON basics

Postman usage

API fundamentals

For experienced testers, interviewers focus more on:

Real-time scenarios

Negative testing

API chaining

Business-rule validation

Authentication handling

Backend debugging

This guide covers theory, practical examples, real-time validations, and scenario-based interview questions specifically designed for manual API testing roles.

What Is API Testing? (Clear & Simple)

API testing is a type of software testing that validates the functionality, reliability, performance, and security of APIs (Application Programming Interfaces) by sending requests and verifying responses.

Instead of testing the graphical user interface (UI), API testing focuses on backend communication between systems. APIs act as intermediaries that allow different software applications to exchange data and communicate with each other.

API testing verifies whether APIs:

Return correct responses

Process requests accurately

Handle errors properly

Maintain security standards

Perform efficiently under load conditions

Why API Testing Is Important

Modern applications depend heavily on APIs for communication between:

Web applications

Mobile applications

Databases

Third-party services

Cloud platforms

If APIs fail, important business operations may stop functioning properly.

Areas Validated in API Testing

Functional Validation

Checks whether APIs work according to business requirements.

Data Validation

Ensures API responses contain accurate data.

Error Handling

Validates how APIs behave under invalid conditions.

Security Validation

Checks authentication and authorization mechanisms.

Performance Validation

Measures response time and scalability.

Example

Sending a GET request to:

/users/1

and validating whether the correct user details are returned in the response.

Real-Time Scenario

In a banking application, API testing verifies whether account balance APIs return accurate balance information after successful authentication.

REST vs SOAP vs GraphQL

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

70+ API Manual Testing Interview Questions & Answers

Basic API Manual Testing Questions (Freshers)

What is API Manual Testing?

API manual testing involves testing APIs by manually sending requests and validating responses using tools like Postman.

Instead of validating frontend UI, API testing focuses on backend systems.

API Manual Testing Includes

Sending API requests

Validating responses

Checking status codes

Verifying business logic

Testing authentication

Performing negative testing

Why is API Testing Important?

API testing validates core business logic and data flow early in development.

Backend defects found during API testing are usually easier and cheaper to fix compared to UI defects.

Advantages of API Testing

Faster Execution

APIs execute faster because no browser or UI rendering is required.

Early Defect Detection

Defects can be identified before frontend development is completed.

Better Backend Validation

API testing directly validates:

Business rules

Data handling

Authentication

Integrations

More Stable Testing

API tests are generally more stable than UI tests because they are less dependent on frontend changes.

What is an API?

API (Application Programming Interface) allows two applications to communicate with each other.

APIs enable systems to exchange:

Data

Requests

Responses

Services

Real-Time API Examples

Examples of APIs used daily:

Payment APIs

OTP APIs

Maps APIs

Food delivery APIs

Modern applications heavily depend on APIs.

What Tools are Used for Manual API Testing?

Common tools include:

Postman

SoapUI

Browser Developer Tools

curl

Among these, Postman is the most commonly used tool in QA interviews.

What is an Endpoint?

An endpoint is a URL where an API receives requests.

Example

GET /api/users/101

This endpoint retrieves user details.

What is a Request Payload?

Request payload is the data sent to the server during:

POST requests

PUT requests

PATCH requests

Example

{
“email”: “test@example.com“,
“password”: “Pass@123”
}

What is a Response Payload?

Response payload is the data returned by the server after processing the request.

Example

{
“id”: 101,
“name”: “Srushti”
}

What are HTTP Headers?

Headers contain metadata about API requests and responses.

Common Headers

Content-Type

Authorization

Example

Content-Type: application/json
Authorization: Bearer token123

What is Content-Type?

Content-Type specifies the request or response format.

Common Formats

application/json

application/xml

multipart/form-data

What is Statelessness in REST?

REST APIs are stateless.

This means:

Each request is independent

Server does not store client session data

Every request contains all required information

REST API Interview Questions

Which HTTP Methods are Commonly Used?

The most common HTTP methods are:

POST

PATCH

DELETE

Difference Between GET and POST

GET

Used to retrieve data.

Example

GET /users/101

POST

Used to create new resources.

Example

POST /users

Difference Between PUT and PATCH

PUT

Replaces the entire resource.

PATCH

Updates only selected fields of the resource.

