Reading Minecraft Logs

This page is intended mostly for non-technical users - those who have never programmed before, and aren't used to seeing stack traces. It may still be useful for developers or those who know more of what they are doing.

What is all this code???

Chances are, you see something like this:

Reported exception thrown!
net.minecraft.class_148: Rendering screen
    at net.minecraft.client.gui.screen.Screen.wrapScreenError:L436
    at net.minecraft.client.Mouse.onMouseButton:L107
    at net.minecraft.client.Mouse.method_22686:L196
    at net.minecraft.util.thread.ThreadExecutor.execute:L108
    at net.minecraft.client.Mouse.method_22684:L196
    at org.lwjgl.glfw.GLFWMouseButtonCallbackI.callback:L43
    at org.lwjgl.system.JNI.invokeV:native
    at org.lwjgl.glfw.GLFW.glfwWaitEventsTimeout:L3509
    at com.mojang.blaze3d.systems.RenderSystem.limitDisplayFPS:L184
    at net.minecraft.client.MinecraftClient.render:L1310
    at net.minecraft.client.MinecraftClient.run:L882
    at net.minecraft.client.main.Main.main:L256
    at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch:L514
    at net.fabricmc.loader.impl.launch.knot.Knot.launch:L72
    at net.fabricmc.loader.impl.launch.knot.KnotClient.main:L23
    at jdk.internal.reflect.DirectMethodHandleAccessor.invoke:unknown
    at java.lang.reflect.Method.invoke:unknown
    at com.modrinth.theseus.MinecraftLaunch.relaunch:L63
    at com.modrinth.theseus.MinecraftLaunch.main:L28
Caused by: java.nio.BufferOverflowException
    at java.nio.Buffer.nextPutIndex:unknown
    at java.nio.DirectByteBuffer.putFloat:unknown
    at com.mr_toad.gpu_booster.api.util.BatchVertexConsumer.method_22912:L75
    at net.minecraft.client.render.VertexConsumer.vertex:L531
    at net.minecraft.client.font.GlyphRenderer.handler$fko000$gpu_booster$batchDraw:L537
    at knot/net.minecraft.class_382.method_2025(class_382.java) 
    at net.minecraft.client.font.TextRenderer.drawGlyph:L303
    at net.minecraft.client.font.TextRenderer$Drawer.accept:class_327:232
    at net.minecraft.text.TextVisitFactory.visitRegularCharacter:L17
    at net.minecraft.text.TextVisitFactory.visitForwards:L43
    at net.minecraft.text.OrderedText.method_30753:L23
    at net.minecraft.client.font.TextRenderer.drawLayer:L298
    at net.minecraft.client.font.TextRenderer.drawInternal:L151
    at net.minecraft.client.font.TextRenderer.draw:L84
    at net.minecraft.client.gui.DrawContext.drawText:L290
    at net.minecraft.client.gui.DrawContext.drawTextWithShadow:L286
    at net.minecraft.client.gui.DrawContext.drawCenteredTextWithShadow:L268
    at com.aetherteam.aether.client.event.hooks.GuiHooks.drawTrivia:L182
    at com.aetherteam.aether.client.event.listeners.GuiListener.onGuiDraw:L65
    at com.aetherteam.aether.client.event.listeners.GuiListener.lambda$listen$0:L31
    at net.fabricmc.fabric.impl.client.screen.ScreenEventFactory.lambda$createAfterRenderEvent$4:L48
    at net.minecraft.client.render.GameRenderer.wrapOperation$fbl000$fabric-screen-api-v1$onRenderScreen(GameRenderer.java:9587) 
    at knot/net.minecraft.class_757.mixinextras$bridge$wrapOperation$fbl000$fabric-screen-api-v1$onRenderScreen$344(class_757.java) 
    at net.minecraft.client.render.GameRenderer.wrapOperation$fgg000$fancymenu$wrapRenderScreenFancyMenu:L11071
    at knot/net.minecraft.class_757.mixinextras$bridge$wrapOperation$fgg000$fancymenu$wrapRenderScreenFancyMenu$346(class_757.java) 
    at net.minecraft.client.render.GameRenderer.wrapOperation$ggi000$konkrete$wrapRenderScreen_Konkrete:L14623
    at net.minecraft.client.render.GameRenderer.render:L913
    at net.minecraft.client.MinecraftClient.render:L1285
    at net.minecraft.client.MinecraftClient.setScreenAndRender:L2322
    at net.minecraft.client.gui.screen.world.CreateWorldScreen.showMessage:L412
    at net.minecraft.client.gui.screen.world.CreateWorldScreen.create:L305
    at net.minecraft.client.gui.screen.world.SelectWorldScreen.method_19944:L54
    at net.minecraft.client.gui.widget.ButtonWidget.onPress:L96
    at net.minecraft.client.gui.widget.PressableWidget.onClick:L48
    at net.minecraft.client.gui.widget.ClickableWidget.mouseClicked:L141
    at net.minecraft.client.gui.ParentElement.mouseClicked:L38
    at net.minecraft.client.Mouse.method_1611:L107
    at net.minecraft.client.gui.screen.Screen.wrapScreenError:L431
    ... 18 more

