diff --git a/LANCommander.SDK.Tests/Install/GameInstallationSharedDirectoryTests.cs b/LANCommander.SDK.Tests/Install/GameInstallationSharedDirectoryTests.cs
new file mode 100644
index 00000000..de127a7f
--- /dev/null
+++ b/LANCommander.SDK.Tests/Install/GameInstallationSharedDirectoryTests.cs
@@ -0,0 +1,197 @@
+using System.Reflection;
+using System.Text;
+using LANCommander.SDK.Enums;
+using LANCommander.SDK.Helpers;
+using LANCommander.SDK.Services;
+using SdkGame = LANCommander.SDK.Models.Game;
+using ManifestGame = LANCommander.SDK.Models.Manifest.Game;
+
+namespace LANCommander.SDK.Tests.Install;
+
+/// <summary>
+/// Simulates two independent users (separate authentication sessions / launcher databases)
+/// installing the same game into an overlapping install root.
+///
+/// Scenario:
+///   - User A installs "Quake" with install root {root}\Games, landing in {root}\Games\Quake.
+///   - User B, on a completely separate GameClient session, chooses the same {root}\Games root.
+///   - User B's install should:
+///       1. Resolve to (detect) User A's existing installation directory.
+///       2. Validate the files already present in that directory.
+///
+/// Both users are modelled as distinct <see cref="GameClient"/> instances. GameClient's
+/// directory resolution (<see cref="GameClient.GetInstallDirectory"/>) and local file
+/// verification are file-system only for a main game, so no server/API is required.
+/// </summary>
+public class GameInstallationSharedDirectoryTests : IDisposable
+{
+    private readonly string _tempDir;
+    private readonly string _sharedRoot;
+    private readonly Guid _quakeId = Guid.NewGuid();
+
+    // Files User A "installs" into the shared directory, relative to the install directory.
+    private static readonly string[] GameFiles =
+    {
+        "quake.exe",
+        "id1/pak0.pak",
+        "id1/config.cfg",
+    };
+
+    // User A's session/launcher.
+    private readonly GameClient _userAClient = CreateClient();
+
+    // User B's session/launcher — a separate authentication session and launcher DB.
+    private readonly GameClient _userBClient = CreateClient();
+
+    // Resolved install directory produced by User A's install ({sharedRoot}\Quake).
+    private readonly string _userAInstallDirectory;
+
+    public GameInstallationSharedDirectoryTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"lc-shared-install-tests-{Guid.NewGuid()}");
+        // Stands in for "C:\Games" — the install root both users select.
+        _sharedRoot = Path.Combine(_tempDir, "Games");
+        Directory.CreateDirectory(_sharedRoot);
+
+        // ── User A performs the initial install ──────────────────────────────
+        _userAInstallDirectory = _userAClient.GetInstallDirectory(MakeQuake(), _sharedRoot)
+            .GetAwaiter().GetResult();
+
+        InstallGameFiles(_userAInstallDirectory);
+        WriteFileList(_userAInstallDirectory, _quakeId, GameFiles);
+
+        ManifestHelper.Write(
+            new ManifestGame
+            {
+                Id = _quakeId,
+                Title = "Quake",
+                Type = GameType.MainGame,
+                Version = "1.0.0",
+            },
+            _userAInstallDirectory);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempDir))
+            Directory.Delete(_tempDir, true);
+    }
+
+    // ── Detection ─────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task UserB_ResolvesToUserAExistingInstallDirectory()
+    {
+        // User A installed to {sharedRoot}\Quake.
+        Assert.Equal(Path.Combine(_sharedRoot, "Quake"), _userAInstallDirectory);
+
+        // User B, on a separate session, selects the same shared root and should resolve
+        // to the exact same directory User A already installed into.
+        var userBInstallDirectory = await _userBClient.GetInstallDirectory(MakeQuake(), _sharedRoot);
+
+        Assert.Equal(_userAInstallDirectory, userBInstallDirectory);
+        Assert.True(Directory.Exists(userBInstallDirectory));
+    }
+
+    [Fact]
+    public async Task UserB_DetectsExistingInstallation_ViaManifest()
+    {
+        var userBInstallDirectory = await _userBClient.GetInstallDirectory(MakeQuake(), _sharedRoot);
+
+        // The existing installation is detected through the manifest User A wrote.
