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Explore top API testing interview questions and answers for freshers, intermediates, and experienced professionals to master backend validation.
Published on: September 23, 2025
API testing is a crucial aspect of software quality assurance, ensuring that applications communicate correctly, handle data reliably, and meet functional and performance expectations.
Understanding API testing interview questions is essential for candidates, as these questions not only reinforce core concepts but also prepare testers to tackle real-world challenges in backend validation and integration testing.
Note: We have compiled all API Testing Interview Questions for you in a template format. Check it out now!
Here are some essential API testing interview questions for freshers. These questions cover the fundamental concepts of APIs, assess knowledge of HTTP methods, request/response handling, status codes, and data formats like JSON and XML, and provide insights into how well candidates understand the building blocks of API testing and practical validation using tools like Postman.
API testing is a software testing technique used to verify that Application Programming Interfaces (APIs) function correctly in terms of logic, performance, and security. It focuses on sending requests directly to the service layer and validating the responses, without depending on the user interface.
This makes it faster, more reliable, and easier to automate compared to UI testing. This is the basic question asked in many of the API testing interview questions because it forms the foundation of the topic.
APIs can be categorized based on different factors:
This is an important concept and is often included in API testing interview questions to test whether you understand the different categories and their use cases.
API testing offers multiple benefits:
Since understanding benefits shows practical knowledge, this is a very common point asked in API testing interview questions.
When testing APIs, several things need to be validated:
This is the type of verification-related query that appears regularly in API testing interview questions because it shows if you know what exactly to test in an API.
An API request and response are the building blocks of communication between a client and a server. Understanding their components is crucial for designing, testing, and debugging APIs effectively.
An API request usually contains:
An API response includes:
This forms a standard explanation, and it is often one of the fundamental API testing interview questions.
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API testing validates business logic and data flow at the backend, while UI testing checks the user interface and overall user experience.
API's are faster, more reliable, and easier to automate, whereas UI testing is slower and more prone to frequent changes. Tools also differ.
Postman and REST Assured are used for APIs, while Selenium and Cypress are used for UI tests.
This difference is commonly asked in API testing interview questions since it highlights whether you can clearly distinguish the two approaches.
Feature | API Testing | UI Testing |
---|---|---|
Definition | Validates the business logic and data responses of APIs | Validates the graphical user interface (UI) elements of the application |
Focus Area | Backend logic, request/response structure, status codes, and data accuracy | Visual elements, layout, usability, and user interactions |
Test Level | Performed at the service layer (below the UI) | Performed at the presentation layer |
Speed | Faster as it skips rendering the UI | Slower due to browser rendering and user flow simulation |
Tools Used | Postman, REST Assured, SoapUI, JMeter | Selenium, Cypress, Playwright |
Data Validation | Verifies data integrity between systems | Focuses on data visibility and display correctness |
Error Localization | Easier to pinpoint bugs in logic or data processing | May require more time to trace errors across layers |
Dependency on UI | No dependency; can be done early in development | Heavily dependent on UI availability |
Automation Complexity | Easier to automate and maintain | More complex due to frequent UI changes |
REST APIs are lightweight, stateless, and use multiple data formats like JSON or XML. They are faster and widely used in web and mobile applications.
SOAP APIs, on the other hand, are XML-based, protocol-driven, and provide built-in security, making them suitable for enterprise applications.
REST relies mainly on HTTP, while SOAP can use multiple protocols like HTTP, SMTP, and TCP.
Comparing REST and SOAP is one of the most frequent API testing interview questions because it demonstrates your ability to understand and explain practical differences.
