diff --git a/src/Dragon-ISO.Engine/Discovery/NdiDiscoveryService.cs b/src/Dragon-ISO.Engine/Discovery/NdiDiscoveryService.cs
index 4a0e4a2..f8b7e58 100644
--- a/src/Dragon-ISO.Engine/Discovery/NdiDiscoveryService.cs
+++ b/src/Dragon-ISO.Engine/Discovery/NdiDiscoveryService.cs
@@ -187,23 +187,42 @@ public sealed class NdiDiscoveryService
     }
 
     /// <summary>
-    /// Dispose the current finder and create a fresh one against the cached
-    /// discovery groups. Clears the seen-set so all currently-visible sources
-    /// will re-fire as <see cref="DiscoveryEvent.Added"/> on the next poll.
+    /// Create a fresh finder against the cached discovery groups and, only once it
+    /// is successfully in hand, dispose the old one and clear the seen-set so all
+    /// currently-visible sources re-fire as <see cref="DiscoveryEvent.Added"/> on
+    /// the next poll.
+    ///
+    /// Ordering matters: we build the replacement <em>before</em> disposing the
+    /// incumbent. If <see cref="INdiInterop.CreateFinder"/> throws (the failure mode
+    /// this method's catch exists for), the incumbent finder is left untouched and
+    /// fully usable, the seen-set is preserved (no spurious re-Add storm), and the
+    /// loop simply keeps polling the existing finder until the next rebuild attempt.
+    ///
+    /// Returns true if the finder was actually swapped; false if the rebuild failed
+    /// and the existing finder was retained. Internal for unit-testing the failure
+    /// path.
     /// </summary>
-    private void RebuildFinder(string reason)
+    internal bool RebuildFinder(string reason)
     {
+        NdiFindHandle replacement;
         try
         {
             _logger.LogInformation("Rebuilding NDI finder ({Reason}).", reason);
-            _finder.Dispose();
-            _finder = _interop.CreateFinder(_discoveryGroups);
-            _previous.Clear();
+            replacement = _interop.CreateFinder(_discoveryGroups);
         }
         catch (Exception ex)
         {
             _logger.LogWarning(ex, "Finder rebuild failed ({Reason}); continuing with existing finder.", reason);
+            return false;
         }
+
+        // New finder is live — retire the old one and reset discovery state.
+        var old = _finder;
+        _finder = replacement;
+        try { old.Dispose(); }
+        catch (Exception ex) { _logger.LogWarning(ex, "Disposing previous finder threw after rebuild ({Reason}).", reason); }
+        _previous.Clear();
+        return true;
     }
 
     /// <summary>