diff --git a/client/src/pdd_u2_purchase_adapter.py b/client/src/pdd_u2_purchase_adapter.py
index 2242687..e88e820 100644
--- a/client/src/pdd_u2_purchase_adapter.py
+++ b/client/src/pdd_u2_purchase_adapter.py
@@ -1168,6 +1168,8 @@ class U2PddPurchaseAdapter(PddPurchaseAdapter):
                 diagnostics={"selection_failure": "target_not_visible"},
             )
 
+        self._validate_size_candidate_dimension(dimension)
+
         available_values = tuple(
             item for item in dimension.values if item.available
         )
@@ -1213,6 +1215,39 @@ class U2PddPurchaseAdapter(PddPurchaseAdapter):
         )
         raise PddPurchaseSpecResolutionRequired(snapshot)
 
+    @staticmethod
+    def _validate_size_candidate_dimension(dimension: SpecDimension) -> None:
+        """候选进入快照前满足 #254 的数量、文字和维度边界。"""
+
+        def valid_text(value: str) -> bool:
+            return (
+                1 <= len(value) <= 191
+                and not any(ord(char) < 32 or 127 <= ord(char) <= 159 for char in value)
+            )
+
+        texts = [item.text for item in dimension.values]
+        available_count = sum(item.available for item in dimension.values)
+        invalid_reason = ""
+        if dimension.key != "size" or not valid_text(dimension.name):
+            invalid_reason = "dimension_invalid"
+        elif len(set(texts)) != len(texts):
+            invalid_reason = "candidate_duplicated"
+        elif any(not valid_text(text) for text in texts):
+            invalid_reason = "candidate_text_invalid"
+        elif available_count > 100:
+            invalid_reason = "candidate_count_exceeded"
+        if invalid_reason:
+            raise PddPurchaseError(
+                "PURCHASE_SIZE_CANDIDATE_SCAN_INCOMPLETE",
+                "完整尺码候选不符合规格解析边界，已停止采购",
+                step="purchase_select_options",
+                diagnostics={
+                    "selection_failure": "target_not_visible",
+                    "candidate_validation": invalid_reason,
+                    "candidate_count": available_count,
+                },
+            )
+
     def _collect_size_candidates(self, device: Any) -> Optional[SpecDimension]:
         """使用采集模块同一套第二规格遍历，不建立新连接也不保存 XML。"""
 
diff --git a/client/test/test_pdd_collect_service.py b/client/test/test_pdd_collect_service.py
index d618862..5e635d2 100644
--- a/client/test/test_pdd_collect_service.py
+++ b/client/test/test_pdd_collect_service.py
@@ -1537,7 +1537,7 @@ class PddCollectParserTest(unittest.TestCase):
             sleeper=lambda _seconds: None,
         )
 
-        result = service._collect_size_dimension(device)
+        result = service.collect_second_dimension_candidates(device)
 
         self.assertIsNotNone(result)
         self.assertEqual(result.name, "套餐（16）")
diff --git a/client/test/test_pdd_u2_purchase_adapter.py b/client/test/test_pdd_u2_purchase_adapter.py
index 5df98a5..e23410c 100644
--- a/client/test/test_pdd_u2_purchase_adapter.py
+++ b/client/test/test_pdd_u2_purchase_adapter.py
@@ -1052,6 +1052,54 @@ class U2PddPurchaseAdapterTest(unittest.TestCase):
         )
         adapter.close()
 
+    def test_invalid_candidate_list_never_requests_remote_resolution(self):
+        for values, expected_reason in (
+            (
+                (DimensionValue("M", True), DimensionValue("M", False)),
+                "candidate_duplicated",
+            ),
+            (
+                tuple(
+                    DimensionValue(f"尺码{index}", True)
+                    for index in range(101)
+                ),
+                "candidate_count_exceeded",
+            ),
+        ):
+            with self.subTest(expected_reason=expected_reason):
+                device = FakeDevice()
+                adapter = U2PddPurchaseAdapter(
+                    "USB-001",
+                    device_service=PddDeviceService(
+                        connector=lambda _serial: device
+                    ),
+                    sleeper=lambda _seconds: None,
+                    select_color_fn=lambda *_args, **_kwargs: True,
+                    select_size_fn=lambda *_args, **_kwargs: False,
+                    size_candidate_collector=lambda _device, items=values: (
+                        SpecDimension("size", "尺码", items)
+                    ),
+                )
+                adapter.open_goods(GOODS_URL)
+
+                with self.assertRaises(PddPurchaseError) as raised:
+                    adapter.select_options(
+                        {"color": "黑色", "size": "不存在"}
+                    )
+
+                self.assertNotIsInstance(
+                    raised.exception, PddPurchaseSpecResolutionRequired
+                )
+                self.assertEqual(
+                    raised.exception.code,
+                    "PURCHASE_SIZE_CANDIDATE_SCAN_INCOMPLETE",
+                )
+                self.assertEqual(
+                    raised.exception.diagnostics["candidate_validation"],
+                    expected_reason,
+                )
+                adapter.close()
+
     def test_size_without_selected_color_never_requests_remote_resolution(self):
         device = FakeDevice()
         scans = []
diff --git a/docs/client/02-architecture.md b/docs/client/02-architecture.md
index c62db67..e486a0c 100644
--- a/docs/client/02-architecture.md
+++ b/docs/client/02-architecture.md
@@ -432,7 +432,8 @@ PDD URL，包名和页面类型仍以最新控件树确认。
 第二规格只读遍历。遍历完成后，Adapter 在内存中保留全部观察项及可用状态，并仅为
 可购买项按页面顺序生成 `c1`、`c2` 等候选和 `spec-resolution-v1` 快照哈希。只有完整
 遍历才能抛出 `PddPurchaseSpecResolutionRequired`；遍历不完整、页面丢失、没有可购买项，
-或完整列表中仍存在目标的精确/繁简等价文字，都返回普通采购错误。该 Adapter 不调用
+候选重复、超过 100 条、文字或维度不合法，或完整列表中仍存在目标的精确/繁简等价文字，
+都返回普通采购错误。该 Adapter 不调用
 Admin，也不点击任何远端解析结果；网络请求和解析后复核属于后续工单。
 
 部分 PDD 页面在点击商品页购买入口后直接进入包含规格和数量的