diff --git a/app/MindWork AI Studio/Tools/IConfidenceExtensions.cs b/app/MindWork AI Studio/Tools/IConfidenceExtensions.cs
index f6f15bfd..df35f7f3 100644
--- a/app/MindWork AI Studio/Tools/IConfidenceExtensions.cs	
+++ b/app/MindWork AI Studio/Tools/IConfidenceExtensions.cs	
@@ -2,6 +2,8 @@ namespace AIStudio.Tools;
 
 public static class IConfidenceExtensions
 {
+    private static readonly ILogger<IConfidence> LOGGER = Program.LOGGER_FACTORY.CreateLogger<IConfidence>();
+
     public static TargetWindow DetermineTargetWindow<T>(this IReadOnlyList<T> items, TargetWindowStrategy strategy, int numMaximumItems = 30) where T : IConfidence 
     {
         switch (strategy)
@@ -53,8 +55,18 @@ public static class IConfidenceExtensions
     {
         if(!targetWindow.IsValid())
         {
-            var logger = Program.SERVICE_PROVIDER.GetService<ILogger<IConfidence>>()!;
-            logger.LogWarning("The target window is invalid. Returning 0f as threshold.");
+            LOGGER.LogWarning("The target window is invalid. Returning 0f as threshold.");
+            return 0f;
+        }
+
+        //
+        // Without items there is no threshold to find, and the Min and Max calls below would throw
+        // on an empty sequence. Every caller checks this today, which is precisely how such a guard
+        // goes missing once a new caller arrives. It belongs here, next to the calls it protects:
+        //
+        if(items.Count == 0)
+        {
+            LOGGER.LogWarning("There are no items to determine a confidence threshold for. Returning 0f as threshold.");
             return 0f;
         }
 
@@ -91,10 +103,7 @@ public static class IConfidenceExtensions
             }
         }
         else
-        {
-            var logger = Program.SERVICE_PROVIDER.GetService<ILogger<IConfidence>>()!;
-            logger.LogWarning("The confidence values are too close. Returning 0f as threshold.");
-        }
+            LOGGER.LogWarning("The confidence values are too close. Returning 0f as threshold.");
         
         return threshold;
     }
diff --git a/app/MindWork AI Studio/Tools/RAG/DataSourceSelectionProcesses/AgenticSrcSelWithDynHeur.cs b/app/MindWork AI Studio/Tools/RAG/DataSourceSelectionProcesses/AgenticSrcSelWithDynHeur.cs
index 094a4bee..5a9cc5d2 100644
--- a/app/MindWork AI Studio/Tools/RAG/DataSourceSelectionProcesses/AgenticSrcSelWithDynHeur.cs	
+++ b/app/MindWork AI Studio/Tools/RAG/DataSourceSelectionProcesses/AgenticSrcSelWithDynHeur.cs	
@@ -50,7 +50,7 @@ public class AgenticSrcSelWithDynHeur : IDataSourceSelectionProcess
             }
 
             // Log the selected data sources:
-            var selectedDataSourceInfo = aiSelectedDataSources.Select(ds => $"[Id={ds.Id}, reason={ds.Reason}, confidence={ds.Confidence}]").Aggregate((a, b) => $"'{a}', '{b}'");
+            var selectedDataSourceInfo = string.Join(", ", aiSelectedDataSources.Select(ds => $"'[Id={ds.Id}, reason={ds.Reason}, confidence={ds.Confidence}]'"));
             LOGGER.LogInformation($"The AI selected the data sources automatically. {aiSelectedDataSources.Count} data source(s) are selected: {selectedDataSourceInfo}.");
 
             //
diff --git a/app/Tests/Tools/ConfidenceThresholdTests.cs b/app/Tests/Tools/ConfidenceThresholdTests.cs
new file mode 100644
index 00000000..dbd4cbc9
--- /dev/null
+++ b/app/Tests/Tools/ConfidenceThresholdTests.cs
@@ -0,0 +1,58 @@
+using AIStudio.Tools;
+
+namespace AIStudio.Tests.Tools;
+
+/// <summary>
+/// Checks that determining a confidence threshold survives a list with nothing in it.
+/// </summary>
+/// <remarks>
+/// GetConfidenceThreshold reaches for Min and Max, both of which throw on an empty sequence. Until
+/// v26.9.1 the only thing standing between them and that exception was a count check at each of the
+/// two call sites. The RAG process already lost such a guard once -- a log line was written before
+/// the check that was meant to protect it, which is how every new chat produced an
+/// InvalidOperationException nobody noticed, because the RAG process catches and logs everything.
+/// The threshold is now asked to defend itself, and these tests hold it to that.
+/// </remarks>
+[TestFixture]
+public sealed class ConfidenceThresholdTests
+{
+    private readonly record struct Decision(float Confidence) : IConfidence;
+
+    [Test]
+    public void AnEmptyListYieldsAThresholdInsteadOfThrowing()
+    {
+        IReadOnlyList<Decision> nothingWasDecided = [];
+        var targetWindow = new TargetWindow(1, 2, 3, 0f);
+
+        Assert.That(nothingWasDecided.GetConfidenceThreshold(targetWindow), Is.EqualTo(0f), "A threshold of zero keeps everything that follows, which is the harmless answer for a list that holds nothing to filter.");
+    }
+
+    [Test]
+    public void AnEmptyListIsAnsweredEvenWhenTheWindowDemandsItems()
+    {
+        //
+        // The window asks for between five and ten items while not a single one exists. The guard
+        // has to hold regardless of what the window wants:
+        //
+        IReadOnlyList<Decision> nothingWasDecided = [];
+        var demandingWindow = new TargetWindow(4, 5, 10, 0.5f);
+
+        Assert.That(nothingWasDecided.GetConfidenceThreshold(demandingWindow), Is.EqualTo(0f), "The number of items the window asks for cannot conjure items to measure.");
+    }
+
+    [Test]
+    public void ASpreadOfDecisionsIsNarrowedToTheTargetWindow()
+    {
+        //
+        // Guarding the empty case must not change what the threshold does with actual items. Two
+        // weak decisions and three strong ones, with a window asking for two to three:
+        //
+        IReadOnlyList<Decision> decisions = [new(0.1f), new(0.2f), new(0.9f), new(0.95f), new(1.0f)];
+        var targetWindow = new TargetWindow(1, 2, 3, 0f);
+
+        var threshold = decisions.GetConfidenceThreshold(targetWindow);
+        var survivors = decisions.Count(decision => decision.Confidence >= threshold);
+
+        Assert.That(survivors, Is.InRange(targetWindow.TargetWindowMin, targetWindow.TargetWindowMax), "The threshold exists to cut a list down to the size the window asks for, and the three strong decisions are what should be left.");
+    }
+}
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