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Fixing "Error renaming ... while normalizing UTF8 encoding" in Syncthing on macOS

Problem Description

At first, I noticed that many files on my Mac would automatically gain a .temp suffix — even after I manually renamed them back to their original names, the .temp extension would soon reappear.
This behavior happened repeatedly and seriously affected normal use.

After checking the Syncthing control panel, I found multiple error messages such as:

Error renaming "... .ipynb.tmp" to "... .ipynb" while normalizing UTF8 encoding: file does not exist. You will want to rename this file back manually.

This indicated that Syncthing attempted to rename temporary .tmp files created during UTF-8 filename normalization but failed to do so, leaving the .tmp files behind and generating warnings.

The issue mainly occurred in project folders containing files with:

  • Japanese characters (日本語ファイル名)
  • Spaces
  • Very long paths

These conditions often lead to encoding inconsistencies on macOS file systems, which caused Syncthing to mis-handle file renaming operations.

🔍 Root Cause​

Syncthing automatically performs UTF-8 normalization when syncing files between systems that use different Unicode formats.

  • macOS uses NFD (Normalization Form D)
  • Windows and Linux use NFC (Normalization Form C)

When Syncthing detects a filename that needs “normalization,” it:

  1. Creates a temporary file like filename.ext.tmp
  2. Tries to rename it back to the normalized version

However, on macOS this often fails because:

  • The file is being indexed by Finder or iCloud Drive
  • The system’s Unicode normalization conflicts with Syncthing’s
  • The filename is too long or contains special characters

Hence the log:

Error renaming ... while normalizing UTF8 encoding: file does not exist

⚙️ Solution — Disable Auto Normalization​

You can stop Syncthing from automatically renaming filenames by disabling the autoNormalize option.

Steps:​

  1. Open the Web GUI → Actions → Advanced
  2. Go to the Folders section
  3. Select the folder that triggers the error
  4. Scroll down to find: 
    autoNormalize: true
  5. Change it to: 
    autoNormalize: false
  6. Click Save and restart Syncthing.

📍 The full path in the GUI is: Actions → Advanced → Folders → [Your Folder] → autoNormalize

After this change, Syncthing will stop renaming files during UTF-8 normalization.
No more .tmp suffixes or rename failures.

⚠️ Potential Side Effects​

Disabling UTF-8 normalization can lead to hidden filename conflicts when syncing across different operating systems.

For example:

  • A filename that appears identical on macOS and Windows might have different internal encodings (NFD vs NFC).
  • Syncthing will treat them as different files, resulting in duplicates or sync loops.

So while this fix prevents Syncthing from renaming your files automatically, it shifts the burden of filename consistency onto you.
If you sync between macOS and Windows/Linux, you’ll need to ensure that filenames are consistent and avoid mixed Unicode characters.

✅ Summary​

  • Issue: Syncthing fails to rename .tmp files due to UTF-8 normalization on macOS.
  • Fix: Disable autoNormalize in Advanced → Folders.
  • Risk: May cause invisible filename encoding mismatches across systems.

In my case, since all devices are macOS-based and share the same filesystem encoding, turning off autoNormalize completely resolved the problem without side effects.

Posted on: 2025-10-23
Environment: macOS + Syncthing v1.29+