diff --git a/AIUsageTracker.Tests/UI/WindowsStartupServiceTests.cs b/AIUsageTracker.Tests/UI/WindowsStartupServiceTests.cs
new file mode 100644
index 00000000..1dcb98bd
--- /dev/null
+++ b/AIUsageTracker.Tests/UI/WindowsStartupServiceTests.cs
@@ -0,0 +1,113 @@
+// <copyright file="WindowsStartupServiceTests.cs" company="AIUsageTracker">
+// Copyright (c) AIUsageTracker. All rights reserved.
+// </copyright>
+
+using AIUsageTracker.UI.Slim;
+using Microsoft.Win32;
+
+namespace AIUsageTracker.Tests.UI;
+
+public sealed class WindowsStartupServiceTests : IDisposable
+{
+    private const string ValueName = "AI Usage Tracker";
+    private readonly string _registryPath = @"Software\AIUsageTracker.Tests\Startup\" + Guid.NewGuid().ToString("N");
+    private readonly string _directory = Path.Combine(Path.GetTempPath(), "AIUsageTracker-startup-tests", Guid.NewGuid().ToString("N"));
+    private readonly string _variableName = "AI_TRACKER_STARTUP_TEST_" + Guid.NewGuid().ToString("N");
+    private readonly RegistryKey _key;
+    private readonly string _exePath;
+    private readonly string _portableCommand;
+
+    public WindowsStartupServiceTests()
+    {
+        Directory.CreateDirectory(this._directory);
+        this._exePath = Path.Combine(this._directory, "AIUsageTracker.exe");
+        File.WriteAllText(this._exePath, string.Empty);
+        Environment.SetEnvironmentVariable(this._variableName, this._directory);
+        this._portableCommand = $"\"%{this._variableName}%\\AIUsageTracker.exe\"";
+        this._key = Registry.CurrentUser.CreateSubKey(this._registryPath);
+    }
+
+    public void Dispose()
+    {
+        this._key.Dispose();
+        Registry.CurrentUser.DeleteSubKeyTree(this._registryPath, throwOnMissingSubKey: false);
+        Environment.SetEnvironmentVariable(this._variableName, null);
+        Directory.Delete(this._directory, recursive: true);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void Apply_Enable_PreservesMatchingExpandableCommand(bool changeCase)
+    {
+        var command = changeCase ? this._portableCommand.ToUpperInvariant() : this._portableCommand;
+        this._key.SetValue(ValueName, command, RegistryValueKind.ExpandString);
+
+        WindowsStartupService.Apply(true, this._key, this._exePath);
+
+        Assert.Equal(command, this._key.GetValue(ValueName, null, RegistryValueOptions.DoNotExpandEnvironmentNames));
+        Assert.Equal(RegistryValueKind.ExpandString, this._key.GetValueKind(ValueName));
+    }
+
+    [Theory]
+    [InlineData("\"%MISSING_AI_TRACKER_STARTUP_ROOT%\\AIUsageTracker.exe\"")]
+    [InlineData("\"C:\\Old Install\\AIUsageTracker.exe\"")]
+    public void Apply_Enable_ReplacesStaleExpandableCommand(string command)
+    {
+        this._key.SetValue(ValueName, command, RegistryValueKind.ExpandString);
+
+        WindowsStartupService.Apply(true, this._key, this._exePath);
+
+        Assert.Equal($"\"{this._exePath}\"", this._key.GetValue(ValueName, null, RegistryValueOptions.DoNotExpandEnvironmentNames));
+        Assert.Equal(RegistryValueKind.String, this._key.GetValueKind(ValueName));
+    }
+
+    [Fact]
+    public void Apply_Enable_DoesNotPreserveUnexpectedArguments()
+    {
+        this._key.SetValue(ValueName, this._portableCommand + " --unexpected", RegistryValueKind.ExpandString);
+
+        WindowsStartupService.Apply(true, this._key, this._exePath);
+
+        Assert.Equal($"\"{this._exePath}\"", this._key.GetValue(ValueName));
+        Assert.Equal(RegistryValueKind.String, this._key.GetValueKind(ValueName));
+    }
+
+    [Fact]
+    public void Apply_Enable_ReplacesNonExpandableTokenString()
+    {
+        this._key.SetValue(ValueName, this._portableCommand, RegistryValueKind.String);
+
+        WindowsStartupService.Apply(true, this._key, this._exePath);
+
+        Assert.Equal($"\"{this._exePath}\"", this._key.GetValue(ValueName));
+        Assert.Equal(RegistryValueKind.String, this._key.GetValueKind(ValueName));
+    }
+
+    [Fact]
+    public void Apply_Enable_CreatesRegistrationWhenAbsent()
+    {
+        WindowsStartupService.Apply(true, this._key, this._exePath);
+
+        Assert.Equal($"\"{this._exePath}\"", this._key.GetValue(ValueName));
+        Assert.Equal(RegistryValueKind.String, this._key.GetValueKind(ValueName));
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void Apply_DisableOrMissingExecutable_RemovesOnlyAppRegistration(bool startUi)
+    {
+        this._key.SetValue(ValueName, this._portableCommand, RegistryValueKind.ExpandString);
+        this._key.SetValue("Another App", "unchanged");
+        if (startUi)
+        {
+            File.Delete(this._exePath);
+        }
+
+        WindowsStartupService.Apply(startUi, this._key, this._exePath);
+
+        Assert.Null(this._key.GetValue(ValueName));
+        Assert.Equal("unchanged", this._key.GetValue("Another App"));
+    }
+}
diff --git a/AIUsageTracker.UI.Slim/WindowsStartupService.cs b/AIUsageTracker.UI.Slim/WindowsStartupService.cs
index c2173b9a..48cb3946 100644
--- a/AIUsageTracker.UI.Slim/WindowsStartupService.cs
+++ b/AIUsageTracker.UI.Slim/WindowsStartupService.cs
@@ -28,10 +28,24 @@ public static void Apply(bool startUi)
         }
 
         var exePath = Path.Combine(AppContext.BaseDirectory, "AIUsageTracker.exe");
+        Apply(startUi, key, exePath);
+    }
 
+    internal static void Apply(bool startUi, RegistryKey key, string exePath)
+    {
         if (startUi && File.Exists(exePath))
         {
-            key.SetValue(UiValueName, $"\"{exePath}\"");
+            var command = $"\"{exePath}\"";
+            var existing = key.GetValue(UiValueName, null, RegistryValueOptions.DoNotExpandEnvironmentNames) as string;
+            if (existing != null &&
+                key.GetValueKind(UiValueName) == RegistryValueKind.ExpandString &&
+                string.Equals(Environment.ExpandEnvironmentVariables(existing), command, StringComparison.OrdinalIgnoreCase))
+            {
+                // Keep the owner's portable registration only while it targets this installation.
+                return;
+            }
+
+            key.SetValue(UiValueName, command, RegistryValueKind.String);
         }
         else
         {