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.

Why API Testing Is Important for QA Professionals

API testing has become one of the most important skills in software testing because modern applications rely heavily on backend communication. Even when the UI looks correct, the actual business logic, database operations, authentication, and integrations happen through APIs.

A QA engineer who understands API testing can:

Detect defects earlier in the development cycle

Validate business logic independently from UI

Verify backend data accuracy

Test integrations between multiple systems

Improve overall application quality

This is why companies now expect testers to have at least basic API testing knowledge, even for manual 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 (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.

QA API Testing Interview Questions and Answers (90+)

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

1. What is an API?

An API (Application Programming Interface) allows two software systems to communicate with each other. It acts as a bridge between applications so they can exchange data and functionality without directly accessing each other’s internal code.

For example, when a mobile app displays weather information, it usually gets the data from a weather API. The app sends a request, and the API returns the response in a structured format like JSON or XML. APIs are widely used in web applications, mobile apps, payment gateways, banking systems, social media integrations, and cloud services.

2. What is API testing?

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

Unlike UI testing, API testing focuses on the communication layer between systems. Testers verify whether APIs behave correctly under different scenarios, including valid and invalid requests.

API testing generally includes:

Verifying response data

Checking HTTP status codes

Validating request and response headers

Ensuring schema correctness

Testing authentication and authorization

Measuring response time and performance

API testing helps identify backend issues early before they affect the frontend application.

3. Why is API testing important for QA?

API testing is important because APIs drive multiple applications, and one API defect can impact the entire system.

Modern applications are highly dependent on backend services and microservices architecture. If an API fails, connected systems such as websites, mobile apps, third-party integrations, and databases may also fail.

API testing helps QA teams:

Detect defects earlier

Validate business logic

Improve application stability

Reduce UI dependency

Increase test coverage

Verify system integrations

It is faster and more reliable than UI testing because it bypasses the frontend layer.

4. Difference between API testing and UI testing?

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

API Testing

Validates backend services

Checks requests and responses

Faster execution

Independent of UI changes

Focuses on business logic and data

UI Testing

Validates user interface behavior

Checks buttons, forms, and navigation

Slower execution

Affected by UI design changes

Focuses on user experience

API testing is generally more stable and efficient for validating core functionality.

5. What types of APIs have you tested?

Mostly REST APIs, with some exposure to SOAP APIs.

REST APIs

REST APIs are lightweight and commonly use JSON format. They are widely used in modern web and mobile applications because they are simple and scalable.

SOAP APIs

SOAP APIs use XML messaging and follow strict standards. They are commonly used in enterprise systems such as banking and insurance applications.

6. What are HTTP methods?

HTTP methods define the type of operation performed on the server. Common HTTP methods include:

POST

PATCH

DELETE

Each method represents a specific action on API resources.

7. What is GET request?

A GET request is used to retrieve data from the server.

It does not modify data and is mainly used for reading information. GET requests are generally safe and idempotent.

Example

GET /users/101

This request retrieves details of user 101.

8. What is POST request?

A POST request is used to create new data on the server.

The request body contains payload data that the server processes and stores.

Example

POST /users

This request creates a new user record.

9. Difference between PUT and PATCH?

PUT

PUT is used for a full update of resources. It replaces the entire existing object with new data.

PATCH

PATCH is used for a partial update. It modifies only specific fields without replacing the entire resource.

Example

PUT updates all user details

PATCH updates only email or phone number

10. What is DELETE request?

A DELETE request is used to remove data from the server.

After successful deletion, the API may return:

200 OK

202 Accepted

204 No Content

depending on implementation.

11. What is an endpoint?

An endpoint is a specific URL representing an API resource.

It identifies where requests should be sent.

Example

/users/101

This endpoint refers to user ID 101.

12. What is request payload?

Request payload is the data sent to the API in the request body.

It is commonly used in POST, PUT, and PATCH requests.

Example JSON Payload

{
“name”: “John”,
“email”: “john@example.com”
}

13. What is response body?

The response body is the data returned by the API after processing the request.

It usually contains:

Requested data

Success messages

Error messages

Status details

Responses are commonly returned in JSON format.

14. What is stateless API?

A stateless API means each request is independent and contains all required information.

