diff --git a/LobbyCompatibility/Enums/LobbyDiffResult.cs b/LobbyCompatibility/Enums/LobbyDiffResult.cs
index 1e6266c..342988e 100644
--- a/LobbyCompatibility/Enums/LobbyDiffResult.cs
+++ b/LobbyCompatibility/Enums/LobbyDiffResult.cs
@@ -17,7 +17,12 @@ public enum LobbyDiffResult
     Incompatible,
 
     /// <summary>
-    ///     Mod list does not exist, and lobby might be compatible.
+    ///     Mod list is compatible, except for unknown mods.
+    /// </summary>
+    PresumedCompatible,
+
+    /// <summary>
+    ///     Mod list information is not available, server does not have LobbyCompatibility installed, or other unknown cause.
     ///     Also applies to vanilla lobbies.
     /// </summary>
     Unknown
diff --git a/LobbyCompatibility/Features/LobbyHelper.cs b/LobbyCompatibility/Features/LobbyHelper.cs
index 91721fa..a594a42 100644
--- a/LobbyCompatibility/Features/LobbyHelper.cs
+++ b/LobbyCompatibility/Features/LobbyHelper.cs
@@ -13,12 +13,11 @@ namespace LobbyCompatibility.Features;
 /// </summary>
 internal static class LobbyHelper
 {
-    public static LobbyDiff LatestLobbyDiff { get; private set; } = new(new List<PluginDiff>());
     public static Dictionary<string, string> LatestLobbyRequestStringFilters = new();
     public static LobbyDistanceFilter? LatestLobbyRequestDistanceFilter;
-
-    private static Dictionary<ulong, LobbyDiff> _lobbyDiffCache { get; set; } = new();
     private static List<PluginInfoRecord>? _clientPlugins;
+    public static LobbyDiff LatestLobbyDiff { get; private set; } = new(new List<PluginDiff>());
+    private static Dictionary<ulong, LobbyDiff> LobbyDiffCache { get; } = new();
 
     /// <summary>
     ///     Get a <see cref="LobbyDiff" /> from a <see cref="Lobby" />.
@@ -35,7 +34,7 @@ internal static class LobbyHelper
     /// <returns> The <see cref="LobbyDiff" />. </returns>
     internal static LobbyDiff GetLobbyDiff(Lobby lobby, string? lobbyPluginString)
     {
-        if (_lobbyDiffCache.TryGetValue(lobby.Id, out var cachedLobbyDiff))
+        if (LobbyDiffCache.TryGetValue(lobby.Id, out var cachedLobbyDiff))
             return cachedLobbyDiff;
         
         var lobbyPlugins = PluginHelper
@@ -106,10 +105,12 @@ internal static LobbyDiff GetLobbyDiff(Lobby lobby, string? lobbyPluginString)
                     clientPlugin.Version, null));
         }
 
-        LatestLobbyDiff = new LobbyDiff(pluginDiffs);
+        var lobbyCompatibilityPresent = lobbyPlugins.Any();
+
+        LatestLobbyDiff = new LobbyDiff(pluginDiffs, lobbyCompatibilityPresent);
 
         // Add to cache to avoid making multiple unnecessary GetData() calls
-        _lobbyDiffCache.Add(lobby.Id, LatestLobbyDiff);
+        LobbyDiffCache.Add(lobby.Id, LatestLobbyDiff);
 
         return LatestLobbyDiff;
     }
@@ -151,17 +152,16 @@ public static string GetCompatibilityHeader(PluginDiffResult pluginDiffResult)
     /// <param name="filteredLobbies"> A custom list of lobbies, with special search filters applied. </param>
     /// <param name="currentFilter"> The <see cref="ModdedLobbyFilter" /> to filter the lobbies against. </param>
     /// <returns> A <see cref="Lobby" /> array with the <see cref="ModdedLobbyFilter" /> applied. </returns>
-    public static Lobby[] FilterLobbies(Lobby[] normalLobbies, Lobby[]? filteredLobbies, ModdedLobbyFilter currentFilter)
+    public static Lobby[] FilterLobbies(Lobby[] normalLobbies, Lobby[]? filteredLobbies,
+        ModdedLobbyFilter currentFilter)
     {
         List<Lobby> allLobbies = new();
 
         if (filteredLobbies != null)
-        {
             // Remove duplicate "normal" lobbies if they were also caught by the hashfilter
             normalLobbies = normalLobbies
                 .Where(lobby => !filteredLobbies.Any(check => lobby.Equals(check)))
                 .ToArray();
-        }
 
