API testing tools can assure you that your APIs function and perform as expected, are easy to integrate and boast robust security. All essential to know before you deploy to production.
With that in mind, we’ve rounded up a selection of the best API testing tools for you to consider, whatever your budget.
What are API testing tools?
API testing tools can test the application logic of your APIs and check that they function as expected and perform as planned. Testing tools can also cover integration, interoperability and security. They support you to catch any issues, so you can ensure your APIs are production-ready before you deploy them.
When to use API testing tools
This testing phase focuses on functionality, performance and security rather than user interface (UI), so you can undertake it rapidly and early in the API development process. You don’t need to wait to finish your UI work before you test that your API is delivering as expected. You can even start your API testing before you start coding.
Also important to note is that testing isn’t only for new API products – you’ll need to test your APIs every time you make changes to any integrated component to ensure the changes haven’t broken anything.
How do API testing tools work?
Tools for API testing work by sending requests to one or more endpoints. They then look at the responses and compare them to what was expected. If the results are as expected, the rest is successful. If not, you have some investigating to do – followed by retesting.
To test your APIs:
- Create a test case. This is where you define what you expect the test results to be. You can also set specific API requests and any additional parameters or conditions you want to test.
- Configure access to the API you want to test. You’ll need to provide the base URL, authentication credentials, request headers and so on.
- Run your first tests. The tool will send API requests in line with the test case conditions and configuration you have set. It will then analyse the responses to ensure they are complete and correct and determine how they measure up against the expected results. To do this, your API testing tool will examine everything from the content and structure of the response data to headers and status codes.
- Analyse the results. You can view the results in your chosen tool’s logs and reports.
Types of API tools for testing
Whether you’re choosing super expensive or free API testing tools, be clear on what you need before you choose a tool. Let’s run through some of your options.
API load testing
API load testing tools examine how your API will perform under the usual expected loads and peak loads. This is an important way to understand what your API can and cannot handle regarding concurrent users and requests.
Load testing is usually a key part of the work undertaken by API performance testing tools. These tools also carry out stress, scalability, and endurance testing to comprehensively overview your API’s performance limitations.
API functional testing
Functional testing tools look at your API’s code-level functionality to reassure you that each of the fundamental aspects of your API works as defined in the API’s documentation and that your API returns the expected results.
Examples of the tasks undertaken during functional API testing include validating that endpoints are accessible and respond as expected and that requests and responses behave as expected (including status codes, headers, response content, data format and so on).
During functional testing, you can examine how your API handles invalid and valid values. This is super useful in uncovering any issues related to input validation and data handling. You can also test boundary conditions and edge cases.
API testing tools cover many other functional areas, including authentication, authorisation and error handling.
API integration testing
Tools for API integration testing focus on the interaction between APIs and software components, along with data exchange. This includes examining the data flow between APIs (including input, transformation and output).
Integration testing also looks at interoperability to ensure that your APIs can communicate effectively with other APIs, understand their formats and protocols and interact correctly in relation to critical integration points such as data validation, authentication and authorisation.
If you’re architecting a system with multiple, complex integrations, this will be an essential part of your API testing process. In such a scenario, you’ll also need to carefully consider the API management architectural and deployment patterns you use to ensure your requirements around performance, resilience, multi-region deployment and other factors are all met.
API testing tools for integration should cover full, end-to-end scenarios when testing, including examining error handling when things don’t pan out as expected. This lets you test entire scenarios, such as an end-to-end user journey.
API security testing
Security testing is about checking your APIs are protected against threats, don’t leak data and follow security-related best practices. All essential when building reliable API products.
Security testing looks at authorisation and authentication mechanisms, along with access control. It ensures users can only access the data and functionalities they are authorised to access and only perform the actions they are permitted to perform.
Furthermore, API security testing checks that your APIs handle sensitive data securely, that data is encrypted in transit and that it is well protected from tampering or unauthorised access. Security testing also includes checks against injection attacks (in relation to how APIs validate and sanitise input data) and scanning for other vulnerabilities and weaknesses.
