HTTP Toolkit is described as 'beautiful, cross-platform & open-source HTTP(S) debugging proxy, analyzer & client, with built-in support for modern tools from Docker to Android to GraphQL' and is a HTTP(S) Debugger in the Development category. There are more than 25 alternatives to HTTP Toolkit for a variety of platforms, including Mac, Windows, Linux, Online / Web-based and Google Chrome. The best alternative is Insomnia REST Client, which is both free and Open Source. Other great apps like HTTP Toolkit are Postman (Freemium), Fiddler (Freemium), mitmproxy (Free, Open Source) and Paw (Paid).
Postman is a tool that facilitates the development and use of APIs. Postman provides all the functionality needed to create or simulate API specifications and use the Postman Collection Runner to create tests.
Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data.
Easily craft your HTTP requests, try them, archive them. Setup HTTP Headers, URL parameters, form-encoded POST key-values or text. Get easy-to-read responses with well formatted headers and body. Sync your projects with your team through Paw for Teams.
Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet. This includes requests, responses and the HTTP headers (which contain the cookies and caching information).