diff --git a/src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/DiscoveryDataAggregator.cs b/src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/DiscoveryDataAggregator.cs
index 581c5b058d..11ea0294a7 100644
--- a/src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/DiscoveryDataAggregator.cs
+++ b/src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/DiscoveryDataAggregator.cs
@@ -191,34 +191,39 @@ public void MarkSourcesWithStatus(IEnumerable<string?>? sources, DiscoveryStatus
                 continue;
             }
 
-            _sourcesWithDiscoveryStatus.AddOrUpdate(source,
-                _ =>
+            MarkSourceWithStatus(source, status);
+        }
+    }
+
+    private void MarkSourceWithStatus(string source, DiscoveryStatus status)
+    {
+        _sourcesWithDiscoveryStatus.AddOrUpdate(source,
+            _ =>
+            {
+                if (status != DiscoveryStatus.NotDiscovered)
                 {
-                    if (status != DiscoveryStatus.NotDiscovered)
-                    {
-                        EqtTrace.Warning($"DiscoveryDataAggregator.MarkSourcesWithStatus: Undiscovered {source} added with status: '{status}'.");
-                    }
-                    else
-                    {
-                        EqtTrace.Verbose($"DiscoveryDataAggregator.MarkSourcesWithStatus: Adding {source} with status: '{status}'.");
-                    }
-
-                    return status;
-                },
-                (_, previousStatus) =>
+                    EqtTrace.Warning($"DiscoveryDataAggregator.MarkSourceWithStatus: Undiscovered {source} added with status: '{status}'.");
+                }
+                else
                 {
-                    if (previousStatus == DiscoveryStatus.FullyDiscovered && status != DiscoveryStatus.FullyDiscovered
-                        || previousStatus == DiscoveryStatus.PartiallyDiscovered && (status == DiscoveryStatus.NotDiscovered || status == DiscoveryStatus.SkippedDiscovery))
-                    {
-                        EqtTrace.Warning($"DiscoveryDataAggregator.MarkSourcesWithStatus: Downgrading source {source} status from '{previousStatus}' to '{status}'.");
-                    }
-                    else if (previousStatus != status)
-                    {
-                        EqtTrace.Verbose($"DiscoveryDataAggregator.MarkSourcesWithStatus: Upgrading {source} status from '{previousStatus}' to '{status}'.");
-                    }
-                    return status;
-                });
-        }
+                    EqtTrace.Verbose($"DiscoveryDataAggregator.MarkSourceWithStatus: Adding {source} with status: '{status}'.");
+                }
+
+                return status;
+            },
+            (_, previousStatus) =>
+            {
+                if (previousStatus == DiscoveryStatus.FullyDiscovered && status != DiscoveryStatus.FullyDiscovered
+                    || previousStatus == DiscoveryStatus.PartiallyDiscovered && (status == DiscoveryStatus.NotDiscovered || status == DiscoveryStatus.SkippedDiscovery))
+                {
+                    EqtTrace.Warning($"DiscoveryDataAggregator.MarkSourceWithStatus: Downgrading source {source} status from '{previousStatus}' to '{status}'.");
+                }
+                else if (previousStatus != status)
+                {
+                    EqtTrace.Verbose($"DiscoveryDataAggregator.MarkSourceWithStatus: Upgrading {source} status from '{previousStatus}' to '{status}'.");
+                }
+                return status;
+            });
     }
 
     /// <summary>
@@ -245,6 +250,12 @@ public void MarkSourcesWithStatus(IEnumerable<string?>? sources, DiscoveryStatus
 
         foreach (var testCase in testCases)
         {
+            if (_isMessageSent == 1)
+            {
+                EqtTrace.Verbose("DiscoveryDataAggregator.MarkSourcesBasedOnDiscoveredTestCases: Message was already sent so skipping remaining source updates.");
+                return previousSource;
+            }
+
             var currentSource = testCase.Source;
 
             // We rely on the fact that sources are processed in a sequential way, which
@@ -252,13 +263,13 @@ public void MarkSourcesWithStatus(IEnumerable<string?>? sources, DiscoveryStatus
             // assume that the previous source was fully discovered.
             if (previousSource is null || previousSource == currentSource)
             {
-                MarkSourcesWithStatus(new[] { currentSource }, DiscoveryStatus.PartiallyDiscovered);
+                MarkSourceWithStatus(currentSource, DiscoveryStatus.PartiallyDiscovered);
             }
             else if (currentSource != previousSource)
             {
                 EqtTrace.Verbose($"DiscoveryDataAggregator.MarkSourcesBasedOnDiscoveredTestCases: Discovered test source changed from {previousSource} to {currentSource}.");
-                MarkSourcesWithStatus(new[] { previousSource }, DiscoveryStatus.FullyDiscovered);
-                MarkSourcesWithStatus(new[] { currentSource }, DiscoveryStatus.PartiallyDiscovered);
+                MarkSourceWithStatus(previousSource, DiscoveryStatus.FullyDiscovered);
+                MarkSourceWithStatus(currentSource, DiscoveryStatus.PartiallyDiscovered);
             }
 
             previousSource = currentSource;
diff --git a/test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/Client/Parallel/DiscoveryDataAggregatorRegressionTests.cs b/test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/Client/Parallel/DiscoveryDataAggregatorRegressionTests.cs
index ffd5ba683a..56939b2738 100644
--- a/test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/Client/Parallel/DiscoveryDataAggregatorRegressionTests.cs
+++ b/test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/Client/Parallel/DiscoveryDataAggregatorRegressionTests.cs
@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 
 using Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Client.Parallel;
+using Microsoft.VisualStudio.TestPlatform.ObjectModel;
 using Microsoft.VisualStudio.TestPlatform.ObjectModel.Engine;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 
@@ -125,4 +126,25 @@ public void MarkSourcesWithStatus_AfterMessageSent_ShouldSkipUpdate()
         var result = aggregator.GetSourcesWithStatus(DiscoveryStatus.NotDiscovered);
         Assert.HasCount(1, result);
     }
+
+    // Pins the _isMessageSent guard for MarkSourcesBasedOnDiscoveredTestCases
+    [TestMethod]
+    public void MarkSourcesBasedOnDiscoveredTestCases_AfterMessageSent_ShouldSkipUpdate()
+    {
+        var aggregator = new DiscoveryDataAggregator();
+        aggregator.MarkSourcesWithStatus(new[] { "test.dll" }, DiscoveryStatus.NotDiscovered);
+
+        // Mark message as sent before the batch arrives
+        aggregator.TryAggregateIsMessageSent();
+
+        // This batch should be entirely skipped since message was already sent
+        var testCases = new[] { new TestCase { Source = "test.dll" } };
+        aggregator.MarkSourcesBasedOnDiscoveredTestCases(null, testCases);
+
+        // Status should still be NotDiscovered, not PartiallyDiscovered
+        var notDiscovered = aggregator.GetSourcesWithStatus(DiscoveryStatus.NotDiscovered);
+        var partiallyDiscovered = aggregator.GetSourcesWithStatus(DiscoveryStatus.PartiallyDiscovered);
+        Assert.HasCount(1, notDiscovered);
+        Assert.IsEmpty(partiallyDiscovered);
+    }
 }