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# OneBlock 5.1.3 *This file is for **Forge 26.2**.* Critical fix for **Minecraft 1.20.1, 1.20.2, 1.20.3 and 1.20.4**. If you play on one of those versions, update before you launch the game again. **The mod was corrupting worlds on save.** Every time the game saved, the part of `level.dat` that describes the world's dimensions was silently dropped. Nothing looked wrong while you played — the world only failed the *next* time you tried to open it, with "Failed to load level data" or a crash straight back to the world list. The backup copy Minecraft keeps next to it was damaged the same way, because every save since the world was made had gone through the same path. This was not limited to OneBlock worlds. Having the mod installed was enough: a plain survival world, a world from another modpack, any world opened once with an affected build was left unable to reopen. The fix removes the cause entirely, and worlds saved from 5.1.3 onwards are written correctly. **Already have a world that won't open?** 5.1.3 cannot repair it — the information is gone from the file, not hidden. Restore it from a launcher backup or a copy made before the world last shut down. The Modrinth App, CurseForge and Prism all keep world backups; if you have one from before the corruption, it will open normally once 5.1.3 is installed. Minecraft 1.20.5 and newer were never affected — that includes every 1.21 and 26.x build. Those are republished here unchanged so the version list stays in step. **CodxLib is required.** Install the build matching your Minecraft version and loader. On Fabric it needs to be 1.3.4 or newer.