API Testing Interview Questions And Answers – 2025

API testing ensures that APIs function correctly in functionality, performance, security, and reliability. Unlike UI testing, which focuses on appearance, API testing verifies backend operations. With APIs being crucial in modern software, mastering API testing is essential for QA professionals. Companies like Google, Facebook, and Amazon prioritize API testing for high software standards.

This interview guide covers the most asked API Testing Interview Questions for beginners and experienced professionals (3, 5, and 8 years). It includes REST vs. SOAP, testing tools, authentication, performance testing, and Postman examples to help you excel in API testing interviews.

API Testing Interview Questions For Freshers

1. What is an API and Types of APIs?

An Application Programming Interface (API) is useful for communication between different software systems. It acts as an interface between two applications and allows two software systems to communicate with one another.

• It is a collection of functions that can be executed by another software program.
• It takes the request from the user and gives the response without exposing the internal details.

APIs (Application Programming Interfaces) allow different software systems to communicate with each other. They come in different types, depending on how they are used and accessed.

1. Web APIs: A Web API (also known as Web Service) is one of the most common types of APIs. It works over the internet and follows HTTP protocols. Web APIs are open-source, meaning they can be accessed by multiple clients like phones, tablets, or computers to exchange data.

2. Local APIs: Local APIs allow applications within the same system to communicate with each other. Developers use them for middleware services—software that connects different applications.

• Examples: TAPI (Telephony Application Programming Interface) and .NET API are popular Local APIs used in different operating systems.

3. Program APIs: Program APIs help one software program interact with another, even if they are not on the same system. They often use Remote Procedural Calls (RPCs) to make a remote program appear as if it’s running locally.

• Example: SOAP API (Simple Object Access Protocol) is a well-known type of Program API.

2. What is API Testing?

API testing is a process of testing a Application Programming Interface (API) which is used to verify and check if API meets the expectations for functionality, performance, and security.

API testing process

3. List the differences between API and Web Service.

Here is the Differences between Web Services and Web API

4. How do APIs work?

APIs (Application Programming Interfaces) work as intermediaries that allow different software systems to communicate with each other. Here’s how they function:

• Client Request: A client (e.g., a web app) sends a request to an API using an HTTP method like GET, POST, PUT, or DELETE.
• Server Processing: The API processes the request, retrieves or modifies data in a database, and prepares a response.
• Response Back to Client: The API sends the response (usually in JSON or XML format) back to the client, which then uses this data for display or further processing.

5. What are the different types of API testing?

Here are the different types of API testing :

Types of API testing

1. Functional Testing: This aims to verify the API functions correctly and meets the required specifications.
2. UI Testing: This aims to validate that the API is returning the expected results and in the correct format.
3. Load Testing: This type of testing aims to test the APIs performance and stability under stressful conditions.
4. Security Testing: This type of testing aims to identify the security-related vulnerabilities and flaws in the API and ensures that the API meets the required security standards.
5. Validation Testing: This type of testing aims to ensure that the API is returning the expected results and in the correct format.
6. Runtime and Error Detection: This type of testing aims to ensure that the API can handle runtime errors and exceptions by testing for network timeouts, memory leaks, etc.
7. Penetration Testing: This is a type of security testing that involves simulating attacks from hackers to detect vulnerabilities and weaknesses in the API.
8. API Hacking: It is a security testing technique that exploits vulnerabilities in an API, Attackers can target API endpoints to gain access to data or disrupt services.
9. Fuzz Testing: It involves giving invalid inputs into the API to test its ability to handle unexpected input and recover from errors.

6. What protocols can be tested using API Testing?

API testing can be used to test various protocols that facilitate communication between software systems. Some commonly tested protocols include:

• HTTP/HTTPS: Used for RESTful APIs, ensuring secure and structured web communication.
• SOAP (Simple Object Access Protocol): A protocol that uses XML-based messaging for web services.
• REST (Representational State Transfer): A widely used web service architecture over HTTP.
• GraphQL: A query language for APIs that allows fetching specific data efficiently.
• WebSockets: For real-time communication between a client and a server

7. What are the advantages of API Testing?

• Faster bug fixes: API testing works quickly to provide a solution, thus it helps to diagnose problems earlier in the development of the system allowing for efficient bug detection and removal.
• Reduced costs: API testing provides quick release of results, easier to maintain, thus reducing the cost of testing.
• Universal language support: API testing provides support for many different languages like Javascript, Ruby, Python, and PHP. Formats like XML, and JSON are supported during API testing.
• Better test coverage: API testing provides better test coverage as it drills down to the layers beneath the UI to ensure databases are communicating correctly with one another.

