# Sync modes

> What Settings Saver does at launch and while you play in each sync mode (Off, Auto, Auto + always apply), and what the one-time apply is.

URL: https://xichil.dev/wiki/settings-saver/v0/mechanics/sync-modes/ · Updated: 2026-11-06 · Applies to: Fabric, Quilt, Legacy Fabric, Forge, NeoForge, MC 1.7.10 - 26.3

Sync is **opt-in** and set **per instance**. The mode decides what Settings Saver does on its own; the
Save and Load buttons work the same in every mode.

## The three modes

| Mode | At launch | While you play |
| --- | --- | --- |
| **Off** (default) | The profile is applied once, to a new instance's first launch. | Nothing. Use Save and Load. |
| **Auto** | Fills in the settings of an instance that has no `options.txt`. | Settings you change are saved to the profile. |
| **Auto + always apply** | Backs up the instance's `options.txt`, then applies the profile over it, at every launch. | As Auto. |

Change the mode with the **Sync** button in the Settings Saver menu: each click moves to the next
mode, and hovering it explains the current one. The mode is stored as `sync=` in the instance's
[config file](https://xichil.dev/wiki/settings-saver/v0/config/instance/index.md). The change takes effect at once:

- **Off to Auto** starts saving the changes you make from now on. Settings you changed before are not
  saved: press **Save settings to profile** first if you want them in the profile.
- **Auto to Off** stops saving at once.
- Changing the mode never applies the profile.

## The one-time apply

A new instance has no `options.txt` until the game writes one. On that first launch Settings Saver
converts the instance's profile to this Minecraft version's format and writes it as the instance's
`options.txt` before the game reads it. The game then starts with your settings, language and
keybinds already in place.

With sync **Off**, this happens once per instance. The instance's config then records
`appliedOnce=true`, and later launches leave the instance alone even if you delete its
`options.txt`. The one-time apply is used up on the first launch with Settings Saver, even when nothing
was applied:

- The instance already had an `options.txt` (an existing instance you just added the mod to). It is
  never overwritten without your say: press **Load** when you want the profile.
- The profile was empty. The `default` profile stays empty until you press **Save** once (or turn on
  Auto and change a setting). Instances never fill a profile on their own.

In the **Auto** modes there is no one-time limit: whenever an instance starts without an
`options.txt`, it is filled in from the profile.

## Auto: saving changes as you play

Minecraft rewrites `options.txt` whenever it saves an option you changed. In the Auto modes a
background thread checks the file every 2 seconds. Once the file has changed and then stayed the same
for one more check, Settings Saver compares it with the state it last synced and saves **only the
settings you changed** into the profile. A final check runs when the game closes.

Because only changes are saved, an instance never undoes something you changed in another instance,
and an older version never replaces a setting it doesn't fully understand. See
[Converting between versions](https://xichil.dev/wiki/settings-saver/v0/mechanics/version-conversion/index.md) and
[Running several instances at once](https://xichil.dev/wiki/settings-saver/v0/mechanics/multiple-instances/index.md).

Settings that the profile doesn't have yet but the game wrote anyway (for example options added by a
newer Minecraft version) are added to the profile. They never replace a value the profile already has.

[Excluded settings](https://xichil.dev/wiki/settings-saver/v0/guides/excluded-settings/index.md) are never saved.

## Auto + always apply

This mode makes the instance follow the profile at every launch, which helps when you change settings
in one instance and want the others to pick them up next time they start:

1. The instance's `options.txt` is copied to the [backups](https://xichil.dev/wiki/settings-saver/v0/guides/backups/index.md).
2. The profile's settings are written over the instance's, in this version's format. Excluded
   settings, and settings the profile doesn't have, keep the instance's values.
3. While you play, changes are saved as in Auto.

## When nothing is applied

Settings Saver applies a profile only when it knows the running Minecraft version, because it must
convert to that version's format. If it can't detect the version, it writes nothing and logs
`Minecraft version unknown, can't convert saved settings; not applying them.` Auto still saves
changes from 1.10 and newer, whose `options.txt` names its own version.

#### Forge

On Forge 1.13.2 to 1.15.2 (Forge 31.1 and older), these Forge versions start mods only after the game
has read `options.txt`. Settings Saver still writes the file, then has the game reload it once the
title screen is up, so on a new instance's first launch the title screen can switch to your language
a few seconds after it appears. Forge 1.7.10 to 1.12.2, and Forge 1.15.2 (31.2 and newer) and later,
apply the profile before the game reads the file.