Minecraft keeps crashing: find the log, then read the three lines that matter
Almost every Minecraft crash is already explained, in writing, in a file on your disk. The problem is that the file is thousands of lines long and the useful part is about three of them. This page shows you where the file is, which three lines to read, and the handful of causes behind most crashes.
Where the log actually is
| What | Where | Use it when |
|---|---|---|
| latest.log | logs/latest.log | Almost always the best file. It has the startup sequence, the mod list and the crash, so it shows what was loaded as well as what broke. |
| Older logs | logs/2026-08-03-1.log.gz | The crash happened earlier and you have restarted since. Extract the .gz first. |
| Crash reports | crash-reports/crash-2026-08-03_14.20.11-server.txt | Written only for a hard crash. Tidier than latest.log but carries less context. |
On a server both folders sit in the server root, next to server.properties. On a host panel the file manager will show them at the top level.
On a client they are inside the instance folder: .minecraft/logs/ for the vanilla launcher, or the instance directory for Prism, MultiMC, CurseForge or Modrinth App. If the game closes instantly on launch, the log is still written, so it is there even when you saw nothing.
The three lines that matter
Open the file and look for these, in this order. You can ignore everything else on a first pass.
- The
Description:line near the top of a crash report. One sentence saying what the game was doing when it died, for example "Exception ticking entity" or "Initializing game". - The first
Caused by:line. A report often stacks several. The last "Caused by" is the root, and it is the one to read. - The topmost
at ...frame that names a mod rather thannet.minecraftorjava.. Package names look likeat com.example.coolmod.SomeClass, andcoolmodthere is your suspect. This single trick resolves most modded crashes.
If the topmost frames are all net.minecraft, the crash is usually memory or the world rather than one mod, which the next section covers.
The causes behind most crashes
1. Out of memory
java.lang.OutOfMemoryError: Java heap space. The single most common modded crash, and it is a sizing problem, not a broken mod. There are four different flavours and they need different fixes: heap space means raise -Xmx, Metaspace means add -XX:MaxMetaspaceSize (raising -Xmx will not help), GC overhead means effectively the same as heap, and direct buffer memory means off-heap and raising -Xmx can make it worse. See how much RAM a pack actually needs.
2. The wrong Java version
UnsupportedClassVersionError ... class file version 65.0. Decode it: 52 is Java 8, 61 is Java 17, 65 is Java 21, 69 is Java 25. Match Java to the Minecraft version rather than installing the newest: 1.16 and earlier want Java 8, 1.17 to 1.20.4 want Java 17, 1.20.5 to 1.21.11 want Java 21, and 26.1 and newer want Java 25. That last one is a hard requirement, not a preference: 26.1 refuses to start on Java 21 and says so outright. In the other direction newer is not safer, because older packs break on new Java.
3. A missing library mod
Missing or unsupported mandatory dependencies or NoClassDefFoundError. The loader names the mod and the version range it wants on the following lines. Usual suspects are Fabric API, Architectury, Cloth Config and Kotlin for Forge. Install the version range it asks for, not simply the latest.
4. A mixin conflict
Mixin apply failed. Two mods tried to patch the same code, or a mod is built for a different Minecraft version. The mixin name contains the owning mod id. This one almost never means your world is damaged.
5. A client-only mod on the server
Shaders, minimaps and HUD mods will stop a server booting. This is the classic failure after copying a client pack wholesale into a server folder. Remove them from the server's mods/; they belong only in the player's. If you are not sure which of your mods are client-only, the modpack server readiness checker will tell you: point it at your mods/ folder and it identifies each jar by hash, so it works whether the pack came from CurseForge, Modrinth or a zip someone sent you.
6. A duplicate jar
Duplicate mods found. Two versions of the same mod are sitting in mods/, usually because an update was added without deleting the old file. Delete the older one.
If you would rather not read it yourself
Several tools will read the log for you. They are not equivalent, and the differences matter more than the marketing does, so here is the honest comparison.
| Tool | Good | Catch |
|---|---|---|
| Our log analyzer | Runs in your browser so the log is never uploaded, no account, no monthly limit, no file size cap, and it checks the heap you configured against your mod count. | Ours, so weigh it accordingly. Pattern-based, so a brand new failure mode may not be recognised yet. |
| MCDoctor.ai | The widest detection of the web tools, 110+ documented error types, and clear step-by-step fixes. | Your log is uploaded to their servers. Free tier is a handful of analyses per month and needs an account, and files are capped at 5-10 MB, which a modded latest.log can exceed. |
| mccrashreader.com | Clean interface, severity ranking, names the affected mods. | Free tier is 3 analyses per month, then paid. Sign-in required, and analysis happens on their side. |
| Crash Assistant (mod) | By far the most capable, 14.6M downloads. Pops up automatically after a crash and reads things no website can, including JVM hs_err files and Windows Event Logs. | Client-only, and it must be installed before the crash. On a server it does nothing. |
| CrashDetector (mod) | 220+ checks, works on client and server, ranks the mods it finds in the stack trace. | Also needs installing in advance, and has known problems on Java 8. |
If you run a server, the two mods are the strongest option for a client-side crash but cannot help with a server that will not boot. For that, a log reader is the practical route.
When it is not a crash at all
Two things get mistaken for crashes often enough to be worth naming. Can't keep up! Is the server overloaded? is the server running behind, not dying, and a couple of those after a restart or a backup is normal. A server that shuts down cleanly after everyone leaves is not crashing either, that is a sleep or idle setting, which is how free hosting is designed to work.
Related guides
- How much RAM a modpack server needs
- Aikar's flags: JVM tuning that actually helps
- Fixing server lag
- How much does a Minecraft server cost?
- Aternos free limits, and when free stops being enough
- Modpack server readiness checker - will this pack run as a dedicated server, and what does it need?
If the answer turns out to be "this pack needs more memory than the box has", Supercraft Minecraft hosting runs on AMD EPYC hardware with NVMe storage and in-place plan upgrades, so you can resize when a modpack grows rather than rebuilding. Live pricing for your region is on the plan page.