8. What are the disadvantages of API Testing?

• Technical skills: API testing requires skilled and trained testers
• Time-consuming: API testing is time-consuming as it requires time to develop and execute test scripts for APIs.
• Limited documentation: Limited documentation of the API makes it difficult for the testers to understand how the API should behave in response to a particular input.
• Cost: API testing using automated tools can be expensive and requires a significant budget thus increasing the cost of testing.
• Security: APIs can introduce security vulnerabilities if not properly tested and secured.

9. List the differences between API Testing and Unit Testing.

Here is the Differences between API Testing and Unit Testing

10. What needs to be verified in API testing?

• API testing helps to verify the correctness of data and responses. The API should return the return correct data and resposnses according to the specified requirements and expected behavior.
• API testing should verify the functionality and behavior. The API should perform its intended functions accurately.
• HTTP status codes of the API needs to verified.
• Error codes if the APIs are expected to throw an error.
• Authorization details.

11. What is Latency in API testing?

Latency in API testing refers to the time delay between sending a request to an API and receiving a response from the server. It is a key performance metric that measures the responsiveness of an API, typically expressed in milliseconds (ms). High latency can impact user experience and system efficiency.

12. What is an API framework?

API framework is a foundation on which a software developer can build applications for a specific platform. It is defined by the configuration file which consists the list of all APIs that is reqquired to be activated and activated for a particular program run.

13. Mention common HTTP methods used in API testing.

Here are the common HTTP methods used in API testing

• POST: It is used to create new data or resources on an API.
• GET: It is used to retrieve data or resources from an API.
• PUT: It is used to update existing data or resources on an API.
• PATCH: It is used to update existing data or resources from an API partially.
• DELETE: It is used to delete data or resources from an API.

14. What is the role of headers in API testing?

Headers in API testing plays an important role in providing additional information that is not included in the request body. Headers are important in controlling the behavior of the server in handling the request, to provide authentication information, or to provide metadata about the request or response.

15. What is the purpose of authentication in API testing?

API authentication is a combination of technology or process that verifies the identities of users who want access to an API. It involves the use of software protocol to verify identity of the requester before granting access to protected resources.

API Testing Interview Questions for Intermediate

16. What is API versioning, and why is it important in API testing?

API versioning involves specifying a version number in the API endpoint or headers to ensure the backward compatibility. It manages the changes in the API over time.

• It ensures the correct version of the API is being tested.
• It ensures that the changes in the API do not break the existing client applications.

17. What is the purpose of HTTP status codes in API testing?

HTTP status codes indicate the result of an API request, helping testers understand whether the request was successful, failed, or requires action.

• Indicate Response Status: Shows if the request succeeded (200 OK), failed (400 Bad Request), or needs authentication (401 Unauthorized).
• Aid Debugging: Helps identify issues like server errors (500 Internal Server Error).
• Improve API Communication: Ensures clear communication between client and server.
• Enhance Automated Testing: Used in test validations to verify expected API behavior.

18. What is the purpose of the request and response body in API testing?

The request body is used to send and receive data such as input parameters, or data to create/ update resources via the REST API. The response body is the data API send to the client.

• Request Body: Contains data sent by the client to the API (e.g., user credentials, form data, JSON payload). Example: Sending user details in a POST request.
• Response Body: Contains data returned by the API, such as success messages, error details, or requested information. Example: Returning user profile details in a GET request.

19. What is the purpose of the request and response headers in API testing?

Purpose of Request and Response Headers in API Testing:

• Request Headers: Provide metadata about the request, such as authentication (Authorization), content type (Content-Type), and caching control.
• Response Headers: Contain metadata about the server response, including status info, content type, and security policies.

20. What is API mocking?

API mocking is practice of simulating the behavior of an API endpoint during testing without actually invoking the real API. It is beneficial during the development stage.

• Request Headers: Provide metadata about the request, such as authentication (Authorization), content type (Content-Type), and caching control.
• Response Headers: Contain metadata about the server response, including status info, content type, and security policies.

21. Why API Mocking is used in API Testing?

• Mock APIs are useful when applications have dependencies with external APIs.
• Mock APIs are useful for testers to plan and validate test executions and for developers to do unit testing and identify the initial development stages.
• Mock APIs are useful in situations where the complete API needs to be made available for consumer testing before committing it to development stage.

22. What are the advantages of using API mocking in API testing?

Here are some advantages of using API mocking in API testing:

• Development stage: API mocking is useful to simulate external dependencies and unexpected behavior.
• Functional stage: API mocking is useful to prepare the API to handle extreme cases detect potential failures and verify whether the API works as expected or not.
• Non-functional stage: API mocking makes it possible to evaluate the performance and response times of APIs in various scenarios.
• Allows API demonstration: API mocking allows developers to demonstrate an API to clients before deploying the software.

23. What is API security testing, and why is it important?

API security testing is the process of testing the vulnerabilities in the API. This is done through penetration testing or manual scanning of the APIs. API security testing is important:

• Cloud-based applications rely on APIs to exchange data and interact with each other. Any security vulnerabilities can have far-reaching consequences.
• Online operations and businesses rely on APIs to integrate different systems and services. This causes potential security risks.
• Organizations rely on security solutions that are built for web apps to detect and secure from API threats. Such solutions cannot detect unique vulnerabilities and gaps in the APIs.

24. How do you ensure the security of APIs in your tests?

To secure APIs during testing, follow these best practices:

• Authentication & Authorization: Verify security mechanisms like: OAuth 2.0, JWT (JSON Web Token), API Keys, Basic Auth
• Input Validation & Sanitization: Prevent SQL Injection and XSS attacks by validating input data.
• Secure Headers & Encryption: Use HTTPS (TLS/SSL) and set security headers like: Strict-Transport-Security, Content-Security-Policy
• Rate Limiting & Throttling: Prevent DDoS attacks by limiting API request rates.
• Token Expiry & Refresh Mechanism: Ensure JWTs expire and implement refresh tokens securely.
• Access Control (RBAC): Restrict API access based on user roles and permissions.
• Logging & Monitoring: Track API activity using logging tools for anomaly detection.
• Automated Security Testing: Use tools like OWASP ZAP, Postman Security Tests, Burp Suite to identify vulnerabilities.

25. What are the different types of error responses in API testing?

Error responses in API testing are categorized using HTTP status codes to indicate different types of failures. Here are the different types:

• Validation errors: Validation errors occur when the API request does not meet the validation criteria defined by the API.
• HTTP error status codes: These can be client-side status codes or server-side status codes. There are standard HTTP status codes like 4xx or 5xx that indicate errors in the API request or response.
• Custom error messages: These are the customer error messages that are returned by the API in case of errors or exceptions.

26. How do you handle error responses in your API tests?

Handling Error Responses in API Testing:

• Validate HTTP Status Codes: Ensure correct 4xx (client errors) and 5xx (server errors) are returned.
• Check Error Messages: Verify meaningful, user-friendly error messages.
• Test Edge Cases: Send invalid inputs, missing parameters, and malformed requests.
• Implement Retry Mechanism: Retry requests for temporary failures (5xx errors).
• Monitor & Log Errors: Use tools like Postman, JMeter to track failures.
• Respect Rate Limits (429): Implement delays or follow Retry-After headers.
• Secure Error Handling: Ensure no sensitive data is leaked in error responses.

27. What is input validation?

Input validation is the process of verifying user input to ensure it is correct, secure, and meets expected criteria before processing. It helps prevent errors, data corruption, and security vulnerabilities like SQL Injection and XSS attacks. Input validation can be performed on both client-side and server-side for better security and data integrity.

28. Why input validation is important in API testing?

Input validation is important in API testing as it prevents malicious data from entering the system. It is important to prevent injection attacks and other malicious activities. This can be done manually or using automated tools.

29. What is cross-site request forgery (CSRF)?

Cross-site request forgery is an attack that forces user top execute unwanted action in the web application in which they are authorized to perform actions. This attack exploits the trust the web application has in an authenticated user.

API Testing Interview Questions for Experienced

30. How CSRF can be prevented in API testing?

• CSRF tokens: CSRF can be prevented by using CSRF tokens. These are the random tokens that need to be unique per user session and should be of large random value to make it difficult to guess.
• SameSite cookies: SameSite is a browser security mechanism that determines when a website’s cookies are included in the requests originating from other websites.
• Referer-based validation: HTTP referer header can be used to defend against CSRF attacks by verifying the request originated from the application’s own domain.

