11 Testing and Building Tools for APIs

An API works as an intermediary between two apps that allows them to communicate with each other. Most companies have built APIs, which stands for application programming interfaces, using different tools for either themselves or their customers.

Perhaps the most crucial and time-consuming part of building APIs is manual testing and debugging. According to Statista, these processes took up 22.2% of API-related working time in 2019. On the other hand, automated testing consumed just 11.4% of that time. To save time and finances, you need the best tools to help you automate API testing. Take a look at these 11 API tools for building and testing processes that will ensure you save both:

This post is contributed content by Anzhela Sychyk of TheAppSolutions. It is intended as a roundup of useful API developer tools. It is not sponsored nor does it reflect any official position by Nordic APIs on top API building or testing vendors.


Swagger is an all-encompassing toolset that assists in API development throughout all the stages. That is designing, building, documenting, testing, and standardizing APIs. Individuals, teams, and large enterprises can use Swagger to make development much more straightforward. With this toolset, you can build API code in almost any language, model APIs based on specification-based standards, and, most importantly, test RESTful APIs with no overheads. This instrument supports OpenAPI workflows and features a variety of open source tools. Swagger tools are classified as either Pro or Open Source.  Other features for Swagger tools include:

  • Hosted API documentation;
  • API standardization;
  • API lifecycle integration;
  • API mocking;
  • Setting and enforcing API style guidelines;
  • Centralized Definition Storage.


One of the most widely automated testing tools for API is SoapUI. The instrument is popular in resting REST and SOAP APIs. What makes it great is that it is an open-source tool that operates across platforms. However, it also has a professional version. SoapUI is a leading tool of its kind that supports functional and non-functional testing and standard protocols (SOAP, REST, HTTP, HTTPs, AMF, JDBC, etc.) for testing all types of APIs. The app is both functional and user-friendly. The pro version includes a handy editor. It has so far been downloaded by millions of users, including large clients such as MasterCard, Apple, BMW, among others. Additional SoapUI features are:

  • API functional testing;
  • Data-driven testing;
  • Advanced API mocking;
  • API performance testing;
  • API monitoring;
  • Asynchronous Testing;
  • Test reporting.


Postman is a multifunctional instrument that can build as well as test API. It first began as a browser extension for Chrome. Now, it has versions for Windows as well as Mac. In a bid to make the development process easy, Postman gives you all the tools you need to design and mock, document, debug, perform automated testing, monitor, and publish APIs in a single platform. In general, the platform allows you to develop, test, and manage your product.  Postman supports several platforms like Swagger, OpenAPI, and RAML that you can use to design APIs. It’s a wonderful choice for development teams as it supports real-time collaboration across workspaces. Elements that you can develop with Postman include API mock servers, environments, and documentation. You can also conduct contract tests, third party API integration tests, and performance tests. Some main features include:

  • API running and documentation;
  • API testing and monitoring;
  • Great for technical and non-technical users;
  • Works on Linux, Mac, Windows, and Chrome.

API Fortress

API Fortress a microservices-based, actors-powered platform that integrates with API design platforms such as Postman. API Fortress enables mass functional test generation from payloads or spec files, including Swagger/OAS and RAML. This allows API Fortress users to do “test and build” APIs in unified workflows.

  • Functional end-to-end testing
  • Ability to mock or record APIs
  • Stress testing and usage simulations
  • Monitoring to validate uptime and performance

Katalon Studio

For fast, automated testing across platforms, Katalon is the go-to free license tool. It is ranked highly by both Capterra and Gartner Peer Insights. What makes Katalon the most popular testing platform out there is that it runs automated tests for the web, API, Desktop, and mobile applications.  This API software is built on top of open-source automation frameworks like Appium and Selenium. With Katalon Studio, the tester doesn’t need to have a lot of technical expertise. The technicalities are hidden behind an easy to use UI. However, there’s also the scripting mode for more technically-capable users to debug, highlight syntax, and get code suggestions.  Some features and advantages of Katalon Studio are:

  • Simple installation;
  • Records & generates test scripts for mobile and web apps;
  • User-friendly and rich UI with menus, tables, and tree views;
  • Uses Groovy language;
  • Provides intuitive reports that can be exported to HTML, PDF


For a Java-specific tool, Rest-Assured is a convenient open-source REST testing platform. As you may know, conducting tests and validations is much more difficult in Java than in Groovy, Ruby, etc. To simplify the testing and validation of response processes, this tool eliminates the need to make boilerplate code for testing complicated responses. The REST-Assured library supports HTTP.   Important features include:

  • Compatible with Java programs;
  • Supports BDD syntax;
  • Eliminates boilerplate requirements.