+        Assert.True(ManifestHelper.Exists(userBInstallDirectory, _quakeId));
+    }
+
+    // ── Validation ────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task UserB_ValidatesExistingFiles_AllPresent()
+    {
+        var userBInstallDirectory = await _userBClient.GetInstallDirectory(MakeQuake(), _sharedRoot);
+
+        var verified = await VerifyLocalFiles(_userBClient, userBInstallDirectory, _quakeId);
+
+        // Every file User A installed is validated as present and can be skipped on re-download.
+        Assert.Equal(GameFiles.Length, verified.Count);
+        foreach (var file in GameFiles)
+            Assert.Contains(file, verified);
+    }
+
+    [Fact]
+    public async Task UserB_Validation_DetectsMissingFile()
+    {
+        var userBInstallDirectory = await _userBClient.GetInstallDirectory(MakeQuake(), _sharedRoot);
+
+        // Simulate a partially-present/corrupted existing install by removing a file.
+        File.Delete(Path.Combine(userBInstallDirectory, "id1/pak0.pak".Replace('/', Path.DirectorySeparatorChar)));
+
+        var verified = await VerifyLocalFiles(_userBClient, userBInstallDirectory, _quakeId);
+
+        // The missing file is not validated, so User B's install would re-fetch it.
+        Assert.DoesNotContain("id1/pak0.pak", verified);
+        Assert.Contains("quake.exe", verified);
+        Assert.Equal(GameFiles.Length - 1, verified.Count);
+    }
+
+    // ── Helpers ────────────────────────────────────────────────────────────────
+
+    private SdkGame MakeQuake() => new()
+    {
+        Id = _quakeId,
+        Title = "Quake",
+        Type = GameType.MainGame,
+        BaseGameId = Guid.Empty,
+    };
+
+    /// <summary>
+    /// Builds a GameClient whose dependencies are unused for main-game directory
+    /// resolution and local file verification, so they are safe to leave null.
+    /// </summary>
+    private static GameClient CreateClient() =>
+        new(null!, null!, null!, null!, null!, null!, null!, null!, null!, null!, null!, null!);
+
+    private static void InstallGameFiles(string installDirectory)
+    {
+        foreach (var file in GameFiles)
+        {
+            var localPath = Path.Combine(installDirectory, file.Replace('/', Path.DirectorySeparatorChar));
+            Directory.CreateDirectory(Path.GetDirectoryName(localPath)!);
+            File.WriteAllText(localPath, $"contents of {file}");
+        }
+    }
+
+    /// <summary>
+    /// Writes a FileList.txt matching the production format ("path | CRC32HEX")
+    /// that GameClient produces after an extraction.
+    /// </summary>
+    private static void WriteFileList(string installDirectory, Guid gameId, IEnumerable<string> files)
+    {
+        var fileListPath = GameClient.GetMetadataFilePath(installDirectory, gameId, "FileList.txt");
+        Directory.CreateDirectory(Path.GetDirectoryName(fileListPath)!);
+
+        var builder = new StringBuilder();
+        foreach (var file in files)
+            builder.AppendLine($"{file} | DEADBEEF");
+
+        File.WriteAllText(fileListPath, builder.ToString());
+    }
+
+    /// <summary>
+    /// Invokes GameClient's private local-file verification (the VerifyFiles install task),
+    /// which reads FileList.txt and returns entries confirmed present on disk.
+    /// </summary>
+    private static async Task<HashSet<string>> VerifyLocalFiles(GameClient client, string installDirectory, Guid gameId)
+    {
+        var method = typeof(GameClient).GetMethod(
+            "VerifyLocalFilesAsync",
+            BindingFlags.NonPublic | BindingFlags.Instance);
+
+        Assert.NotNull(method);
+
+        var task = (Task<HashSet<string>>)method!.Invoke(
+            client,
+            new object[] { installDirectory, gameId, CancellationToken.None })!;
+
+        return await task;
+    }
+}