Few software applications exist completely in isolation. Instead, they use application programming interfaces (APIs) to communicate with other software systems. APIs enable an application to fetch data from external sources and share its own functionality with other apps.

If you want to make your application available for integrations, your organization will need at least one of its own APIs. There are several ways to go about building an API for your business, but no matter your approach, it needs to be tested.

Proper API testing will ensure that your API performs as expected when faced with a wide variety of requests, including ones you expect and those you don’t. The process typically entails sending requests to your API and monitoring the responses. Specific API testing approaches include:

  • functional tests, which check that an API returns the right response for a given request.
  • load tests, which gauge how an API handles a large volume of requests over a short period.
  • security tests, which assess how an API responds to and resists cyberattacks.

Effective API testing requires specialized testing software that can send custom requests to an API and record the API’s responses. All API testing tools let testers submit requests manually — in other words, you write out the request, send it, and see how the API reacts. However, it would take a while to test any API this way, which is why many testing tools also support automated tests. Automated testing systems run many scenarios quickly with minimal human input.

Automated testing has become increasingly popular due to the new “shift left” approach to software development. With this approach, teams conduct structured testing early and regularly through the development cycle, not just when the product is near completion. A good API testing tool, especially one with automated testing, saves time and helps testers shift left to catch bugs earlier.

There are numerous tools on the market you can use to evaluate your APIs. To help your search, we’ve compiled 10 of the best options below to ensure your software is on par with expectations and allows for seamless integrations.

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1. Postman

logo for the API testing tool Postman

If you’re familiar with API testing, you’ve likely heard of Postman. It’s a comprehensive option for designing, testing, documenting, and monitoring REST APIs. Postman began as a simple extension for Google Chrome and has since expanded to a leading tool for macOS, Windows, and Linux.

Postman serves as an HTTP client capable of both automated and manual testing, allowing you to autopilot routine tests while trying specific use cases yourself. Postman also lets you build collections, API descriptions that you can execute quickly and share with fellow Postman users. Collections eliminate the friction of writing out the same API calls repeatedly.

Postman is also acclaimed for its ease-of-use. The entire application is designed for all skill levels, from beginner testers unfamiliar with command-line interfaces to experts seeking a more powerful, customizable solution. The user interface is very intuitive compared to other testing solutions, and Postman doesn’t require testers to adopt the same language as developers, which leaves more time to focus on testing.

Postman’s pricing plans include a free version with limited features, paid plans for individual teams ($12/user/month) and businesses ($24/user/month), and an enterprise option.

2. SoapUI

logo for the API testing tool SoapUI

Advanced testers and developers looking for a different popular solution should consider SoapUI, an API testing tool for REST and SOAP APIs as well as other API frameworks. With this tool, you can run functional, performance, and security tests.

SoapUI was built for experienced users seeking to build more complex automated tests, plus manual calls when necessary. However, junior testers and developers can also get up to speed thanks to SoapUI’s graphical testing interface. Rather than coding the API calls, users may drag-and-drop elements to build out their tests.

SoapUI comes in two variants, SoapUI Open Source, a free tool, and ReadyAPI, its paid tool. SoapUI Open Source is capable of everything described above, but ReadyAPI provides more means to construct, deploy, and monitor your tests. ReadyAPI pricing starts at $644 for a one-year license.

3. Katalon Studio

logo for the API testing tool Katalon Studio

Katalon Studio is a free software testing tool that combines manual and automated testing for web APIs, desktop applications, and mobile apps. Another well-known tool in the space, Katalon appeals to users looking for a comprehensive solution for deployments across multiple platforms — no need to purchase several different tools for testing different deployments. For web APIs, Katalon Studio can perform both REST and SOAP requests.

Katalon Studio is also built to be accessible. Its graphical user interface helps users construct automated and exploratory tests from its project templates and from stored elements that you create. There’s also a library of built-in automation keywords to speed up the coding process.

Katalon Studio is free. There’s also an enterprise upgrade for $759 per license per year with all the capabilities of Katalon Studio plus additional features.

4. Assertible

logo for the API testing tool Assertible

Though a relative newcomer to the space, Assertible has proven a strong contender for user-friendly API testing. Use Assertible to build, store, and automate tests of your REST API or HTTP service — all its technologies are focused on making your API high-performing and reliable.

Of particular note is this tool’s attention to up-to-date testing. Assertible can be set to conduct automatic tests after you deploy a new version of your API and quickly flag problems, saving friction in your build and testing process. Plus, Assertible automatically syncs your tests with changes to your API, so you don’t need to update tests yourself after every deployment.

Assertible showcases impressive integrations with other applications like GitHub, for storing your assets, and Slack, for notifications regarding performance.

