API testing became absolutely critical with the rise in cloud applications and interconnected platforms. The majority of services we use daily rely on multiple interconnected APIS. If one of them does not work properly, the entire service can be compromised. Fortunately, several API testing tools on the market can help us ensure everything goes as smoothly as possible. This article features the six best API testing tools. Still, first, we will explain what an API is and the benefits of API testing.

Table of contents

What is an API?
What is API Testing?
What are API Testing tools?
The best API testing tools
    ➤  1. Apigee
    ➤  2. SoapUI
    ➤  3. Katalon
    ➤  4. Postman
    ➤  5. Tricentis
    ➤  6. Assertible

What is an API?

APIs (Application Programming Interfaces) allow applications to interact and communicate with each other by establishing precise rules and determinations. More specifically, they are responsible for stipulating the types of requests that an application can make to another and for defining the following three aspects: how to make those requests, in what data format, and what practices the user must follow.

Thanks to APIs, an application connected to the internet can send the request (with data) to a server, and, in turn, the server will retrieve the request and interpret it. When the necessary actions are executed, it sends a response back to the application's user interface with information.

How APIs work | Imaginary Cloud

What is API Testing?

API testing consists of software that tests how the APIs were developed. It analyzes whether the APIs correctly fulfill their purpose in order to ensure the application's functionality, reliability, security, and performance are not compromised. Moreover, API testing mainly entails testing SOAP web services or REST APIs, using JSON or XML message payloads, which are sent through HTTPS, HTTP, MQ, and JMS.

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

  • if an API is returning the expected response and in the correct format;
  • if it reacts adequately to edge cases (e.g., failures and unexpected inputs);
  • whether it responds securely to possible security attacks;
  • The time it takes to deliver a response.

What are API Testing tools?

What are API Testing tools? API Testing tools refer to software that aids in the testing of application programming interfaces (APIs). Examples include Postman for manually testing APIs, Rest-Assured for testing REST services in Java, and SoapUI for both REST and SOAP services.

The best API testing tools

1. Apigee

Apigee is a very interesting cross-cloud API testing software powered by JavaScript that allows developers and testers to access its features via different editors.

This tool is compatible with APIs that contain large amounts of data, being ideal for robust and complex digital businesses. Plus, it is able to easily identify issues that are compromising performance by analyzing the API traffic, response times, and potential error rates. Moreover, Apigee also enables the creation of proxies from Open API Specification.

Apigee provides a free trial, which allows taking it for a test ride before making any decisions. Afterward, to continue using it, organizations and testers must choose the most suitable package. The existing packages are Evaluation, Standard, Enterprise, and Enterprise Plus.

Apigee API testting tool

2. SoapUI

SoapUI is a software tool 100% dedicated to API testing that enables executing tests on SOAP APIs and also on web services REST, considering that it is headless. It is often one of the preferred tools by testers since it allows them to create complex scenarios and further supports asynchronous testing. Plus, it also stands out for its great data-driven testing and easiness of use.

SoapUI offers two packages, one free and the other paid (the pro package). The free solution is fairly easy to use, and testers can reutilize scripts. In the pro package, it is additionally possible to create custom code, extract data (from databases, files, and excels), and support native CI/CD integrations.

SoapUI API testing tool


Katalon is a popular test automation tool for APIs, as well as web applications and desktop and mobile apps. It supports SOAP and REST requests and provides multiple parameterization features and commands. Additionally, Katalon follows a data-driven approach, supports CI/CD integrations, and allows testers to execute automated and exploratory testing.

One of the greatest advantages that this software has is that, despite the numerous useful features it offers, it is not very hard to use, being suitable even for non-techies.

Katalon offers a very good and complete free package. 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) that further executes tests in CI/CD pipelines or via the command-line interface.

Katalon API testing tool.


Although it was built as a plugin for Google Chrome, Postman can also be used with Windows and Mac and is an excellent tool for testing API services, including monitorization, debugging, running requests, etc.

Postman does not require developers or testers to learn a new language and further supports various formats (such as Swagger and RAML). Moreover, it also provides built-in tools, workspaces, and collections. Contrarily to CURL, Postman is not a tool based on the command line; thus, developers and testers do not need to paste the text into the command line window.

Postman is a free API testing tool. For those who wish to benefit from additional features, then it costs 12$ per month for each user.

Postman API testing tool.


Tricentis is one of the top choices for those who wish to integrate it into the Agile and DevOps Cycle. It follows model-based test automation, which facilitates script maintenance, and provides a fast time for regression testing.

One of the primary pros is that it supports multiple protocols, such as REST, SOAP, HTTP(s), MQ, JMS, NET TCP, IBM MQ, etc. Another great plus is that Tricentis allows end-to-end testing across mobile, cross-browser, and packaged apps.

Regarding the pricing, it may vary according to the specifications of each package.

Tricentis API testing tool.


Developers and testers widely perceive Assertible as one of the top API testing tools with a high focus on reliability, considering it supports API tests at every step that goes from continuous integration to the delivery pipeline.

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

This API testing tool offers three different packages. The standard version costs 25$ per month; the startup plan is 50$ per month; the business plan is the most expensive package (and the one that offers more features), which costs 100$ per month. Moreover, there is also a free personal plan that is suitable for executing more manageable API tests and understanding how Assertible works.

Assertible API testing tool


When choosing an API testing tool is important to consider how each of the different options available in the market might be more suitable for certain purposes. Sometimes developers and testing teams may even opt to use more than one tool, considering that it can be complicated to find the perfect and ideal tool. For this reason, teams should prioritize the "must-have" features they seek.

Therefore, even though all the mentioned solutions in this article hold a high-quality reputation, the best API testing tool will always be the one that better fulfills the organization's or project's needs.

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