diff --git a/SharepointToolbox.Tests/ViewModels/GlobalSiteSelectionTests.cs b/SharepointToolbox.Tests/ViewModels/GlobalSiteSelectionTests.cs
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+using System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.Messaging;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Moq;
+using SharepointToolbox.Core.Messages;
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Infrastructure.Auth;
+using SharepointToolbox.Infrastructure.Persistence;
+using SharepointToolbox.Services;
+using SharepointToolbox.Services.Export;
+using SharepointToolbox.ViewModels;
+using SharepointToolbox.ViewModels.Tabs;
+
+namespace SharepointToolbox.Tests.ViewModels;
+
+/// <summary>
+/// Unit tests for the global site selection flow (Phase 6).
+/// Covers: message broadcast, base class reception, single-site pre-fill,
+/// multi-site pre-populate, local override, override reset, tenant switch clear,
+/// and toolbar label update.
+/// Requirements: SITE-01, SITE-02
+/// </summary>
+public class GlobalSiteSelectionTests
+{
+    // ── Helper: minimal concrete subclass of FeatureViewModelBase ────────────
+
+    private class TestFeatureViewModel : FeatureViewModelBase
+    {
+        public TestFeatureViewModel(ILogger<FeatureViewModelBase> logger) : base(logger) { }
+
+        protected override Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress)
+            => Task.CompletedTask;
+
+        /// <summary>Expose protected GlobalSites for assertions.</summary>
+        public IReadOnlyList<SiteInfo> TestGlobalSites => GlobalSites;
+    }
+
+    // ── Reset messenger between tests to avoid cross-test contamination ──────
+
+    public GlobalSiteSelectionTests()
+    {
+        WeakReferenceMessenger.Default.Reset();
+    }
+
+    // ── Helper factories ─────────────────────────────────────────────────────
+
+    private static StorageViewModel CreateStorageViewModel()
+        => new(
+            Mock.Of<IStorageService>(),
+            Mock.Of<ISessionManager>(),
+            NullLogger<FeatureViewModelBase>.Instance);
+
+    private static PermissionsViewModel CreatePermissionsViewModel()
+        => new(
+            Mock.Of<IPermissionsService>(),
+            Mock.Of<ISiteListService>(),
+            Mock.Of<ISessionManager>(),
+            NullLogger<FeatureViewModelBase>.Instance);
+
+    private static TransferViewModel CreateTransferViewModel()
+        => new(
+            Mock.Of<IFileTransferService>(),
+            Mock.Of<ISessionManager>(),
+            new BulkResultCsvExportService(),
+            NullLogger<FeatureViewModelBase>.Instance);
+
+    private static MainWindowViewModel CreateMainWindowViewModel()
+    {
+        var tempFile = Path.GetTempFileName();
+        var profileRepo = new ProfileRepository(tempFile);
+        var profileService = new ProfileService(profileRepo);
+        var sessionManager = new SessionManager(new MsalClientFactory());
+        return new MainWindowViewModel(
+            profileService,
+            sessionManager,
+            NullLogger<MainWindowViewModel>.Instance);
+    }
+
+    private static IReadOnlyList<SiteInfo> TwoSites() =>
+        new List<SiteInfo>
+        {
+            new("https://contoso.sharepoint.com/sites/hr", "HR"),
+            new("https://contoso.sharepoint.com/sites/finance", "Finance")
+        }.AsReadOnly();
+
+    // ── Test 1: GlobalSitesChangedMessage carries site list ──────────────────
+
+    [Fact]
+    public void GlobalSitesChangedMessage_WhenSent_ReceiverGetsSites()
+    {
+        // Arrange
+        IReadOnlyList<SiteInfo>? received = null;
+        WeakReferenceMessenger.Default.Register<GlobalSitesChangedMessage>(
+            this, (_, m) => received = m.Value);
+        var sites = TwoSites();
+
+        // Act
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+
+        // Assert
+        Assert.NotNull(received);
+        Assert.Equal(2, received!.Count);
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", received[0].Url);
+        Assert.Equal("https://contoso.sharepoint.com/sites/finance", received[1].Url);
