diff --git a/backend/app/api/thresholds.py b/backend/app/api/thresholds.py
index 9e3d09f1..4123dd4b 100644
--- a/backend/app/api/thresholds.py
+++ b/backend/app/api/thresholds.py
@@ -94,11 +94,19 @@ def suggest_by_type(kpi):
 
 
 def _kpi_type(kpi) -> str:
-    if kpi.kpi_code in KPI_TYPES["lower_better"]:
+    """KPI → 推荐类型（**保持历史默认语义**）。
+
+    原实现为 if higher_better / elif lower_better / **else → middle_best**，
+    即「不在任何清单里的编码」默认走适中型（如 C_REBATE_RATE 渠补率、
+    BH_SALES_10 销售-任富海，其历史生成的 '52.33~107.67' / '偏离…%' 即出自该分支）。
+    此处默认值必须同为 middle_best，否则默认分支的推荐口径会被改变。
+    """
+    code = getattr(kpi, "kpi_code", None)
+    if code in KPI_TYPES["higher_better"]:
+        return "higher_better"
+    if code in KPI_TYPES["lower_better"]:
         return "lower_better"
-    if kpi.kpi_code in KPI_TYPES["middle_best"]:
-        return "middle_best"
-    return "higher_better"
+    return "middle_best"
 
 
 def _sanity_check(suggestion: dict) -> dict:
diff --git a/backend/tests/test_threshold_parser.py b/backend/tests/test_threshold_parser.py
index 197a8005..1b909a03 100644
--- a/backend/tests/test_threshold_parser.py
+++ b/backend/tests/test_threshold_parser.py
@@ -185,6 +185,34 @@ class TestSuggestFormat:
         for level in ("green", "yellow", "red"):
             assert is_valid_threshold(s2[level]["label"]), s2[level]["label"]
 
+    def test_kpi_type_default_preserves_legacy_middle_best(self):
+        """_kpi_type 默认分支必须保持历史语义 = middle_best（原实现的 else 分支）。"""
+        from app.api.thresholds import _kpi_type
+
+        class K:
+            def __init__(self, code):
+                self.kpi_code = code
+
+        assert _kpi_type(K("SALES_TOTAL")) == "higher_better"
+        assert _kpi_type(K("COST_CONTROL_RATE")) == "lower_better"
+        assert _kpi_type(K("C_REBATE_RATE")) == "middle_best"      # 渠补率：历史为适中型
+        assert _kpi_type(K("BH_SALES_10")) == "middle_best"        # 销售-任富海：历史为适中型
+        assert _kpi_type(K("MADE_UP_CODE")) == "middle_best"       # 未知编码 → 默认适中型
+
+    def test_c_rebate_rate_suggestion_is_middle_best_range(self):
+        """回归：C_REBATE_RATE 建议仍走适中型区间口径（不得退化为更高的单边界）。"""
+        from app.api.thresholds import _kpi_type, build_suggestion
+
+        class K:
+            kpi_code = "C_REBATE_RATE"
+
+        t = _kpi_type(K())
+        s = build_suggestion(t, 80.0, tolerance=27.67)
+        assert t == "middle_best"
+        assert s["green"]["label"] == "52.33~107.67"
+        assert is_valid_threshold(s["yellow"]["label"])
+        assert is_valid_threshold(s["red"]["label"])
+
     def test_middle_best_zero_tolerance_guard(self):
         """容忍度退化时不得生成 A~A 之类不可用表达式"""
         s = build_suggestion("middle_best", 0.0, tolerance=0.0)