PATCH is generally more efficient for partial updates.

What is JSON?

JSON (JavaScript Object Notation) is a lightweight data-interchange format widely used in REST APIs.

Example

{
“id”: 101,
“name”: “Srushti”,
“role”: “QA Engineer”
}

Why JSON is Popular

Lightweight

Easy to read

Easy to parse

Faster communication

What is a Query Parameter?

Query parameters are values passed after ? in the URL.

Example

GET /users?page=1&size=10

Common Uses

Pagination

Filtering

Sorting

What is a Path Parameter?

Path parameter is a dynamic value inside endpoint path.

Example

GET /users/101

Here:

101 is the path parameter.

What is Pagination?

Pagination means splitting large responses into multiple pages.

Example

GET /users?page=2&size=20

Pagination improves response handling and performance.

What is API Versioning?

API versioning manages changes using:

Headers

This helps maintain backward compatibility.

What is Idempotency?

Idempotency means multiple identical requests produce the same result.

Common Idempotent Methods

Example

GET /users/101

Calling this repeatedly should not change backend data.

What is Caching in REST APIs?

Caching stores responses temporarily to reduce server load and improve performance.

Benefits

Faster responses

Reduced backend processing

Better scalability

Manual API Testing with Postman

What is Postman?

Postman is a tool used for manual and automated API testing.

It helps testers:

Send requests

Validate responses

Manage collections

Test APIs efficiently

What is a Postman Collection?

A collection is a group of related API requests.

Benefits

Better organization

Reusability

Easier execution

What is a Postman Environment?

A Postman environment stores variables for different environments such as:

Staging

Production

Example

What are Environment Variables?

Environment variables are dynamic values used across requests.

Examples include:

Tokens

Base URLs

User IDs

How Do You Validate Response Manually in Postman?

Manual validation includes checking:

Status code

Response body

Response headers

Business data

Example Status Code Validation

pm.response.to.have.status(200);

Example Response Validation

pm.expect(pm.response.json().name).to.eql(“Srushti”);

How Do You Test Headers in Postman?

Validate headers such as:

Content-Type

Authorization

Custom headers

Header validation ensures proper request and response handling.

How Do You Test Negative Scenarios Manually?

Negative testing includes:

Invalid payloads

Missing headers

Wrong endpoints

Invalid authentication

Incorrect data types

Negative testing improves system reliability.

What is API Chaining?

API chaining means using response data from one API in another request.

Example Flow

Store token

Use token in secured APIs

This is common in real-world API testing.

How Do You Handle Authentication APIs Manually?

Authentication APIs are tested using:

Bearer tokens

API keys

Basic authentication

Example

Authorization: Bearer token123

What is Newman?

Newman is a command-line tool used to run Postman collections.

Basic awareness of Newman is useful in interviews because it supports:

Collection execution

Automation

CI/CD integration

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

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

Response message should match expected value

Response time should be acceptable

Scenario-Based API Manual Testing Questions

API Returns 200 but Incorrect Data — What Do You Do?

Validate:

Business logic

Database consistency

Response schema

Backend mappings

Raise a functional defect if backend behavior is incorrect.

How Do You Test Authentication APIs?

Validate:

Valid token

Invalid token

Expired token

Missing token

Authentication testing is critical for API security.

How Do You Test Rate Limiting?

Send multiple rapid requests and validate:

HTTP 429 response

Proper throttling behavior

How Do You Test API Performance?

Measure:

Response time

SLA compliance

Backend latency

Performance issues can affect scalability.

How Do You Test File Upload APIs?

Validate:

File size

File format

Upload success response

Invalid file handling

How Do You Test Backward Compatibility?

Validate older API versions still work after upgrades.

Example

/v1/users

/v2/users

Older clients should continue functioning correctly.

How Do You Debug Failing APIs?

Inspect:

Request payload

Headers

Authentication

Response body

Status codes

Debugging ability is highly important for manual API testers.

SOAP API Basics

What is SOAP?

SOAP (Simple Object Access Protocol) is a protocol used for XML-based communication.

SOAP is commonly used in enterprise systems.

What is WSDL?

WSDL is an XML document describing SOAP services.

It contains:

Endpoints

Operations

Request structure

Response structure

SOAP & XML Interview Questions What is SOAP?

