diff --git a/tests/test_mapping.py b/tests/test_mapping.py
index 33ca964..24eb77d 100644
--- a/tests/test_mapping.py
+++ b/tests/test_mapping.py
@@ -2,7 +2,7 @@
 
 from vaultengine import config as cfg
 from vaultengine.mapping import Vault, is_token_like
-from vaultengine.spans import (CAT_DATE, CAT_ID, CAT_ORG, CAT_PERSON, Span)
+from vaultengine.spans import (CAT_DATE, CAT_ORG, CAT_PERSON, Span)
 
 
 class MappingTests(unittest.TestCase):
@@ -94,5 +94,24 @@ def test_save_load(self):
         self.assertEqual(v2.rehydrate_text(v.apply("王五在此")), "王五在此")
 
 
+    def test_ascii_word_boundary_guards(self):
+        v = Vault()
+        pairs = v.assign([
+            Span("Jack", CAT_PERSON, source="llm"),
+            Span("张三", CAT_PERSON, source="llm"),
+        ])
+        text = "Jack went to jackpot with 张三, hijack, and 张三李四."
+        out = v.apply(text, pairs)
+        # Jack -> P-n1 (assuming it's P-n1)
+        # jackpot -> unchanged (partial hit prevented)
+        # hijack -> unchanged (partial hit prevented)
+        # 张三 -> P-n2
+        # 张三李四 -> P-n2李四 (CJK has no word boundary check)
+        self.assertNotIn("Jack went", out)
+        self.assertIn("jackpot", out)
+        self.assertIn("hijack", out)
+        self.assertIn("P-n2李四", out)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/vaultengine/mapping.py b/vaultengine/mapping.py
index 93a6967..dd7a1ef 100644
--- a/vaultengine/mapping.py
+++ b/vaultengine/mapping.py
@@ -136,7 +136,19 @@ def apply(self, text: str, pairs: Optional[Dict[str, str]] = None) -> str:
         pairs = pairs if pairs is not None else self.replacement_pairs()
         for surface in sorted(pairs, key=len, reverse=True):
             if surface and surface in text:
-                text = text.replace(surface, pairs[surface])
+                token = pairs[surface]
+                # Check if first and last characters are ASCII alphanumeric or underscore
+                first = surface[0]
+                last = surface[-1]
+                is_ascii_word = (
+                    ("a" <= first <= "z" or "A" <= first <= "Z" or "0" <= first <= "9" or first == "_") and
+                    ("a" <= last <= "z" or "A" <= last <= "Z" or "0" <= last <= "9" or last == "_")
+                )
+                if is_ascii_word:
+                    pattern = re.compile(r"\b" + re.escape(surface) + r"\b")
+                    text = pattern.sub(token, text)
+                else:
+                    text = text.replace(surface, token)
         return text
 
     def replacement_pairs(self) -> Dict[str, str]: