The 6 best software tools to test an API

APIs are interfaces that define how software, applications, and services should interact with each other. They allow users to take advantage of the digital interactions offered by computers and mobile devices. Today, many people unknowingly use APIs to meet their business or personal needs. But to ensure the proper functioning of a software or an application, each programming interface must be systematically tested.

Indeed, API testing has become absolutely essential with the rise of cloud applications and interconnected platforms. The majority of services used daily rely on several interconnected APIs. If one of them is not working properly, the whole service can be compromised. Luckily, there are several API testing tools on the market that can help make everything run as smoothly as possible without noticeably impacting app usage.


What is an API test?

API testing software helps determine if APIs meet functionality, reliability, security, and performance requirements.

An API is of good quality if it returns the appropriate response of the expected format at the right time.

Also, API testing mainly involves testing SOAP web services or REST APIs, using JSON or XML message payloads, which are sent over HTTPS, HTTP, MQ, and JMS.

Indeed, during an API test, several elements must be identified and verified: data accuracy, HTTP status codes, error codes, response time, authorization checks, tests not functional like benchmarks, bugs or other suspicious behavior. For users to be able to use an application, it is therefore necessary to ensure that the proposed API is of good quality and efficient. It is possible to perform an API test on a software system that has several programming interfaces.

API testing tools allow testers to verify many aspects, including:

  • whether an API returns the expected response and in the correct format;
  • whether it responds adequately to edge cases (eg failures and unexpected inputs);
  • whether it reacts securely to possible security attacks;
  • the time required to provide a response.

Testers test APIs designed by a development team as well as APIs used in an application such as third-party APIs.

To test programming interfaces, testers use different types of API tests:

  • unit testing;
  • functional testing;
  • load testing;
  • security testing;
  • reliability testing;
  • runtime error detection;
  • web user interface testing;
  • penetration testing;
  • validation tests;
  • fuzz tests.


Why test an API?

APIs are very useful for the daily life of their users and companies. Testing them is absolutely essential since they become the main interface of the application logic.

API testing helps to determine the actual quality and performance by making many calls simultaneously, but also to reduce risk. In short, they allow companies to ensure that the APIs used meet their expectations.

Testing an API saves valuable time. Automating API tests requires less code, which ensures faster test coverage. In general, API testing takes less time to complete full UI regression testing. They therefore allow more efficient detection and correction of errors, but also faster release of the product.

Through API testing, companies can test the basic functionality of software or an application. APIs can then be tested without a user interface.

For companies, API testing is also a good way to reduce costs. They help catch minor bugs that may become more significant during GUI testing. It is therefore more profitable for companies to find and fix these bugs upstream, during API testing.

API testing has the advantage of being language independent. Indeed, to share information, APIs use formats like XML and JSON. To develop a test automation system, it is therefore possible to use any language.

Testing an API helps businesses catch errors users may be making. By analyzing this data, companies can act in a predictive way. The information is then used to anticipateper recurring user errors and to optimize the interface accordingly.

Among the features they offer, the APIs give the possibility to create more complete automation scripts, which improves the efficiency of tests.

With API testing, it is possible to search for security vulnerabilities by providing a specific set of parameters to mimic possible methods of phishing attacks. It is also possible to check how the API can react in the event of such attacks.

That's not all, the API is the bridge that allows data to be extracted with more or less complex processing. It is advisable to check the time it takes to get a response from the API to monitor the performance of the overall application. If it does not meet the criteria, then optimizations must be performed.

Finally, API testing is fast and makes it easy to validate business logic, security, compliance, performance, and other aspects of the application.



On LoadView, companies can test APIs with thousands of concurrent connections. The tests are 100% cloud-based managed in real browsers. LoadView enables powerful and dynamic API testing on any type of system or infrastructure.

This tool supports RESTful APIs, SOAP APIs, and Web APIs that require multi-step execution or authentication. LoadView allows you to make many API calls to detect problems and fix them.

