Setup

Download Plugin SDK

The SDK is available on the Overview page.

Installation

SDK Setup

1. (Windows only) Unblock FusionPluginSDK-x.x.x.zip by right-click > Properties > Unblock before unzipping it.
2. Unzip the archive into a folder inside or next to the Fusion Unity project

MyFusionProject
├─Assets
├─Library
├─..
└─PluginSDK
    

MyFusionProject
├─Assets
├─Library
└─..
PluginSDK
    

3. Place your .license file in Fusion.Plugin.Custom/Config/. Without it, the server will log a license error and refuse to start games.

Unity Setup

1. Inside your Unity project, create a FusionPluginProjectSettings asset via right-click > Create > Fusion > Plugin Project Settings.
2. Set the Plugin SDK Path field to the directory you extracted the SDK into in the previous step.

3. Press Export. It should complete without errors.
4. Press Build and Run. This builds your custom plugin and starts the local PhotonServer with your plugin loaded in a new terminal window.
5. To get your game to connect to the local server, open PhotonAppSettings and set the server IP address to 127.0.0.1.

6. Run your game and connect. You should see a log line's output in the terminal as the player connects.

IDE Setup

The Build and Run button is convenient but obscures the underlying C# solution. To work on the plugin code directly, open the solution file at PluginSDK/Fusion.Plugin.Custom.sln — either by opening it from your file explorer, or via the Open Plugin Project button in the FusionPluginProjectSettings inspector.

Once the solution is open in your IDE, you can launch the server and plugin straight from the IDE's run/debug button — there's no need to come back to Unity and press Build and Run. This is also the path for attaching a debugger, setting breakpoints, and inspecting plugin state at runtime.

Have a go at extending CustomServer, this is done by placing a new partial in the Fusion.Plugin.Custom assembly, we recommend naming it CustomServer.User.cs

C#

namespace Fusion.Plugin;

partial class CustomServer {
  public override void OnPlayerJoined(NetworkRunner runner, PlayerRef player) {
    base.OnPlayerJoined(runner, player);

    Log.Info($"Adding player {player}");

  }
}

Run your server from the IDE, or from the button in Unity and reconnect. You should now see the following log line when the player connects

15:01:58.317 [Plugin.Custom] [8 ] INFO  - adding player [Player:1]

SDK structure

Inside the extracted SDK you'll find:

Next steps

You now have a custom plugin running and connected. To extend it with game-specific behaviour — overriding server-side callbacks, intercepting RPCs, making properties plugin-authoritative, baking custom data in from Unity — see Features.

Last updated on 21/07/2026