- Pixelfed requires
redirect_urisparameter for OAuth2, while Mastodon usesredirect_uri. I am not exactly sure if Mastodon would work with Pixelfed's parameter, but when usingredirect_uriparameter for Pixelfed, even withredirect_urisassigned, it results in error. - GET /api/v1/accounts/relationships HTTP/1.1 endpoint on Mastodon uses
idparameter (in docs it is reffered asid[]because you can assign it multiple times), but in Pixelfed it is named literally asid[].
In general, Mastodon API is ~comptaible with Pixelfed API.
You can check massive API differences (endpoints which are unsupported) on official website here.