Tell whoever asked for a page what the search could not cover

app/MindWork AI Studio/Settings/DataModel/DataSourceERI_V1.cs

@@ -85,7 +85,7 @@ public readonly record struct DataSourceERI_V1 : IERIDataSource
             _ => string.Empty
         };
 
-        return await this.RetrieveDataAsync(latestUserPrompt, lastUserPrompt.ToERIContentType, thread, this.MaxMatches, token);
+        return await this.RetrieveDataAsync(latestUserPrompt, lastUserPrompt.ToERIContentType, thread, this.MaxMatches, token) ?? [];
     }
 
     /// <inheritdoc />
@@ -103,11 +103,15 @@ public readonly record struct DataSourceERI_V1 : IERIDataSource
         // returns fewer matches than asked for ends the paging early, which errs on the safe side.
         //
         var contexts = await this.RetrieveDataAsync(query, ContentType.TEXT, thread, window, token);
+        if (contexts is null)
+            return RetrievalPage.EMPTY with { Gaps = [RetrievalGap.NOT_SEARCHED] };
+
         var (pageContexts, hasMore) = RetrievalPaging.Cut(contexts, page, this.MaxMatches);
         return new RetrievalPage(pageContexts, hasMore);
     }
 
-    private async Task<IReadOnlyList<IRetrievalContext>> RetrieveDataAsync(string latestUserPrompt, ContentType latestUserPromptType, ChatThread thread, int maxMatches, CancellationToken token)
+    /// <returns>What the ERI server found, or null when it could not be searched.</returns>
+    private async Task<IReadOnlyList<IRetrievalContext>?> RetrieveDataAsync(string latestUserPrompt, ContentType latestUserPromptType, ChatThread thread, int maxMatches, CancellationToken token)
     {
         // Important: Do not dispose the RustService here, as it is a singleton.
         var rustService = Program.SERVICE_PROVIDER.GetRequiredService<RustService>();
@@ -175,11 +179,11 @@ public readonly record struct DataSourceERI_V1 : IERIDataSource
             }
 
             logger.LogWarning($"Was not able to retrieve data from the ERI data source '{this.Name}'. Message: {retrievalResponse.Message}");
-            return [];
+            return null;
         }
 
         logger.LogWarning($"Was not able to authenticate with the ERI data source '{this.Name}'. Message: {authResponse.Message}");
-        return [];
+        return null;
     }
 
     public static bool TryParseConfiguration(int idx, LuaTable table, Guid configPluginId, out DataSourceERI_V1 dataSource)

app/MindWork AI Studio/Settings/IDataSource.cs

@@ -43,12 +43,16 @@ public interface IDataSource : IConfigurationObject
     /// lets the model work it out from the conversation, and search as often as it takes. The first
     /// page holds what the retrieval above finds for the same text. How the pages are cut is
     /// described in RetrievalPaging.
+    ///
+    /// Since the user did not write the query, the user is not told about problems with it. They
+    /// arrive in RetrievalPage.Gaps instead, together with everything else which kept the search
+    /// from covering the whole data source.
     /// </remarks>
     /// <param name="query">What to search for.</param>
     /// <param name="page">The page to retrieve, from 1 up to RetrievalPaging.GetLastPage for MaxMatches.</param>
     /// <param name="thread">The chat thread.</param>
     /// <param name="token">The cancellation token.</param>
-    /// <returns>The retrieved data contexts of this page, and whether the next page is worth asking for.</returns>
+    /// <returns>The retrieved data contexts of this page, whether the next page is worth asking for, and what the search could not cover.</returns>
     /// <exception cref="ArgumentOutOfRangeException">The page is below 1 or beyond the last page.</exception>
     public Task<RetrievalPage> RetrieveDataAsync(string query, int page, ChatThread thread, CancellationToken token = default);
 }