+    }
+
+    // ── Test 2: FeatureViewModelBase updates GlobalSites on message receive ──
+
+    [Fact]
+    public void FeatureViewModelBase_OnGlobalSitesChangedMessage_UpdatesGlobalSitesProperty()
+    {
+        // Arrange
+        var vm = new TestFeatureViewModel(NullLogger<FeatureViewModelBase>.Instance);
+        var sites = TwoSites();
+
+        // Act
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+
+        // Assert
+        Assert.Equal(2, vm.TestGlobalSites.Count);
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", vm.TestGlobalSites[0].Url);
+        Assert.Equal("https://contoso.sharepoint.com/sites/finance", vm.TestGlobalSites[1].Url);
+    }
+
+    // ── Test 3: StorageViewModel pre-fills SiteUrl from first global site ────
+
+    [Fact]
+    public void OnGlobalSitesChanged_WithSites_PreFillsSiteUrlOnStorageTab()
+    {
+        // Arrange
+        var vm = CreateStorageViewModel();
+        var sites = TwoSites();
+
+        // Act
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+
+        // Assert
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", vm.SiteUrl);
+    }
+
+    // ── Test 4: StorageViewModel local override prevents global update ────────
+
+    [Fact]
+    public void OnGlobalSitesChanged_AfterLocalSiteEntry_DoesNotOverrideSiteUrl()
+    {
+        // Arrange
+        var vm = CreateStorageViewModel();
+        // User types a local URL — this sets the local override flag
+        vm.SiteUrl = "https://contoso.sharepoint.com/sites/custom";
+
+        // Act: global sites message arrives
+        var sites = TwoSites();
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+
+        // Assert: local override takes precedence
+        Assert.Equal("https://contoso.sharepoint.com/sites/custom", vm.SiteUrl);
+    }
+
+    // ── Test 5: StorageViewModel clearing SiteUrl reverts to global ──────────
+
+    [Fact]
+    public void OnSiteUrlChanged_WhenClearedAfterLocalOverride_RevertsToGlobalSite()
+    {
+        // Arrange: establish global sites first, then set a local override
+        var vm = CreateStorageViewModel();
+        var sites = TwoSites();
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+        // Confirm global was applied
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", vm.SiteUrl);
+
+        // User types their own URL — sets local override
+        vm.SiteUrl = "https://contoso.sharepoint.com/sites/custom";
+        Assert.Equal("https://contoso.sharepoint.com/sites/custom", vm.SiteUrl);
+
+        // Act: user clears the SiteUrl field
+        vm.SiteUrl = string.Empty;
+
+        // Assert: override is cleared and global site is re-applied
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", vm.SiteUrl);
+    }
+
+    // ── Test 6: PermissionsViewModel pre-populates SelectedSites from global ─
+
+    [Fact]
+    public void OnGlobalSitesChanged_WithSites_PrePopulatesSelectedSitesOnPermissionsTab()
+    {
+        // Arrange
+        var vm = CreatePermissionsViewModel();
+        var sites = TwoSites();
+
+        // Act
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+
+        // Assert
+        Assert.Equal(2, vm.SelectedSites.Count);
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", vm.SelectedSites[0].Url);
+        Assert.Equal("https://contoso.sharepoint.com/sites/finance", vm.SelectedSites[1].Url);
+    }
+
+    // ── Test 7: PermissionsViewModel local picker override prevents global ───
+
+    [Fact]
+    public void OnGlobalSitesChanged_AfterLocalPickerOverride_DoesNotChangeSelectedSites()
+    {
+        // Arrange: add a site locally to simulate local site picker usage
+        var vm = CreatePermissionsViewModel();
+
+        // Simulate what ExecuteOpenSitePicker does when user picks sites locally:
+        // set _hasLocalSiteOverride = true (via reflection since it's private) and add a site.
+        // We do this by using the TenantSwitchedMessage pattern to first let global sites
+        // populate, then simulate a local override by directly reflecting the flag.
