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A Syncthing device appearing as Connected does not mean every folder on that device will synchronize. Device configuration establishes which Syncthing instances may communicate; each folder separately specifies which configured devices participate in sharing it. Syncthing’s setup guide presents adding a remote device and selecting folders as distinct choices. (Syncthing: Getting Started)
For a particular folder to synchronize in the standard two-device workflow:
- Each device must be configured with the other device’s ID.
- The devices must establish a connection.
- The folder must be assigned to the intended remote device.
- Both devices must participate in the corresponding shared folder.
The first two steps concern the device relationship. The last two concern one particular data set. A working connection satisfies a prerequisite for synchronization; it does not authorize or configure every local folder.
Three separate relationships
Syncthing setup becomes easier to reason about when divided into three layers:
- Device identity: Which Syncthing instance is this?
- Mutual device configuration: Have both instances been configured with each other’s identity?
- Folder participation: Is this particular folder shared with the other device?
These relationships allow selective sharing. A home server could share a Photos folder with a laptop and a Documents folder with a desktop. Adding both computers as remote devices would not automatically assign both folders to both computers. Syncthing’s folder configuration lists the devices intended to share each folder. (Syncthing Configuration)
That distinction is useful both during initial setup and when troubleshooting. “The other device is listed” and “this folder is shared with the other device” describe different states.
A device ID identifies the peer
Syncthing calls the machine whose configuration or status you are viewing the local device. The other machine is a remote device. These terms are relative: viewed from the other machine, that machine is local and the first one is remote. (Syncthing: Getting Started)
Each Syncthing instance has a device ID based on its key material. You enter that ID when adding the instance as a remote device. The private key must remain private, but the device ID is the identifier used to configure the peer. (Syncthing: Getting Started; Syncthing Configuration)
A device ID therefore identifies which peer you are configuring. It does not select folders for that peer.
Device configuration must be mutual
In Syncthing’s documented two-device setup, each device must be configured with the other device’s ID. If a laptop has the server’s ID but the server has not been configured with the laptop’s ID, the mutual-configuration requirement has not been met, so the documented connection cannot occur yet. (Syncthing: Getting Started)
Mutual configuration is necessary, but it does not guarantee an immediate connection. Syncthing’s documentation advises checking firewall setup and errors in the GUI or console when configured devices do not connect. It also cautions that configuration changes may not be reflected instantly. (Syncthing: Getting Started)
A configured device can also appear in the interface while disconnected. Seeing its name in the device list therefore proves neither a current connection nor participation in a particular folder. (Syncthing: Getting Started)
Folder sharing is another setting
Each folder has its own device selection. A local folder that has not been shared with another device is shown as Unshared, independently of whether another configured device can connect. (Syncthing: Getting Started)
Both devices also need corresponding configuration for the shared folder. Older versioned GUI documentation states explicitly that the Folder ID must be the same across participating devices because it uniquely identifies the folder. Folder labels can differ, so two folders having similar display names does not establish that they represent the same shared folder. (Syncthing GUI documentation, v1.19.0)
That matching-ID statement comes from the v1.19 documentation and should be treated as support for the folder-identity concept, not as a guide to the exact layout of the current interface. Current releases and third-party Syncthing wrappers may present configuration controls differently.
The local storage path is a separate detail again. Syncthing’s current configuration documentation says a folder path identifies where the directory is stored on that device and is not sent to other devices. The corresponding shared folder can therefore use different local paths on a server and laptop. (Syncthing Configuration)
State table: connection versus sharing
The following table summarizes the standard two-device workflow. Here, “unknown” means not configured as peers; it is not a promised GUI status label.
| State | Device configuration | Connection consequence | Result for this folder |
|---|---|---|---|
| Devices unknown | Neither side has configured the other | Mutual-configuration requirement is unmet | Cannot synchronize |
| Configured on one side | A has B’s ID, but B has not configured A | Documented mutual connection cannot occur yet | Cannot synchronize |
| Configured mutually but disconnected | Each side has the other’s ID | Connection is permitted, not guaranteed | Cannot synchronize while disconnected |
| Connected without folder sharing | Devices are mutually configured and connected | Peers can communicate | Folder remains unshared with that peer |
| Connected with corresponding shared folder | Both devices participate in the folder, using the same Folder ID under the versioned documentation | Peers can communicate | Synchronization is possible if the folder is active and error-free |
This is a compact model, not an exhaustive Syncthing state machine. Advanced paths involving introducers or automatic folder acceptance can change how configurations are added, but they do not erase the distinction between a device relationship and per-folder participation.
What to check when files do not move
Check the configuration in dependency order:
- Verify both device IDs. Confirm that each device is configured with the other.
- Check the actual connection status. A listed device may still be disconnected.
- Open the affected folder’s settings. Confirm that the intended remote device is selected for that folder.
- Inspect the corresponding folder on the other device. Confirm that both devices participate in the same shared-folder configuration.
- Compare Folder IDs, not only labels. The explicit matching requirement in the supplied evidence comes from the v1.19 GUI documentation.
- Look for folder pauses or errors. The versioned GUI documentation identifies both Paused and error-related Stopped states. (Syncthing GUI documentation, v1.19.0)
Once the documented setup is complete, Syncthing describes files added to the shared directory on either device as synchronizing to the other. That is documented expected behavior, not a transfer result measured for this article. (Syncthing: Getting Started)
The practical mental model is simple: the device ID answers which peer, mutual configuration determines whether the peers may connect, and the folder’s device selection determines which data set may synchronize.
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