Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsShort answer: a server-side mod is installed on the server, a client-side mod is installed on each player’s game, and a content mod that adds blocks, items or gameplay normally belongs on both. The mod’s project page must identify its environment, and the Minecraft version and mod loader must match exactly.
The three installation categories
“Server-side” and “client-side” describe where a mod is allowed to run, not whether it uses Forge, NeoForge or Fabric. Environment, loader and Minecraft version are separate compatibility checks. A mod can be marked for one side, both sides, or—if its metadata is wrong—fail before the game starts.
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| Mod category | Where it is installed | Typical purpose | What other players need |
|---|---|---|---|
| Server-only | Dedicated server | Server utility, administration or performance work that does not change the client’s visuals | Nothing, provided the mod truly has no client-side requirement |
| Client-only | Every player’s game client that wants the feature | Visual or interface features such as shaders and minimaps | The server does not need the mod |
| Both sides | Server and every connecting client | Content or gameplay additions, including new blocks and items | All clients must use the compatible mod and version |
This three-way distinction is summarized in the installation guidance from UltraServers. The practical rule is simple: if the mod changes what the server must know about the world, install it on the server and clients; if it only changes how a player sees or controls the game, keep it on the client; if it performs work exclusively for the server, keep it there.
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What “side” means in Minecraft
Physical sides
The physical side is the program that is actually running. A dedicated server is one physical program, while the Minecraft launcher starts a client program. A player’s single-player game combines a client with an integrated server inside the same application.
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Logical sides
The logical side is what a level or thread is doing at that moment: handling game-world authority (logical server) or presenting and controlling the player view (logical client). NeoForge documents this physical/logical split in its current sides documentation at docs.neoforged.net/docs/concepts/sides. Forge describes the same model and notes that checking a Level object establishes which logical side that level belongs to; see Forge’s Sides page.
The distinction explains why a single-player test can be misleading. Both logical sides exist inside one physical client process, so code that is unsafe on a dedicated server may appear to work in single-player. Always test a mod intended for multiplayer on an actual dedicated server as well as in a local world.
Why the distinction matters to mod authors and players
Client classes contain rendering, keyboard, screen and other names that do not exist in a dedicated-server process. Forge warns: “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 warning appears on a Forge documentation page that is several years old, so check the current loader documentation for implementation changes, but the failure mode remains a useful diagnostic principle.
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How Forge, NeoForge and Fabric express environment
Forge and NeoForge
Forge removed the old sidedness setting from mods.toml. Mods are expected to be written safely for the physical client and the physical server rather than relying on a declaration that prevents all unsuitable code from loading. NeoForge’s sides documentation likewise focuses on physical and logical execution. In practice, read the mod’s project page and release notes for its supported environment, then verify that its loader and Minecraft version match your installation.
For these loaders, the most dangerous mistake is not merely putting a file in the wrong folder. A common class or entrypoint can refer to client-only names before a side check runs. On a dedicated server that reference can trigger a class-loading crash. A server-only utility should avoid client classes entirely; a mod that adds gameplay must keep its shared code safe on both sides.
Fabric metadata
Fabric declares an environment in fabric.mod.json. The environment value can be * (all environments, and the default), client, or server; it may be a string or an array. The specification is documented on the Fabric Wiki.
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- A
clientmod is not loaded on a dedicated server. - A
servermod is not loaded on clients, including the client used for single-player or LAN. *means the mod can load in all environments and is the normal declaration for code intended for both.
Fabric also separates entrypoints. main runs on both sides, while client and server entrypoints run only on their matching side. Mixin entries can carry their own environment key. The current entrypoint guidance is in Fabric’s fabric.mod.json documentation.
There is an unresolved documentation inconsistency around integrated servers. The Fabric specification says the server environment excludes integrated servers, while the Fabric guide says server-only mods do not load in single-player or LAN. Treat that behavior as version- and implementation-dependent rather than settled; confirm the specific mod’s documentation and test the exact loader build you use.
A reliable decision process before installing a mod
- Open the mod’s project page. Look for an explicit client, server, or both-sides environment label. Do not infer the label solely from the mod’s name or screenshots.
- Confirm the Minecraft version. A file built for one game version may fail or behave incorrectly on another, even when the feature appears unchanged.
- Confirm the loader. Forge, NeoForge and Fabric are different loading systems. Environment is not a substitute for a loader match.
- Read dependency and installation notes. A client visual mod can still require a library on the client; a gameplay mod can require libraries on both server and clients.
- Classify the feature by authority. If it creates or changes world data, recipes, entities, blocks, items or rules, plan for both sides. If it only draws information or changes controls, plan for clients. If it manages the server without changing client presentation, plan for the server.
- Install one change at a time. Keep a known-good copy of the server and client mod lists so a new crash can be tied to one file.
- Test the intended topology. Test client-only additions in a multiplayer connection, server-only additions on the dedicated server, and both-sides content with a clean client and server pair.
