an API for Signal. Not official and not nearly as secure as first party Signal clients
Signal does not offer any sort of official API. Their server software is open source and anyone can figure out the REST endpoints, but building the payloads turns out to be a little trickier. Signal, unlike traditional messaging applications, puts a number requirements around encryption and account data storage on the client software. signald aims to handle all of the Signal-specific requirements, exposing an API that sends and receives unencrypted messages with other software on the same machine.
signald does not come with a user friendly frontend. Users should select an existing client to use signald with.
Developers wanting to learn how to write code to interact with signald, check out getting started page.