Assertible offers four pricing plans, starting with the free plan with strict limits on the number of services you may test and the number of tests you can conduct per service. Above that, there are plans for $25, $50, and $100 per month, each permitting more users and tests.

5. Tricentis Tosca

logo for the API testing tool Tricentis Tosca

Tricentis develops enterprise-level software for testing and analytics — Tricentis Tosca is its continuous testing platform, built to function within the Agile software development framework. It’s designed to allow testing at all stages of your development cycle, with an interface that lets anyone probe your API’s functionality.

Regardless of your API’s build and environment, Tricentis aims to provide an enterprise solution custom-fit to your application. This tool supports functional tests, load tests, security tests, and exploratory tests via SOAP, REST, and other protocols.

Tricentis Tosca also places heavy emphasis on its AI-based user interface test automation. The software uses machine learning to develop test cases based on user interface mockups that you submit, reducing the amount of manual tests required.

6. API Fortress

logo for the API testing tool API Fortress

Another prominent enterprise-level API testing option, API Fortress combines the range of tools necessary with a simple user interface that experts and beginners alike can learn quickly.

API Fortress works as a centralized cloud-based platform where development and testing teams can work in one place. Inside their browser, users can run functional tests, execute load tests, and automate tests on REST and SOAP APIs. The API Fortress reporting dashboard consolidates results into a cohesive display so you can understand your tests at a glance.

API Fortress also helps you keep a close eye on your APIs with its monitoring capabilities — the program can continuously run unlimited tests and alert you when problems arise. Results data are stored on its servers and won’t disrupt your app’s performance.

7. Mocklets

logo for the API testing tool Mocklets

On a tighter budget? You might consider Mocklets, an API simulator for creating mock-HTTP-based APIs. Use this software to build a simulation of a future API to conduct testing while your actual API is in development.

With Mocklets, you can set up an API in a controlled environment, run security tests without risking harm to your existing application, and monitor your mock-API activity in real-time. You can also easily toggle the responses sent from your mock-API instead of needing to reconfigure the whole thing. Mock-APIs adhere to Open API specifications to ensure they behave like the real thing.

Mocklets is comparatively cheap amongst other paid options here. There’s a free version with 10 mock APIs and 5 test files, and a paid version for $5 per month allowing up to 125 mock APIs and test files, plus more collaborators. Finally, larger organizations interested in this approach to API testing may try Mocklets Enterprise.

8. Swagger

logo for the API testing tool Swagger

Swagger builds open-source tools for API development encompassing every stage of the build process, from planning and design to testing and monitoring. When implementing a web API from the ground up, Swagger helps ensure your API adheres closely to protocol.

For a quick and easy manual API test, you can try Inspector, Swagger’s free REST, SOAP, and GraphQL client. Inspector lets you submit manual calls to verify that your API returns the right responses. It also lets you generate user documentation from previous successful tests.

For more advanced testing, including automation and collaboration tools, Swagger integrates with ReadyAPI, the paid arm of SoapUI.

9. Apache JMeter

logo for the API testing tool JMeter

You may be familiar with Apache HTTP Server, the most popular open-source web server software used today — your servers may even run on it. The Apache Software Foundation also makes a free piece of API testing software called JMeter.

JMeter was originally developed for API load testing, but can now handle performance testing as well. JMeter works with a variety of protocols, including REST and SOAP. However, this tool isn’t for beginners — it requires a skilled Java developer to integrate into your testing environment.

JMeter’s functionality can compete with paid options, as it supports caching and recording of requests/responses, distributed tests, and dozens of plugins to extend its capabilities.

10. REST Assured

logo for the API testing tool Rest Assured

Rest Assured is another free and open-source testing resource for developers. It’s a Java library that streamlines HTTP requests to REST APIs.

If you want to implement a DIY approach to an HTTP client, you can construct custom requests in the Java language with REST Assured’s syntax, saving you time spent programming lengthy instructions from the ground up.

Without knowledge of constructing API calls or the Java programming language, this tool won’t get you far. However, if you want to sidestep a more commercial option, consider this one instead.

Finding the Right Testing Tool

Much like a high-quality website conveys professionalism in an online brand, a functional API conveys the competency of your development team. By allowing third-party applications to easily integrate with your service, you’ll extend your reach and build out your user base. As we’ve discussed, this isn’t possible without adequate testing, and adequate testing is very difficult without the right tool.

All the testing tools I’ve listed vary by cost, purpose, and learning curve. Fortunately, several are free or at least offer free trials, so it’s worth taking the time to feel out your options in this space — you’ll save a lot of time in the long-run, especially with fewer bugs to address.

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Originally published Dec 23, 2020 7:00:00 AM, updated June 27 2022


Application Programming Interface (API)