And that means a whole lot of nothing to you. So let's break it down.

OK, so now we know what the program is saying. But where's the problem really coming from?

Look for Mod IDs

All mods have an "id." This is usually a shortened form of their name. Fabric uses it to tell if two mod .jar files are the same mod (and different versions), or just different mods entirely. Let's look at a few examples:

As you can see, some are a little more understandable than others. Very rarely, you'll find mods that have ids that are entirely different from their name - usually this is because they renamed the mod, but mod ids aren't supposed to change, so they kept the old id.

What can you find in our example that looks like a mod id?

Caused by: java.nio.BufferOverflowException
    at java.nio.Buffer.nextPutIndex:unknown
    at java.nio.DirectByteBuffer.putFloat:unknown
    at com.mr_toad.gpu_booster.api.util.BatchVertexConsumer.method_22912:L75 // gpu_booster is a mod id
    at net.minecraft.client.render.VertexConsumer.vertex:L531
    at net.minecraft.client.font.GlyphRenderer.handler$fko000$gpu_booster$batchDraw:L537
    at knot/net.minecraft.class_382.method_2025(class_382.java) 
    at net.minecraft.client.font.TextRenderer.drawGlyph:L303
    at net.minecraft.client.font.TextRenderer$Drawer.accept:class_327:232
    at net.minecraft.text.TextVisitFactory.visitRegularCharacter:L17
    at net.minecraft.text.TextVisitFactory.visitForwards:L43
    at net.minecraft.text.OrderedText.method_30753:L23
    at net.minecraft.client.font.TextRenderer.drawLayer:L298
    at net.minecraft.client.font.TextRenderer.drawInternal:L151
    at net.minecraft.client.font.TextRenderer.draw:L84
    at net.minecraft.client.gui.DrawContext.drawText:L290
    at net.minecraft.client.gui.DrawContext.drawTextWithShadow:L286
    at net.minecraft.client.gui.DrawContext.drawCenteredTextWithShadow:L268
    at com.aetherteam.aether.client.event.hooks.GuiHooks.drawTrivia:L182 // aether is a mod id
    at com.aetherteam.aether.client.event.listeners.GuiListener.onGuiDraw:L65
    at com.aetherteam.aether.client.event.listeners.GuiListener.lambda$listen$0:L31
    at net.fabricmc.fabric.impl.client.screen.ScreenEventFactory.lambda$createAfterRenderEvent$4:L48
    at net.minecraft.client.render.GameRenderer.wrapOperation$fbl000$fabric-screen-api-v1$onRenderScreen(GameRenderer.java:9587)  // fabric-screen-api-v1 is a mod id
    at knot/net.minecraft.class_757.mixinextras$bridge$wrapOperation$fbl000$fabric-screen-api-v1$onRenderScreen$344(class_757.java)  // fabric-screen-api-v1 is a mod id
    at net.minecraft.client.render.GameRenderer.wrapOperation$fgg000$fancymenu$wrapRenderScreenFancyMenu:L11071 // fancymenu is a mod id
    at knot/net.minecraft.class_757.mixinextras$bridge$wrapOperation$fgg000$fancymenu$wrapRenderScreenFancyMenu$346(class_757.java)  // fancymenu is a mod id
    at net.minecraft.client.render.GameRenderer.wrapOperation$ggi000$konkrete$wrapRenderScreen_Konkrete:L14623 // konkrete is a mod id
    at net.minecraft.client.render.GameRenderer.render:L913
    at net.minecraft.client.MinecraftClient.render:L1285
    at net.minecraft.client.MinecraftClient.setScreenAndRender:L2322
    at net.minecraft.client.gui.screen.world.CreateWorldScreen.showMessage:L412
    at net.minecraft.client.gui.screen.world.CreateWorldScreen.create:L305
    at net.minecraft.client.gui.screen.world.SelectWorldScreen.method_19944:L54
    at net.minecraft.client.gui.widget.ButtonWidget.onPress:L96
    at net.minecraft.client.gui.widget.PressableWidget.onClick:L48
    at net.minecraft.client.gui.widget.ClickableWidget.mouseClicked:L141
    at net.minecraft.client.gui.ParentElement.mouseClicked:L38
    at net.minecraft.client.Mouse.method_1611:L107
    at net.minecraft.client.gui.screen.Screen.wrapScreenError:L431
    ... 18 more