Feature | REST API | SOAP API |
---|---|---|
Full Form | Representational State Transfer | Simple Object Access Protocol |
Data Format | Supports multiple formats: JSON (preferred), XML, HTML, etc. | Strictly XML only |
Ease of Use | Lightweight, simple, faster | Heavier, complex, slower |
Transport Protocol | Uses HTTP/HTTPS | Uses multiple protocols: HTTP, SMTP, TCP, etc. |
Performance | High performance, better suited for web apps | Slower due to XML overhead |
Security | Relies on HTTPS, OAuth, etc. | Built-in WS-Security for enterprise-level security |
Statefulness | Stateless (each call is independent) | Can be stateful or stateless |
Best Suited For | Public APIs, web/mobile apps | Enterprise-level, formal contracts and high security needs |
Some of the most common HTTP methods include:
This list is very important, and interviewers often bring it up in API testing interview questions to check your familiarity with HTTP methods.
HTTP status codes communicate the result of an API request. They help indicate whether the request was successful, failed, or requires additional action.
hey are essential for debugging since they clearly differentiate between client-side and server-side issues. Status codes also support automation and flow control in test scripts, while enforcing security through codes like 401 Unauthorized or 403 Forbidden.
Because status codes form the basis of request validation, they are frequently included in API testing interview questions.
Some of the most popular API testing tools include:
Input validation is the process of checking that the data sent to an API, whether through request body, headers, query parameters, or path parameters, is correct, safe, and conforms to expected formats.
In API testing, input validation ensures that the API accepts only valid data, handles invalid or malicious inputs gracefully, and behaves predictably under different conditions.
Input validation is crucial because it:
This is a commonly asked question in many API testing interview questions, as it shows whether you understand how to safeguard APIs and ensure robustness.
Postman is a comprehensive platform for designing, testing, and automating APIs. It allows testers to send requests, validate responses, and create automated test scripts without complex setup.
This is particularly useful in the early stages of API development, providing quick endpoint validation, debugging, and visualization of request-response cycles.
Key contributions of Postman include:
This is one of the most frequent API testing interview questions because Postman is widely used in real-world API testing workflows.
Understanding this difference is a standard API testing interview question, as it evaluates whether you can distinguish general APIs from network-based web services.
Here are the differences between Web API and Web Service:
Parameters | API | Web Service |
---|---|---|
Definition | A set of protocols and tools that allow software components to communicate. | A type of API that operates over a network using standardized web protocols. |
Communication | Can work over any protocol (HTTP, FTP, etc.) | Specifically works over HTTP, SOAP, or REST |
Functionality | Can be used for local or remote communication | Strictly used for remote communication over a network |
Data Format | Supports multiple formats: JSON, XML, etc. | Typically uses XML (especially in SOAP) |
APIs act as intermediaries that allow two software systems to communicate. When a client, like a mobile app or web browser, sends a request, the API processes it, interacts with the backend or database, and returns a response, often in JSON or XML.
This enables modular development, allowing developers to access specific functionality without understanding the internal workings of the other system. Having core understanding of how API works is very important and is the commonly asked question in most of the API testing interview questions.
An endpoint is a URL where a client sends requests to access specific server resources or functionality, such as /users/123. API testers validate endpoints by checking:
This is one of the basic questions asked in many API testing interview questions because endpoint validation is central to API testing.
JSON (JavaScript Object Notation) is a lightweight, human-readable format that represents data as key-value pairs. It is easy to parse and widely used in modern web and mobile applications.
XML (eXtensible Markup Language) is a more verbose markup language that uses custom tags to structure data. While it is flexible and supports complex schemas, it is heavier and slower to parse compared to JSON.
In REST APIs, JSON is generally preferred because it is compact, faster to parse, consumes less bandwidth, and integrates seamlessly with JavaScript-based frontends. When working with mobile app testing, JSON is even more common due to its efficiency and compatibility.
You can also explore supporting documentation on commonly used APIs for mobile app testing to see how these data formats are applied in practice.
This is a frequently asked topic in API testing interview questions because understanding data formats is essential for both designing APIs and validating their responses effectively.
Common authentication mechanisms include:
Handling authentication is an essential topic often asked in API testing interview questions, as security validation is critical for API reliability.
