Muster is a macOS app that keeps one master address book and syncs it to the address books you already use: iCloud, Google, Nextcloud, mailbox.org, Zoho, and any other CardDAV server.

It exists because my contacts were spread across five accounts, each with a slightly different version of the same person, and every tool I tried either merged them badly or made me pick a winner.

What it does

One master copy. Your contacts live in a store on your Mac. Every server is a copy of some part of it, and the store decides what is true.

You choose who goes where. Contacts belong to sets, and each address book receives the sets you route to it. Work contacts can stay off your phone.

It shows you the diff before it writes. Every sync is a dry run first, listing what would be created, updated or deleted on each server. Nothing is pushed until you approve it.

Nothing is thrown away. When two servers disagree about a field, the difference goes into a review queue with what each side holds and what it held last time. You decide, once, and the answer is carried out on the next sync.

Where your data is

On your Mac, in ~/Library/Application Support/contact-manager/store.

It is a git repository, so every change is a commit you can read and undo. Field values are encrypted at rest with a key held in the macOS Keychain, so the files on disk show field names and no phone numbers.

Muster has no server. I run nothing, I receive nothing, and there is no account to create. The only machines your contacts reach are the address books you configure yourself.

Status

In development. It will be sold on the Mac App Store. I use it daily on my own library of about 900 contacts.