Sweet! We've found some mod ids. Now it's time to apply some intuition to find the culprit.

Rules of Thumb

These rules can help you guess at what mod to try removing (or updating). They're not foolproof, and are generally for crashes where all dependencies are met (meaning you've installed all the mods that other mods need).

1. Higher up = more likely. Remember that at... is the program saying "and I was told to do that by..." Chances are, the uppermost mod is at fault.
2. Relevant error types. This one is more for the technical. In this case, I happen to know that a BufferOverflowException is caused by something messing very close to memory, such as an optimization mod or something that messes with rendering or hardware.
3. mixinextras...wrapOperation = less likely. This is used by mods that are trying to be careful about what they change, but it will show up in the logs a lot because of how it works, even if the mod doesn't choose to change anything this time around.
4. It's probably not Fabric API. Fabric API does have bugs, but they're usually fixed quickly. Most of Fabric API is automatically tested, and all of it is reviewed by multiple people before being published.

So what's up with this log?

In our example, we've got quite a bit to work with. Right before crashing, we see Reported exception thrown!. Minecraft knows it has crashed! That's pretty helpful. We know it was Rendering screen, and that the error was Caused by: java.nio.BufferOverflowException. Just three lines down, we see at com.mr_toad.gpu_booster.api.util.... Looks like GPU Booster is probably the cause. The user removed GPU Booster from their mods, and now the game works!

Now the error has changed

Don't fret! That's a good thing. That almost always means that you've fixed a problem with your modpack! Now you do it again. If it doesn't change, then you probably didn't guess the right mod.

What if I don't see my error listed here?

This is where you'll need to apply some intuition and guesswork. Below are a few common cases and the most common fixes. Ask for help if you need it! #player-support in The Fabric Project Discord is a great place to start.

Failed to load registries due to above errors

This is usually a problem with datapacks OR a mod that has a datapack inside it (most mods do!). Chances are, you're seeing this in a crash report - take a look at the latest.log if you haven't already. You're looking for a line like this:

java.lang.IllegalStateException: Failed to parse cobbleverse:worldgen/structure/blaine.json from pack COBBLEVERSE-DP-v26.zip

In this case, we can see that the datapack " COBBLEVERSE-DP-v26.zip" is causing the crash. Most of the Cobblemon datapack issues that I see are because the datapack doesn't support the current version of Cobblemon. Removing the datapack fixes this issue.

Sometimes you'll see "...from pack modid". That means it's from a datapack inside a mod with the id "modid".