Support AMCI in v18.0

AmongUs.props

@@ -1,7 +1,7 @@
 <Project>
     <ItemGroup>
         <PackageReference Include="BepInEx.Unity.IL2CPP" Version="6.0.0-be.735" Private="false" ExcludeAssets="runtime;native" />
-        <PackageReference Include="AmongUs.GameLibs.Steam" Version="2025.4.15" PrivateAssets="all" />
+        <PackageReference Include="AmongUs.GameLibs.Steam" Version="2026.8.18" PrivateAssets="all" />
 
         <PackageReference Include="BepInEx.AutoPlugin" Version="1.1.0" PrivateAssets="all" />
         <PackageReference Include="BepInEx.IL2CPP.MSBuild" Version="2.1.0-rc.1" PrivateAssets="all" ExcludeAssets="runtime" />

Reactor/Networking/AmciMods.cs

@@ -0,0 +1,139 @@
+#pragma warning disable CA5350
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Security.Cryptography;
+using BepInEx;
+using BepInEx.Unity.IL2CPP;
+using Reactor.Networking.Attributes;
+
+namespace Reactor.Networking;
+
+/// <summary>
+/// Tracks the AMCI mod registrations of the loaded plugins and drives the vanilla
+/// <see cref="CurrentModRegistration"/> integration.
+/// </summary>
+public static class AmciMods
+{
+    private static readonly Dictionary<string, Guid> _guidByModId = new();
+
+    /// <summary>
+    /// Gets the loaded mod ids and their registered AMCI GUIDs.
+    /// </summary>
+    public static IReadOnlyDictionary<string, Guid> Registered => _guidByModId;
+
+    /// <summary>
+    /// Gets the calculated active AMCI GUID (Single mod GUID or deterministic Composite GUID for multiple mods),
+    /// or null when no loaded mod registered one.
+    /// </summary>
+    public static Guid? Primary { get; private set; }
+
+    /// <summary>
+    /// Gets a value indicating whether AMCI is currently active: at least one mod GUID is registered.
+    /// </summary>
+    public static bool IsEnabled => Primary != null;
+
+    /// <summary>
+    /// Applies the current AMCI state to the vanilla client.
+    /// </summary>
+    public static void Apply()
+    {
+        var primary = Primary;
+        CurrentModRegistration.ModRegistrationGuidString = IsEnabled && primary != null ? primary.Value.ToString() : string.Empty;
+    }
+
+    internal static void Initialize()
+    {
+        foreach (var pluginInfo in IL2CPPChainloader.Instance.Plugins.Values)
+        {
+            if (pluginInfo.Instance != null)
+            {
+                Register(pluginInfo, (BasePlugin) pluginInfo.Instance);
+            }
+        }
+
+        IL2CPPChainloader.Instance.PluginLoad += (pluginInfo, _, plugin) => Register(pluginInfo, plugin);
+
+        IL2CPPChainloader.Instance.Finished += () =>
+        {
+            foreach (var pluginInfo in IL2CPPChainloader.Instance.Plugins.Values)
+            {
+                if (pluginInfo.Instance != null)
+                {
+                    Register(pluginInfo, (BasePlugin) pluginInfo.Instance);
+                }
+            }
+
+            RefreshPrimary();
+            Apply();
+            Info($"AMCI initialized: registered={_guidByModId.Count} activeGuid={Primary} CurrentModRegistration={CurrentModRegistration.ModRegistrationGuidString}");
+        };
+    }
+
+    private static void Register(PluginInfo pluginInfo, BasePlugin plugin)
+    {
+        if (plugin == null)
+        {
+            return;
+        }
+
+        var pluginType = plugin.GetType();
+        if (AmciIgnoreAttribute.IsIgnored(pluginType))
+        {
+            return;
+        }
+
+        var guid = AmciModGuidAttribute.GetGuid(pluginType);
+        if (guid.HasValue)
+        {
+            _guidByModId[pluginInfo.Metadata.GUID] = guid.Value;
+        }
+    }
+
+    private static void RefreshPrimary()
+    {
+        if (_guidByModId.Count == 0)
+        {
+            Primary = null;
+        }
+        else if (_guidByModId.Count == 1)
+        {
+            Primary = _guidByModId.Values.First();
+        }
+        else
+        {
+            Primary = ComputeCompositeGuid(_guidByModId.Values);
+        }
+    }
+
+    private static Guid ComputeCompositeGuid(IEnumerable<Guid> guids)
+    {
+        var sortedGuids = guids
+            .Distinct()
+            .OrderBy(g => g.ToString(), StringComparer.OrdinalIgnoreCase)
+            .ToArray();
+
+        if (sortedGuids.Length == 1)
+        {
+            return sortedGuids[0];
+        }
+
+        var buffer = new byte[sortedGuids.Length * 16];
+        for (var i = 0; i < sortedGuids.Length; i++)
+        {
+            var bytes = sortedGuids[i].ToByteArray();
+            Buffer.BlockCopy(bytes, 0, buffer, i * 16, 16);
+        }
+
+        var hash = SHA1.HashData(buffer);
+        var guidBytes = new byte[16];
+        Array.Copy(hash, 0, guidBytes, 0, 16);
+
+        // Conform to RFC 4122 version 4
+        guidBytes[6] = (byte) ((guidBytes[6] & 0x0F) | 0x40); // version 4
+        guidBytes[8] = (byte) ((guidBytes[8] & 0x3F) | 0x80); // variant RFC 4122
+
+        return new Guid(guidBytes);
+    }
+}