31. What is API contract testing?

API contract testing is a type of testing that aims to monitor the API conversation that takes place between the API consumer and the API producer. It can be performed using tools like Postman, Swagger, etc.

32. Why API contract testing important?

API contract testing ensures that APIs follow a defined structure (contract) between the client and server, preventing breaking changes.

• Ensures Consistency: Verifies API responses match the expected format (JSON, XML).
• Detects Breaking Changes: Prevents issues when updating APIs.
• Improves Reliability: Ensures backward compatibility across services.
• Speeds Up Development: Allows frontend and backend teams to work independently.
• Enhances API Stability: Reduces failures in integrations by enforcing standards.

33. What is API performance testing?

API Performance Testing evaluates an API’s speed, scalability, and reliability under different loads and conditions. It ensures that the API responds efficiently and remains stable under varying traffic levels.

• Response Time: Measures how quickly the API responds.
• Throughput: Checks the number of requests handled per second.
• Load Testing: Simulates high user traffic to check performance.
• Stress Testing: Determines API behavior under extreme conditions.
• Spike Testing: Evaluates API response to sudden traffic surges.

34. Why API performance testing important?

API performance testing ensures that an API is fast, scalable, and reliable under different workloads.

• API performance testing helps to improve the API’s overall performance and stability.
• It helps to provide insight into the API’s overall performance thus helping to identify the areas of strength and weakness.
• It helps to identify the performance issues and determine the impact of changes.
• It helps to ensure that the API is flexible and can handle the demands of the real world.

35. What is API monitoring?

API monitoring is the process of continuously tracking an API’s availability, performance, and functionality to detect issues in real-time.

• Uptime & Downtime: Ensures the API is always accessible.
• Response Time: Tracks API speed and latency.
• Error Rates: Detects failed requests or unexpected responses.
• Traffic & Load: Monitors API usage patterns.
• Security Threats: Identifies potential vulnerabilities.

36. Why API monitoring important?

API monitoring ensures that APIs remain reliable, fast, and secure by tracking their performance in real-time.

• They provide measurements of how long a routine takes to execute, how often it is called, and how much of total time is spent in executing the transaction.
• It is important to ensure availability, performance, and security of the APIs.
• It helps to guarantee a dependable and effective user experience.
• It helps to track the availability of the critical APIs.

37. What is API virtualization?

API virtualization is the process of using a tool that creates a virtual copy of the API mirroring all of the specifications of the production API and using this virtual copy for testing.

• Allows early testing before the actual API is built.
• Reduces dependency on live systems by simulating unavailable APIs.
• Improves development speed by enabling teams to work in parallel.
• Supports performance testing under various conditions.

38. Why API virtualization is used in API testing?

• It is used in API testing as it allows for testing much earlier in the development process, removing the key bottlenecks that would otherwise delay production.
• It is helpful to isolate dependencies, simulate responses, and ensure consistent behavior.

39. What is the purpose of API documentation in API testing?

API documentation is a technical document that describes the API in detail. It includes instructions on how to effectively use and integrate API and provides updates regarding API’s lifecycle such as new versions. It is primary resource explaining what is possible with the API and how to get started with the API.

40. How do you prioritize API test cases for regression testing?

• Critical Functionality First: Focus on core APIs that impact business operations.
• Frequently Used APIs: Test APIs that are accessed most often by users.
• Recently Changed or Updated APIs: Prioritize APIs modified in recent updates.
• APIs with High Bug History: Retest APIs that previously had defects.
• Security and Authentication APIs: Ensure no vulnerabilities are introduced.
• Performance-Critical APIs: Validate APIs affecting system speed and scalability.
• Dependent APIs: Test APIs that interact with multiple services or components.

41. How to handle versioning in API testing?

• Test Multiple Versions: Ensure backward compatibility by testing old and new API versions.
• Use Version-Specific Endpoints: Validate API calls with versioned URLs (e.g., /v1/users vs. /v2/users).
• Check Header-Based Versioning: Verify API responses when versioning is done via headers (Accept: application/vnd.api.v2+json).
• Compare Responses: Ensure consistency between different versions while testing improvements.
• Validate Deprecated Features: Test error handling for removed or updated functionalities.
• Automate Version-Based Tests: Maintain separate test suites for each API version.

