diff --git a/src/AcDream.App/Rendering/ICameraCollisionProbe.cs b/src/AcDream.App/Rendering/ICameraCollisionProbe.cs
index 52fad70..0f05c9a 100644
--- a/src/AcDream.App/Rendering/ICameraCollisionProbe.cs
+++ b/src/AcDream.App/Rendering/ICameraCollisionProbe.cs
@@ -2,9 +2,17 @@ using System.Numerics;
 
 namespace AcDream.App.Rendering;
 
+/// <summary>
+/// Result of a camera spring-arm sweep: the collided eye position AND the cell the swept
+/// viewer-sphere ended in (retail <c>viewer_cell = sphere_path.curr_cell</c>, update_viewer
+/// pc:92871). The cell is graph-tracked by the transition — no AABB, no grace frames — so it
+/// is the robust per-frame "which cell is the camera in?" answer that roots the render.
+/// </summary>
+public readonly record struct CameraSweepResult(Vector3 Eye, uint ViewerCellId);
+
 /// <summary>
 /// Sweeps a small sphere from the camera pivot (player head) toward the
-/// desired eye and returns the stopped (non-penetrating) eye. The seam that
+/// desired eye and returns the stopped (non-penetrating) eye + its cell. The seam that
 /// lets <see cref="RetailChaseCamera"/> collide its eye without depending on
 /// the physics engine directly (and stay unit-testable with a fake).
 /// </summary>
@@ -13,8 +21,9 @@ public interface ICameraCollisionProbe
     /// <summary>
     /// Roll a collision sphere from <paramref name="pivot"/> to
     /// <paramref name="desiredEye"/>; return the position it reaches without
-    /// penetrating geometry. Returns <paramref name="desiredEye"/> unchanged
+    /// penetrating geometry AND the cell it ended in. Returns
+    /// <paramref name="desiredEye"/> + <paramref name="cellId"/> unchanged
     /// when nothing blocks the path or when <paramref name="cellId"/> is 0.
     /// </summary>
-    Vector3 SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId);
+    CameraSweepResult SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId);
 }
diff --git a/src/AcDream.App/Rendering/PhysicsCameraCollisionProbe.cs b/src/AcDream.App/Rendering/PhysicsCameraCollisionProbe.cs
index 8f94e6e..4dc98c8 100644
--- a/src/AcDream.App/Rendering/PhysicsCameraCollisionProbe.cs
+++ b/src/AcDream.App/Rendering/PhysicsCameraCollisionProbe.cs
@@ -21,10 +21,10 @@ public sealed class PhysicsCameraCollisionProbe : ICameraCollisionProbe
 
     public PhysicsCameraCollisionProbe(PhysicsEngine physics) => _physics = physics;
 
-    public Vector3 SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId)
+    public CameraSweepResult SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId)
     {
-        // No starting cell → nothing to sweep against; keep the desired eye.
-        if (cellId == 0) return desiredEye;
+        // No starting cell → nothing to sweep against; keep the desired eye + cell.
+        if (cellId == 0) return new CameraSweepResult(desiredEye, cellId);
 
         // SpherePath.InitPath puts sphere0's center at pathPos + (0,0,radius)
         // (the player foot-capsule convention). Retail's viewer_sphere center is
@@ -80,7 +80,10 @@ public sealed class PhysicsCameraCollisionProbe : ICameraCollisionProbe
                 $"collNormValid={r.CollisionNormalValid}");
         }
 
-        return eye;
+        // Phase W single-viewpoint V1 (2026-06-03): surface the swept cell (r.CellId =
+        // sp.CurCellId) as the viewer cell — retail viewer_cell = sphere_path.curr_cell
+        // (update_viewer pc:92871). Graph-tracked, no AABB/grace → the U.4c flap source is gone.
+        return new CameraSweepResult(eye, r.CellId);
     }
 
     /// <summary>Eye/pivot point → InitPath path point (subtract the sphere-center offset).</summary>
diff --git a/src/AcDream.App/Rendering/RetailChaseCamera.cs b/src/AcDream.App/Rendering/RetailChaseCamera.cs
index 79de8f4..993999a 100644
--- a/src/AcDream.App/Rendering/RetailChaseCamera.cs
+++ b/src/AcDream.App/Rendering/RetailChaseCamera.cs
@@ -153,7 +153,7 @@ public sealed class RetailChaseCamera : ICamera
         //     leave _dampedEye as the clean, uncollided sought position.
         Vector3 publishedEye = _dampedEye;
         if (CameraDiagnostics.CollideCamera && CollisionProbe is not null)
-            publishedEye = CollisionProbe.SweepEye(pivotWorld, _dampedEye, cellId, selfEntityId);
+            publishedEye = CollisionProbe.SweepEye(pivotWorld, _dampedEye, cellId, selfEntityId).Eye;
 
         // 6. Publish renderer surface (from the collided eye; rotation stays the
         //    smoothly-damped look direction toward the pivot).
diff --git a/tests/AcDream.App.Tests/Rendering/CameraCollisionIndoorTests.cs b/tests/AcDream.App.Tests/Rendering/CameraCollisionIndoorTests.cs
index 0d859c6..d0fb3b6 100644
--- a/tests/AcDream.App.Tests/Rendering/CameraCollisionIndoorTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/CameraCollisionIndoorTests.cs
@@ -140,7 +140,7 @@ public class CameraCollisionIndoorTests
             pivot:          PivotWorld,
             desiredEye:     DesiredEye,
             cellId:         IndoorCellId,
-            selfEntityId:   0u);
+            selfEntityId:   0u).Eye;
 
         // The eye should be stopped before the exterior wall at Y=4.0.
         // Expected stopped eye Y ≈ 4.0 - ViewerSphereRadius = 3.7.
diff --git a/tests/AcDream.App.Tests/Rendering/PhysicsCameraCollisionProbeTests.cs b/tests/AcDream.App.Tests/Rendering/PhysicsCameraCollisionProbeTests.cs
index 5cb7b0c..c756c94 100644
--- a/tests/AcDream.App.Tests/Rendering/PhysicsCameraCollisionProbeTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/PhysicsCameraCollisionProbeTests.cs
@@ -38,6 +38,7 @@ public class PhysicsCameraCollisionProbeTests
 
         var result = probe.SweepEye(pivot, eye, cellId: 0, selfEntityId: 0);
 
-        Assert.Equal(eye, result);
+        Assert.Equal(eye, result.Eye);
+        Assert.Equal(0u, result.ViewerCellId);   // cellId==0 → returned unchanged
     }
 }
diff --git a/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs b/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs
index 8a3063b..0e58afb 100644
--- a/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs
@@ -452,10 +452,11 @@ public class RetailChaseCameraTests
     {
         public int Calls;
         public Vector3 ReturnEye;
-        public Vector3 SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId)
+        public uint ReturnCell;
+        public CameraSweepResult SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId)
         {
             Calls++;
-            return ReturnEye;
+            return new CameraSweepResult(ReturnEye, ReturnCell);
         }
     }
 
@@ -538,10 +539,10 @@ public class RetailChaseCameraTests
     {
         public int Calls;
         public Vector3 ClampEye;
-        public Vector3 SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId)
+        public CameraSweepResult SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId)
         {
             Calls++;
-            return Calls == 1 ? ClampEye : desiredEye;
+            return new CameraSweepResult(Calls == 1 ? ClampEye : desiredEye, cellId);
         }
     }