Reactor/Networking/Attributes/AmciIgnoreAttribute.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Linq;
+using System.Reflection;
+
+namespace Reactor.Networking.Attributes;
+
+/// <summary>
+/// Marks a plugin to be ignored during AMCI mod GUID registration and composite calculation.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class)]
+public sealed class AmciIgnoreAttribute : Attribute
+{
+    internal static bool IsIgnored(Type type)
+    {
+        if (type.GetCustomAttribute<AmciIgnoreAttribute>() != null)
+        {
+            return true;
+        }
+
+        var metadata = type.Assembly.GetCustomAttributes<AssemblyMetadataAttribute>().SingleOrDefault(x => x.Key == "Reactor.AmciIgnore");
+        if (metadata is { Value: not null } && bool.TryParse(metadata.Value, out var ignored))
+        {
+            return ignored;
+        }
+
+        return false;
+    }
+}

Reactor/Networking/Attributes/AmciModGuidAttribute.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Linq;
+using System.Reflection;
+
+namespace Reactor.Networking.Attributes;
+
+/// <summary>
+/// Registers the annotated plugin with the official AMCI  protocol.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class)]
+public sealed class AmciModGuidAttribute : Attribute
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="AmciModGuidAttribute"/> class.
+    /// </summary>
+    /// <param name="guid">The self-assigned v4 mod GUID of the mod.</param>
+    public AmciModGuidAttribute(string guid)
+    {
+        Guid = Guid.Parse(guid);
+    }
+
+    public Guid Guid { get; }
+
+    internal static Guid? GetGuid(Type type)
+    {
+        var attribute = type.GetCustomAttribute<AmciModGuidAttribute>();
+        if (attribute != null)
+        {
+            return attribute.Guid;
+        }
+
+        var metadataAttribute = type.Assembly.GetCustomAttributes<AssemblyMetadataAttribute>().SingleOrDefault(x => x.Key == "Reactor.AmciModGuid");
+        if (metadataAttribute is { Value: not null } && Guid.TryParse(metadataAttribute.Value, out var guid))
+        {
+            return guid;
+        }
+
+        return null;
+    }
+}