The server does not store client session information between requests. REST APIs are typically stateless, which improves scalability and performance.

15. What is idempotency?

Idempotency means repeating the same request multiple times gives the same result.

Example

Deleting the same resource repeatedly should still produce the same final state.

Common Idempotent Methods

DELETE

POST is usually not idempotent.

16. What is authentication?

Authentication is the process of verifying user identity using:

Tokens

API keys

Username and password

JWT tokens

It ensures only valid users can access APIs.

17. What is authorization?

Authorization verifies user access permissions after authentication.

It determines what actions a user is allowed to perform.

Example

Admin can delete records

Normal user can only view records

18. What authentication types have you tested?

The authentication types tested include:

Bearer Token

Basic Authentication

API Key

Bearer Token

Used with JWT or OAuth tokens.

Basic Auth

Uses username and password encoded in Base64.

API Key

Uses unique keys passed in headers or query parameters.

19. What is JWT?

JWT (JSON Web Token) is used for secure authentication.

It contains encoded user information and is digitally signed to prevent tampering.

JWT Structure

Header

Payload

Signature

JWT is commonly used in REST APIs for session management and authorization.

20. What is negative API testing?

Negative API testing means testing APIs with invalid or unexpected inputs.

The goal is to verify proper error handling and system stability.

Examples

Invalid data types

Missing mandatory fields

Invalid authentication tokens

Unsupported HTTP methods

HTTP Status Codes – Must Know for QA API Testing

200 – OK

Indicates a successful GET request.

201 – Created

Indicates successful resource creation after POST request.

204 – No Content

Indicates successful request with no response body, commonly used in DELETE operations.

400 – Bad Request

Returned when request data is invalid.

401 – Unauthorized

Returned when authentication fails.

403 – Forbidden

Returned when user lacks access permission.

404 – Not Found

Returned when endpoint or resource does not exist.

409 – Conflict

Returned when duplicate or conflicting data exists.

422 – Unprocessable Entity

Returned when business validation fails.

500 – Internal Server Error

Returned when backend server encounters unexpected failure.

Section 2: API Validation & Testing Types (Q21–Q45)

21. What validations do you perform in API testing?

Common validations include:

Status code validation

Response body validation

Header validation

Schema validation

Response time validation

These validations ensure the API works correctly functionally and technically.

22. Is validating only status code enough?

No. Validating only status code is not enough.

Even if the API returns 200 OK, the response data may still be incorrect. QA engineers must validate:

Response content

Business rules

Database updates

Headers

Performance

23. What is positive API testing?

Positive API testing means testing APIs with valid input data.

The goal is to verify that APIs behave correctly under expected conditions.

Example

Sending valid user details and verifying successful user creation.

24. What is negative API testing?

Negative API testing means testing APIs with invalid or missing parameters.

It verifies whether proper error messages and status codes are returned.

25. What is boundary value testing in APIs?

Boundary value testing checks minimum and maximum allowed values.

Examples

Minimum password length

Maximum character limit

Numeric range limits

Boundary testing helps identify edge-case defects.

26. What is API regression testing?

API regression testing means re-testing APIs after code changes.

Its purpose is to ensure new changes do not break existing functionality.

27. What is API smoke testing?

API smoke testing is a basic health check of APIs.

It verifies whether critical APIs are functioning correctly before detailed testing begins.

28. What is API security testing?

API security testing validates:

Authentication

Authorization

Data protection

Token security

The objective is to identify vulnerabilities and prevent unauthorized access.

29. What is API performance testing?

API performance testing measures:

Response time

Throughput

Stability under load

It helps ensure APIs perform efficiently under expected traffic conditions.

30. What is API rate limiting?

API rate limiting restricts the number of requests allowed per user or client within a specific time period.

It protects backend systems from overload and abuse.

31. What is pagination testing?

Pagination testing validates page-wise API responses.

It ensures:

Correct page size

Proper navigation

Accurate total records

Consistent ordering

32. What is filtering testing?

Filtering testing validates query parameters used to filter API data.

Example

/users?country=India

The API should return only users from India.

33. What is sorting testing?

Sorting testing validates the order of response data.

Examples

Ascending order

Descending order

Alphabetical sorting

Date-based sorting

34. What is schema validation?

Schema validation ensures the response structure matches the API contract.