         if (currentFilter == ModdedLobbyFilter.VanillaAndUnknownOnly)
         {
@@ -173,11 +173,13 @@ public static Lobby[] FilterLobbies(Lobby[] normalLobbies, Lobby[]? filteredLobb
             // Add only lobbies that are vanilla/unknown
             allLobbies.AddRange(FilterLobbiesByDiffResult(normalLobbies, LobbyDiffResult.Unknown));
         }
-        else if (filteredLobbies != null && (currentFilter == ModdedLobbyFilter.CompatibleFirst || currentFilter == ModdedLobbyFilter.CompatibleOnly))
+        else if (filteredLobbies != null && currentFilter is ModdedLobbyFilter.CompatibleFirst or ModdedLobbyFilter.CompatibleOnly)
         {
             // Lobbies returned by the hashfilter are not 100% going to always be compatible, so we'll still need to filter them
-            var (compatibleFilteredLobbies, otherFilteredLobbies) = SplitLobbiesByDiffResult(filteredLobbies, LobbyDiffResult.Compatible);
-            var (compatibleNormalLobbies, otherNormalLobbies) = SplitLobbiesByDiffResult(normalLobbies, LobbyDiffResult.Compatible);
+            var (compatibleFilteredLobbies, otherFilteredLobbies) =
+                SplitLobbiesByDiffResult(filteredLobbies, LobbyDiffResult.Compatible);
+            var (compatibleNormalLobbies, otherNormalLobbies) =
+                SplitLobbiesByDiffResult(normalLobbies, LobbyDiffResult.Compatible);
 
             // Add filtered lobbies that are 100% compatible first, then any extra compatible lobbies not caught by the hashfilter
             allLobbies.AddRange(compatibleFilteredLobbies);
@@ -193,7 +195,7 @@ public static Lobby[] FilterLobbies(Lobby[] normalLobbies, Lobby[]? filteredLobb
         else if (filteredLobbies == null && currentFilter == ModdedLobbyFilter.CompatibleOnly)
         {
             // Handle the special case where we're sorting for compatible only and nothing comes up, so we need to force return nothing
-            allLobbies = new();
+            allLobbies = new List<Lobby>();
         }
         else
         {
@@ -205,12 +207,17 @@ public static Lobby[] FilterLobbies(Lobby[] normalLobbies, Lobby[]? filteredLobb
     }
 
     /// <summary>
-    ///     Splits a <see cref="Lobby" /> IEnumerable into two arrays based on of it matches <see cref="LobbyDiffResult" /> or not.
+    ///     Splits a <see cref="Lobby" /> IEnumerable into two arrays based on of it matches <see cref="LobbyDiffResult" /> or
+    ///     not.
     /// </summary>
     /// <param name="lobbies"> The lobbies. </param>
     /// <param name="filteredLobbyDiffResult"> The <see cref="LobbyDiffResult" /> to match against. </param>
-    /// <returns> A tuple containing two <see cref="Lobby"/> IEnumerables. matchedLobbies contains the lobbies that match the LobbyDiffResult, and unmatchedLobbies contains everything else. </returns>
-    private static (IEnumerable<Lobby> matchedLobbies, IEnumerable<Lobby> unmatchedLobbies) SplitLobbiesByDiffResult(IEnumerable<Lobby> lobbies, LobbyDiffResult filteredLobbyDiffResult)
+    /// <returns>
+    ///     A tuple containing two <see cref="Lobby" /> IEnumerables. matchedLobbies contains the lobbies that match the
+    ///     LobbyDiffResult, and unmatchedLobbies contains everything else.
+    /// </returns>
+    private static (IEnumerable<Lobby> matchedLobbies, IEnumerable<Lobby> unmatchedLobbies) SplitLobbiesByDiffResult(
+        IEnumerable<Lobby> lobbies, LobbyDiffResult filteredLobbyDiffResult)
     {
         List<Lobby> matchedLobbies = new();
         List<Lobby> unmatchedLobbies = new();
@@ -233,11 +240,14 @@ private static (IEnumerable<Lobby> matchedLobbies, IEnumerable<Lobby> unmatchedL
     /// <param name="lobbies"> The lobbies. </param>
     /// <param name="filteredLobbyDiffResult"> The <see cref="LobbyDiffResult" /> to match against. </param>
     /// <returns> A filtered <see cref="Lobby" /> IEnumerable. </returns>
-    private static IEnumerable<Lobby> FilterLobbiesByDiffResult(IEnumerable<Lobby> lobbies, LobbyDiffResult filteredLobbyDiffResult)
-        => SplitLobbiesByDiffResult(lobbies, filteredLobbyDiffResult).matchedLobbies;
+    private static IEnumerable<Lobby> FilterLobbiesByDiffResult(IEnumerable<Lobby> lobbies,
+        LobbyDiffResult filteredLobbyDiffResult)
+    {
+        return SplitLobbiesByDiffResult(lobbies, filteredLobbyDiffResult).matchedLobbies;
+    }
 
