diff --git a/src/AcDream.Core/Physics/BSPQuery.cs b/src/AcDream.Core/Physics/BSPQuery.cs
index e5cddc0..10033e3 100644
--- a/src/AcDream.Core/Physics/BSPQuery.cs
+++ b/src/AcDream.Core/Physics/BSPQuery.cs
@@ -1230,7 +1230,17 @@ public static class BSPQuery
             var worldNormal = TransformNormal(polyHit.Plane.Normal, localToWorld);
             var worldVertices = TransformVertices(polyHit.Vertices, localToWorld, scale, worldOrigin);
             var worldPlane = BuildWorldPlane(worldNormal, worldVertices);
-            collisions.SetContactPlane(worldPlane, path.CheckCellId, false);
+
+            // A6.P3 #98 (2026-05-23): gate ContactPlane assignment by Normal.Z.
+            // Retail's cdb capture shows CP is only ever set to flat polygons
+            // (Normal.Z = 1.0); the cellar ramp (Normal.Z = 0.695) is never CP.
+            // Setting CP=ramp made AdjustOffset slope-project forward motion
+            // and caused the sphere to wedge at the ramp top when step-up's
+            // downward probe couldn't find cottage floor (which is ABOVE).
+            // Polygons that fail this gate still drive collision detection
+            // and walkable-polygon tracking — they just don't become CP.
+            if (worldNormal.Z >= PhysicsGlobals.ContactPlaneFlatThreshold)
+                collisions.SetContactPlane(worldPlane, path.CheckCellId, false);
 
             path.SetWalkable(worldPlane, worldVertices, Vector3.UnitZ);
 
@@ -1770,7 +1780,14 @@ public static class BSPQuery
                 var worldNormal = TransformNormal(hitPoly.Plane.Normal, localToWorld);
                 var worldVertices = TransformVertices(hitPoly.Vertices, localToWorld, scale, worldOrigin);
                 var worldPlane = BuildWorldPlane(worldNormal, worldVertices);
-                collisions.SetContactPlane(worldPlane, path.CheckCellId, false);
+
+                // A6.P3 #98 (2026-05-23): same Normal.Z gate as
+                // AdjustSphereToPlane above — only flat polygons become CP.
+                // Sloped walkable polygons (cellar ramp etc.) drive collision
+                // and walkable tracking but don't override the resting CP.
+                if (worldNormal.Z >= PhysicsGlobals.ContactPlaneFlatThreshold)
+                    collisions.SetContactPlane(worldPlane, path.CheckCellId, false);
+
                 path.SetWalkable(worldPlane, worldVertices, Vector3.UnitZ);
 
                 if (PhysicsDiagnostics.ProbeBuildingEnabled || PhysicsDiagnostics.ProbeIndoorBspEnabled)
diff --git a/src/AcDream.Core/Physics/TransitionTypes.cs b/src/AcDream.Core/Physics/TransitionTypes.cs
index 7d5b2b2..5d6d076 100644
--- a/src/AcDream.Core/Physics/TransitionTypes.cs
+++ b/src/AcDream.Core/Physics/TransitionTypes.cs
@@ -557,6 +557,18 @@ public static class PhysicsGlobals
     public const float EpsilonSq  = EPSILON * EPSILON;
     public const float LandingZ   = 0.0871557f;
     public const float FloorZ     = 0.6642f;
+    /// <summary>
+    /// A6.P3 #98 (2026-05-23). Threshold for "flat enough to become a ContactPlane".
+    /// Retail's cdb capture (cellar_up_capture_1) shows 161 set_contact_plane writes
+    /// across 5 seconds of cellar climbing — every single one has Normal.Z = 1.0
+    /// exactly. The cellar ramp polygon (Normal.Z = 0.695, walkable per FloorZ but
+    /// sloped at ~46°) is never set as CP. Acdream's BSPQuery.AdjustSphereToPlane
+    /// previously set CP unconditionally on any walkable polygon, causing the
+    /// sphere to wedge at the top of the cellar ramp (issue #98). 0.99 admits
+    /// near-axis floor irregularities (slopes up to ~8°) while excluding genuine
+    /// ramps.
+    /// </summary>
+    public const float ContactPlaneFlatThreshold = 0.99f;
     public const float DefaultStepHeight = 0.01f;
     public const float Gravity    = -9.8f;
     public const float MaxVelocity = 50.0f;