It checks:

Required fields

Data types

Nested objects

Array structures

35. What is API mocking?

API mocking simulates API responses when the backend is unavailable.

It allows frontend and QA teams to continue testing independently.

36. What is API rollback?

API rollback means reverting operations when a failure occurs.

It ensures system consistency and prevents partial updates.

37. What is data consistency testing?

Data consistency testing ensures the same data appears correctly across systems.

Example

Data created through API should match database records and UI display.

38. What is API concurrency testing?

API concurrency testing checks how APIs behave when multiple requests are sent simultaneously.

It helps identify:

Race conditions

Data conflicts

Performance bottlenecks

39. What is API caching?

API caching stores responses temporarily to improve performance and reduce server load.

Cached responses help APIs return data faster for repeated requests.

40. What is content-type validation?

Content-type validation ensures response format is correct.

Common Content Types

application/json

application/xml

41. What is header validation?

Header validation checks important headers such as:

Authorization

Content-Type

Cache-Control

Headers are essential for security and communication standards.

42. What is response time SLA?

Response time SLA (Service Level Agreement) defines the maximum acceptable API response time.

Example

An API must respond within 2 seconds under normal load.

43. What is contract testing?

Contract testing validates the agreement between API provider and consumer.

It ensures request and response structures remain compatible between systems.

44. What is API monitoring?

API monitoring tracks:

API uptime

Failures

Response time

Availability

Monitoring helps detect issues quickly in production environments.

45. What is API throttling?

API throttling limits traffic to protect backend systems from excessive requests.

It helps maintain system stability and prevents server overload.

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

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;

});

Rest Assured (Java – Basic)

given()

.contentType(“application/json”)

.body(payload)

.when()

.post(“/login”)

.then()

.statusCode(200);

Python Requests (Basic)

import requests

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

assert response.status_code == 200

Scenario-Based QA API Testing Interview Questions (15)

These are very common in QA interviews:

API returns 200 but wrong data – what do you validate?

Token expired but API still accessible – defect type?

API works in Postman but not in UI – why?

API slow only in production – possible causes?

Duplicate records created – what validation missed?

Unauthorized user accesses secured API – issue?

API crashes for special characters – what testing?

Same request returns different responses – why?

Payment deducted but order not created – what testing?

API returns null fields – how do you handle?

API schema changes suddenly – impact on QA?

API fails only in CI pipeline – possible reasons?

API returns XML instead of JSON – issue?

Partial data saved after failure – what testing missed?

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

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

FAQs – QA API Testing Interview Questions

Q1. Is API testing mandatory for QA roles?
Yes, API testing is almost mandatory for most modern QA roles, especially for:

Manual QA

SDET

Automation QA

Backend QA

Integration Testing roles

Today, most applications are built using APIs and microservices, so companies expect QA engineers to validate backend functionality, not just UI behavior.

Why API Testing Is Important for QA

1. Applications Depend on APIs

Mobile apps, web apps, payment systems, and third-party integrations all communicate through APIs.

If the API fails:

UI may break

Data may become incorrect

Transactions may fail

2. Faster Defect Detection

API testing helps find bugs before the UI is ready.

QA teams can validate:

Business logic

Authentication

Database data

Response validation

Error handling

3. Interview Expectation

Most QA interviews now include API-related questions such as:

HTTP methods

Status codes

Authentication

Postman usage

JSON validation

CRUD operations

Even for manual testing roles, basic API knowledge is usually expected.

Q2. Is Postman enough for QA API interviews?
For many Manual QA interviews, yes — Postman is often enough to clear the API testing round if you understand it properly.

But it depends on the role level.

When Postman Is Usually Enough

Manual QA / Functional QA Roles

Most companies expect you to know:

Creating API requests

GET, POST, PUT, DELETE methods

Status codes

Headers

Authentication (Bearer token, Basic Auth)

JSON request/response validation

Collections

Environment variables

Basic test scripts

If you can confidently explain:

how you tested APIs,

what validations you performed,

how you handled negative testing,

then Postman knowledge is usually sufficient.

Q3. REST or SOAP – which is more important?
For most modern API testing interviews and real-world projects, REST is much more important than SOAP.