API Specification Review is the step where you examine the API documentation and requirements before testing. It ensures that the purpose, workflow, endpoints, features, and expected responses are well defined.
This review helps plan the testing process smoothly, reduces ambiguities, and prevents missing test scenarios. This is a common question in API testing interview questions, as it highlights your understanding of planning and preparation before executing tests.
Both 401 and 403 are HTTP status codes related to access control, but 401 indicates authentication issues, while 403 indicates insufficient permissions even after authentication.
Status Code | Meaning | Common Causes | Example |
---|---|---|---|
401 Unauthorized | Client is not authenticated, or credentials are invalid | Missing/invalid Authorization headers, expired tokens, wrong API keys | User tries to access a protected endpoint without logging in or with an invalid token |
403 Forbidden | Client is authenticated but does not have permission to access the resource | Role-based restrictions, ACL rules, IP-based blocks | Logged-in user attempts to access an admin-only route without admin privileges |
This distinction is frequently asked in API testing interview questions, as testers need to verify both authentication and authorization behavior in APIs.
Headers carry metadata that provides context for the API request. They tell the server how to process the request, such as content type, authorization, or caching instructions. While headers do not contain the main data payload, they are essential for proper communication between client and server. This topic frequently appears in API testing interview questions because headers are a critical part of request validation and security checks.
Negative testing in the context of APIs refers to the practice of intentionally sending invalid, unexpected, or malformed inputs to an API to verify how well it handles error conditions and edge cases.
The goal is not to confirm that the API works under normal conditions, but to ensure it fails gracefully and securely when things go wrong.
Query parameters and path parameters are ways to pass information to an API, but they serve different purposes in the URL structure. Path parameters identify specific resources, while query parameters modify or filter the data returned.
Use Cases:
Use Cases:
Here are some intermediate-level API testing interview questions designed for professionals with hands-on experience.
These questions delve into more advanced topics in API testing, including versioning, mocking and virtualization, pagination, performance, and security, helping candidates demonstrate practical skills and problem-solving abilities in real-world API validation scenarios.
API mocking is the practice of creating a simulated version of an API that returns predefined responses. It is especially useful when the real API is unavailable, under development, or when you want to isolate tests from backend dependencies.
It is used for:
Functional and non-functional API testing serve different purposes: functional testing ensures the API behaves as expected, while non-functional testing evaluates performance, security, and reliability under various conditions.
This distinction is often part of API testing interview questions, as it tests your understanding of both the behavior and the performance aspects of an API.
API performance testing evaluates response times, throughput, and resource usage to ensure the API handles expected and peak traffic efficiently.
For example:
This topic is commonly asked in API testing interview questions, as performance validation ensures the reliability and scalability of APIs.
Boundary Value Analysis is a black-box technique used to identify errors at the edges of input ranges rather than within them. For APIs that accept numeric or range-based inputs, BVA tests values just below, at, and just above the allowed limits.
Example: If an API accepts values between 1 and 100, BVA test cases include 0, 1, 100, and 101.
This technique is often included in API testing interview questions, as it ensures APIs handle edge cases correctly and prevent common off-by-one errors.
API versioning involves labeling API releases (v1, v2, v1.1) to manage changes while maintaining backward compatibility.
In API testing, versioning helps:
This is a frequent topic in API testing interview questions, highlighting how testers ensure stability across evolving APIs.
A payload is the actual data sent in a request or returned in a response. It carries the meaningful content, such as user input, JSON objects, or XML data. Proper payload validation ensures APIs process only correct and secure data.
This concept appears often in API testing interview questions, as it’s essential to verify the accuracy of data exchanged between client and server.
API virtualization creates a simulated version of an API to mimic real behavior, allowing development and testing to continue even when the actual API is unavailable or unstable.
It is useful when:
Virtual APIs can simulate success, failure, or timeout responses, enabling error-handling and automated testing independent of real services. Tools like WireMock, MockServer, and Postman Mock Servers are commonly used.
