diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index da6711e..5868fe0 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -625,6 +625,33 @@ public sealed class PhysicsEngine
         bool collisionNormalValid = ci.CollisionNormalValid;
         Vector3 collisionNormal   = ci.CollisionNormal;
 
+        // #42 diagnostic (2026-05-05): trace airborne sweeps to identify the
+        // source of the ~1m XY drift on retail-observed stationary jumps.
+        // Gated on ACDREAM_AIRBORNE_DIAG=1 and !isOnGround. One line per
+        // resolve call. deltaXY = post - target tells us how much the sweep
+        // diverged from the requested target; for a clean stationary +Z
+        // jump we expect (0,0). cp=valid with a tilted normal would confirm
+        // H1 (initial-overlap depenetration → next-step AdjustOffset projects
+        // the +Z offset along a non-+Z normal). User repros at flat plaza /
+        // east hillside / north hillside; if drift direction tracks terrain
+        // orientation, H1 is the cause; if it tracks actor facing, H2 / H3.
+        if (!isOnGround
+            && Environment.GetEnvironmentVariable("ACDREAM_AIRBORNE_DIAG") == "1")
+        {
+            var post = sp.CheckPos;
+            float dx = post.X - targetPos.X;
+            float dy = post.Y - targetPos.Y;
+            string cpInfo = ci.ContactPlaneValid
+                ? $"valid cpN=({ci.ContactPlane.Normal.X:F3},{ci.ContactPlane.Normal.Y:F3},{ci.ContactPlane.Normal.Z:F3})"
+                : "none";
+            Console.WriteLine(
+                $"[SWEEP] airborne pre=({currentPos.X:F3},{currentPos.Y:F3},{currentPos.Z:F3}) " +
+                $"target=({targetPos.X:F3},{targetPos.Y:F3},{targetPos.Z:F3}) " +
+                $"post=({post.X:F3},{post.Y:F3},{post.Z:F3}) " +
+                $"cell={cellId:X8}->{sp.CheckCellId:X8} ok={ok} " +
+                $"deltaXY=({dx:F3},{dy:F3}) cp={cpInfo}");
+        }
+
         if (ok)
         {
             bool onGround = ci.ContactPlaneValid