42. List the challenges faced while performing API testing.

• Building irrelevant tests: Building tests without considering how the APIs will be consumed may be quicker in short term.
• Initial setup of API testing: Setting up an API testing setup requires certain level of expertise and dedication among the team members.
• Not including API dependencies: Failure to include API dependencies as a part of API testing strategy can be a critical API testing challenge.
• Not validating data: It might be possible that API tests pass successfully but APIs are not returning the correct data in their responses.

43. What are the different bugs that can be found in API testing?

Different Bugs Found in API Testing:

• Duplicate or missing functionality.
• Improper messaging.
• Multi-threading issues.
• Security and performance issues.
• Reliability issues.

44. What do you understand by Input injection?

Input Injection is a type of security vulnerability where an attacker manipulates input data to exploit a system, often leading to unauthorized access, data corruption, or system crashes.

45. Is it possible to hack API while testing?

Yes, it is possible to hack API while testing as requests are being sent over the internet which mostly follows HTTP protocol which is a text-based protocol. Hence, it is important to perform security testing of the APIs to ensure safer systems.

Common API Vulnerabilities Exploited:

• Broken Authentication: Weak tokens or missing authentication.
• SQL Injection: Malicious queries to access or modify databases.
• Cross-Site Scripting (XSS): Injecting scripts into API responses.
• Insecure Endpoints: Exposed sensitive data due to improper access controls.
• Rate Limiting Bypass: Exploiting APIs by sending excessive requests.

46. How do you perform API Load Testing?

API load testing is a type of performance testing that is done to check the application’s capability to perform under various user loads. It is done by simulating many users hitting the API at the same time to identify if the application is capable of handling the load by maintaining the consistency in the response times and not impacting the functionality.

47. What is the test environment of API?

The API test environment is a setup where APIs are tested under controlled conditions before deployment. It includes:

• Server & Database: A dedicated test server and database to mimic production.
• API Endpoints: Staging or sandbox URLs separate from live systems.
• Authentication & Security Setup: API keys, OAuth, JWT tokens for access control.
• Mock Services & Virtualization: Simulated dependencies for testing APIs in isolation.
• Testing Tools: Tools like Postman, JMeter, SoapUI for automation and performance testing.
• Logging & Monitoring: Tracks API requests, responses, and failures.

48. What is the procedure to perform API testing?

The API testing flow basically includes three main steps:

• Send the request with the necessary input data.
• Get the response having the output data.
• verify that the response returned is as expected in the requirement.

49. When writing API document, what must be considered?

Key Considerations When Writing API Documentation

• Clear API Overview: Explain API purpose, features, and use cases.
• Endpoint Details: List all endpoints (GET /users, POST /orders) with descriptions.
• Request & Response Format: Provide JSON/XML examples with expected parameters.
• Authentication & Security: Explain how to use API keys, OAuth, or JWT for access.
• Error Handling: Document status codes (400 Bad Request, 500 Internal Server Error) and responses.
• Rate Limits & Throttling: Define request limits and handling of 429 Too Many Requests.

50. What tools could be used for API testing?

• Katalon: This is easy-to-use tool that supports REST, SOAP requests, and SSL client certificates. It also enables test import from Swagger, Postman, WADL, and WSDL.
• JMeter: Apache JMeter is an open-source, 100% Java application that is created for performance testing. This tool enables automatic working with CSV files.
• REST-assured: REST-assured is an open-source Java domain-specific language that enables testing REST services. It supports POST, GET, PUT, DELETE, PATCH, and HEAD requests.
• SoapUI: SoapUI is an automated testing tools for Soap and REST APIs. It is an open-source tool that allows to create tests effortlessly with drag and drop, point-and-click.
• Postman: Postman is an application that is used for API testing. It is a standalone platform that is used to build, test, design, modify, and document APIs.

Conclusion

API testing ensures functionality, security, and performance in modern applications. This guide covered key interview questions, best practices, and essential tools for beginners and experienced professionals. Mastering API testing helps improve software reliability and prepares you for real-world challenges.

API Testing Interview Questions for 2025: Top 50 Interview Questions

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Article Tags :

• Interview Questions
• Software Testing
• Web Technologies
• Interview-Questions
• Web-API

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