Fabric API

Fabric API is the foundation for Fabric mods. It doesn't show anything in-game by itself, and players won't notice anything from it — but without it, most other mods don't work. It's a set of tools that mod authors use so they don't have to write everything from scratch again.

Practical consequence: when a player downloads a mod and it reports after starting that it needs Fabric API (or the game simply crashes), the solution is almost always to install Fabric API.

Fabric API belongs on both sides depending on what you use. A client-side mod (for example Sodium) usually doesn't need Fabric API, but many mods do — and those then require Fabric API on the player's side too. A server-side mod that needs Fabric API needs it on the server. If you're not sure, install it on both sides; you won't break anything.

The version must match the game version exactly. Fabric API is released separately for each Minecraft version, and the wrong version is the most common reason a modded server won't start or a player's game crashes when connecting.

For a server, it's also a rule for players: if the server has its own mods with Fabric API, players must install the same version, otherwise they won't be able to connect. It's worth writing this on the website and on Discord — it's the most common question on modded servers.

1. Decide where you need Fabric API: on the server, for players, or on both sides (most often both).
2. Download Fabric API from modrinth.com/mod/fabric-api in the version exactly for your game version.
3. On the server: stop the server, upload the .jar to the mods folder, and start it.
4. For players: the file belongs in the mods folder in their game (on Windows %appdata%\.minecraft\mods).
5. Players must have the same game version and Fabric API as the server — otherwise they will not be able to connect.
6. Check the server console: Fabric API must load without errors and without an incompatible version message.
7. Try connecting from a client with the same version — it must go through without an error during connection.
8. If the server reports a missing dependency, install the mod that requires it as well (Fabric API itself does not add anything).
9. Tell players on the website and on Discord which mods they need and in which version — it will save half the questions.
10. When updating the game, replace Fabric API for everyone who has it: server and players. Version mismatch is the most common cause of connection problems.
11. Update Fabric API together with the game and other mods — doing it one by one means more troubleshooting.

Fabric API does nothing on its own — it is the foundation for other mods. If you install it and nothing changes, that is correct.
The most common mistake on modded servers is a version mismatch: the server has one game version and the player another. Tell players exactly what they should have.
Install it on both sides when you are not sure. An extra presence will not hurt anything, a missing dependency will bring the mod down.
The Fabric API version must match the game version. After updating Minecraft, updating only the game is not enough — the mod must have a new version too.
If a mod reports a missing dependency after installation, look for the name of the missing mod in the error. The console will state it precisely, including the version it expects.
Fabric API is only for Fabric. Do not install it on NeoForge or Forge — they have their own system and the mod would not load.
Update mods together. If you update one and not the other, you create a combination the author did not test.