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Minecraft 26.x world upgrade doctor

Java Edition 26.1 moved almost every folder inside a world. Drop yours and this says which layout it has, exactly which paths the upgrade will move, and, if it has already run, whether anything was left behind in the old places.

The upgrade is one way. A converted world cannot be opened by an older release, so the copy you take before starting the new server is the only way back. Nothing here is uploaded: only file and folder names are read, in your browser.


The folder named after your world, the one holding level.dat. On a server that is usually world/.

What 26.1 moved

BeforeAfter 26.1
region/dimensions/minecraft/overworld/region/
DIM-1/dimensions/minecraft/the_nether/
DIM1/dimensions/minecraft/the_end/
playerdata/players/data/
advancements/players/advancements/
stats/players/stats/
data/scoreboard.datdata/minecraft/scoreboard.dat
data/map_1.datdata/minecraft/maps/1.dat
data/idcounts.datdata/minecraft/maps/last_id.dat
resources.zipresourcepacks/resources.zip

26.1 also requires Java 25 and raised the default allocation from 2 GB to 4 GB. A server started on Java 21 refuses to boot rather than degrading, which is a separate failure from anything to do with the world; the log analyzer names it from the log line.

Want this on a real server? Supercraft hosts Minecraft →

Frequently asked questions

What changed about Minecraft world folders in 26.1?

Java Edition 26.1 reorganised world storage. Dimensions moved into a dimensions folder: DIM-1 became dimensions/minecraft/the_nether and DIM1 became dimensions/minecraft/the_end, with the overworld under dimensions/minecraft/overworld. Player files moved under players: playerdata became players/data, advancements became players/advancements and stats became players/stats. The data folder is namespaced now, so scoreboard.dat is at data/minecraft/scoreboard.dat and maps moved to data/minecraft/maps with map_1.dat renamed to 1.dat and idcounts.dat renamed to last_id.dat. A world resource pack moved to resourcepacks/resources.zip.

Can I go back to an older Minecraft after upgrading a world?

No. The conversion is one way: once a world has been converted to the 26.1 layout an older release cannot open it. Copy the whole world folder somewhere else before you first start the new server, and keep that copy until you are certain you are staying on the new version.

My world folder has both DIM-1 and a dimensions folder. Is it broken?

Not necessarily. Both layouts sitting side by side is what a world looks like just after conversion, and it is also what an interrupted conversion looks like. Check that the new locations hold the data before deleting anything from the old ones, and restore your backup rather than guessing if the new directories look incomplete.

Does this upload my world?

No. Only the list of file names and folder names is read, in your browser. No file content is opened and nothing is sent anywhere. A Minecraft world is often several gigabytes and none of it needs to leave your machine to answer which layout it uses.

This tool: v1.4 · updated 20 Oct 2026