This is a notable topic in API testing interview questions, as it ensures testing can proceed without dependency on external or incomplete services.
Pagination breaks large datasets into smaller, manageable "pages," avoiding overload for clients or servers. Instead of returning all records at once, APIs return limited results with metadata to fetch the next set.
This approach ensures that all properties and methods within the interface consistently use the defined type.
Common methods:
During testing, you check that the correct number of items is returned, navigation links work, and edge cases (last page, no data) are handled correctly.
This is often asked in API testing interview questions, as proper pagination ensures performance and usability for large datasets.
SOAP APIs rely on strict XML and WSDL contracts, making setup, payload construction, and validation more complex than REST, which is lightweight and JSON-based. Testing SOAP often requires specialized tools like SoapUI, and validating responses involves XPath or XML schema assertions.
Other challenges include statefulness, which can affect test isolation and repeatability, and rigid error handling through standardized fault structures.
To address these:
This topic appears in API testing interview questions because testers need to handle SOAP-specific complexities while ensuring accurate validation.
To manage these challenges, I rely on contract testing with WSDL validation, use automated test suites in SoapUI or RestAssured with XML support, and integrate these into CI pipelines for regression coverage. I also maintain clear documentation for each operation and its expected structure to reduce onboarding friction for new testers.
In TypeScript, both interfaces and abstract classes are used to define the structure that other classes can implement or extend. An interface defines a contract or a shape for an object. It cannot contain any implementation, only method and property declarations.
An abstract class, on the other hand, is a partially implemented class. It can have both implemented methods and abstract methods that must be overridden in derived classes.
Simulating network latency or unstable conditions helps validate API behavior under poor connectivity. I use several approaches:
Testing for latency is often discussed in API testing interview questions, as it ensures APIs remain stable under real-world network conditions.
Rate limiting prevents excessive requests to APIs. To test it, I simulate multiple rapid requests and monitor responses.
Tools: Postman/Newman, JMeter, Python requests with time.sleep(), K6 for performance-based testing.
This is a relevant topic in API testing interview questions, as rate limiting is crucial for API reliability and preventing misuse.
Here are some advanced API testing interview questions tailored for experienced professionals. These questions focus on complex, real-world challenges such as API security (JWT, OAuth, CSRF), microservices testing, contract validation, stateful workflows, caching, and mutation testing
They are designed to evaluate not just your technical expertise, but also your ability to make strategic decisions, optimize automation, and ensure reliability in large-scale distributed systemskey skills for senior QA and automation engineering roles.
Cross-Site Request Forgery (CSRF) is a security vulnerability where a malicious website tricks a user's browser into performing unintended, authenticated requests to another site where the user is already logged in.
Since browsers automatically include cookies or session identifiers, these forged requests are processed as legitimate. This can allow attackers to perform actions like changing account settings, initiating transactions, or deleting data without the user’s consent.
Preventing CSRF in API Testing:
Practical Testing: In Postman, you can craft requests with/without tokens, spoof headers, or simulate cross-origin requests to verify CSRF protection enforcement.
Handling API timeouts and retries ensures your API tests are resilient against network latency, unstable endpoints, or third-party service failures.
Techniques:
pm.test("Response time < 5s", () => pm.expect(pm.response.responseTime).to.be.below(5000));
This approach ensures APIs are tested for reliability under real-world conditions, making it an important API testing interview question.
API contract testing validates that an API’s response structure, data types, and status codes conform to the defined contract (OpenAPI, Swagger, RAML, or WSDL).
Steps:
APIs can reveal a variety of issues impacting functionality, performance, security, and reliability. Identifying these bugs is a core part of API testing and is frequently asked in interview questions.
