Mods



Mods (short for modifications) change Minecraft's game content from what it originally was.

Purpose
Most mods add content to the game to alter gameplay, change the creative feel, or give the player more options in how they interact with the Minecraft world. Most people who create mods for Minecraft (known as modders) use Minecraft Coder Pack and either ModLoader, Minecraft Forge, or Fabric to do so. Some mods may be bigger expansions, others add more settings and options to optimize speed, graphics, or gameplay of the game. Server mods or plugins mainly give server admins more options and ease of use, and most mods for single-player have a server version that allows or optimizes the mod in multiplayer. Ideally, the creator of a mod updates the mod whenever the game updates - bringing more content, bug fixes, or optimizations. Otherwise, the mod may not work with a newer version.

Mods are also available for via jailbreaking on iOS and through apps such as "BlockLauncher", "MCPE Master", or "Toolbox for Minecraft: PE" on Android devices. Mods are not available for the Legacy Console Edition.

An official way of creating mods is available for called Add-ons.

Client-based
Client mods are modifications to your game files themselves. They are not custom clients, as they require modification of the Minecraft .jar file. They require mod loaders such as "Minecraft Forge" or "Fabric" and install a separate profile.

Most of the common mods in Minecraft 1.14 and higher require hooks that require Fabric API.

As the Minecraft server software will ignore custom content from the client side, most of the client mods which add new content to the game do not work in multiplayer unless a modification has also been installed on the server. Some authors of client mods have made server versions of their mods available, and others have not. A few of the Loader/API type client mods (e.g. ModLoader and AudioMod) and many of the functional client mods (e.g. OptiFine or TooManyItems) have effect in multiplayer straight away, without any modification on server side.

Mod list on Minecraft Forums

Server-based
Server mods are modifications to the official Minecraft server software. They are commonly designed to make the administration of servers easier by implementing tiered privileges for commands (such as kicking, banning, etc.). They are frequently implemented as "wrappers" which do not modify the main server .jar file, instead of monitoring its output and sending commands to it.

Mod packs
Mod packs are collections of mods that have been put together and configured so that they will all work together. Mod packs are often centered around a general theme like tech, quests, or magic. Mod packs often have either custom launchers or installers that make installing and running the mod pack easy. Some of the more popular mod packs are Feed The Beast, Tekkit, and Hexxit. There are of course also smaller mod packs, such as e.g. Crafting Dead and Foolcraft, most of which using the special Minecraft Technic Launcher from Technic Pack. Most mod packs can be found on custom launchers, which make it easy to install and launch various mod packs. The most popular of these are Technic, ATlauncher, and the new Twitch Desktop App. In addition to making it easy to install mod pack clients, certain launchers can also download server mod packs.

Bugs
If players installed a mod, it may stop the game from working properly. If they encounter a problem while using mods and want to report it, first remove all of the mods and see if the problem still happens. If the problem continues to happen, it can be reported on the bug tracker. Otherwise, try to reproduce the problem with the fewest possible number of mods, and then report the problem to the authors of those mods.

If Minecraft crashes, a modified game is flagged in the crash report.

The crash report text will include a line near the bottom which will say one of:

Is Modded: Probably not. Jar signature remains and client brand is untouched Is Modded: Very likely; Jar signature invalidated Is Modded: Definitely: client brand changed to (present loader, such as "fml,forge", "modloader", or "fabric") Is Modded: Unknown (can't tell)

A shortened example crash report is given below:

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The code that checks for mods is fairly simple, and it's not always correct; it may say 'probably not' even with mods installed. However, it's very difficult to get the 'very likely' message if you haven't modified your Minecraft .jar file somehow, so that's essentially a 'yes'. There's also a 'definitely' message, seen when a Bukkit server crashes and under other similar circumstances, like when the Minecraft Forge API is installed.

'Probably not' appears when the client/server brand appears to the in-game check to be unaltered (often termed 'vanilla') and the  folder is still there. 'Very likely' appears when the  folder is not present but the client/server brand seems to be vanilla. 'Definitely' plus the client name appears when the client is not vanilla. For example, you might see something like:

Is Modded: Definitely: Client brand changed to 'fml,forge'

Because of issues including (but not limited to) the bugs above, it is recommended that you use mods in a new world.