Apiary is a comprehensive API platform that allows users to design, develop, and document API. It offers an avenue for creating, testing, and implementing a production-ready API way quicker. In practice, creating an API requires the identification of a schema for input and output. However, in Apiary, the application programming interface can be made with input and output set as mocked.  As such, the mocked data will fulfill the specifics of the app without the need for modifying the code while the data is integrated and analyzed. Creating an application programming interface has several stages. However, Apiary provides each of the steps for the dev department to handle.  Some of the main features include:

  • Server mock;
  • GitHub Sync;
  • Traffic Inspector;
  • Command Line Tools.

API Blueprint

API Blueprint is a powerful high-level API description language for website application programming interfaces. It stands out from the rest due to its simplicity and accessibility, making operations quite straightforward. Among users, API Blueprint is massively appreciated for its concise but expressive syntax. It enables users to create and also prototype APIs to be developed swiftly. And if nothing else, this tool allows for the documenting and testing of deployed mission-critical application programming interfaces. Some of the companies that use API Blueprint include the Karnov Group, Socrata, and SalesLoft. The key features of API Blueprint include:

  • Regular Markdown syntax;
  • Conforms to the GitHub Flavored Markdown syntax


PyRestTest is a Python-based REST API testing platform. It supports tests in JSON or YAML config files. For this reason, there is no need to write code. Some of the notable features of this API tool include:

  • It has few dependencies making it easy to deploy on-server for smoke tests/ health checks;
  • Returns exit codes upon fail;
  • Enables the generation and validating of mechanisms for creating test scenarios.

Karate DSL

With Karate DSL, you can make a test that can sequence calls to any form of web-service and ensure that the feedback received is as desired. It is a relatively new addition to testing frameworks and is based on the cucumber library. This tool enables users to craft useful tests for web services using a domain-specific language.  Some of its notable features include:

  • Can run tests and provide reports like any standard Java project;
  • Tests can be written without the need for Java knowledge;
  • Tests can be written straightforwardly, even for non-coders.
  • Enables the re-using of payload-data and user-defined functions across tests.


Insomnia is a cross-platform GraphQL and REST client that works on Mac, Windows, and Linux. It allows users to create HTTP requests by specifying their URL, payload, headers, and authorization in a single place. Moreover, it enables the viewing of response details such as cookies, status code, body, headers, etc. Users highly prefer this tool since it allows users to organize pretty much everything, thanks to functionalities such as drag-and-drop requests, data importing, and exporting.  With Insomnia, users can generate snippets of HTTP code for over 30 libraries, among them Curl, NodeJS, Go, Swift, Python, Java, C, etc. Some of the features provided by Insomnia include:

  • A fantastic interface;
  • Reusable values.

So far, Insomnia has made quite a good name for itself. Already, some of the biggest names in the tech sector use it. These include Netflix, Cisco, Kayak, 1800 contacts, box, and LogDNA  H2: AI and ML  Artificial Intelligence and Machine Learning are impacting the API testing process in new ways. These new-age technologies, when implemented in API testing, minimize the chances of risks occurring while also improving the overall experience. Some of the areas where these technologies play a vital role in all of this include configuration, data collection, feature extraction, data verification, tracking, machine learning code (smart application,) and so forth.  However, before using machine learning and artificial intelligence as API testing tools, it is essential to do the following:

  • Identify all patterns in test automation;
  • Determine the credibility in data for perpetual integration;
  • Infuse human creativity to complex algorithms.

Honorable Mentions

    • Per our policy for tooling roundup posts, vendors may comment to be included in this Honorable Mentions list.

Final Thoughts

These are the eleven of some of the best tools you can use to test and build APIs swiftly today. However, it is essential to appreciate the fact that while all tools provide the same functionality, there are significant differences in their approaches. As such, it makes sense to try out the different tools to determine the one that will suit your purpose and your business.