diff --git a/src/AcDream.Core/Physics/PhysicsEngine.cs b/src/AcDream.Core/Physics/PhysicsEngine.cs
index c938c36..bb26c54 100644
--- a/src/AcDream.Core/Physics/PhysicsEngine.cs
+++ b/src/AcDream.Core/Physics/PhysicsEngine.cs
@@ -398,12 +398,27 @@ public sealed class PhysicsEngine
         if (isOnGround)
             transition.ObjectInfo.State |= ObjectInfoState.Contact | ObjectInfoState.OnWalkable;
 
-        // Seed the transition's CollisionInfo with the previous frame's
-        // contact plane (retail PhysicsObj field). Without this, every
-        // ResolveWithTransition call starts with a fresh plane, AdjustOffset's
-        // "Have a contact plane" branch never fires, and slope projection
-        // never happens.
-        if (body is not null && body.ContactPlaneValid)
+        // K-fix7 (2026-04-26): only seed the contact plane when the body
+        // is actually grounded. Pre-seeding while AIRBORNE caused
+        // AdjustOffset's "Have a contact plane / Moving away from plane"
+        // branch to fire on every jump step — which calls
+        // Plane::snap_to_plane on the offset and ZEROES the Z component,
+        // killing all upward jump motion (the body's Z velocity stayed
+        // ~9 m/s but Position.Z never advanced because every step's
+        // offset got snapped flat). Retail's CTransition::init at
+        // 0x509dd0 (named-retail line 271954) explicitly clears
+        // contact_plane_valid = 0 at the start of every transition
+        // resolve, then ValidateWalkable re-establishes it during the
+        // sweep when the sphere bottom is within EPSILON of the terrain
+        // plane — so for grounded motion the plane is set fresh every
+        // resolve, and for airborne motion no plane interferes.
+        //
+        // We KEEP the seeding when isOnGround for slope-walking
+        // continuity (the original concern that motivated the seed) —
+        // walking up a hill needs the previous step's slope to project
+        // movement properly. Airborne / jumping must start with no
+        // plane so AdjustOffset preserves Z.
+        if (isOnGround && body is not null && body.ContactPlaneValid)
         {
             transition.CollisionInfo.SetContactPlane(
                 body.ContactPlane,