+        var globalSites = TwoSites();
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(globalSites));
+        // Global applied — SelectedSites has 2 entries
+        Assert.Equal(2, vm.SelectedSites.Count);
+
+        // Simulate local override: clear and add a local site, then set the override flag
+        var localSite = new SiteInfo("https://contoso.sharepoint.com/sites/local", "Local");
+        vm.SelectedSites.Clear();
+        vm.SelectedSites.Add(localSite);
+
+        // Use reflection to set the private _hasLocalSiteOverride flag (same as site picker would)
+        var field = typeof(PermissionsViewModel)
+            .GetField("_hasLocalSiteOverride", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
+        field!.SetValue(vm, true);
+
+        // Act: new global sites arrive
+        var newGlobalSites = new List<SiteInfo>
+        {
+            new("https://contoso.sharepoint.com/sites/new1", "New1")
+        }.AsReadOnly();
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(newGlobalSites));
+
+        // Assert: local override prevents global from replacing SelectedSites
+        Assert.Single(vm.SelectedSites);
+        Assert.Equal("https://contoso.sharepoint.com/sites/local", vm.SelectedSites[0].Url);
+    }
+
+    // ── Test 8: Tenant switch resets local override; new global sites are applied ─
+
+    [Fact]
+    public void TenantSwitched_AfterLocalOverride_ResetsOverrideSoNewGlobalSitesAreApplied()
+    {
+        // Arrange: user has a local override (different from global)
+        var vm = CreateStorageViewModel();
+        var sites = TwoSites();
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+        // Confirm global was applied
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", vm.SiteUrl);
+
+        // User types a different URL — sets local override
+        vm.SiteUrl = "https://contoso.sharepoint.com/sites/custom";
+
+        // New global sites message should NOT change SiteUrl because of local override
+        var updatedGlobal = new List<SiteInfo>
+        {
+            new("https://contoso.sharepoint.com/sites/marketing", "Marketing")
+        }.AsReadOnly();
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(updatedGlobal));
+        Assert.Equal("https://contoso.sharepoint.com/sites/custom", vm.SiteUrl); // override still active
+
+        // Act: tenant switch clears the override
+        var newProfile = new TenantProfile
+        {
+            Name = "NewTenant",
+            TenantUrl = "https://newtenant.sharepoint.com",
+            ClientId = "new-client-id"
+        };
+        WeakReferenceMessenger.Default.Send(new TenantSwitchedMessage(newProfile));
+
+        // Send new global sites after tenant switch — override should be gone
+        var newSites = new List<SiteInfo>
+        {
+            new("https://newtenant.sharepoint.com/sites/sales", "Sales")
+        }.AsReadOnly();
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(newSites));
+
+        // Assert: new global site is applied (override was reset by tenant switch)
+        Assert.Equal("https://newtenant.sharepoint.com/sites/sales", vm.SiteUrl);
+    }
+
+    // ── Test 9: TransferViewModel pre-fills SourceSiteUrl from first global ──
+
+    [Fact]
+    public void OnGlobalSitesChanged_WithSites_PreFillsSourceSiteUrlOnTransferTab()
+    {
+        // Arrange
+        var vm = CreateTransferViewModel();
+        var sites = TwoSites();
+
+        // Act
+        WeakReferenceMessenger.Default.Send(new GlobalSitesChangedMessage(sites));
+
+        // Assert: only SourceSiteUrl is pre-filled (first global site)
+        Assert.Equal("https://contoso.sharepoint.com/sites/hr", vm.SourceSiteUrl);
+    }
+
+    // ── Test 10: MainWindowViewModel GlobalSitesSelectedLabel updates with count
+
+    [Fact]
+    public void GlobalSitesSelectedLabel_WhenSitesAdded_ReflectsCount()
+    {
+        // Arrange
+        var vm = CreateMainWindowViewModel();
+        // Initially no sites selected
+        var initialLabel = vm.GlobalSitesSelectedLabel;
+        Assert.DoesNotContain("1", initialLabel); // Should say "none" equivalent
+
+        // Act: add two sites
+        vm.GlobalSelectedSites.Add(new SiteInfo("https://contoso.sharepoint.com/sites/hr", "HR"));
+        vm.GlobalSelectedSites.Add(new SiteInfo("https://contoso.sharepoint.com/sites/finance", "Finance"));
+
+        // Assert: label reflects the count
+        var label = vm.GlobalSitesSelectedLabel;
+        Assert.Contains("2", label);
+        // Ensure label is non-empty (different from the initial "none" state)
+        Assert.NotEqual(initialLabel, label);
+    }
+}