Are your APIs working and performing as they should be? Are you sure?
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. We’ve looked at tools for different needs and different budgets, so you can find the ideal testing tool to suit your use case.
What are API testing tools?
API testing tools can:
- Test the application logic of your APIs
- Check that they function as expected
- Test that they perform as planned
- Cover integration, interoperability and security
- Support you to catch issues, to ensure your APIs are production-ready before you deploy them
When to use API testing tools
You can undertake testing focused on functionality, performance and security:
- Early in the API development process
- Before you finish your user interface (UI) work
- Before you start coding
You should undertake testing:
- For new API products
- 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 test 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 analyze 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.
- Analyze 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
- Help you 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.
Functional API testing includes:
- Validating that endpoints are accessible and respond as expected
- Checking that requests and responses behave as expected (including status codes, headers, response content, data format and so on)
- Examining how your API handles invalid and valid values (this is super useful in uncovering any issues related to input validation and data handling)
- Testing boundary conditions and edge cases
- Testing many other functional areas, including authentication, authorization and error handling
API integration testing
Tools for API integration testing:
- Focus on the interaction between APIs and software components, along with data exchange
- Examine the data flow between APIs (including input, transformation and output)
- Look at interoperability to ensure 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 authorization
- Should cover full, end-to-end scenarios, including examining error handling when things don’t pan out as expected, so you can test entire scenarios, such as an end-to-end user journey
If you’re architecting a system with multiple, complex integrations, this will be an essential part of your API testing process. 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 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 authorization and authentication mechanisms, along with access control
- Ensures users can only access the data and functionalities they are authorized to access and only perform the actions they are permitted to perform
- Checks that your APIs handle sensitive data securely, that data is encrypted in transit and that it is well protected from tampering or unauthorized access
- Checks against injection attacks (in relation to how APIs validate and sanitize input data)
- Scans 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. Consider:
- 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
- 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 license 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
- Cost: Many open source tools are free.
- Superbly customizable: Modify the code to suit your needs, tailoring your testing solution very specifically while enjoying full visibility into the source code.
- Community: Enjoy community support for troubleshooting, looking for resources, community improvements, bug fixes and even new features
- Freedom: Avoid vendor lock-in
- Easy integration: Open source tools are usually designed to play nicely with others
- Flexibility: Use your testing tools with different environments and operating systems
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 favorite 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 centralized 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!
We’ve seen how invaluable testing tools can be when it comes to testing the load, functionality, integration and security of your APIs. We’ve also looked at a range of free and paid tools that can help ensure your APIs perform and function as they should.
In addition to using the right API testing tools, ensure 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.
