Swagger useful for documenting REST APIs into a JSON format that can then be executed in a web browser against a live API.
ngrep is useful when you need to see exactly what is being sent between client and web server (or proxy server) it will show you traffic in real time on a given port.
for example sudo ngrep -W byline -d lo port 9001 will show what is happening on some arbitrary REST API configured for port 9001
mitmproxy/mitmdump (mitm - Man In The Middle) is an intercepting and traffic capture proxy that will do many, many things including capturing and saving web traffic.
For example to capture all traffic destined for a server on port 9001, use this command which configures mitmdump as a forwarding proxy on port 7001 and writes traffic to "outfile".
mitmdump -p 7001 -P http://127.0.0.1:9001 -w outfile