Online UUID Fix
Online UUID Fix is a Fabric server-side mod that replaces the offline UUIDs of existing players with the real online UUIDs issued by Mojang and Microsoft.
What it does
On an offline-mode server, vanilla Minecraft derives each player's UUID from the username with a local algorithm, so the same player can receive a different UUID when the server switches to online mode. At login, the mod takes over UUID assignment and queries the Mojang API for the player's genuine UUID, reverting to the standard offline UUID only when that API cannot be reached. Each resolved UUID is logged on the first login.
The mod is for server owners who already run in offline mode and plan to move to online mode, and who want returning players to keep their data. Players whose data is stored under offline UUIDs need their player files renamed before the switch, or they start fresh. The mod runs only on the server, so players do not install anything. Its source code is open under the GNU General Public License v3.0 only, and files target 1.21.11 for Fabric Loader 0.18.2+ with JDK 21.
Key features
Real UUID lookup
At login the mod requests the player's UUID from the Mojang API and uses it instead of the offline value.
Fallback to offline UUID
When the API cannot be reached, the vanilla offline UUID is used so the login still succeeds.
Playerdata renaming guide
Renaming files under world/playerdata to the online UUID, with the server stopped, keeps existing player data attached to the account.
In-game command option
A command can look up a player's online UUID while the server is running, as an alternative to the manual lookup.
Resolved UUID logging
Each UUID resolved on a player's first login is written to the server log.
Compatibility
Latest build for each game version and loader.
Only full game releases are shown. This mod publishes no snapshot builds.
Use cases
Who it’s for: Server owners running Minecraft 1.21.11 on Fabric in offline mode who plan to enable online mode and need existing players to keep their data.
How it compares
AccountSwitcher is a client-side only Fabric mod for 1.21.4, with no open source code under All rights reserved, so it works on a different side and version than this server mod. DiscordLogin is server-side only on Fabric for 1.21 – 1.21.11 under a custom license, and handles login rather than UUIDs. LAN Properties is open source under the Apache License 2.0 and runs on client and server across several loaders, so it covers a different function.
| Mod | Loaders | Game versions | Environment | Updated |
|---|---|---|---|---|
| Online UUID Fix | Fabric | 1.21.11 | Server-side only | 2026-03-25 |
| AccountSwitcher | Fabric | 1.21.4 | Client-side only | 2026-06-29 |
| DiscordLogin | Fabric | 1.21 – 1.21.11 | Server-side only | 2026-06-29 |
| LAN Properties | Fabric, Forge, NeoForge, Quilt, Legacy Fabric and Ornithe | 1.12 – 1.21.8 | Client and server | 2026-02-10 |
Questions
Does Online UUID Fix need to be installed on clients?
What happens to players who already have data on the server?
Which Fabric versions does the mod require?
In the catalog
Online UUID Fix is one of 8,016 Management mods on ModsFinder, and one of 5,930 Management mods for Fabric. 19,862 mods support Minecraft 1.21.11.
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What the author says
A Fabric server-side mod for offline-mode servers that assigns players their real Mojang/Microsoft UUID instead of the vanilla offline UUID. When a server runs in offline mode (online-mode=false), Minecraft generates a UUID from the player's username using a local hash. This means the same player gets a different…
Official listing
- Modrinth page
- modrinth.com/mod/online-uuid-fix
- Owner
- mateusfilipe2k2
- Published
- 2026-03-25
- Updated on Modrinth
- 2026-03-25
- Facts verified
- 2026-10-15