SOAP (Simple Object Access Protocol) is a protocol used for exchanging XML-based messages between systems.

SOAP APIs use XML format for communication.

SOAP is commonly used in enterprise-level applications because it provides:

Strong security

Standardized messaging

Structured communication

Reliable transactions

Characteristics of SOAP

SOAP APIs are:

XML-based

Protocol-driven

Structured

Secure

Platform-independent

SOAP is heavily used in enterprise integrations.

Real-Time Examples of SOAP Usage

SOAP APIs are commonly used in:

Banking transactions

Insurance claim systems

Airline booking systems

Healthcare systems

Enterprise ERP integrations

What is WSDL?

WSDL (Web Services Description Language) is an XML file that describes SOAP services.

It acts like a contract between client and server.

WSDL contains details about:

Endpoints

Operations

Request formats

Response formats

Data types

Why WSDL is Important

WSDL helps testers and developers understand:

Available SOAP operations

XML request structure

XML response structure

API communication rules

How Do You Test SOAP APIs Manually?

SOAP APIs are tested manually by:

Sending XML requests

Validating XML responses

Verifying status codes

Checking business logic

Validating XML schema

Tools commonly used include:

Postman

SoapUI

curl

SOAP Request Example

<soapenv:Envelope>

  <soapenv:Body>

     <getUser>

        <id>101</id>

     </getUser>

  </soapenv:Body>

</soapenv:Envelope>

SOAP Response Example

<response>

<status>SUCCESS</status>

</response>

How Do You Validate XML Response?

XML responses are validated by checking:

XML nodes

Node values

XML structure

Schema compliance

Important XML Validations

Validate Node Existence

Check whether required nodes exist.

Example

<status>

<userId>

<message>

Validate Node Values

Verify whether values match expected business behavior.

Example

<status>SUCCESS</status>

Validate XML Structure

Check whether XML hierarchy and nesting are correct.

Validate XML Schema

Ensure response follows expected XML schema definition.

SOAP Fault Validation

SOAP APIs return SOAP Fault messages for failures.

Validate:

Fault code

Fault string

Proper error handling

SOAP fault validation is very important in enterprise projects.

HTTP Status Codes – Must Know for Interviews

Understanding HTTP status codes is extremely important in API testing interviews.

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

Common Status Code Interview Questions

What Does 200 Mean?

The request was processed successfully.

What Does 201 Mean?

A new resource was successfully created.

Difference Between 401 and 403

401 Unauthorized

Authentication failed.

403 Forbidden

User is authenticated but lacks permission.

What Causes 400 Errors?

Possible causes include:

Invalid payload

Missing fields

Incorrect XML structure

Validation failure

Why is 500 Important?

500 indicates backend server failure.

Possible causes:

Application crash

Database issues

Unhandled exceptions

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

Validate Status Code

Expected:

Status code = 201

Validate ID Field

Ensure:

id should not be null

Validate Response Message

Verify:

Message text is correct

Validate Business Logic

Ensure backend behavior matches expected functionality.

Why Manual Validation is Important

Manual validation helps testers verify:

Functional correctness

Business behavior

Backend processing

Response accuracy

Automation Awareness (Interview Advantage)

Even for manual testing roles, interviewers often like candidates who have basic automation awareness.

This shows:

Learning mindset

Technical growth potential

Modern testing awareness

Postman Scripts (Basic Awareness)

Example

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

This validates successful API execution.

SoapUI (Overview)

SoapUI is commonly used for:

REST APIs

SOAP APIs

XML validations

Mock services

Common SoapUI Features

JSONPath Assertions

Used for REST API validation.

XPath Assertions

Used for XML validation.

Mock Services

Used when backend services are unavailable.

Rest Assured (Java – Awareness)

Rest Assured is a Java-based API automation library.

Example

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

Why Rest Assured is Popular

Rest Assured provides:

Readable syntax

Automation support

CI/CD integration

Framework compatibility

Python Requests (Awareness)

Python is also commonly used for API automation.

Example

import requests

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

Why Automation Awareness Helps

Even basic awareness of automation tools creates stronger interview impression because modern projects increasingly combine:

Manual testing

API automation

CI/CD pipelines

Scenario-Based API Manual Testing Questions

Modern interviewers heavily focus on scenario-based questions because they evaluate practical thinking and debugging ability.

