diff --git a/src/Core/Log.cs b/src/Core/Log.cs
index 310e23f..01bb7a8 100644
--- a/src/Core/Log.cs
+++ b/src/Core/Log.cs
@@ -1,3 +1,4 @@
+using System;
 using UnityModManagerNet;
 
 namespace S3.Core;
@@ -8,11 +9,56 @@ namespace S3.Core;
 /// </summary>
 public static class Log
 {
+    private const int RingN = 200;
+    private static readonly object RingLock = new();
+    private static readonly string[] Ring = new string[RingN];
+    private static int _ringCount;
+
     private static UnityModManager.ModEntry.ModLogger? _logger;
 
     public static void Init(UnityModManager.ModEntry modEntry) => _logger = modEntry.Logger;
 
-    public static void Info(string msg)  => _logger?.Log(msg);
-    public static void Warn(string msg)  => _logger?.Warning(msg);
-    public static void Error(string msg) => _logger?.Error(msg);
+    public static void Info(string msg)
+    {
+        Push("INF", msg);
+        _logger?.Log(msg);
+    }
+
+    public static void Warn(string msg)
+    {
+        Push("WRN", msg);
+        _logger?.Warning(msg);
+    }
+
+    public static void Error(string msg)
+    {
+        Push("ERR", msg);
+        _logger?.Error(msg);
+    }
+
+    public static string[] Tail(int count)
+    {
+        if (count < 1) count = 1;
+        if (count > RingN) count = RingN;
+        lock (RingLock)
+        {
+            int have = Math.Min(_ringCount, RingN);
+            int take = Math.Min(count, have);
+            var result = new string[take];
+            int start = _ringCount - take;
+            for (int i = 0; i < take; i++)
+                result[i] = Ring[(start + i) % RingN];
+            return result;
+        }
+    }
+
+    static void Push(string level, string msg)
+    {
+        string line = DateTime.Now.ToString("HH:mm:ss") + " [" + level + "] " + msg;
+        lock (RingLock)
+        {
+            Ring[_ringCount % RingN] = line;
+            _ringCount++;
+        }
+    }
 }
diff --git a/src/Core/ModuleRegistry.cs b/src/Core/ModuleRegistry.cs
index 9133aca..1c34e72 100644
--- a/src/Core/ModuleRegistry.cs
+++ b/src/Core/ModuleRegistry.cs
@@ -10,6 +10,7 @@ namespace S3.Core;
 public sealed class ModuleRegistry
 {
     private readonly List<IModule> _modules = new();
+    private readonly HashSet<IModule> _active = new();
 
     public IReadOnlyList<IModule> Modules => _modules;
 
@@ -29,6 +30,7 @@ public sealed class ModuleRegistry
             try
             {
                 m.OnEnable();
+                _active.Add(m);
                 Log.Info($"[{m.Id}] enabled.");
             }
             catch (Exception e)
@@ -38,6 +40,66 @@ public sealed class ModuleRegistry
         }
     }
 
+    public bool IsActive(IModule module) => _active.Contains(module);
+
+    public bool IsActive(string id)
+    {
+        IModule? module = Find(id);
+        return module != null && _active.Contains(module);
+    }
+
+    public IModule? Find(string id)
+    {
+        foreach (IModule module in _modules)
+            if (string.Equals(module.Id, id, StringComparison.OrdinalIgnoreCase))
+                return module;
+        return null;
+    }
+
+    /// <summary>
+    /// Changes a module's live state without changing its persisted Enabled setting.
+    /// Callers that want the change persisted must update Enabled and SaveSettings.
+    /// </summary>
+    public bool TrySetActive(string id, bool active, out string message)
+    {
+        IModule? module = Find(id);
+        if (module == null)
+        {
+            message = "unknown module: " + id;
+            return false;
+        }
+
+        bool current = _active.Contains(module);
+        if (current == active)
+        {
+            message = $"{module.Id} active={active} (unchanged)";
+            return true;
+        }
+
+        try
+        {
+            if (active)
+            {
+                module.OnEnable();
+                _active.Add(module);
+            }
+            else
+            {
+                module.OnDisable();
+                _active.Remove(module);
+            }
+            message = $"{module.Id} active={active}";
+            Log.Info($"[{module.Id}] live active={active}.");
+            return true;
+        }
+        catch (Exception e)
+        {
+            message = $"{module.Id} active={current}; transition failed: {e.Message}";
+            Log.Error($"[{module.Id}] live transition to active={active} failed: {e}");
+            return false;
+        }
+    }
+
     /// <summary>Persist every module's settings (called from UMM's OnSaveGUI).</summary>
     public void SaveAll()
     {
diff --git a/src/Main.cs b/src/Main.cs
index faef95c..127fa84 100644
--- a/src/Main.cs
+++ b/src/Main.cs
@@ -16,6 +16,7 @@ namespace S3;
 public static class Main
 {
     internal static UnityModManager.ModEntry ModEntry { get; private set; } = null!;
+    public static ModuleRegistry Registry { get; private set; } = null!;
 
     private static ModuleRegistry _registry = null!;
 
@@ -26,6 +27,7 @@ public static class Main
         SettingsStore.Init(modEntry.Path);
 
         _registry = new ModuleRegistry();
+        Registry = _registry;
         // Modules are registered here. Each module loads its own settings in its
         // constructor. Order here is display order in the settings panel.
         _registry.Register(new Modules.PhysicsOptimizer.PhysicsOptimizerModule());