diff --git a/backend/engine/cdsl_engine/runtime.py b/backend/engine/cdsl_engine/runtime.py
index c370ad6f..80c1d1ae 100644
--- a/backend/engine/cdsl_engine/runtime.py
+++ b/backend/engine/cdsl_engine/runtime.py
@@ -702,9 +702,20 @@ def _mirrored_node(node: FeaturePlanNode, instance_id: str, plane: PlaneSpec) ->
         host_frame.get(key) is not None for key in ("origin_mm", "x_dir", "normal")
     )
     if positions_are_local:
-        for key in ("origin_mm", "x_dir", "y_dir", "normal"):
+        for key in ("origin_mm", "x_dir", "normal"):
             if host_frame.get(key):
                 host_frame[key] = _reflect_point(host_frame[key], plane, vector=key != "origin_mm")
+        # #1 y_dir 保留：PlaneSpec 现在会尊重显式正交 y_dir。镜像后 frame 的
+        # canonical y 轴必须是 n×x（x 已反射 → y 反转），否则反射后的 frame
+        # 会保留反射前的 y_dir，与下方"局部坐标 v 取反"双重翻转。
+        x_reflected = host_frame.get("x_dir")
+        n_reflected = host_frame.get("normal")
+        if x_reflected is not None and n_reflected is not None:
+            host_frame["y_dir"] = [
+                n_reflected[1] * x_reflected[2] - n_reflected[2] * x_reflected[1],
+                n_reflected[2] * x_reflected[0] - n_reflected[0] * x_reflected[2],
+                n_reflected[0] * x_reflected[1] - n_reflected[1] * x_reflected[0],
+            ]
         # See _mirrored_sketch: the canonical reflected plane reverses local
         # y, so local hole coordinates must do the same.
         for position in params.get("positions") or []:
diff --git a/backend/engine/cdsl_engine/runtime_types.py b/backend/engine/cdsl_engine/runtime_types.py
index 242c1f39..def85b0a 100644
--- a/backend/engine/cdsl_engine/runtime_types.py
+++ b/backend/engine/cdsl_engine/runtime_types.py
@@ -7,6 +7,7 @@ any geometry adapter without importing OCC objects.
 
 from __future__ import annotations
 
+import warnings
 from dataclasses import dataclass, field
 from math import sqrt
 from typing import Any, Iterable
@@ -14,6 +15,10 @@ from typing import Any, Iterable
 
 Vector3 = tuple[float, float, float]
 
+# y_dir 与 x_dir / normal 点积的绝对值不超过该值时，认为 y_dir 是正交的，
+# 予以保留；否则视为偏斜数据，正交化并显式警告。
+_Y_DIR_ORTHOGONALITY_TOL = 1e-6
+
 
 def _vector3(value: Any, *, field_name: str) -> Vector3:
     if not isinstance(value, (list, tuple)) or len(value) != 3:
@@ -176,7 +181,25 @@ class PlaneSpec:
         x_raw = _vector3(value.get("x_dir"), field_name="plane.x_dir")
         projected_x = tuple(x_raw[index] - _dot(x_raw, normal) * normal[index] for index in range(3))
         x_dir = _unit(projected_x, field_name="plane.x_dir")
-        y_dir = _unit(_cross(normal, x_dir), field_name="plane.y_dir")
+        generated = _unit(_cross(normal, x_dir), field_name="plane.y_dir")
+        y_raw = value.get("y_dir")
+        if y_raw is None:
+            # y_dir 缺失：用 normal × x_dir 补全右手系（默认行为）。
+            y_dir = generated
+        else:
+            y_vec = _unit(_vector3(y_raw, field_name="plane.y_dir"), field_name="plane.y_dir")
+            if abs(_dot(y_vec, x_dir)) <= _Y_DIR_ORTHOGONALITY_TOL and abs(_dot(y_vec, normal)) <= _Y_DIR_ORTHOGONALITY_TOL:
+                # 输入 y_dir 与 x_dir / normal 正交：尊重文档作者给的坐标方向，
+                # 不再静默丢弃（SolidWorks 导出的非标准 y_dir 得以保留）。
+                y_dir = y_vec
+            else:
+                # 偏斜 y_dir：正交化并显式警告，避免"静默丢语义"。
+                warnings.warn(
+                    f"plane y_dir {list(y_vec)} is not orthogonal to x_dir/normal; re-orthogonalized to {list(generated)}",
+                    UserWarning,
+                    stacklevel=2,
+                )
+                y_dir = generated
         return cls(origin_mm=origin, x_dir=x_dir, y_dir=y_dir, normal=normal)
 
     def as_dict(self) -> dict[str, list[float]]:
diff --git a/backend/engine/cdsl_engine/sketch_solver.py b/backend/engine/cdsl_engine/sketch_solver.py
index c9c5e4a6..29bff622 100644
--- a/backend/engine/cdsl_engine/sketch_solver.py
+++ b/backend/engine/cdsl_engine/sketch_solver.py
@@ -44,14 +44,33 @@ def _to_3d(workplane: _Ctx, u: float, v: float) -> list[float]:
     origin = workplane.get("origin_mm") or [0, 0, 0]
     x_dir = workplane.get("x_dir") or [1, 0, 0]
     normal = workplane.get("normal") or [0, 0, 1]
-    y_dir = [
-        normal[1] * x_dir[2] - normal[2] * x_dir[1],
-        normal[2] * x_dir[0] - normal[0] * x_dir[2],
-        normal[0] * x_dir[1] - normal[1] * x_dir[0],
-    ]
+    y_raw = workplane.get("y_dir")
+    y_dir = _default_y_dir(x_dir, normal)
+    if y_raw:
+        magnitude = math.sqrt(sum(component * component for component in y_raw))
+        if magnitude > 1e-12:
+            y_unit = [component / magnitude for component in y_raw]
+            # 与 PlaneSpec.from_mapping 同策略：只有与 x_dir / normal 正交的
+            # y_dir 才尊重（SolidWorks 导出的 y_dir==x_dir 占位数据与 X 平行，
+            # 直接使用会让轮廓塌缩成一条线，必须回退到 normal×x_dir）。
+            if abs(_dot(y_unit, x_dir)) <= 1e-6 and abs(_dot(y_unit, normal)) <= 1e-6:
+                y_dir = y_unit
     return [origin[0] + u * x_dir[0] + v * y_dir[0], origin[1] + u * x_dir[1] + v * y_dir[1], origin[2] + u * x_dir[2] + v * y_dir[2]]
 
 
+def _dot(left: Iterable[float], right: Iterable[float]) -> float:
+    return sum(a * b for a, b in zip(left, right))
+
+
+def _default_y_dir(x_dir: Iterable[float], normal: Iterable[float]) -> list[float]:
+    x, n = list(x_dir), list(normal)
+    return [
+        n[1] * x[2] - n[2] * x[1],
+        n[2] * x[0] - n[0] * x[2],
+        n[0] * x[1] - n[1] * x[0],
+    ]
+
+
 def _transform_contours(contours: list[_Ctx], workplane: _Ctx) -> list[_Ctx]:
     transformed: list[_Ctx] = []
     normal = workplane.get("normal") or [0, 0, 1]