If your instance on **Minecraft 1.20.5, 1.20.6, 1.21 – 1.21.4+** exits immediately after clicking Launch, open the **Minecraft Log** tab. An `UnsupportedClassVersionError` or `Incompatible magic value` means the instance is overriding the global runtime with the wrong Java version.
Each instance has its own version, mods, settings and worlds.
3-step repair in Prism Launcher
Right-click the failing instance → **Edit** → **Settings** → **Java**.
Tick **Java installation**, click **Auto-detect** (or download via Prism), and choose **Eclipse Adoptium Temurin 21 / Microsoft OpenJDK 21**.
Click **Test** to confirm the runtime starts cleanly, then launch your world. Need a fresh launcher install? Visit /download.
Frequently asked questions
What if clicking Test says 'Could not reserve enough space for object heap'?
You either selected a 32-bit Java installation or set Maximum Memory higher than the free physical RAM currently available on Windows.
Should I check 'Skip Java compatibility checks'?
Only enable that toggle for special backported modpacks (such as GTNH Java 17/21 builds on 1.7.10); for normal packs, switch to the matching Java version instead.
Mojang upgraded Minecraft 1.20.5 and 1.21+ to require a 64-bit Java 21 runtime; launching with Java 17 or Java 8 causes an immediate incompatible class version crash. Applies to Minecraft 1.20.5, 1.20.6, 1.21 – 1.21.4+.