     /// <summary>
-    ///     Gets the error message for when no lobbies are found using a <see cref="ModdedLobbyFilter"/>.
+    ///     Gets the error message for when no lobbies are found using a <see cref="ModdedLobbyFilter" />.
     /// </summary>
     /// <param name="moddedLobbyFilter"> The <see cref="ModdedLobbyFilter" /> to get the error message for. </param>
     public static string GetEmptyLobbyListString(ModdedLobbyFilter moddedLobbyFilter)
@@ -249,4 +259,4 @@ public static string GetEmptyLobbyListString(ModdedLobbyFilter moddedLobbyFilter
             _ => "No available servers to join."
         };
     }
-}
+}
\ No newline at end of file
diff --git a/LobbyCompatibility/Features/PluginHelper.cs b/LobbyCompatibility/Features/PluginHelper.cs
index 4ba4812..ebbe0cc 100644
--- a/LobbyCompatibility/Features/PluginHelper.cs
+++ b/LobbyCompatibility/Features/PluginHelper.cs
@@ -198,12 +198,15 @@ internal static bool MatchesTargetRequirements(IEnumerable<PluginInfoRecord> tar
     }
 
     /// <summary>
-    ///     Checks if client is allowed to join vanilla lobbies.
+    ///     Checks if client is allowed to join vanilla or unknown lobbies.
     /// </summary>
-    /// <returns> True if client is allowed to join vanilla lobbies, false otherwise. </returns>
+    /// <returns> True if client is allowed to join vanilla or unknown lobbies, false otherwise. </returns>
+    /// <remarks> This means the client is allowed to have unknown or clientside mods. </remarks>
     internal static bool CanJoinVanillaLobbies()
     {
-        return GetAllPluginInfo().All(plugin => plugin.CompatibilityLevel is CompatibilityLevel.ClientOnly or null);
+        return GetAllPluginInfo().All(plugin =>
+            plugin.CompatibilityLevel != CompatibilityLevel.Everyone &&
+            plugin.CompatibilityLevel != CompatibilityLevel.ClientOptional);
     }
 
     /// <summary>
@@ -284,4 +287,4 @@ private static string GetRequiredPluginsChecksum()
 
         return _cachedChecksum = stringBuilder.ToString();
     }
-}
\ No newline at end of file
+}
diff --git a/LobbyCompatibility/Models/LobbyDiff.cs b/LobbyCompatibility/Models/LobbyDiff.cs
index 1639a19..310d14f 100644
--- a/LobbyCompatibility/Models/LobbyDiff.cs
+++ b/LobbyCompatibility/Models/LobbyDiff.cs
@@ -8,7 +8,8 @@ namespace LobbyCompatibility.Models;
 ///     Container for diffs between lobby and client plugins.
 /// </summary>
 /// <param name="PluginDiffs"> The diffs between the lobby and client plugins. </param>
-public record LobbyDiff(List<PluginDiff> PluginDiffs)
+/// <param name="LobbyCompatibilityPresent"> Whether or not the lobby has this mod installed. </param>
+public record LobbyDiff(List<PluginDiff> PluginDiffs, bool LobbyCompatibilityPresent = true)
 {
     private LobbyDiffResult? _cachedResult;
 
@@ -30,20 +31,18 @@ internal LobbyDiffResult GetModdedLobbyType()
         if (_cachedResult != null)
             return (LobbyDiffResult)_cachedResult;
 
-        if (PluginDiffs.Count == 0)
-        {
-            _cachedResult = LobbyDiffResult.Unknown;
-            return LobbyDiffResult.Unknown;
-        }
+        if (!LobbyCompatibilityPresent)
+            return (LobbyDiffResult)(_cachedResult = LobbyDiffResult.Unknown);
+
+        var unknownFound = PluginDiffs.Any(pluginDiff => pluginDiff.PluginDiffResult == PluginDiffResult.Unknown);
 
         if (PluginDiffs.Any(pluginDiff => pluginDiff.PluginDiffResult != PluginDiffResult.Compatible &&
                                           pluginDiff.PluginDiffResult != PluginDiffResult.Unknown))
-        {
-            _cachedResult = LobbyDiffResult.Incompatible;
-            return LobbyDiffResult.Incompatible;
-        }
+            return (LobbyDiffResult)(_cachedResult = LobbyDiffResult.Incompatible);
+
+        if (unknownFound)
+            return (LobbyDiffResult)(_cachedResult = LobbyDiffResult.PresumedCompatible);
 
-        _cachedResult = LobbyDiffResult.Compatible;
-        return LobbyDiffResult.Compatible;
+        return (LobbyDiffResult)(_cachedResult = LobbyDiffResult.Compatible);
     }
 }
\ No newline at end of file