Today, most applications use REST APIs because they are:

Lightweight

Faster

Easier to integrate

Easier to automate

Better suited for web and mobile applications

However, basic SOAP knowledge is still useful because some enterprise and legacy systems continue using SOAP services.

Why REST Is More Important Today

REST APIs are heavily used in:

Web applications

Mobile applications

Microservices architecture

Cloud platforms

SaaS applications

Third-party integrations

Most modern backend systems are REST-based.

That is why interviewers focus heavily on REST concepts during API testing interviews.

Q4. Biggest mistake QA candidates make?
The biggest mistake candidates make in API testing interviews is focusing only on tools and status codes instead of understanding real business behavior and backend validation.

Many candidates think:

“I sent the request in Postman and got 200 OK, so the API works.”

But interviewers expect much deeper analysis, especially for candidates with around 2 years of experience.

1. Trusting Only 200 OK

This is the most common mistake.

Candidates often validate only:

Status code = 200

But APIs can still return incorrect business data.

Example

{
“total”: -500
}

The API technically succeeded, but the business logic is wrong.

Interviewers expect validation of:

Response body

Business calculations

Database updates

Schema

Headers

Workflow behavior

Not just status codes.

2. Knowing Only Basic Postman Usage

Many candidates only know:

Sending requests

Checking response

Viewing status code

But at 2 years experience, interviewers expect more advanced usage such as:

Assertions

API chaining

Dynamic variables

Pre-request scripts

Environment variables

Collection Runner

Negative testing

Example

pm.expect(r.total).to.eql(r.subtotal – r.discount + r.tax);

This demonstrates business validation thinking.

3. Ignoring Business Logic

API testing is not only technical testing.

Interviewers expect candidates to validate:

Discounts

Tax calculations

Order workflows

Payment handling

Access permissions

Duplicate prevention

Example Questions

Can duplicate orders happen?

Can unauthorized users access APIs?

Are invalid transactions blocked?

Does rollback work properly?

Business logic validation is one of the most important interview areas.

4. No Negative Testing Mindset

Many candidates test only happy paths.

Strong candidates always test:

Invalid payloads

Missing fields

Expired tokens

Invalid authentication

Boundary values

Special characters

Empty requests

Negative testing shows deeper understanding of API behavior.

5. Weak Debugging Approach

Weak answer:

“I will report the defect.”

Strong answer:

Check logs

Verify request payload

Compare database records

Validate headers

Analyze backend logic

Reproduce the issue

Check dependent services

Interviewers heavily evaluate troubleshooting ability at this level.

6. No Real-Time Scenario Thinking

Many candidates memorize definitions but struggle with practical questions.

Common interview scenarios:

API returns 200 but wrong data — what do you do?

Payment deducted but order not created — what testing applies?

Retry creates duplicate records — how prevent it?

Interviewers prefer practical thinking over memorized theory.

7. Weak Understanding of Authentication

Candidates commonly confuse:

Authentication

Authorization

JWT tokens

Bearer tokens

401 vs 403

These are among the most frequently asked API interview topics.

You should clearly understand:

How tokens work

How tokens expire

How tokens are passed

Role-based access control

8. No Automation Awareness

Some candidates think API testing means only manual testing in Postman.

But modern projects increasingly expect:

Basic automation knowledge

Assertions

API automation awareness

CI/CD basics

Even simple knowledge of:

Rest Assured

Python requests

Newman

creates a stronger profile.

9. Weak Assertions

Some candidates validate only:

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

Interviewers expect stronger validations such as:

Schema validation

Field validation

Business rule validation

Header validation

Range validation

Assertions should validate meaningful behavior, not just technical success.

10. Explaining “What” but Not “Why”

Weak answer:

“I validated response fields.”

Better answer:

“I validated totals and discounts because incorrect calculations may cause financial defects.”

Interviewers value reasoning and risk awareness.

Q5. How should freshers prepare?
If you want to prepare quickly for Infosys API testing interviews, focus only on the topics that Infosys interviewers repeatedly ask.

Do not try to learn everything deeply at once.

Infosys interviews usually focus more on:

API fundamentals

Practical testing knowledge

Real-time scenarios

Postman usage

Functional validation

Basic automation awareness

Introduction – Why API Testing Is Important in QA Interviews

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