Recipe UI is a free and open source alternative to Postman, which you can use to construct API calls and analyze the response. This too lets you debug and understand REST APIs and it has some extra features that you will like. If you are a developer or a network engineer then you can use this tools just like Postman and it supports dynamic fields for API URLs. The desktop that it offers is cross-platform and can be used on macOS, Linux, and Windows.
If you have used Postman, then you will surely like Recipe UI. Or, if you are in search of an open source Postman alternative, then again this is one of the best options. You can open many sessions and then prepare the API request parameters such as body and URL. Also, for developers, it offers a cURL command generator that can be translated into various programming languages snippet.
Using this Postman Alternative on Desktop:
You can download it from here for your platform. And if you are interested in its source code, then you can check out its GitHub repository. Launch it after installing it and you will find its interface to be like this.
Now, it has some collection of ready-made APIs such as Reddit that you can start using right away. Or, you can start a session from scratch with everything customized.
Now, the main session page will open. From here, you can make an API call and then see the response. You will see the API response body here with the JSON data returned.
Similarly, you can start another session and then do anything you like. Here I have used a public API in the above screenshots but you can use a private API in the same way. You can supply the custom authentication tokens and other body parameters.
As a developer or network engineer if you are looking for an open source alternative to Postman then you are at the right place. React UI is one of the best tools you can use to debug and test REST APIs for now. What I liked best about it is the built-in type safety and cURL command generator and converter.