Why Exit Code 1 is not the diagnosis
Launchers use Exit Code 1 as a general failure result for many unrelated problems. A missing Fabric dependency, an outdated Forge mod, the wrong Java runtime, a failed Mixin, and a graphics driver error can all end with exactly the same launcher message.
Avoid reinstalling Minecraft immediately. Reinstallation rarely changes the incompatible files inside a separate modded instance, and it can erase useful evidence. Start with the newest log from the instance that actually crashed.
Find the log that contains the cause
Open the affected instance in your launcher, choose its folder, and look inside the logs directory for latest.log. If Minecraft created a crash-reports directory, use the newest timestamped text file as well. MultiMC, Prism Launcher, CurseForge, Modrinth App, and the official launcher may store instances in different locations, so opening the folder from the launcher is safer than guessing.
The final line is not always the cause. Search upward for the first occurrence of “Caused by”, “requires”, “missing”, “Mixin apply failed”, “NoClassDefFoundError”, or “UnsupportedClassVersionError”. Later errors are often consequences of that first failure.
A safe troubleshooting order
First confirm that Minecraft, the mod loader, and every mod target the same game version. Then check the Java version, missing dependencies, and duplicate mod files. Update only the mod named near the first meaningful exception. Changing dozens of files at once makes it impossible to learn which change fixed the instance.
Back up the world before testing a changed mod set. Removing content mods from an existing world can delete blocks, items, or entities. When possible, reproduce the crash in a temporary copy or a fresh test world.
Quick troubleshooting checklist
- ✓Upload latest.log, not a launcher screenshot
- ✓Match Minecraft and loader versions
- ✓Check the required Java version
- ✓Install missing dependencies
- ✓Remove duplicate or wrong-loader JARs
- ✓Test one change at a time
Every launch creates a new result. After making one change, use the newly generated latest.log rather than continuing from an older report.