Guides7 min read
Running a modded Minecraft server without the crashes
Why modpacks fail differently from vanilla, how to read the crash report you actually got, and the three settings that cause most of it.
A vanilla server that falls over is almost always out of memory. A modpack that falls over could be out of memory, missing a dependency, running the wrong Java, or arguing with itself about a block ID. The fix is not more RAM. The fix is reading which of those happened, and the server tells you.
Read the crash report, not the last line
The last line of a crash is usually the symptom. The cause is above it, in the first entry of the stack that names a mod rather than the game. Scroll up until you find a package name that is not net.minecraft - that mod is where to start, and nine times out of ten the report says so plainly a few lines further down under 'Suspected Mods'.
- java.lang.OutOfMemoryError - genuinely out of memory. Raise the tier or cut the view distance.
- NoSuchMethodError or NoClassDefFoundError - a mod is built against a version of another mod you do not have. A dependency problem, not a memory one.
- Mixin apply failed - two mods are patching the same code. One of them has to go.
- UnsupportedClassVersionError - the pack wants a newer Java than the one running it.
Java version is not a detail
Modpacks pin a Java version far more tightly than vanilla does. A 1.20 pack generally wants Java 17 or 21; a 1.12 pack wants Java 8 and will not start on anything newer. The Startup tab carries the version, and changing it is a restart rather than a reinstall - so it is the cheapest thing to rule out first.
The three settings that cause most of it
- 1.View distance. It multiplies memory by players. A modpack at view distance 12 with fifteen players is a different server from the same pack at 8.
- 2.Chunk pregeneration. Generating world on demand while people play is where most 'random' lag spikes come from. Pregenerate the area you expect to be used.
- 3.Entity limits. Farms and mob spawners are the usual cause of a server that runs fine for a week and then does not.
If a pack crashes on first boot it is almost never memory. If it crashes after an hour of play, it usually is.