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Short answer: a server-side mod runs on the Minecraft server, a client-side mod runs in each player’s game, and a content or gameplay mod usually has to be installed in both places. A mod’s environment is separate from its Minecraft version and loader, so check all three on the project page before copying files. Installing a client-only class on a dedicated server, or sending players to a server whose required content mod is missing, can prevent the game from starting or joining.
This guide explains the physical and logical “sides” used by Forge, NeoForge and Fabric, shows how to classify a mod, and gives a recovery path for the most common crashes.
What “server-side” and “client-side” mean
In multiplayer Minecraft, the server owns the shared world state: terrain, entities, inventories, game rules and most authoritative gameplay decisions. Every player connects with a client, the game program that renders the world, accepts input and displays interfaces.
A mod can target either program, both programs, or (in some cases) different logical responsibilities inside one program. “Side” therefore describes where code is allowed to run, not whether a mod file is installed in a particular folder.
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Server-side (server-only) mods
A server-side mod changes or assists the server without adding code that connecting clients must have. Typical examples are administration, backups, performance, permissions, logging or world-management utilities. Players can often join with an unmodified client, provided the mod does not add blocks, items, entities, screens or other data that the client must understand.
Client-side (client-only) mods
A client-side mod changes the local presentation or controls: a minimap, shader support, HUD, key-binding helper, zoom feature or other visual interface. It normally belongs in the client’s mods folder and does not belong on a dedicated server. The server does not need to know about a purely local display feature.
Both-sides (client-and-server) mods
A content or gameplay mod that adds blocks, items, entities, dimensions, recipes or networked mechanics generally must be installed on the server and on every connecting client. Both sides need the same mod identity and compatible version so they can register and exchange the added content.
| Mod category | Installed on dedicated server? | Installed on player clients? | Typical result |
|---|---|---|---|
| Server-only | Yes | No, unless the project says otherwise | Server administration, rules, optimization or world services |
| Client-only | No | Yes, for each player who wants the feature | Rendering, HUD, controls or visual effects |
| Both sides | Yes | Yes, matching versions and loader | New content or synchronized gameplay |
These categories are practical guidance rather than a substitute for the mod author’s declaration. A host guide summarizes the same distinction for server administration and client visual mods at UltraServers’ client-side mod documentation.
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Physical side versus logical side
Forge and NeoForge distinguish two related ideas. The physical side is the program process you launched: the physical client or the physical dedicated server. The logical side is the role a level or thread is performing: logical client work handles presentation and input, while logical server work owns authoritative game state.
That distinction matters in singleplayer. A singleplayer game launches a client process that also contains an integrated server. Code can therefore run on both logical sides even though there is no separate server computer. A dedicated server has no client rendering environment at all.
Forge’s documentation explains that a Level object can be used to establish which logical side owns that level. The same documentation also says Forge removed the old sidedness setting from mods.toml; mods are expected to work when loaded on either the physical client or the physical server. Read the current Forge sides documentation for the API details.
Why a mod can crash before the world loads
Java classes are loaded as code paths require them. If code that executes on a dedicated server references a client-only class—such as a renderer, keyboard handler or graphical screen—the server can fail while loading that class because those client names do not exist there. Forge documents the risk this way: “if any class you write references those names in any way, they will crash the game when that respective class is loaded in an environment where those names do not exist.” The page is several years old, so treat it as a conceptual warning rather than a statement about every current loader implementation.
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The practical rule is simple: a client-only class must not be referenced from code that the dedicated server can load. A third-party troubleshooting guide describes the resulting symptom when a client-only Fabric mod is placed on a server: a startup crash (mcdoctor.ai’s Fabric client-mod troubleshooting page).
How to tell which side a mod needs
- Open the mod’s official project page. Look for an environment label such as client, server, both, client and server or universal. Do not infer the environment from the mod’s name alone.
- Confirm the Minecraft version. A file for one release may not work on another, even when the feature is unchanged. Match the exact version range stated by the author.
- Confirm the loader. Forge, NeoForge and Fabric are different mod-loading platforms. A project page normally identifies which loader the file targets. Do not assume a Fabric file is interchangeable with a Forge or NeoForge file.
- Read dependency and installation notes. A mod may require a library on the same side, a particular Java runtime, or a separate resource pack. Install every required dependency in the location the author specifies.
- Check the release file’s metadata when documentation is unclear. Fabric declares an environment in
fabric.mod.json; the project page and loader-specific metadata are more reliable than a filename such as “client” or “common.”
Environment, loader and Minecraft version are three separate checks. A mod can target the correct version but the wrong loader, or the correct loader but only the client environment.
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Fabric uses the environment field in fabric.mod.json. The field accepts a string or an array and can contain:
| Value | Meaning | Loading behavior |
|---|---|---|
"*" |
All environments; this is the default | Eligible for both client and dedicated-server environments |
"client" |
Game client only | Not loaded on a dedicated server |
"server" |
Dedicated server only | Not loaded on clients, including the client side of ordinary play |
The specification and documentation are available in the Fabric Wiki reference and the Fabric documentation. Fabric also separates entry points: main runs on both sides, while client and server entry points run only on their matching side. Mixin entries can carry their own environment key.
