diff --git a/app/MindWork AI Studio/Tools/DataSourceReindexWarning.cs b/app/MindWork AI Studio/Tools/DataSourceReindexWarning.cs
new file mode 100644
index 00000000..34c4a584
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+++ b/app/MindWork AI Studio/Tools/DataSourceReindexWarning.cs	
@@ -0,0 +1,147 @@
+using System.Text;
+
+using AIStudio.Dialogs;
+using AIStudio.Settings;
+using AIStudio.Tools.PluginSystem;
+using AIStudio.Tools.Services;
+
+namespace AIStudio.Tools;
+
+/// <summary>
+/// Asks before an edit makes the prepared documents of data sources useless.
+/// </summary>
+/// <remarks>
+/// Kept here rather than in the dialogs which ask -- the embedding provider dialog and the two data
+/// source dialogs -- so the sentence naming what a rebuild costs cannot drift apart between them.
+/// That is the same reason DataSourceRepair sits next to it, and both name the same two costs.
+///
+/// Nothing is asked when nothing is lost. A data source only reaches the question when the edit
+/// really changes its embedding signature and when the index already holds something for it, so
+/// renaming an embedding provider or editing a data source nobody has indexed yet stays silent.
+/// </remarks>
+public static class DataSourceReindexWarning
+{
+    /// <summary>
+    /// How many data sources are named before the rest is only counted.
+    /// </summary>
+    private const int MAX_NAMED_DATA_SOURCES = 10;
+
+    private static string TB(string fallbackEN) => I18N.I.T(fallbackEN, typeof(DataSourceReindexWarning).Namespace, nameof(DataSourceReindexWarning));
+
+    /// <summary>
+    /// Asks before an edited embedding provider is saved.
+    /// </summary>
+    /// <param name="dialogService">The dialog service to ask with.</param>
+    /// <param name="settingsManager">The settings, read for the data sources behind the provider.</param>
+    /// <param name="embeddingService">The service which knows what the index holds.</param>
+    /// <param name="before">The embedding provider as it is stored.</param>
+    /// <param name="after">The embedding provider as it would be stored.</param>
+    /// <param name="token">The cancellation token.</param>
+    /// <returns>True when the edit may be saved.</returns>
+    public static async Task<bool> ConfirmEmbeddingProviderChangeAsync(IDialogService dialogService, SettingsManager settingsManager, DataSourceEmbeddingService embeddingService,
+        EmbeddingProvider before, EmbeddingProvider after, CancellationToken token = default)
+    {
+        // Nothing was stored under this id, so no data source can point at it:
+        if (before == EmbeddingProvider.NONE)
+            return true;
+
+        var candidates = GetDataSourcesUsing(settingsManager, before.Id)
+            .Where(dataSource => EmbeddingChangeImpact.AffectsStoredIndex(dataSource, before, after))
+            .Cast<IDataSource>()
+            .ToList();
+
+        if (candidates.Count == 0)
+            return true;
+
+        var affected = await embeddingService.GetDataSourcesWithStoredIndexAsync(candidates, token);
+        return await ConfirmAsync(dialogService, affected, !after.IsSelfHosted);
+    }
+
+    /// <summary>
+    /// Asks before an edited data source is saved.
+    /// </summary>
+    /// <param name="dialogService">The dialog service to ask with.</param>
+    /// <param name="settingsManager">The settings, read for the embedding provider of the data source.</param>
+    /// <param name="embeddingService">The service which knows what the index holds.</param>
+    /// <param name="before">The data source as it is stored.</param>
+    /// <param name="after">The data source as it would be stored.</param>
+    /// <param name="token">The cancellation token.</param>
+    /// <returns>True when the edit may be saved.</returns>
+    public static async Task<bool> ConfirmDataSourceChangeAsync(IDialogService dialogService, SettingsManager settingsManager, DataSourceEmbeddingService embeddingService,
+        IInternalDataSource before, IInternalDataSource after, CancellationToken token = default)
+    {
+        // Without a provider nothing is embedded at all, so nothing can be lost:
+        if (!DataSourceEmbeddingProviders.TryResolve(settingsManager, after, out var embeddingProvider))
+            return true;
+
+        if (!EmbeddingChangeImpact.AffectsStoredIndex(embeddingProvider, before, after))
+            return true;
+
+        var affected = await embeddingService.GetDataSourcesWithStoredIndexAsync([after], token);
+        return await ConfirmAsync(dialogService, affected, !embeddingProvider.IsSelfHosted);
+    }
+
+    /// <summary>
+    /// The data sources which are indexed with a given embedding provider.
+    /// </summary>
+    /// <param name="settingsManager">The settings holding the data sources.</param>
+    /// <param name="embeddingProviderId">The id of the embedding provider.</param>
+    /// <returns>The data sources pointing at that embedding provider.</returns>
+    private static IReadOnlyList<IInternalDataSource> GetDataSourcesUsing(SettingsManager settingsManager, string embeddingProviderId) =>
+        settingsManager.ConfigurationData.DataSources
+            .OfType<IInternalDataSource>()
+            .Where(dataSource => embeddingProviderId.Equals(dataSource.EmbeddingId, StringComparison.OrdinalIgnoreCase))
+            .ToList();
+
+    /// <summary>
+    /// Names data sources as a Markdown list, counting the rest when there are too many to name.
+    /// </summary>
+    /// <param name="dataSources">The data sources to name.</param>
+    /// <returns>The Markdown list.</returns>
+    private static string FormatDataSourceNames(IReadOnlyList<IDataSource> dataSources)
+    {
+        var names = dataSources
+            .Select(dataSource => dataSource.Name)
+            .OrderBy(name => name, StringComparer.OrdinalIgnoreCase)
+            .ToList();
+
+        var lines = names.Take(MAX_NAMED_DATA_SOURCES).Select(name => $"- {name}").ToList();
+        if (names.Count > MAX_NAMED_DATA_SOURCES)
+            lines.Add($"- {string.Format(TB("and {0} more."), (names.Count - MAX_NAMED_DATA_SOURCES).CompactCount())}");
+
+        return string.Join(Environment.NewLine, lines);
+    }
+
+    private static async Task<bool> ConfirmAsync(IDialogService dialogService, IReadOnlyList<IDataSource> affected, bool usesCloudEmbedding)
+    {
+        if (affected.Count == 0)
+            return true;
+
+        var body = new StringBuilder();
+
+        // Counted rather than put into a plural form: the I18N has no mechanism for one.
+        body.AppendLine(string.Format(TB("This change makes the prepared documents of the following data sources unusable ({0}):"), affected.Count.CompactCount()));
+        body.AppendLine();
+        body.AppendLine(FormatDataSourceNames(affected));
+        body.AppendLine();
+        body.AppendLine(TB("Everything prepared for them is thrown away, and every one of their documents goes to your embedding provider once more. With a large data source, this takes a while."));
+
+        if (usesCloudEmbedding)
+        {
+            body.AppendLine();
+            body.AppendLine(TB("Your embedding provider runs in the cloud, so preparing everything again costs money."));
+        }
+
+        body.AppendLine();
+        body.AppendLine(TB("Do you want to apply this change anyway?"));
+
+        var dialogParameters = new DialogParameters<ConfirmDialog>
+        {
+            { x => x.MarkdownBody, body.ToString() },
+        };
+
+        var dialogReference = await dialogService.ShowAsync<ConfirmDialog>(TB("Documents Will Be Prepared Again"), dialogParameters, Dialogs.DialogOptions.FULLSCREEN);
+        var dialogResult = await dialogReference.Result;
+        return dialogResult is not null && !dialogResult.Canceled;
+    }
+}
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