How Java .dat files work
Java Edition stores many save files as GZip-compressed NBT (Named Binary Tag), big-endian. level.dat holds world-wide fields under a Data compound. Server player state is UUID.dat under players/data (or playerdata on older layouts).
Recent versions moved game rules, weather, world gen settings, and more into data/minecraft/*.dat. This tool focuses on fields that still live in level.dat / player.dat per the Minecraft Wiki.
Bedrock level.dat uses a different header and little-endian NBT. Not supported in v1.
What is inside level.dat?
level.dat is a compressed NBT document whose root is a Data compound. It stores world-wide settings: the world name, game rules, difficulty, spawn point, weather state, and on single-player worlds the player's own data. Knowing what lives here helps you edit a save without corrupting it.
Player files (UUID.dat)
On servers, each player's state is stored separately as <uuid>.dat - inventory, position, health, food, XP, and game mode. The file name is the player's Mojang UUID, not their username. Always take the player offline before editing or replacing this file, otherwise the server overwrites your changes when the player saves.
The data/minecraft split (26.1+)
Modern worlds moved game rules, weather, world generation settings and world clocks into data/minecraft/*.dat files. level.dat still holds the core fields, but fields such as gamerules may no longer exist there. This editor only touches fields that still belong in level.dat per the Minecraft Wiki, and preserves any unknown tags exactly as they are.
How to use this editor
Open the page in any modern browser, drag level.dat or a player UUID.dat into the drop zone, change the fields you need, then press Download .dat. Save the new file with the exact same name and path as the original: level.dat goes in the world root folder, player files go under players/data/. Make a backup of the whole world folder first - a wrong write can corrupt a save.
Backup workflow
The safe order is: close the world or server, copy the entire world folder to a backup location, edit the .dat file, replace the original, then open the world again. If anything looks wrong, restore the backup - no damage done.
Safety notes
Everything runs locally: your file never leaves the browser. Be careful with files from strangers - a malicious .dat can exploit older game versions. Never trust a random 'free' level.dat you found online and load it blindly.
Common questions
Does this editor work on Bedrock Edition?
No. Bedrock level.dat uses a different header and little-endian NBT, so Bedrock saves are not supported yet.
Why does the editor show 'Unknown NBT compound'?
The file was recognized as NBT, but its root structure did not match level.dat or player.dat. You can still download it unchanged - unknown tags are preserved and the file stays intact.
Is my world data uploaded anywhere?
No. The file is read and written entirely in your browser, so nothing is ever uploaded to a server.