Bearer is described as 'developer tool that helps engineering teams monitor API requests, track performance, detect anomalies, and fix issues for their critical API usage. The solution monitors all outgoing API requests without proxying traffic'. There are more than 25 alternatives to Bearer for a variety of platforms, including Windows, Online / Web-based, Mac, Linux and Self-Hosted solutions. The best alternative is Insomnia REST Client, which is both free and Open Source. Other great apps like Bearer are soapUI (Free, Open Source), Hoppscotch (Free, Open Source), Advanced REST Client (ARC) (Free, Open Source) and Beeceptor (Freemium).
STORM is a free and open source tool for testing web services. It is written mostly in F#. (I love this language!) STORM allows you to 1. Test web services written using any technology (.NET , Java, etc.)