diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 9881d15b..97b22e5f 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -9669,6 +9669,10 @@ public sealed class GameWindow : IDisposable
         DisableClipDistances();
     }
 
+    // #119-residual [viewer] probe state: print-on-change signature so a
+    // stable climb is silent and every flood/root flip emits exactly one line.
+    private string? _lastViewerProbeSig;
+
     private void EmitRetailPViewDiagnostics(
         AcDream.App.Rendering.RetailPViewFrameResult result,
         LoadedCell clipRoot,
@@ -9677,6 +9681,21 @@ public sealed class GameWindow : IDisposable
         System.Numerics.Vector3 camPos,
         System.Numerics.Vector3 playerViewPos)
     {
+        // #119-residual: the capture half of the tower capture→replay loop.
+        // One line per change of (root, flood, outPolys, playerCell); the eye
+        // at mm precision rides every line so the harness can replay the
+        // exact production (eye, root) pairs. ACDREAM_PROBE_VIEWER=1.
+        if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeViewerEnabled)
+        {
+            string sig = System.FormattableString.Invariant(
+                $"root=0x{clipRoot.CellId:X8}{(clipRoot.IsOutdoorNode ? "(OUT)" : "")} flood={result.PortalFrame.OrderedVisibleCells.Count} outPolys={result.PortalFrame.OutsideView.Polygons.Count} pCell=0x{playerCellId:X8}");
+            if (sig != _lastViewerProbeSig)
+            {
+                _lastViewerProbeSig = sig;
+                Console.WriteLine(System.FormattableString.Invariant(
+                    $"[viewer] {sig} eye=({camPos.X:F3},{camPos.Y:F3},{camPos.Z:F3}) viewerCell=0x{viewerCellId:X8}"));
+            }
+        }
         if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeVisibilityEnabled)
             AcDream.Core.Rendering.RenderingDiagnostics.EmitVis(
                 clipRoot.CellId,
diff --git a/src/AcDream.Core/Rendering/RenderingDiagnostics.cs b/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
index 7e77496b..49302d9c 100644
--- a/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
+++ b/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
@@ -96,6 +96,19 @@ public static class RenderingDiagnostics
     public static bool ProbeVisibilityEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_VIS") == "1";
 
+    /// <summary>
+    /// #119-residual viewer/flood capture (2026-06-11): one <c>[viewer]</c>
+    /// line per CHANGE of (root cell, flood size, OutsideView poly count,
+    /// player cell), with the projection EYE at mm precision on every line —
+    /// the capture half of the tower-ascent capture→replay loop
+    /// (TowerAscentReplayTests replays the captured pairs deterministically).
+    /// Light: silent while the visibility state is stable; a tower climb
+    /// emits a few dozen lines. Initial state from
+    /// <c>ACDREAM_PROBE_VIEWER=1</c>.
+    /// </summary>
+    public static bool ProbeViewerEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_VIEWER") == "1";
+
     /// <summary>
     /// Phase U.4c (2026-05-31) flap-convergence probe. When true, the portal
     /// visibility pass emits, EVERY frame the camera root is an indoor cell, a