How to choose the right API testing tool
Any API testing tool you choose will be part of your long-term workflows, so finding a tool you’re happy with is important. You’ll need to balance aspects such as the vendor’s and tool’s reputations, integration with other tools and software, automation, what degree of complex scenario testing the tool enables you to undertake and the cost.
Best API testing tools
Here are some of the best API testing tools for 2024.
| Tool | Details | Cost |
|---|---|---|
| ReadyAPI | A low-code API testing platform from SmartBear with an emphasis on creating test automation across any workflow. A centralized interface makes it easy to create, manage and execute performance, functional and security tests. A 14-day fully featured free trial gives you plenty of time to assess whether ReadyAPI is the tool for you. | Starts at €911 per licence when billed annually. |
| Testsigma | Testsigma makes it easy to test a variety of scenarios and validate APIs in real-time. You can also test how UI and business layers interact with your APIs, using unit, integration and contract tests for extended test coverage. | Prices available on request. |
| REST-Assured | This open source API testing tool makes it easy to test REST services in Java. The end-to-end testing functionality is simple to integrate into your Java-based testing framework. | Free. |
| SoapUI | A headless tool for functional API testing, SoapUI is ideal for testing REST and SOAP APIs and web services, with an easy drag-and-drop process and reusable test scripts. This open source tool from SmartBear provides cross-platform functional, regression, compliance and load testing. | Free. |
Benefits of open source API testing tools
We’ve included a couple of open source API testing tools above. Many open source tools are free, meaning you can free up budget to spend on other things. Open source testing tools are also superbly customisable. You can modify the code to suit your needs, meaning you can tailor your testing solution very specifically. All while enjoying full visibility into the source code.
The community element of open source API testing tools is also important. Whether you’re troubleshooting, looking for resources or in need of a helping hand, it’s good to know there’s an active community to connect with. Community improvements, bug fixes and even new features can all benefit your testing program, too.
Other benefits include avoiding vendor lock-in, enjoying easy integration (as open source tools are usually designed to play nicely with others) and flexibility regarding the environments and operating systems with which you can use your testing tools.
Free API testing tools
You can get free API testing tools even if you don’t go down the open source route. Many proprietary tools offer basic or limited functionality for free, with a payable upgrade option. Just remember to assess carefully whether the free option meets your needs both now and in the future. Otherwise, you could end up using a ‘free’ tool that costs you dearly.
| Tool | Details | Cost |
|---|---|---|
| Postman | Well suited to both manual and automated API testing, Postman is super user friendly, making it a favourite with many developers. You can configure multiple test environments, integrate into CI/CD pipelines, use a data-driven testing approach and more. The Postman testing tool also makes API mocking easy. | Free plan available, along with a wide range of payable plans, starting from $168 per year. |
| Assertible | Focused on automation and reliability, Assertible is a great tool for automating tests throughout your CI/CD pipeline. It integrates with a range of tools, including Slack, Zapier and GitHub, so is easy to build into your overall API testing approach. Its handy Sync feature helps update your tests automatically when specifications change, which can be a great time-saver. | Free for testing two services with up to 10 tests per service. Paid plans start from $300 per year with a one-month free trial. |
| Katalon | A test automation solution for APIs with an easy-to-use UI and centralised reporting. Katalon supports REST, SOAP requests and SSL client certificates, with native CI/CD integrations, data-driven testing and more. Katalon Academy provides free API testing courses and tool tutorials to help you get the best from the testing process. | Free for basic test automation or from $2,099 when billed annually. |
Ready, set, test!
Are you ready to test that your APIs perform and function as they should and do so securely? Using API testing tools is just one aspect of doing so. Make sure you build a sound API testing strategy to tie everything together and ensure you deliver the best API experiences.
It’s also worth checking out our guide to securely exposing your APIs, to ensure you’re adhering to the latest best practices.
