API testing tools: what are they and how do they work?

Do you need to ensure your APIs function and perform as expected, are easy to integrate and boast robust security? If so, it’s time to undertake API testing before you deploy to production.

But which API testing tools can you use to do the job properly? With so many on the market, it can be difficult to know where to start. But worry not! 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 exist to test the application logic of your APIs. To check that they function as expected and perform as planned. Testing tools can also cover various associated issues, such as integration and interoperability. Testing API security is another core function of such tools. The overall goal is to catch any issues so that you can resolve them and ensure your APIs are production-ready before you deploy them.

This testing phase focuses on functionality, performance and security rather than user interface (UI), so it can be carried out fairly 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.

You could undertake API testing manually, but introducing automation into the process provides greater testing efficiency and a faster route to market. Of course, choosing the right automated API testing tools for your particular use case, CI/CD pipeline, API type, budget and so on is important.

How do these 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.

The first step in the testing process is to create a test case. This is where you define what you expect the test results to be – essentially, what good looks like. You can also set specific API requests and any additional parameters or conditions you want to test. There may be an element of manual test case creation here, although these days, many API testing automation tools will take care of this for you.

Next, you will need to configure access to the API you want to test, providing the base URL, authentication credentials, request headers and so on.

Like with most tools used for API testing, this is all you need to do before you can 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. You can view the results in your chosen tool’s logs and reports.

Sending high volumes of requests

API load testing tools will take things further by sending a high volume of requests during the test. This will look at how well the API performs in such a scenario, helping you identify any performance problems or bottlenecks. API stress testing tools that do this can be hugely helpful in heading off any embarrassing potential performance issues before they occur.

On the integration front, API testing tools – some of them, anyway – may be designed to work with version control systems, CI/CD platforms, bug-tracking software and more so that you can build a comprehensive and continuous development and testing environment.

In security terms, tools for API testing can examine a range of potential weaknesses and vulnerabilities. Suppose you’re worried about data leakage, authorisation problems or your API’s vulnerability to injection (and other) attacks. In that case, this is the phase of testing that should provide the peace of mind you need.

Types of API tools for testing

Whether you’re choosing super expensive or free API testing tools, you’ll need to be clear on what you need before you jump in and choose a tool. Let’s run through some of your options quickly.

API load testing

API load testing tools examine how your API will perform under the usual expected loads and peek 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. They aim to test that everything 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 for a deep understanding of how your API behaves in a wide range of scenarios.

API testing tools cover many other functional areas, including authentication, authorisation and error handling. Essentially, functionality testing tools can reassure you that each of the fundamental aspects of your API is working as it should.

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

Many API testing tools list security as their top priority – and for good reason. This element of API testing is about ensuring your APIs are protected against threats, don’t leak data and follow security-related best practices. It is all essential if you’re building reliable API products.

Security testing looks at authorisation and authentication mechanisms. It ensures everything works as it should so that users can only access the data and functionalities they are authorised to access. It also encompasses access control, ensuring that users can only perform the actions they are permitted to.

On the data security front, API 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.

Factors to consider 

There’s a lot to think about when choosing an API testing tool. 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. This continual testing culture means that any tool you choose will be part of your long-term workflows, so finding a tool you’re happy with is important.

In terms of factors to consider, 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 whether you want a free API testing tool or one that you pay for.

Top API testing tools in 2024

API testing tools free up your time to focus on providing the best possible API products, mainly when you can automate tests as part of your overall workflow. With that in mind, what are the tools used for API testing that are going to shine in 2024? Let’s find out…

Best API testing tools

Here are some of the best API testing tools for 2024.

Open source API testing tools

We’ve included a couple of open source API testing tools above. Tyk has long been an advocate of the power of open source. Our commitment to this is one reason our open source API gateway is free to use forever.

For API testing, opting for an open source tool means enjoying several benefits. Many open source tools are free, meaning you can free up hundreds or even thousands from your 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 around your very specific needs. 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

There are still several free API testing tools available, 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 if you need it. 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.

Let’s look at a few options.

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. You’ll also need to build a sound API testing strategy to tie everything together and ensure you deliver the best API experiences. If you need a hand, feel free to chat with the Tyk team.