There are 4 repositories under x-callback-url topic.
Implementation of x-callback-url (Inter app communication) in swift
Implementation of x-callback-url (Inter app communication) on macOS using Python
Automated backlinks for notes in Bear.app, on macOS.
A 100% type safe API to the x-callback-url scheme.
Ulysses has removed this documentation from their website, but support remains in their application for the moment and they were kind enough to provide it when I asked.
Documenting the history/subject of x-callback-urls and URL schemes.
This is an example of using CallbackURLKit. Receiver part of it.
Custom X-Callback-URL-based actions for Terminology dictionary.
This is an example of using CallbackURLKit. Sender part of it.