it works?

MiraAPI/Modifiers/ModifierData.cs

@@ -0,0 +1,21 @@
+using System;
+
+namespace MiraAPI.Modifiers;
+
+/// <summary>
+/// Modifier data for networking.
+/// </summary>
+/// <param name="type">The Type of the modifier.</param>
+/// <param name="args">Parameters for constructor.</param>
+public readonly struct ModifierData(Type type, object[] args)
+{
+    /// <summary>
+    /// Gets the type of the modifier.
+    /// </summary>
+    public Type Type { get; } = type;
+
+    /// <summary>
+    /// Gets the parameters for the constructor.
+    /// </summary>
+    public object[] Args { get; } = args;
+}

MiraAPI/Modifiers/ModifierExtensions.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using MiraAPI.Networking;
 using Reactor.Networking.Attributes;
+using Reactor.Networking.Rpc;
 using Reactor.Utilities;
 
 namespace MiraAPI.Modifiers;
@@ -104,7 +105,7 @@ public static class ModifierExtensions
     /// </summary>
     /// <param name="target">The player to add the modifier to.</param>
     /// <param name="modifierId">The modifier ID.</param>
-    [MethodRpc((uint)MiraRpc.AddModifier)]
+    /// <param name="args">The arguments to initialize the modifier constructor with.</param>
     public static void RpcAddModifier(this PlayerControl target, uint modifierId, params object[] args)
     {
         var type = ModifierManager.GetModifierType(modifierId);
@@ -114,8 +115,7 @@ public static class ModifierExtensions
             return;
         }
 
-        var modifier = ModifierFactory.CreateInstance(type, args);
-        target.GetModifierComponent()?.AddModifier(modifier);
+        Rpc<AddModifierRpc>.Instance.Send(target, new ModifierData(type, args));
     }
 
     /// <summary>
@@ -133,7 +133,6 @@ public static class ModifierExtensions
             return;
         }
 
-        var modifier = ModifierFactory<T>.CreateInstance(args);
-        player.GetModifierComponent()!.AddModifier(modifier);
+        player.RpcAddModifier(id.Value, args);
     }
 }

MiraAPI/Networking/AddModifierRpc.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using Hazel;
+using MiraAPI.Modifiers;
+using Reactor.Networking.Attributes;
+using Reactor.Networking.Rpc;
+using Reactor.Networking.Serialization;
+
+namespace MiraAPI.Networking;
+
+/// <summary>
+/// Remote procedure call for adding a modifier.
+/// </summary>
+/// <param name="plugin">Mira plugin.</param>
+/// <param name="id">RPC ID.</param>
+[RegisterCustomRpc((uint)MiraRpc.AddModifier)]
+public class AddModifierRpc(MiraApiPlugin plugin, uint id) : PlayerCustomRpc<MiraApiPlugin, ModifierData>(plugin, id)
+{
+    private static readonly Dictionary<Type, ParameterInfo[]> ParameterCache = [];
+
+    /// <inheritdoc />
+    public override RpcLocalHandling LocalHandling => RpcLocalHandling.Before;
+
+    /// <inheritdoc />
+    public override void Write(MessageWriter writer, ModifierData data)
+    {
+        var modId = ModifierManager.GetModifierId(data.Type);
+        writer.WritePacked((uint)modId!);
+        MessageSerializer.Serialize(writer, data.Args);
+    }
+
+    /// <inheritdoc />
+    public override ModifierData Read(MessageReader reader)
+    {
+        var modId = reader.ReadPackedUInt32();
+        var modifier = ModifierManager.GetModifierType(modId)!;
+
+        if (!ParameterCache.TryGetValue(modifier, out var paramTypes))
+        {
+            paramTypes = modifier.GetConstructors().OrderBy(x => x.GetParameters().Length).First().GetParameters();
+            ParameterCache[modifier] = paramTypes;
+        }
+
+        var objects = new object[paramTypes.Length];
+        foreach (var paramType in paramTypes)
+        {
+            objects[paramType.Position] = reader.Deserialize(paramType.ParameterType);
+        }
+
+        return new ModifierData(modifier, objects);
+    }
+
+    /// <inheritdoc />
+    public override void Handle(PlayerControl innerNetObject, ModifierData data)
+    {
+        var modifier = ModifierFactory.CreateInstance(data.Type, data.Args);
+        innerNetObject.GetModifierComponent()?.AddModifier(modifier);
+    }
+}