diff --git a/src/AcDream.Core/Physics/BSPQuery.cs b/src/AcDream.Core/Physics/BSPQuery.cs
index 2895b20..eff0222 100644
--- a/src/AcDream.Core/Physics/BSPQuery.cs
+++ b/src/AcDream.Core/Physics/BSPQuery.cs
@@ -1469,11 +1469,18 @@ public static class BSPQuery
             if (hit0 || hitPoly0 is not null)
             {
                 var worldNormal = L2W(hitPoly0!.Plane.Normal);
-                if (engine is not null)
+                // L.2.3b (2026-04-29): recursion guard. Retail
+                // (acclient_2013_pseudo_c.txt:272954) gates step_sphere_up on
+                // `if (sp.step_up == 0 && sp.step_down == 0)`. Without this,
+                // the inner TransitionalInsert spawned by DoStepDown re-enters
+                // FindObjCollisions, hits the same wall, and recursively
+                // re-invokes step-up — churning the contact plane until
+                // numAttempts decays. Mid-recursion we fall back to wall-slide.
+                if (engine is not null && !path.StepUp && !path.StepDown)
                     return StepSphereUp(transition, worldNormal, engine);
 
-                // No engine available (env-cell path without engine param) —
-                // fall back to wall-slide so existing indoor geometry still blocks.
+                // No engine OR step-up/step-down already in progress — fall
+                // back to wall-slide so the inner sphere doesn't recurse.
                 collisions.SetCollisionNormal(worldNormal);
                 collisions.SetSlidingNormal(worldNormal);
                 return TransitionState.Slid;
@@ -1490,7 +1497,8 @@ public static class BSPQuery
                 if (hit1 || hitPoly1 is not null)
                 {
                     var worldNormal = L2W(hitPoly1!.Plane.Normal);
-                    if (engine is not null)
+                    // L.2.3b: same recursion guard as the foot-sphere branch.
+                    if (engine is not null && !path.StepUp && !path.StepDown)
                         return StepSphereUp(transition, worldNormal, engine);
 
                     collisions.SetCollisionNormal(worldNormal);