diff --git a/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs b/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs
index e421e3a4..52e5c364 100644
--- a/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs
+++ b/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs
@@ -112,6 +112,12 @@ public static class PortalVisibilityBuilder
         var popCounts = new Dictionary<uint, int>(); // per-cell pop count for the MaxReprocessPerCell cap
         var trace = PortalBuildTrace.Start(cameraCell, cameraPos);
 
+        // [portal-churn] apparatus (2026-06-08): when ProbePortalChurnEnabled, accumulate re-enqueue churn
+        // + reciprocal pre/post region counts, emitted as one summary line at end of Build. Inert when off.
+        bool churnProbe = AcDream.Core.Rendering.RenderingDiagnostics.ProbePortalChurnEnabled;
+        int churnReenqueues = 0;
+        var churnReciprocal = churnProbe ? new System.Text.StringBuilder(256) : null;
+
         bool pvDump = false;
         if (s_pvDump)
         {
@@ -295,6 +301,9 @@ public static class PortalVisibilityBuilder
                 var preReciprocalClip = eyeInsideOpening ? CloneViewPolygons(clippedRegion) : null;
                 int preReciprocalCount = clippedRegion.Count;
                 ApplyReciprocalClip(clippedRegion, portal.OtherPortalId, neighbour, viewProj);
+                if (churnProbe)
+                    churnReciprocal!.Append(System.FormattableString.Invariant(
+                        $" recip[0x{neighbourId:X8} {preReciprocalCount}->{clippedRegion.Count}]"));
                 if (clippedRegion.Count == 0)
                 {
                     if (preReciprocalClip is null)
@@ -324,6 +333,7 @@ public static class PortalVisibilityBuilder
                     dist = NearestPortalVertexDistance(poly, cell.WorldTransform, cameraPos);
                     todo.Insert(neighbour, dist);
                     inserted = true;
+                    if (churnProbe) churnReenqueues++;
                 }
                 trace?.Add($"portal cell=0x{cell.CellId:X8} p{i}->0x{neighbourId:X8} addCell polys={clippedRegion.Count} clipVerts={clipVerts} recip={preReciprocalCount}->{clippedRegion.Count} grew={grew} queued={inserted} dist={(float.IsNaN(dist) ? "na" : dist.ToString("F2"))}");
             }
@@ -338,6 +348,18 @@ public static class PortalVisibilityBuilder
             EmitFlapProbe(cameraCell, cameraPos, viewProj, frame);
         trace?.Emit(frame);
 
+        if (churnProbe)
+        {
+            int maxPop = 0; uint maxCell = 0; int rePopped = 0;
+            foreach (var kv in popCounts)
+            {
+                if (kv.Value > maxPop) { maxPop = kv.Value; maxCell = kv.Key; }
+                if (kv.Value > 1) rePopped++;
+            }
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[portal-churn] root=0x{cameraCell.CellId:X8} cells={frame.OrderedVisibleCells.Count} reEnqueues={churnReenqueues} rePoppedCells={rePopped} maxPop=0x{maxCell:X8}:{maxPop}") + churnReciprocal);
+        }
+
         return frame;
     }