The tool offers flexible performance testing for DevOps teams. This allows them to create multiple test scenarios for the most complex applications.

With LoadView, companies can gain API performance data to improve development and determine overall system performance.

LoadView is available in several languages, including French. It makes available monthly, yearly, or on-demand subscription plans that work for every business. The cost of a subscription on LoadView starts from $129/month.



JMeter is an open source tool for performing load and performance testing of applications and servers. This tool works at a protocol layer and has a plugin-based architecture. For developers, it can serve as a unit testing tool to test JDBC database connections.

It comes with a wide range of API testing features and additional features to make the procedure more efficient.

JMeter allows companies to use different programming languages, collect test data and replay test results.

The main features of this tool are:

  • Allows the use of multiple programming languages
  • Allows replay of test results
  • Helps the team quickly create exceptional data (parameter values) for API testing via CSV Files.
  • Can be used for dynamic and static resource performance testing
  • Performance and load testing of many different applications, protocols and servers.


Katalon Studio

Katalon Studio is a free automation solution for testing APIs, web apps and services, and mobile apps. This tool is accessible to both beginners and experts. It presents a user interface for creating, running and maintaining tests. The main strength of Katalon Studio lies in its ability to bring together UI and business levels for different operating systems.

Katalon Studio supports SOAP and RESTful requests with different types of commands like GET, POST, PUT and DELETE as well as parameterization possibilities. The tool also supports data-driven approach, CI/CD integration, and AssertJ assertion library.

Additionally, users can also opt for Studio Enterprise ($76/month), which includes extended features and private plugins. As an alternative, there is also the Runtime Engine package ($54/month) which allows the tests to be run in CI/CD systems or via the command line interface. The tool is available in English only.

Katalon Studio also offers:

  • autocompletion, autoformatting and code inspection functions;
  • pre-built and customizable code templates;
  • sample projects provided for immediate reference.



Apigee is a cross-cloud API testing tool for measuring and testing the performance of application programming interfaces.

This tool is compatible with APIs that contain large amounts of data, ideal for robust and complex digital businesses. Additionally, it is able to easily identify performance-robbing issues by analyzing API traffic, response times, and potential error rates. Additionally, Apigee also allows the creation of proxies from the Open API specification.

Apigee offers a free trial version, which allows you to test it before making a decision. Then, to continue using it, testers need to choose the most suitable package. The existing packages are Evaluation, Standard, Enterprise and Enterprise Plus.



The Assertible tool is used to test web services and automate API testing. It also supports running API tests after deployment and validating HTTP requests. Assertible has a Sync feature through which users can update tests as their specifications change.

Updating tests is done manually after adding new parameters or modifying the API request.

Developersand testers generally consider Assertible one of the best API testing tools with a focus on reliability because it supports API testing at every stage, from continuous integration to the delivery process.

Besides, Assertible provides users with some very useful features. It supports integration with GitHub, Zapier, and Slack. Assertible also validates HTTP responses with turnkey assertions.

This API testing tool offers three different formulas. The standard version costs $25 per month; the starter plan is $50 per month; the business plan is the most expensive plan (and the one with more features), costing $100 per month. Additionally, there is also a free personal plan that allows you to run simpler API tests and understand how Assertible works. Tool available in English.



SoapUI is designed for testing REST and SOAP APIs of web services. It makes it quick and easy to create API tests, perform data-driven testing, and reuse scripts. SoapUI is a tool that integrates with 13 API management platforms. It supports REST, SOAP, JMS and IoT APIs.

It is often one of testers' favorite tools, as it allows them to create complex scenarios and supports asynchronous testing. Moreover, it also stands out for its excellent data-driven tests and ease of use.

SoapUI offers two plans, one free and the other paid. The free solution is quite easy to use and testers can reuse the scripts. In the pro plan, additional features allow creating custom code, extracting data (from databases, files and Excel tables) and supporting native CI/CD integrations. Tool available in English.

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