Common Bug Types:
Type | Description |
---|---|
Missing/Duplicate Functionality | Endpoints are absent or redundant, leading to incomplete features. |
Improper Error Messaging | Vague or misleading responses that hinder debugging. |
Data Issues | Inconsistent formats, outdated values, or incomplete payloads. |
Unauthorized Access | Weak authentication or missing authorization checks. |
Performance Bottlenecks | Slow responses, timeouts, or high latency under load. |
Inconsistent Error Handling | Varied error formats/codes across endpoints. |
Compatibility Failures | APIs are not working on expected platforms or frameworks. |
Security Vulnerabilities | Susceptible to injection, token leakage, or weak encryption. |
Reliability Issues | Intermittent failures or downtime. |
Documentation Gaps | Outdated or incomplete API specs are affecting integration. |
Caching improves API performance and reduces server load. In API testing, it’s important to verify that caching is implemented correctly.
Proper caching ensures data consistency, improves response times, and prevents unnecessary backend hits.
API versioning ensures stable integrations while evolving your API. Testing versioning validates that old clients still work with newer versions.
Steps:
Contract-first and code-first testing are two approaches to API development and validation. Each has its advantages depending on whether you prioritize upfront design, consistency, or rapid implementation.
Approach | Description | When to Use |
---|---|---|
Contract-First | Define API contract (OpenAPI, GraphQL, Protobuf) before coding. Mocks, tests, and code are generated from the contract. | Multiple consumers, public APIs, long-term maintainability, strict versioning. |
Code-First | Develop implementation first, generate contract from code. | Rapid prototyping, internal tools, evolving business logic, speed prioritized over upfront design. |
Choosing between them:
Contract-first offers predictability and control, while code-first provides speed and flexibility. For stable, scalable APIs, especially in microservices or cross-team environments, contract-first is often preferred. For exploratory or internal development, code-first may be more pragmatic.
Mutation testing introduces small, deliberate changes (mutations) into API code to check whether existing test cases detect errors. If tests fail as expected, the mutation is “killed,” confirming strong coverage.
Benefits:
Stateful API testing involves validating APIs that maintain user context or session data across multiple requests. This type of testing ensures that workflows requiring authentication, user-specific data, or session continuity behave as expected. To verify session-persistent workflows:
This helps confirm that the application behaves correctly for logged-in users over an extended series of interactions.
Testing webhooks and callback endpoints involves simulating real-world scenarios where your application receives asynchronous events. Effective testing ensures that your system handles callbacks reliably, even under complex or delayed workflows.
Local Testing Tools:
Testing Strategies:
Alongside traditional API testing tools and techniques, modern AI-driven cloud platforms like LambdaTest KaneAI API testing can enhance webhook testing by automatically generating test payloads, predicting potential failure scenarios, and validating asynchronous workflows. This approach reduces manual effort while improving coverage for edge cases and delayed callbacks.
Follow the official support documentation to get started with KaneAI API testing.
Callback Simulation Tips:
Input injection is a critical security vulnerability where malicious or unexpected input is sent to an API to manipulate its behavior, access unauthorized data, or compromise the system. In API testing, verifying input injection vulnerabilities ensures that the API correctly sanitizes and validates all inputs.
Common types of Input Injection:
Type | Description | Example |
---|---|---|
SQL Injection | Sending SQL code in input fields to manipulate database queries | "' OR 1=1 --" in a login API to bypass authentication |
Command Injection | Injecting system commands in API parameters to execute shell commands | ; rm -rf / in a parameter |
Script Injection (XSS) | Injecting JavaScript in API input that could execute in a browser | |
Header Injection | Modifying or injecting headers like Host, Referer, Authorization | Custom Host header to bypass access control |
How I test for Input Injection in API testing: Send invalid, unexpected, or malicious inputs in parameters (query, path, headers, body).
Testing secured APIs involves validating the complete token lifecycle, authorization flow, and session control mechanisms.
Testing APIs in microservices involves strategies to handle distributed dependencies, multiple teams, and parallel development.
