# How it links ## A companion device FastsApp registers with WhatsApp as a linked device over the multi-device protocol, exactly the slot WhatsApp Web and WhatsApp Desktop use, through [whatsapp-rust](https://github.com/oxidezap/whatsapp-rust). Messages are end-to-end encrypted to it like to any device; the pairing happens through the QR code or the pairing code, and the phone lists it under **Linked devices**, where it can be signed out at any time. One connection exists per linked device: running FastsApp twice, or FastsApp and another client on the same slot, has them taking the connection from each other. FastsApp guards against its own copies (a second launch just shows the window of the first), and links on its own slot otherwise. ## What the phone is for - **History.** WhatsApp replays only recent history to a fresh device. FastsApp archives everything it sees from then on, and asks the phone for older messages when you scroll past the archive; the phone must be online to answer those requests, and to re-upload attachments whose server copies expired. - **Nothing else.** 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, the media, and the session keys live on your disk and never leave it; [Settings & Files](/settings-and-files/) lists every path. FastsApp talks to WhatsApp's servers, and to GIPHY only when GIF search is used with an API key. There is no telemetry. Unlinking from Settings tells the phone to forget the device and deletes the local archive and caches.

A companion device

FastsApp registers with WhatsApp as a linked device over the multi-device protocol, exactly the slot WhatsApp Web and WhatsApp Desktop use, through whatsapp-rust. Messages are end-to-end encrypted to it like to any device; the pairing happens through the QR code or the pairing code, and the phone lists it under Linked devices, where it can be signed out at any time.

One connection exists per linked device: running FastsApp twice, or FastsApp and another client on the same slot, has them taking the connection from each other. FastsApp guards against its own copies (a second launch just shows the window of the first), and links on its own slot otherwise.

What the phone is for

• History. WhatsApp replays only recent history to a fresh device. FastsApp archives everything it sees from then on, and asks the phone for older messages when you scroll past the archive; the phone must be online to answer those requests, and to re-upload attachments whose server copies expired.
• Nothing else. 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, the media, and the session keys live on your disk and never leave it; Settings & Files lists every path. FastsApp talks to WhatsApp’s servers, and to GIPHY only when GIF search is used with an API key. There is no telemetry.

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