diff --git a/app/Tests/Settings/SettingsStorageTests.cs b/app/Tests/Settings/SettingsStorageTests.cs
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+++ b/app/Tests/Settings/SettingsStorageTests.cs
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+using System.Text.Json;
+
+using AIStudio.Settings;
+using AIStudio.Settings.DataModel;
+
+using Microsoft.Extensions.Logging.Abstractions;
+
+using Version = AIStudio.Settings.Version;
+
+namespace AIStudio.Tests.Settings;
+
+/// <summary>
+/// Checks what settings operations do to each other when they overlap.
+/// </summary>
+/// <remarks>
+/// The settings are written from everywhere: a timer firing on its own thread, a dialog the user
+/// just closed, a configuration plugin which arrived over the network. Nothing keeps two of those
+/// from meeting, and what they must never leave behind is a settings file nobody can read -- it is
+/// the file the app starts from the next morning. These tests arrange the meeting on purpose and
+/// look at what is on the disk afterward.
+/// </remarks>
+[TestFixture]
+[NonParallelizable]
+public sealed class SettingsStorageTests
+{
+    /// <summary>
+    /// How many operations are set against each other.
+    /// </summary>
+    /// <remarks>
+    /// High enough that the operations genuinely overlap on any machine, low enough that the test
+    /// stays a test. A race which needs more than this to show up would not be one the app meets.
+    /// </remarks>
+    private const int CONCURRENT_OPERATIONS = 50;
+
+    private const string SETTINGS_FILENAME = "settings.json";
+
+    private const string BACKUP_FILENAME = "settings.v6.json";
+
+    private string? previousConfigDirectory;
+    private string? previousDataDirectory;
+    private string testDirectory = string.Empty;
+
+    [SetUp]
+    public void PrepareTestDirectory()
+    {
+        //
+        // Both directories are static state of the whole application, which is why this fixture
+        // does not run alongside others. They are put back in the teardown so that a later test
+        // does not inherit a directory which is gone by then.
+        //
+        this.previousConfigDirectory = SettingsManager.ConfigDirectory;
+        this.previousDataDirectory = SettingsManager.DataDirectory;
+
+        this.testDirectory = Path.Combine(Path.GetTempPath(), $"ai-studio-settings-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(this.testDirectory);
+
+        SettingsManager.ConfigDirectory = this.testDirectory;
+        SettingsManager.DataDirectory = this.testDirectory;
+    }
+
+    [TearDown]
+    public void RemoveTestDirectory()
+    {
+        SettingsManager.ConfigDirectory = this.previousConfigDirectory;
+        SettingsManager.DataDirectory = this.previousDataDirectory;
+
+        try
+        {
+            Directory.Delete(this.testDirectory, true);
+        }
+        catch (IOException)
+        {
+            // A temporary directory we could not remove says nothing about the code under test.
+        }
+    }
+
+    [Test]
+    public async Task OverlappingStoresLeaveBothFilesReadable()
+    {
+        var settingsManager = CreateSettingsManager();
+        await Task.WhenAll(Enumerable.Range(0, CONCURRENT_OPERATIONS).Select(_ => settingsManager.StoreSettings()));
+
+        var settingsPath = Path.Combine(this.testDirectory, SETTINGS_FILENAME);
+        var backupPath = Path.Combine(this.testDirectory, BACKUP_FILENAME);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(File.Exists(settingsPath), Is.True, "The settings file was never written.");
+            Assert.That(File.Exists(backupPath), Is.True, "The settings backup file was never written.");
+            Assert.That(ReadSettingsFile(settingsPath)?.Version, Is.EqualTo(Version.V6), "The settings file could not be read back.");
+            Assert.That(ReadSettingsFile(backupPath)?.Version, Is.EqualTo(Version.V6), "The settings backup file could not be read back.");
+        });
+    }
+
+    [Test]
+    public async Task OverlappingStoresLeaveNoTemporaryFilesBehind()
+    {
+        var settingsManager = CreateSettingsManager();
+        await Task.WhenAll(Enumerable.Range(0, CONCURRENT_OPERATIONS).Select(_ => settingsManager.StoreSettings()));
+
+        //
+        // Every store writes its settings next to the previous ones and renames afterwards. The
+        // temporary file carries a name of its own, so two stores cannot collide over it -- but a
+        // store which gave up halfway would leave one lying around, and the next start would find
+        // a configuration directory filling up with them.
+        //
+        var leftovers = Directory.GetFiles(this.testDirectory, "*.tmp-*").Select(Path.GetFileName).ToList();
+        Assert.That(leftovers, Is.Empty, $"Temporary settings files were left behind: {string.Join(", ", leftovers)}.");
+    }
+
+    [Test]
+    public async Task AStoreCannotSlipThroughWhileAReadReconsidersTheWriteBlock()
+    {
+        var settingsPath = Path.Combine(this.testDirectory, SETTINGS_FILENAME);
+
+        //
+        // Settings written by a newer app than this one. Reading them blocks every write, so that
+        // this app cannot replace settings it does not understand with the little it does. What
+        // makes this the interesting case is how a read arrives at that verdict: it clears the
+        // block first and only re-establishes it once it has seen the file. A store meeting that
+        // moment would find nothing standing in its way and overwrite the very file the block
+        // exists for -- which is why a read holds the same lock a store does.
+        //
+        await File.WriteAllTextAsync(settingsPath, """{"Version": "V99"}""");
+
+        var settingsManager = CreateSettingsManager();
+        await settingsManager.TryReadSettingsSnapshot();
+        Assert.That(settingsManager.SettingsWriteBlockReason, Is.EqualTo(SettingsWriteBlockReason.VERSION_NEWER_THAN_APP), "The newer settings file did not block writes in the first place.");
+
+        var operations = new List<Task>();
+        for (var i = 0; i < CONCURRENT_OPERATIONS; i++)
+        {
+            operations.Add(settingsManager.StoreSettings());
+            operations.Add(settingsManager.TryReadSettingsSnapshot());
+        }
+
+        await Task.WhenAll(operations);
+
+        using var settingsDocument = JsonDocument.Parse(await File.ReadAllTextAsync(settingsPath));
+        Assert.Multiple(() =>
+        {
+            Assert.That(settingsDocument.RootElement.GetProperty("Version").GetString(), Is.EqualTo("V99"), "A store overwrote the newer settings file while a read was reconsidering the write block.");
+            Assert.That(settingsManager.SettingsWriteBlockReason, Is.EqualTo(SettingsWriteBlockReason.VERSION_NEWER_THAN_APP), "The write block did not survive the reads which re-established it.");
+        });
+    }
+
+    /// <summary>
+    /// Builds a settings manager the way these tests need it.
+    /// </summary>
+    /// <remarks>
+    /// The rust service is handed in as null on purpose: neither storing nor reading settings ever
+    /// asks it anything. Only the active language is read through it, and that is not what is being
+    /// checked here. Should a future store reach for it, the test says so by failing loudly rather
+    /// than by quietly testing a different thing.
+    /// </remarks>
+    private static SettingsManager CreateSettingsManager() => new(NullLogger<SettingsManager>.Instance, null!);
+
+    private static Data? ReadSettingsFile(string settingsPath) => JsonSerializer.Deserialize<Data>(File.ReadAllText(settingsPath), SettingsManager.JSON_OPTIONS);
+}
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