diff --git a/src/AcDream.App/Rendering/DatLiveEntityProjectionMaterializer.cs b/src/AcDream.App/Rendering/DatLiveEntityProjectionMaterializer.cs
index 13760355..e5f1abe3 100644
--- a/src/AcDream.App/Rendering/DatLiveEntityProjectionMaterializer.cs
+++ b/src/AcDream.App/Rendering/DatLiveEntityProjectionMaterializer.cs
@@ -163,6 +163,8 @@ internal sealed class DatLiveEntityProjectionMaterializer
         CreateObject.ServerPosition position = canonicalSpawn.Position.Value;
         int lbX = (int)((position.LandblockId >> 24) & 0xFFu);
         int lbY = (int)((position.LandblockId >> 16) & 0xFFu);
+        if (AcDream.Core.Physics.PhysicsDiagnostics.ProbeWorldFrameEnabled)
+            ProbeWorldFrameAgreement(position.LandblockId);
         var worldOrigin = new Vector3(
             (lbX - _origin.CenterX) * 192f,
             (lbY - _origin.CenterY) * 192f,
@@ -1185,4 +1187,38 @@ internal sealed class DatLiveEntityProjectionMaterializer
         }
         return nullSlots >= 8;
     }
+
+    /// <summary>
+    /// #283 reachability probe. This method is the App-side counterpart of
+    /// Runtime's <c>TryGetWorldFrameOffset</c>: both convert a
+    /// landblock-local network origin into the streamed world frame, and both
+    /// must use the SAME centre landblock or the entity lands a multiple of
+    /// 192 m from the geometry around it.
+    ///
+    /// Runtime rebases on the accepted teleport Position; App's
+    /// <c>LiveWorldOriginState</c> rebases only after old-window retirement
+    /// completes. Emits one line per DISAGREEMENT — silence across a portal
+    /// run is the evidence that the window is unreachable in practice, and
+    /// that #283 can be closed as a permanent invariant rather than an
+    /// ownership restructure. Measurement only; it never gates placement.
+    /// </summary>
+    private void ProbeWorldFrameAgreement(uint landblockId)
+    {
+        uint runtimeCenter = _runtime.Physics.WorldFrameCenterLandblockId;
+        if (runtimeCenter == 0u || !_origin.IsKnown)
+            return;
+
+        int runtimeCenterX = (int)((runtimeCenter >> 24) & 0xFFu);
+        int runtimeCenterY = (int)((runtimeCenter >> 16) & 0xFFu);
+        if (runtimeCenterX == _origin.CenterX
+            && runtimeCenterY == _origin.CenterY)
+        {
+            return;
+        }
+
+        float deltaX = (runtimeCenterX - _origin.CenterX) * 192f;
+        float deltaY = (runtimeCenterY - _origin.CenterY) * 192f;
+        Console.WriteLine(System.FormattableString.Invariant(
+            $"[world-frame] DISAGREE runtime=({runtimeCenterX},{runtimeCenterY}) app=({_origin.CenterX},{_origin.CenterY}) offsetDelta=({deltaX:F0}m,{deltaY:F0}m) projecting=0x{landblockId:X8}"));
+    }
 }
diff --git a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
index b38e319e..c42d4ebf 100644
--- a/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
+++ b/src/AcDream.Core/Physics/PhysicsDiagnostics.cs
@@ -61,6 +61,27 @@ public static class PhysicsDiagnostics
     public static bool ProbeCellEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_CELL") == "1";
 
+    /// <summary>
+    /// #283 reachability probe (2026-08-03). Runtime rebases its world frame
+    /// the instant an accepted Position carries TeleportAdvanced, while App's
+    /// <c>LiveWorldOriginState</c> rebases only once
+    /// <c>StreamingOriginRecenterCoordinator</c> observes old-window
+    /// retirement completion — many frames later. Between those edges the two
+    /// owners can disagree by the source-to-destination landblock delta, and
+    /// anything converted in that gap lands a multiple of 192 m from the
+    /// geometry App is building.
+    ///
+    /// <para>When true, every projected placement compares Runtime's frame
+    /// centre against App's origin centre and emits one
+    /// <c>[world-frame]</c> line per DISAGREEMENT only — silence means the
+    /// window is never observed. This exists to prove or disprove
+    /// reachability before any ownership is restructured; it is a
+    /// measurement, never a gate. Initial state from
+    /// <c>ACDREAM_PROBE_WORLD_FRAME=1</c>.</para>
+    /// </summary>
+    public static bool ProbeWorldFrameEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_WORLD_FRAME") == "1";
+
     /// <summary>
     /// Stuck-cast/missing-attack investigation (2026-07-30). When true,
     /// every REMOTE ground-contact edge (HitGround / LeaveGround — each of
diff --git a/src/AcDream.Runtime/Physics/RuntimePhysicsState.cs b/src/AcDream.Runtime/Physics/RuntimePhysicsState.cs
index 70fd147e..c6d95943 100644
--- a/src/AcDream.Runtime/Physics/RuntimePhysicsState.cs
+++ b/src/AcDream.Runtime/Physics/RuntimePhysicsState.cs
@@ -546,6 +546,16 @@ public sealed class RuntimePhysicsState : IDisposable
         }
     }
 
+    /// <summary>
+    /// #283: the landblock Runtime currently treats as world-frame (0,0), or
+    /// 0 before the local-player Create publishes one. Read-only and
+    /// diagnostic — App compares it against its own
+    /// <c>LiveWorldOriginState</c> centre to detect the teleport-window
+    /// disagreement described on
+    /// <c>PhysicsDiagnostics.ProbeWorldFrameEnabled</c>.
+    /// </summary>
+    internal uint WorldFrameCenterLandblockId => _worldFrameCenterLandblockId;
+
     /// <summary>
     /// #284: records that this session's local-player Create was accepted,
     /// independently of whether it carried a usable landblock. The frame is