diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3b5386e..ba314e5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ All notable public changes to Nexus Unity are documented here.
 - Consolidated duplicate internal Ollama-review and serialized-property write helpers.
 
 ### Fixed
+- Persist authentication tokens across Unity Editor process restarts in Library token file, preventing persistent HTTP 401 Unauthorized errors for external MCP CLI integrations after restarting Unity (#166).
 - `get_scene_dependencies` now uses `enterChildren = false` after the initial property iteration step, preventing deep recursive traversal into child properties and eliminating duplicated dependency references (#71).
 - `find_objects` now safely constructs name search regexes with a 100ms match timeout and falls back gracefully to literal substring search on invalid regex patterns or match timeouts (#119).
 - Restrict type resolution (`FindType`) to user project assemblies and standard public `UnityEngine` assemblies, disallow internal system/editor assemblies and namespaces, and enforce strict type allowlist constraints for component and ScriptableObject creation/inspection tools (#139).
diff --git a/Editor/MCPServer.Identity.cs b/Editor/MCPServer.Identity.cs
index 14949a0..f2a09c2 100644
--- a/Editor/MCPServer.Identity.cs
+++ b/Editor/MCPServer.Identity.cs
@@ -45,29 +45,68 @@ private static string ReadPackageVersion()
 
         internal static string AuthToken => EnsureAuthToken();
 
+        private static readonly object AuthTokenLock = new object();
+
         private static string EnsureAuthToken()
         {
             if (!string.IsNullOrEmpty(_authToken)) return _authToken;
 
-            if (_mainThreadId != -1 && Thread.CurrentThread.ManagedThreadId != _mainThreadId)
+            lock (AuthTokenLock)
             {
-                return _authToken;
-            }
+                if (!string.IsNullOrEmpty(_authToken)) return _authToken;
+
+                bool isMainThread = _mainThreadId != -1 && Thread.CurrentThread.ManagedThreadId == _mainThreadId;
+
+                if (isMainThread)
+                {
+                    try
+                    {
+                        _authToken = SessionState.GetString(AuthSessionStateKey, string.Empty);
+                    }
+                    catch { }
+                }
+
+                if (string.IsNullOrEmpty(_authToken))
+                {
+                    _authToken = ReadTokenFile();
+                }
 
-            try
-            {
-                _authToken = SessionState.GetString(AuthSessionStateKey, string.Empty);
                 if (string.IsNullOrEmpty(_authToken))
                 {
                     _authToken = Guid.NewGuid().ToString("N") + Guid.NewGuid().ToString("N");
-                    SessionState.SetString(AuthSessionStateKey, _authToken);
+                }
+
+                if (isMainThread)
+                {
+                    try
+                    {
+                        SessionState.SetString(AuthSessionStateKey, _authToken);
+                    }
+                    catch { }
                 }
 
                 WriteTokenFile(_authToken);
+                return _authToken;
             }
-            catch { }
+        }
 
-            return _authToken;
+        internal static string RotateAuthToken()
+        {
+            lock (AuthTokenLock)
+            {
+                _authToken = Guid.NewGuid().ToString("N") + Guid.NewGuid().ToString("N");
+                bool isMainThread = _mainThreadId != -1 && Thread.CurrentThread.ManagedThreadId == _mainThreadId;
+                if (isMainThread)
+                {
+                    try
+                    {
+                        SessionState.SetString(AuthSessionStateKey, _authToken);
+                    }
+                    catch { }
+                }
+                WriteTokenFile(_authToken);
+                return _authToken;
+            }
         }
 
         private static void WriteTokenFile(string token)
@@ -76,10 +115,11 @@ private static void WriteTokenFile(string token)
             {
                 string projectRoot = Path.GetFullPath(Path.Combine(UnityEngine.Application.dataPath, ".."));
                 string libraryDir = Path.Combine(projectRoot, "Library");
-                if (Directory.Exists(libraryDir))
+                if (!Directory.Exists(libraryDir))
                 {
-                    File.WriteAllText(Path.Combine(libraryDir, "NexusUnityAuthToken.txt"), token);
+                    Directory.CreateDirectory(libraryDir);
                 }
+                File.WriteAllText(Path.Combine(libraryDir, "NexusUnityAuthToken.txt"), token);
             }
             catch { }
         }
diff --git a/Tests~/Editor/ServerPortTests.cs b/Tests~/Editor/ServerPortTests.cs
index 3de66a5..3e44ef8 100644
--- a/Tests~/Editor/ServerPortTests.cs
+++ b/Tests~/Editor/ServerPortTests.cs
@@ -58,6 +58,23 @@ public void AuthToken_IsRequiredForNetworkRequests()
             Assert.IsTrue((bool)InvokePrivateMethod("IsAuthorizedToken", token));
         }
 
+        [Test]
+        public void AuthToken_PersistsAcrossSessionStateReset()
+        {
+            string initialToken = MCPServer.AuthToken;
+            Assert.IsFalse(string.IsNullOrEmpty(initialToken), "Auth token should be non-empty");
+
+            var authTokenField = typeof(MCPServer).GetField("_authToken", BindingFlags.NonPublic | BindingFlags.Static);
+            authTokenField?.SetValue(null, null);
+
+            var sessionKeyField = typeof(MCPServer).GetField("AuthSessionStateKey", BindingFlags.NonPublic | BindingFlags.Static);
+            string sessionKey = (string)sessionKeyField?.GetValue(null) ?? "NexusUnity_AuthToken";
+            SessionState.SetString(sessionKey, string.Empty);
+
+            string restoredToken = MCPServer.AuthToken;
+            Assert.AreEqual(initialToken, restoredToken, "AuthToken should persist across SessionState resets");
+        }
+
         [Test]
         public void GetPortOwner_IdentifiesProcess()
         {