Software applications rarely exist in isolation — instead, they use application programming interfaces (APIs) to communicate with other software systems. Therefore, you need API testing tools to ensure your API performs as expected when faced with various requests.
A good API testing tool, especially one with automated testing — that runs many scenarios quickly with minimal human input — saves time and helps testers shift left to catch bugs earlier.
You can use numerous tools on the market to test your APIs. To help your search, we’ve compiled 25 of the best options below to ensure your software is on par with expectations and allows seamless integrations.
Best API Testing Tools
- Katalon Studio
- Tricentis Tosca
- Sauce Labs
- Apache JMeter
- REST Assured
- RoboHydra Server
- Rest Console
- Karate DSL
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 is an HTTP client capable of automated and manual testing, allowing you to autopilot routine tests while trying specific use cases yourself. Postman also lets you build collections and 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 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 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.
Advanced testers and developers looking for a different popular solution should consider SoapUI, an API testing tool for REST and SOAP APIs, and 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.
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 stored elements 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.
Though a relative newcomer to the space, Assertible has proven a strong contender for user-friendly API testing. You can use Assertible to build, store, and automate your REST API or HTTP service tests — 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. For example, 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.
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.
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 you submit, reducing the number of manual tests required.
6. Sauce Labs
Another prominent enterprise-level API testing option, Sauce Labs (formerly API Fortress), combines the range of tools necessary with a simple user interface that experts and beginners alike can learn quickly.
Sauce Labs is 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. In addition, the API Fortress reporting dashboard consolidates results into a cohesive display so you can understand your tests at a glance.
Sauce Labs 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. In addition, results data are stored on its servers and won’t disrupt your app’s performance.
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 ten 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.
Swagger builds open-source tools for API development encompassing every stage of the build process, from planning and design to testing and monitoring. In addition, when implementing a web API from the ground up, Swagger helps ensure your API adheres closely to protocol.
You can try Inspector, Swagger’s free REST, SOAP, and GraphQL client for a quick and easy manual API test. Inspector lets you submit manual calls to verify that your API returns the correct responses. It also enables you to 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.
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 initially 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.
10. 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.
This tool won't get you far without knowledge of constructing API calls or the Java programming language. However, consider this one if you want to sidestep a more commercial option.
11. RoboHydra Server
RoboHydra is a free API testing tool for software that makes HTTP requests. Instead of testing your clients on real servers, you can handle the testing on RoboHydra.
RoboHydra web servers are typically composed of many heads, where a head refers to a piece of software or code that handles requests or triggers certain behaviors for specific URL paths.
You can configure a RoboHydra server to handle test suites, help test race conditions, exploratory testings, and so on.
Since RoboHydra runs as a Node JS package, you’ll need to have Node.js installed.
Apigee is an excellent API management tool provided by Google. Like most other products from Google, this tool is secure, scalable, and reliable.
With Apigee, you can design, secure, publish, analyze, and monetize APIs. But that’s not all. Apigee also features a developer portal that allows developers to create customizable portals where they can test, get API keys, and reduce the time it takes to launch new software.
Apigee’s interface is so simple to use that people from different teams can easily find and access the information they need.
Apigee adopts a flexible pricing system. This API testing tool is free for the first 60 days, where you can run up to 100 thousand API calls per month. After that, you’ll have to contact a Google Cloud sales specialist to find a pricing plan that works for you.
Dredd, like RoboHydra Server, is a command-line tool developers use to validate or test APIs. However, Dredd does more than just test API, as the tool also confirms the validity of your API documentation.
Dredd also supports writing hooks and is free for anyone to use.
No matter your API type, you can test it with RapidAPI’s testing and monitoring solution. RapidAPI testing is supported by RapidAPI studio — the largest API hub worldwide.
RapidAPI’s intuitive user interface makes it easy for developers to manage and monitor their API tests. You can also integrate your tests with the RapidAPI hub to collaborate with other teams.
Security is assured on RapidAPI with the tool’s OAuth 2.0/OpenID Connect, header validation, and injection flaw prevention authentication method.
RapidAPI’s basic plan is free, while it costs $9/month, $59/month, and $159/month for Starter, Pro, and Ultra plans.
