Postcat
Postcat is a scalable API tool platform
...The software supports HTTP REST and WebSocket protocols for communicating with backend services. Users can configure requests, inspect responses, and manage API environments and global variables. Its extensible plugin system allows additional tools and functionality to be incorporated into development workflows. Postcat also provides API design, documentation, collaboration, and sharing capabilities. Available for Windows, macOS, Linux, and web browsers, the project has entered maintenance suspension and no longer receives regular feature updates or security fixes.