Top 50 API Testing Interview Questions and Answers (2025)

Top API Testing Interview Questions and Answers for 2025

1. What is API testing?

API testing refers to the process of testing Application Programming Interfaces (APIs) to ensure their functionality, reliability, performance, and security. 

2. What are the types of APIs?

There are several types of APIs, including:

• Web APIs: These APIs are designed for communication over the web and are widely used for building web applications. Examples include RESTful APIs, SOAP APIs, and GraphQL APIs.
• Operating System APIs: These APIs are used for interaction with a device or server's underlying operating system. Examples include Windows API, iOS API, and Android API.
• Library APIs: These APIs are used for software development and provide pre-built functions and classes for common tasks. Examples include Python's requests library, Java's JDBC API, and . NET's Entity Framework.
• Hardware APIs: These APIs are used for interacting with hardware devices such as sensors, cameras, and printers. Examples include USB APIs, Bluetooth APIs, and GPIO APIs.

3. Mention a few common tools used for API testing.

Some common tools used for API testing include:

• Postman: A popular tool for designing, testing, and documenting APIs. It provides a user-friendly interface for sending requests, inspecting responses, and automating tests.
• Swagger: A widely used tool for designing, building, and documenting RESTful APIs. It provides a visual editor for designing API contracts and generating documentation.
• SoapUI: A comprehensive tool for testing SOAP and RESTful APIs. It supports various protocols, message formats, and authentication methods and provides advanced testing features such as data-driven testing and security testing.

4. What is a RESTful API?

RESTful API (Representational State Transfer) is an architectural style for designing networked applications. It is based on a set of constraints that enable scalability, simplicity, and interoperability. 

5. What is SOAP API?

SOAP API (Simple Object Access Protocol) is a protocol for exchanging structured information to implement web services. It uses XML as its message format and provides a set of rules for message exchange, fault handling, and security. 

6. What is the difference between RESTful API and SOAP API?

The main difference between RESTful API and SOAP API lies in their architectural styles and message formats. RESTful API follows the principles of Representational State Transfer (REST) and typically uses HTTP methods such as GET, POST, PUT, and DELETE, with data represented in formats like JSON or XML. On the other hand, SOAP API uses the Simple Object Access Protocol (SOAP) and typically relies on XML for message exchange, with predefined methods and strict message structures.

7. What is an API endpoint?

An API endpoint refers to a specific URL or URI (Uniform Resource Identifier) that represents a unique resource or service provided by an API. It acts as a point of interaction for clients to send requests and receive responses from an API. 

8. Explain API documentation.

API documentation is a comprehensive guide that provides information on how to use and interact with an API. It typically includes details about API endpoints, request and response formats, authentication and authorization methods, error handling, and other relevant information. 

9. What is an API testing framework?

An API testing framework is a set of predefined rules, conventions, and tools that provide a structured approach to designing, implementing, and executing API tests. 

10. Mention common HTTP methods used in API testing.

Common HTTP methods used in API testing are:

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

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

The purpose of HTTP status codes in API testing is to indicate the outcome of an API request. Status codes are three-digit numbers included in the response message from the server to provide information about the result of the request.

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

Headers in API testing play a significant role in controlling and managing the behavior of API requests and responses.  

13. What is JSON, and why is it commonly used in API testing?

JSON stands for JavaScript Object Notation, and is a lightweight data-interchange format that is commonly used in API testing. JSON is commonly used in RESTful APIs since it enables efficient data serialization and deserialization, making it ideal for API testing.

14. What is XML, and when is it used in API testing?

XML, which stands for Extensible Markup Language, defines the set of rules for encoding documents in a format that is readable by both humans and machines. XML is used in API testing when APIs require data exchange in XML format.

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

The purpose of authentication in API testing is to verify the identity of the requester before granting access to protected resources. 

16. What are some common authentication methods used in API testing?

Some common authentication methods used in API testing include:

• Token-based authentication: This involves issuing a token to the client after successful authentication, which is then included in subsequent API requests for authorization.
• Basic authentication: This involves sending the username and password as part of the API request headers, encoded in base64 format.
• OAuth: This is a widely used authorization framework that allows for the delegation of access rights from one entity to another without sharing credentials.

17. How do you handle API authentication in your tests?

In API tests, authentication is typically handled by including the required authentication parameters, such as tokens, API keys, or OAuth credentials, in the appropriate headers or query parameters of the API requests. 

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

API versioning is the practice of specifying a version number in the API endpoint or headers to ensure backward compatibility and manage changes in the API over time. API versioning is important in API testing to ensure that the correct version of the API is being tested and that changes in the API do not break existing client applications.

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

Different types of error responses in API testing include:

• HTTP error status codes: These are standard HTTP status codes, such as 4xx and 5xx codes, that indicate errors in the API request or response.
• Custom error responses: These are custom error messages or error objects returned by the API in case of errors or exceptions.
• Validation errors: These are errors that occur when the API request does not meet the validation criteria or constraints defined by the API.

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

Error responses in API tests can be handled by checking the response status codes, parsing the custom error messages or error objects returned by the API, and validating against expected error responses. 

21. What is the purpose of query parameters in API testing?

Query parameters in API testing are used to pass additional parameters in the URL of an API request. These parameters are used to customize the behavior of the API request, such as filtering, sorting, or paginating results. 

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

Request and response headers in API testing are used to transmit additional information about the request or response. Request headers can be used to specify a content type, authentication, caching, language preferences, etc., while response headers can provide information about the server, caching, and more.

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

The request and response body in API testing contains the payload or data that is sent in the request and response messages. The request body is used to send data from the client to the server, such as input parameters or data to create/update resources, while the response body contains the data returned by the API in response to a request. 