Paw is an API testing tool designed specifically for devices that run on macOS. With this tool, you can test APIs, compose requests, generate client code, and export API definitions, among many other things.
Paw enhances teamwork and collaboration by allowing users to synchronize their tests so others can make conflict-free changes and get real-time updates.
You can use Paw for a limited free trial period, after which you’ll need to buy the full version. The full version costs $49.99
Pyresttest is a simple python RESTful API testing and micro-benchmarking tool for developers.
This tool is so easy to use that sometimes even those without coding knowledge can work around it. One of Pyresttest’s best features is that it returns exit codes after failed queries.
Its lack of many dependencies makes it easy for developers to use it to deploy code.
Pyresttest costs nothing to use, making it an ideal tool for beginners.
Although primarily designed to test native apps, you can use Appium to write and test APIs.
You can use Appium with native, hybrid, and mobile web apps running on iOS or Android.
Thanks to the Selenium WebDriver API and other language-specific client libraries, Appium works with more programming languages than most API testing tools.
Appium is an open-source project with a budding community of developers willing to share solutions to problems others might have.
It costs nothing to start using Appium.
18. Rest Console
Rest Console is another excellent API testing tool for developers. This tool, as a Google Chrome REST/HTTP client, lets users create and test HTTP requests with rest APIs.
Some key features that make this tool stand out include rate limiting, its response format negotiation, which supports JSON and XML, and different forms of authentication.
Rest Console is free to use.
Hoppscotch is an excellent open-source API request builder. This tool allows users to run and validate REST API request responses.
Hoppscotch’s minimal design makes it easier to use and reduces the time spent on development.
This tool has a documentation generator, dedicated GraphQL, and WebSocket support — all of which make it stand out.
It’s free to use Hoppscotch, but users can always make donations through Patreon or PayPal.
Airborne is an API testing tool that uses REST clients to make HTTP requests. Unlike other tools, Airborne doesn’t have a user interface. Instead, it only provides the text file where you can write code.
Users need to have a fundamental knowledge of Ruby and RSpec to make the most use of Airborne.
Airborne is also a free, open-source API tool.
Taurus is an API testing tool best suited for users who prefer to use text editors and command line. This way, users don’t need to have thorough programming language knowledge, as a basic understanding of Taurus’ YAML structure is enough.
Taurus comes with powerful dashboards that aid collaboration with other team members. This tool is also compatible with the CI/CD pipeline.
To cap it all off, using Taurus comes at zero costs.
RedwoodHQ is a simple, free, open-source API testing tool that works with different tools like Appium, Silk, etc.
You can use RedwoodHQ to test AI SOAP/REST, CLI, etc., as the tool supports Python, C#, and Java/Groovy code.
RedwoodHQ also works well with tools like TeamCity and Jenkins to aid your continuous integration process so teams can run tests faster.
23. Karate DSL
Karate DSL is a unique API testing tool based on the Cucumber/Gherkin standard. It’s one of the few tools that allows users to run API test automation, UI automation, and performance in the same place.
This tool is simple to use as you don’t need Java knowledge to write and run tests.
One of Karate DSL’s outstanding features is its debugger. This debugger can help you replay steps as you’re editing to help spot mistakes, enabling you to save a lot of time.
AccelQ is an API test automation tool that works with SOAP & RESTful services.
This tool has a distinct user interface and offers an experience that makes testing easier. In addition, since it’s web-based, you don’t have to download or install an IDE.
Some of AccelQ’s key features include seamless UI and API automation, chain API tests, and integrated CI workflows.
You can start using AccelQ on a free trial. However, after the trial expires, you’ll need to schedule a call to get pricing that suits your needs.
Last but not least on our list of best API testing tools is Eggplant.
Eggplant is a testing tool tailored to help enterprises across different industries to develop test scripts and handle automated API evaluation.
Eggplant replaces the need for enterprises to use multiple tools to test individual technology layers, which often reduces functionality and leaves some API results unverified.
With Eggplant, teams can ensure their APIs work and are ready for deployment.
Eggplant uses a free trial and quotation-based pricing model.
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. A good API testing tool is vital to your business and strategy. You'll extend your reach and build your user base by allowing third-party applications to integrate easily with your service. 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.
Editor's note: This article was originally publsihed in December 2020 and has been updated for comprehensiveness.