API Returns 200 but Incorrect Data — What Do You Do?

Validate:

Business rules

Database consistency

Response schema

Backend mappings

Raise a functional defect if backend behavior is incorrect.

How Do You Test Authentication Manually?

Validate:

Valid token

Invalid token

Expired token

Missing token

Authentication testing is critical for API security.

Example Authorization Header

Authorization: Bearer token123

How Do You Test Rate Limiting Manually?

Send multiple rapid requests and validate:

HTTP 429 response

Proper throttling behavior

How Do You Test Missing Headers?

Remove required headers and validate error response.

Example Missing Headers

Content-Type

Authorization

How Do You Test Invalid Payload Fields?

Validate APIs using:

Wrong data types

Missing mandatory fields

Invalid formats

Empty values

Negative testing improves reliability.

How Do You Test Dependent APIs?

Use API chaining.

Example Flow

Store token

Use token in secured API

How Do You Test File Upload APIs Manually?

Validate:

File size

File format

Upload success response

Invalid file handling

How Do You Test Backward Compatibility?

Validate older API versions still work after upgrades.

Example

/v1/users

/v2/users

Older clients should continue functioning correctly.

How Do You Debug API Failures?

Inspect:

Request payload

Headers

Authentication

XML or JSON structure

Response body

Status codes

Debugging ability is one of the most important API testing skills.

How Do You Test API Performance Manually?

Measure:

Response time

SLA compliance

Backend latency

Tools like Postman help measure response times manually.

What Interviewers Evaluate

Interviewers commonly assess:

Understanding of API fundamentals

Ability to explain manual testing steps

Knowledge of status codes

Real-time problem-solving skills

Clear communication

Structured explanations

What Makes Strong Candidates Different

Strong candidates explain:

Why validations matter

Business impact of failures

Backend behavior

Real-time testing logic

instead of only describing tools.

Weak Interview Answer

“I checked status code 200.”

Strong Interview Answer

“I validated status code 200 to ensure the API successfully processed the request and returned expected backend business behavior.”

This sounds much more professional.

API Manual Testing Cheat Sheet (Quick Revision)

Important Revision Points

Validate status code and response body

Test positive and negative scenarios

Check headers and payload

Verify business logic

Use Postman efficiently

Validate XML responses

Log detailed defects with request and response

FAQs – API Manual Testing Interview Questions

Q1. Is automation mandatory for API testing roles?
Automation is Not Always Mandatory — But It is Increasingly Important

In many companies, manual API testing roles still exist, especially for:

Freshers

Junior QA roles

Functional testing teams

Manual testing projects

Candidates with strong understanding of:

REST APIs

HTTP methods

Status codes

Postman

Backend validation

can still crack many API testing interviews without advanced automation knowledge.

However, modern software companies increasingly prefer candidates who also understand API automation concepts.

Q2. Is Postman enough for manual API testing?
Yes, Postman is Usually Enough for Manual API Testing

Postman is one of the most popular and widely used tools for manual API testing.

For many manual QA and fresher API testing roles, strong Postman knowledge is often sufficient because interviewers mainly evaluate:

API fundamentals

Request and response validation

HTTP methods

Status codes

Real-time testing scenarios

Postman helps testers validate backend services without depending on frontend UI.

Q3. Do freshers get API manual testing jobs?
Yes, Freshers Do Get API Manual Testing Jobs

API manual testing has become one of the most common entry points into modern QA and software testing careers.

Today’s applications heavily depend on:

REST APIs

Microservices

Mobile backends

Cloud platforms

Third-party integrations

Because of this, many companies actively hire freshers for manual API testing roles.

Q4. Are REST APIs more important than SOAP?
Yes, REST APIs Are More Commonly Used Today

In modern software development, REST APIs are generally more important and more widely used than SOAP APIs.

Most modern applications use REST because it is:

Lightweight

Faster

Easier to integrate

More scalable

Better suited for web and mobile applications

Because of this, interviewers ask far more REST API questions in modern QA and API testing interviews.

However, SOAP is still very important in many enterprise systems.

Introduction – Why API Testing Is Important in Interviews

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