24. How do you handle dynamic values in API testing, such as timestamps or random data?

Dynamic values in API testing, such as timestamps or random data, can be handled by using techniques such as data-driven testing, parameterization, or test data management. 

25. What is API mocking, and why is it used in API testing?

API mocking is the practice of simulating or emulating the behavior of an API endpoint during testing without actually invoking the real API. API mocking is used in API testing to isolate the testing environment from external dependencies.

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

• Test independence: Mocking APIs allows for testing APIs in isolation without relying on external APIs, reducing dependencies and potential failures due to external factors.
• Test repeatability: Mocking APIs ensure consistent responses and behavior during testing, making it easy to reproduce and debug issues.
• Test control: Mocking APIs provide control over the responses and behavior of the API endpoints, allowing for targeted testing of different scenarios or error conditions.

27. How do you perform load testing on APIs?

Load testing on APIs can be performed by simulating a large number of concurrent users or requests to the API endpoint using load testing tools or frameworks. 

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

API security testing is the practice of evaluating the security posture of an API to identify and mitigate potential security risks or vulnerabilities. It is important in API testing to ensure that APIs are secure and protect sensitive data.

29. What are some common security vulnerabilities in APIs?

• Injection attacks: These occur when untrusted data is directly included in API requests or responses, allowing attackers to execute malicious commands or inject malicious code.
• Authentication and authorization vulnerabilities: These occur when authentication or authorization mechanisms are weak or improperly implemented, leading to unauthorized access or privilege escalation.
• Cross-Site Scripting (XSS) attacks: These occur when an API does not properly validate or sanitize user input, allowing malicious scripts to be injected and executed in the responses displayed in web browsers.

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

Some best practices to ensure the security of APIs in your test include:

• Input validation: Ensure that all input data in API requests are properly validated and sanitized to prevent injection attacks.
• Authentication and authorization: Properly implement and validate authentication and authorization mechanisms to ensure that only authorized users can access the API endpoints and perform authorized actions.
• Secure communications: Use secure communication protocols, such as HTTPS, to encrypt data transmitted between the client and server.

31. What is cross-site scripting (XSS), and how can it be prevented in API testing?

Cross-site scripting (XSS) is a type of vulnerability where malicious scripts are injected into web pages. It can be prevented by properly validating and sanitizing user input in API requests.

32. What is cross-site request forgery (CSRF), and how can it be prevented in API testing?

Cross-Site Request Forgery (CSRF) is an attack where unauthorized actions are performed on behalf of authenticated users. It can be prevented by implementing CSRF tokens and verifying requests in API testing.

33. What is input validation, and why is it important in API testing?

Input validation is the process of validating and sanitizing user input to prevent security vulnerabilities. It is important in API testing to prevent injection attacks and other malicious activities.

34. What are some best practices for API testing?

Best practices for API testing include proper input validation, authentication and authorization, error handling, security testing, access controls, and regular updates and patches.

35. How do you handle API versioning in your tests?

API versioning can be handled in tests by including versioning information in API endpoints or headers and ensuring backward compatibility to avoid breaking existing functionality.

36. What are some techniques for handling API timeouts and retries in tests?

Techniques for handling API timeouts and retries in tests include setting appropriate timeout thresholds, implementing retries with backoff strategies, and handling error responses gracefully.

37. What is API contract testing, and why is it important?

API contract testing is important to validate the compatibility between API consumers and providers. It can be performed using tools like Swagger, Postman, or specialized contract testing frameworks.

38. How do you perform API contract testing in your tests?

API contract testing involves verifying that APIs meet predefined expectations and behaviors, ensuring compatibility and reliability.

39. What is API performance testing, and why is it important?

API performance testing is important to measure the response time, throughput, and resource utilization of APIs. It can be performed using tools like JMeter, LoadRunner, or Gatling.

40. How do you measure the performance of APIs in your tests?

API performance testing is important to measure the response time, throughput, and resource utilization of APIs. It can be performed using tools like JMeter, LoadRunner, or Gatling.

41. What is API monitoring, and why is it important?

API monitoring is important to ensure the availability, performance, and security of APIs. 

42. What are some common tools used for API monitoring?

Common tools used for API monitoring include Prometheus, Grafana, and ELK stack.

43. What is API virtualization, and why is it used in API testing?

API virtualization is the process of simulating APIs for testing purposes. It is used in API testing to isolate dependencies, simulate responses, and ensure consistent behavior.

44. What are the benefits of using API virtualization in API testing?

The benefits of using API virtualization in API testing include reduced dependency on external systems, faster and isolated testing, and improved test repeatability.

45. How do you handle versioning and backward compatibility in your API tests?

Versioning and backward compatibility in API tests can be handled by maintaining backward compatibility for existing APIs, providing clear versioning information, and updating tests accordingly.

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

API documentation is important in API testing to understand the API endpoints, their functionalities, and how to use them correctly.

47. How do you generate and maintain API documentation in your tests?

It can be generated and maintained using tools like Swagger, API Blueprint, or OpenAPI.

48. What are some best practices for API test automation?

Best practices for API test automation include using a framework or tool for automation, designing reusable and maintainable test scripts, incorporating test data management, and leveraging continuous integration and delivery (CI/CD) practices.

49. How do you handle API changes and updates in your tests?

API changes and updates can be handled in tests by updating the test scripts, retesting affected functionalities, and ensuring backward compatibility before deploying changes to production.

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

Prioritizing API test cases for regression testing can be based on the criticality of APIs, impact on business functionalities, frequency of API usage, and feedback from stakeholders.

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Conclusion

Equip yourself with in-depth knowledge and insights using these comprehensive API Testing Interview Questions and answers. Boost your confidence and impress your potential employers with your expertise in API testing. 

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