Issue: Syncthing Web GUI “Connection Error” on Brave (501 Not Implemented)
The Problem
Users on Brave (and occasionally other Chromium-based browsers) experience a recurring “Syncthing seems to be down” overlay every few minutes, while the service remains perfectly stable on Firefox or via the command line.
The Root Cause
The issue is a protocol mismatch triggered by the Automatic Upgrade check.
- Many distributions (Apt, Homebrew, etc.) disable the upgrade functionality within the binary.
- The Web GUI still attempts to poll
/rest/system/upgrade. - The Syncthing backend returns a
501 Not Implementedstatus code. - While Firefox ignores this non-critical error, Brave’s security engine (and “Vinci” privacy layer) interprets a
5xxerror from a local resource as a connection failure or a “broken” backend, triggering a session reset and the “Down” overlay.
The Solution
The fix is to prevent the Web GUI from ever requesting the upgrade status, thereby avoiding the 501 error that trips Brave’s security sensors.
Option 1: Environment Variable (Recommended and what worked for me)
Start Syncthing with the STNOUPGRADE flag. This tells the internal web server to disable the upgrade endpoint entirely.
- Linux/macOS:
STNOUPGRADE=1 syncthing - Windows (Shortcut): Target:
C:\path\to\syncthing.exe --no-upgrade
Option 2: Config.xml Manual Edit
If the “Automatic Upgrade” toggle is greyed out in the GUI (as is common with managed packages), edit the configuration file manually:
- Locate
config.xml. - Find the
<autoUpgradeIntervalH>tag. - Set the value to
0:XML<autoUpgradeIntervalH>0</autoUpgradeIntervalH> - Restart Syncthing.
Additional Note for Brave Users
Using the literal IP http://127.0.0.1:8384 instead of http://localhost:8384 can also reduce “Private Network Access” interference, as Chromium treats the numeric IP as a more trusted “Secure Context.”