diff --git a/resources/tester/test_536_3_stab_supports.py b/resources/tester/test_536_3_stab_supports.py
index 6b32ecc95..994aca713 100644
--- a/resources/tester/test_536_3_stab_supports.py
+++ b/resources/tester/test_536_3_stab_supports.py
@@ -316,3 +316,19 @@ def test_station_side_and_result_slot():
 
 if __name__ == "__main__":
     sys.exit(pytest.main([__file__, "-q"]))
+
+
+def test_station_nail_length_vs_fs_monotonic():
+    """측점 꼴 — 네일 3행 길이 3~25 m: 임계 F 가 길이에 따라 줄지 않음(포화 뒤 평평 허용) · 항상 보강 없음 이상 · 탐색 한계 그대로(요구 351)."""
+    base = {c["side"]: c for c in _station(None)["cases"]}["right"]
+    f0 = base["min_F"]["bishop"]
+    heads = [(-4.0, 11.48), (-4.6, 12.98), (-5.2, 14.48)]
+    prev = f0
+    for L in (3.0, 6.0, 7.5, 9.0, 12.0, 25.0):
+        sups = [nail(x, z, 15, L, 1.5, 121.6, 40 * math.pi * 0.1) for x, z in heads]
+        right = {c["side"]: c for c in _station(sups)["cases"]}["right"]
+        assert right["result"]["limits"] == base["result"]["limits"]
+        f = right["min_F"]["bishop"]
+        assert f >= f0 - 1e-9 and f >= prev - 0.005, (L, f0, prev, f)
+        prev = f
+    assert prev > f0