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There is an unresolved documentation discrepancy about integrated servers. The specification describes the server value as excluding integrated servers, while the documentation guide says server-only mods do not load in singleplayer or LAN. Because those statements do not align, do not treat integrated-server behavior as settled from the label alone; follow the mod’s current installation instructions and test the exact loader and game version you use.
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For a dedicated server
- Stop the server and make a verified world backup. Copy the world directory somewhere outside the server installation.
- Install the matching loader and Minecraft version. The server, the loader and the mod must target compatible versions.
- Read the mod page’s environment label. Copy server-only and both-sides files into the server’s
modsdirectory. Leave client-only files out. - Install required libraries and dependencies in the same server
modsdirectory unless the author specifies another location. - Start the server once and inspect the log. A clean startup should reach the normal “server started” state. Stop immediately if the log names a missing dependency, wrong loader or client class.
- Give players the exact client set. For a both-sides mod, every client needs the same mod and compatible version. Players may additionally install client-only visual mods, but those should not be copied to the server.
For a player client
- Install the loader profile that the mod supports, then launch it once to create the expected directories.
- Place the file in the client’s
modsfolder. Use the folder belonging to the profile you actually launch; putting a Fabric file in a Forge profile does not make it compatible. - Install client dependencies listed by the project page. A dependency can itself be required on both sides.
- For a multiplayer server, mirror every required both-sides mod. A client-only mod can remain local, but it cannot supply missing server content.
- Launch and read the first error. Do not repeatedly restart with the same incompatible file; remove or update the named mod, then test again.
What happens in common multiplayer combinations
| Server setup | Client setup | Expected outcome |
|---|---|---|
| Server-only utility | Vanilla or client-only additions | Usually works because clients do not need the utility code |
| Both-sides content mod | Same content mod and compatible version | Normal operation, assuming dependencies match |
| Both-sides content mod | Client lacks it | Join failure, missing-content error or protocol mismatch |
| Client-only visual mod accidentally installed on server | Any client | Possible server startup crash when a client class is loaded |
| Client-only visual mod only on client | That same client | Usually works; the feature is visible only to that player |
“Works on my singleplayer world” does not prove that a mod is safe for a dedicated server. Singleplayer contains an integrated server and a client in one process, so an unsafe reference can remain hidden until the code is loaded in a server-only process.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why a modded server crashes, and how to recover
1. The log names a client class or rendering package
Stop the server and remove the last client-only mod you added. Recheck the project page’s environment label. If the mod is supposed to support both sides, update to a release that matches the server’s Minecraft version and loader; otherwise keep it on clients only.
2. The log reports a missing dependency
Install the dependency in the side where the error occurs, using the exact version range requested. Some libraries are needed on both server and client, while a rendering library may be client-only. Restart only after the dependency set is complete.
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3. The log reports an incompatible loader or version
Remove the file and download the build for the server’s loader and Minecraft version. Do not “solve” this by renaming the file. Forge, NeoForge and Fabric use different loading systems, and a matching filename cannot change its target platform.
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Compare the server’s required mod list with the client’s list. Add missing both-sides mods, remove client copies of server-only utilities when the project says they are server-only, and make sure every required file uses a compatible release. A client-only visual feature should not be used to satisfy a server content requirement.
5. The game crashes only in singleplayer or LAN
Check whether the mod declares a client, server or both environment and whether it has separate entry points. Fabric’s documentation has an unresolved difference over how a server declaration relates to integrated servers, so use the project’s current notes for that specific version rather than assuming that “server” means every local world.
Forge and NeoForge: what developers and administrators should remember
NeoForge’s current sides documentation (version 26.1) presents physical side as the program launched and logical side as what a level or thread is actually running; its overview is at NeoForge’s sides documentation. Forge describes the same split. Neither platform’s modern mods.toml workflow gives you a simple sidedness switch that makes unsafe code safe.
For administrators, that means a mod advertised as “client-only” must still be kept off a dedicated server, even if the loader accepts the file long enough to begin startup. For developers, side checks must protect class loading as well as method execution: moving a client call behind a runtime condition is not enough if the server can still resolve the client-only class while loading common code.
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A practical decision checklist
- Does the mod add or change shared blocks, items, entities, dimensions, recipes or gameplay? Plan for both server and client.
- Does it only render information, add HUD elements, change controls or provide shaders? Treat it as client-only unless the author states otherwise.
- Does it manage backups, permissions, commands, ticking or server performance without new client-visible content? It may be server-only.
- Does the project page explicitly list an environment? Follow that declaration over a community assumption.
- Do Minecraft version, loader, dependencies and Java requirements all match?
- Is the installation a dedicated server, singleplayer world or LAN host? Integrated-server behavior can differ from a dedicated process.
- Have you backed up the world before changing the server’s mod set?
Bottom line
Install server-only mods on the server, client-only mods on the clients, and content or synchronized gameplay mods on both sides. Verify the environment label independently from the Minecraft version and loader, keep client classes out of dedicated-server code paths, and use the startup log to identify the first incompatible file. Forge and NeoForge rely heavily on safe side-aware code, while Fabric records environment and entry-point intent in fabric.mod.json; neither approach removes the need to follow the individual project’s instructions.
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