Installing each type without creating a mismatch
Server-only mods
Install the loader-compatible file in the dedicated server’s mod installation, restart the server, and leave it out of clients unless the project page explicitly lists a client component. Players should connect with an otherwise compatible client. If the server starts but the feature is absent, check that the file is in the active server instance and that the server is running the loader and Minecraft version named by the mod.
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Client-only mods
Install the file in each player’s client instance. Do not copy it to the dedicated server merely to keep mod lists identical. Visual effects, HUD changes and minimaps are typical examples. A player without the mod generally sees the normal game; a player with it sees the additional interface or rendering feature.
Both-sides content and gameplay mods
Install the same compatible release on the server and on every client that joins. Keep the loader, Minecraft version and required dependencies aligned. If only one side has the mod, the connection can fail, the client can reject registry data, or the server can crash while loading content. Updating the server first without distributing the matching client files is a frequent source of “works for the host, fails for everyone else” incidents.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why a server crashes after a client mod is added
The usual cause is that a client-only class is referenced from code the dedicated server loads. Rendering and screen classes are not present in that process, so the JVM can fail during class loading before the mod has an opportunity to check the side. A troubleshooting report from mcdoctor.ai likewise describes a dedicated server crashing when a Fabric client mod is installed there.
Use the first meaningful error in the log, not the final “server stopped” line. A stack trace naming rendering, GUI, key-binding or other client-only packages points toward a client mod on the server or a shared entrypoint that imports client classes. Remove the suspected file, restart, and then verify its environment declaration and project-page instructions before trying another build.
Diagnosing the most common symptoms
The server crashes immediately after adding a mod
- Remove the newest file and restart to confirm causality.
- Check that the file is built for the server’s loader and Minecraft version.
- If the trace names client-only classes, keep that mod on clients only or obtain a server-safe release.
- Check dependencies; a missing library can produce a failure that looks unrelated to sidedness.
The server starts, but the feature does nothing
- Verify that a server-only mod is in the active server instance rather than a different test directory.
- For Fabric, inspect the
environmentand entrypoint values; acliententrypoint will not run on the server. - For a both-sides mod, confirm that the server actually loaded the file and that its required data or configuration was accepted.
Players cannot join after a content mod was installed
- Compare the exact mod filename or release on the server and each client.
- Compare Minecraft and loader versions, not just the mod’s display name.
- Install every listed dependency on the side where its documentation requires it.
- Remove extra client or server files temporarily to isolate an environment mismatch.
It works in single-player but fails on a dedicated server
Single-player contains an integrated server inside the client process, so it does not reproduce the isolation of a dedicated server. Re-test with a dedicated instance. For Fabric’s server environment, consult the specific version’s documentation because the specification and guide differ on integrated-server loading.
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| Question | What to verify | Why it matters |
|---|---|---|
| Where may it run? | Client, server or all environments in the project metadata | Determines whether the file should be present on that physical side |
| Which loading system? | Forge, NeoForge or Fabric | A mod built for one loader is not automatically a mod for another |
| Which game build? | The exact Minecraft version listed by the release | Registry, mappings and APIs can change between versions |
| Which entrypoint or code path? | Fabric entrypoints, mixin environment keys, or loader-specific setup | A correct top-level label cannot rescue an unsafe side-specific class reference |
Do not treat a “client” label as permission to install the file anywhere a client exists. A client-only mod belongs on a client, not on the dedicated server. Conversely, a server utility marked server-only should not be copied to every player’s game just because the filenames are easy to synchronize.
A compact compatibility matrix
| Scenario | Server installation | Client installation | Expected result |
|---|---|---|---|
| Server utility or performance mod | Yes | Usually no | Server behavior changes; clients connect without the utility |
| Shader, minimap or interface enhancement | No | Yes, for players who want it | Only the installing player sees the visual or UI change |
| New block, item or gameplay system | Yes | Yes, every connecting player | World and registry data remain consistent |
Fabric file with environment: client |
No | Yes | Fabric excludes it from a dedicated server |
Fabric file with environment: server |
Yes, where supported | No | Client loading is excluded; integrated-server behavior requires version-specific confirmation |
| Forge or NeoForge mod with unsafe shared client references | Not safe on a dedicated server | Possibly | Can crash when the server loads the referenced class |
Best practices for a stable modded server
- Keep separate, documented server and client mod manifests instead of assuming every file belongs in both.
- Record the Minecraft version, loader version and mod release together whenever you update.
- Back up the world before installing content that adds blocks, items or other persistent data.
- Change one variable at a time and retain the previous working files for rollback.
- Read the project page for environment notes even when the mod’s feature seems obvious.
- When reporting a crash, include the loader, Minecraft version, side that crashed and the first relevant stack-trace cause.
The safest mental model is to ask two questions separately: “Which program must run this code?” and “Which loader and game version can load this build?” Answering both prevents most side-related failures.
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