Layer | Purpose | Tools |
---|---|---|
Unit | Validate internal logic | JUnit, TestNG |
Component | Test service in isolation | RestAssured, Postman |
Integration | Verify inter-service communication | Karate, Newman |
Contract | Ensure API compatibility | Pact, Spring Cloud Contract |
End-to-End (E2E) | Validate business workflows | Cypress, Selenium |
For large-scale distributed systems, platforms like LambdaTestHyperExecute help execute API tests in parallel with reduced flakiness, ensuring faster feedback and more stable pipelines across microservices. It enables distributed test execution at lightning speed while maintaining reliability across multiple services and environments.
LambdaTest is a GenAI-native test orchestration and execution platform that allows you to perform both manual and automated API tests at scale across 3000+ browsers and OS combinations.
This structured approach ensures comprehensive API testing across microservices, handling dependencies, authorization, and session state effectively while maintaining test reliability and coverage.
To get started, check out the support documentation on HyperExecute API Testing.
Handling dynamic data in APIs is a key part of real-world API testing, especially when chaining requests. Extracting values from one API response and using them in subsequent calls is a common topic in API testing interview questions.
let jsonData = pm.response.json();
pm.environment.set("authToken", jsonData.token);
pm.environment.set("userId", jsonData.user.id);
These environment variables can be referenced in subsequent requests:
GET {{baseUrl}}/users/{{userId}}
Authorization: Bearer {{authToken}}
You can also use Pre-request scripts for dynamic setup, validate the variables using assertions, and automate the chain with Collection Runner or Newman CLI. This ensures robust handling of APIs with dependent calls.
To perform data-driven API tests in Postman/Newman:
CSV Example:
username,email
user1,user1@example.com
user2,user2@example.com
JSON Example:
[
{ "username": "user1", "email": "user1@example.com" },
{ "username": "user2", "email": "user2@example.com" }
]
newman run collection.json -d data.csv
Each row or object replaces the placeholders, allowing the same requests to run multiple times with different inputs. This approach ensures scalable, repeatable, and efficient data-driven API testing.
This is a frequently asked question in API testing interviews, as handling dynamic data effectively across multiple requests is a core concept in API testing and evaluates your ability to manage variable values reliably.
Testing APIs that support asynchronous operations is essential to ensure that background jobs or message queues complete reliably and produce correct results. Proper handling of async workflows ensures that delayed processing does not cause data inconsistencies or missed validations.
Managing multiple environments is a crucial aspect of API testing, ensuring that your requests run correctly across Dev, Staging, and Prod without manual changes. Proper environment configuration improves automation efficiency and reduces errors during deployments.
This is one of the most common questions asked in API testing interviews, as it evaluates a candidate’s understanding of environment management and variable handling in automated API tests.
Authenticating against OAuth-protected APIs involves obtaining a valid access token, attaching it to requests, and handling token expiry or refresh flows. This is a key API testing interview question for experienced testers, as managing secure authentication and the token lifecycle is essential in API testing.
Automation scripts or tools like Postman, RestAssured, or Karate DSL can handle token management efficiently.
Rate limiting ensures APIs remain reliable under high traffic by controlling how many requests a client can make in a given time frame. Testing it involves simulating high request volumes and verifying how the API enforces these limits.
This is one of the most common questions asked in API testing interviews, as it’s a core concept in API testing. Knowing and validating the rate limits in an API is crucial for performance and stability.
Mastering the concepts, techniques, and tools involved in API testing is crucial for anyone pursuing a role in quality assurance or backend testing. This comprehensive guide on API testing interview questions and answers is intended to equip you with both foundational knowledge and practical insights needed to tackle real-world scenarios.
From basic definitions to hands-on problem-solving approaches, the API testing interview questions covered here are curated to reflect what top companies look for in a candidate. Review them thoroughly, practice applying them in testing tools like Postman or through automation scripts, and you’ll be well-prepared to stand out in your next interview.
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