Reactor/Networking/Patches/HttpPatches.cs

@@ -92,7 +92,7 @@ public static void Postfix(UnityWebRequest __instance, UnityWebRequestAsyncOpera
 
                     if (__instance.GetMethod() == UnityWebRequest.UnityWebRequestMethod.Get)
                     {
-                        if (responseHeader == null && ModList.IsAnyModRequiredOnAllClients)
+                        if (responseHeader == null && !AmciMods.IsEnabled && ModList.IsAnyModRequiredOnAllClients)
                         {
                             HandshakePopup.Show();
                         }
@@ -110,6 +110,7 @@ private static class GameStartManagerPatch
         public static void Postfix(GameStartManager __instance)
         {
             if (AmongUsClient.Instance.NetworkMode != NetworkModes.OnlineGame) return;
+            if (AmciMods.IsEnabled) return;
             if (ModList.IsAnyModRequiredOnAllClients && !IsCurrentRegionModded())
             {
                 Warning("Vanilla region, locking public toggle");

Reactor/Networking/Patches/ModRegistrationPatch.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using AmongUs.GameOptions;
+using AmongUs.Matchmaking;
+using HarmonyLib;
+using InnerNet;
+
+namespace Reactor.Networking.Patches
+{
+    internal class ModRegistrationPatch
+    {
+        [HarmonyPatch(typeof(CurrentModRegistration), nameof(CurrentModRegistration.UpdateFilterSetWithModRegistrationSettings))]
+        public static class CurrentModRegistrationPatch
+        {
+            public static void Prefix(GameFilterSet filterSet)
+            {
+                for (int i = filterSet.Filters.Count - 1; i >= 0; i--)
+                {
+                    if (filterSet.Filters[i].Key == "mod")
+                    {
+                        filterSet.Filters.RemoveAt(i);
+                    }
+                }
+            }
+        }
+
+        [HarmonyPatch(typeof(InnerNetClient), nameof(InnerNetClient.HostGame), typeof(IGameOptions), typeof(GameFilterOptions))]
+        public static class LocalGamePatch
+        {
+            private static string _savedGuid;
+
+            public static void Prefix()
+            {
+                if (AmongUsClient.Instance != null
+                    && AmongUsClient.Instance.NetworkMode != NetworkModes.OnlineGame)
+                {
+                    _savedGuid = CurrentModRegistration.ModRegistrationGuidString;
+                    CurrentModRegistration.ModRegistrationGuidString = "";
+                }
+            }
+
+            public static void Postfix()
+            {
+                if (_savedGuid != null)
+                {
+                    CurrentModRegistration.ModRegistrationGuidString = _savedGuid;
+                    _savedGuid = null;
+                }
+            }
+        }
+    }
+}

Reactor/ReactorPlugin.cs

@@ -47,6 +47,7 @@ public ReactorPlugin()
 
         RegisterInIl2CppAttribute.Initialize();
         ModList.Initialize();
+        AmciMods.Initialize();
 
         RegisterCustomRpcAttribute.Initialize();
         MessageConverterAttribute.Initialize();

Reactor/packages.lock.json

@@ -4,9 +4,9 @@
     "net6.0": {
       "AmongUs.GameLibs.Steam": {
         "type": "Direct",
-        "requested": "[2025.4.15, )",
-        "resolved": "2025.4.15",
-        "contentHash": "R3RxzHGZOGUJ9nMwrP0cjS6g+sh60QUU506Mo47lh17155enUjH6PtErijihSoU7t3aQpzCIyC67W3XrrHhH4g=="
+        "requested": "[2026.8.18, )",
+        "resolved": "2026.8.18",
+        "contentHash": "sSGu2zGYvLboOc83PzPxQ0yGsK1YrhIO/538Tfzc/5FKfEbGCtCQErcKV0wdiYZCHGpCe561RzzfrBtp46jcgQ=="
       },
       "BepInEx.AutoPlugin": {
         "type": "Direct",