# How it links ## A companion device ZapFast registers as a linked device through [whatsapp-rust](https://github.com/oxidezap/whatsapp-rust). Messages are end-to-end encrypted for each device. Pair with a QR code or phone-number code. New links list ZapFast under **Linked devices**; an existing link may retain the earlier name FastsApp until you link again. There you can unlink it at any time. Each linked device can have one connection. Two clients using the same device keys will disconnect each other. A second ZapFast launch reopens the existing window instead of starting another instance. **Development builds:** when WhatsApp identifies a contact previously known by a private id, ZapFast merges that chat into the contact's phone-number chat. Messages keep their stars, pins, and motion-photo clips. Newer star and pin changes take precedence, including removing a star or pin. ## What the phone is for - **History.** WhatsApp replays only recent history to a fresh device. ZapFast archives new messages and asks the phone for older messages when you scroll past the archive, or in the background when **Fetch older history** is on in Settings > Chats. The phone must be online for these requests and to re-upload expired attachments. - **Other features.** Sending, receiving, receipts, and presence go through WhatsApp's servers directly; the phone can be offline for all of it. ## What stays local The message archive, media, and session keys stay on your computer. [Settings & Files](/settings-and-files/) lists their paths. ZapFast connects to WhatsApp's servers, GIPHY for the GIF picker and search, and Signal's sticker CDN when you import a Signal sticker pack. It has no telemetry. It also asks `api.github.com` once a day whether a newer release exists; you can turn this off in Settings. Downloading an update fetches release files from GitHub. Unlinking from Settings tells the phone to forget the device and deletes the local archive and caches.

A companion device

ZapFast registers as a linked device through whatsapp-rust. Messages are end-to-end encrypted for each device. Pair with a QR code or phone-number code. New links list ZapFast under Linked devices; an existing link may retain the earlier name FastsApp until you link again. There you can unlink it at any time.

Each linked device can have one connection. Two clients using the same device keys will disconnect each other. A second ZapFast launch reopens the existing window instead of starting another instance.

Development builds: when WhatsApp identifies a contact previously known by a private id, ZapFast merges that chat into the contact’s phone-number chat. Messages keep their stars, pins, and motion-photo clips. Newer star and pin changes take precedence, including removing a star or pin.

What the phone is for

• History. WhatsApp replays only recent history to a fresh device. ZapFast archives new messages and asks the phone for older messages when you scroll past the archive, or in the background when Fetch older history is on in Settings > Chats. The phone must be online for these requests and to re-upload expired attachments.
• Other features. Sending, receiving, receipts, and presence go through WhatsApp’s servers directly; the phone can be offline for all of it.

What stays local

The message archive, media, and session keys stay on your computer. Settings & Files lists their paths. ZapFast connects to WhatsApp’s servers, GIPHY for the GIF picker and search, and Signal’s sticker CDN when you import a Signal sticker pack. It has no telemetry. It also asks api.github.com once a day whether a newer release exists; you can turn this off in Settings. Downloading an update fetches release files from GitHub.

Unlinking from Settings tells the phone to forget the device and deletes the local archive and caches.