diff --git a/python/sglang/srt/constrained/grammar_manager.py b/python/sglang/srt/constrained/grammar_manager.py
index 754bc825d..c218956a4 100644
--- a/python/sglang/srt/constrained/grammar_manager.py
+++ b/python/sglang/srt/constrained/grammar_manager.py
@@ -147,7 +147,7 @@ class GrammarManager:
                     key = ("regex", req.sampling_params.regex)
                 elif req.sampling_params.ebnf is not None:
                     key = ("ebnf", req.sampling_params.ebnf)
-                elif req.sampling_params.structural_tag:
+                elif req.sampling_params.structural_tag is not None:
                     key = ("structural_tag", req.sampling_params.structural_tag)
 
                 value, cache_hit = self.grammar_backend.get_cached_or_future_value(
diff --git a/python/sglang/srt/sampling/sampling_params.py b/python/sglang/srt/sampling/sampling_params.py
index 77b72aab7..39f6a363b 100644
--- a/python/sglang/srt/sampling/sampling_params.py
+++ b/python/sglang/srt/sampling/sampling_params.py
@@ -134,6 +134,12 @@ class SamplingParams(msgspec.Struct, kw_only=True, array_like=True):
             self.no_stop_trim if self.no_stop_trim is not None else False
         )
 
+        # An empty grammar constraint means "unset", not "constrain to nothing".
+        self.json_schema = self.json_schema or None
+        self.regex = self.regex or None
+        self.ebnf = self.ebnf or None
+        self.structural_tag = self.structural_tag or None
+
         # Process some special cases
         if 0 <= self.temperature < _SAMPLING_EPS:
             # top_k = 1 means greedy sampling
diff --git a/test/registered/unit/constrained/test_grammar_manager.py b/test/registered/unit/constrained/test_grammar_manager.py
index e2abddc07..9f5c96524 100644
--- a/test/registered/unit/constrained/test_grammar_manager.py
+++ b/test/registered/unit/constrained/test_grammar_manager.py
@@ -189,6 +189,21 @@ class TestProcessReqWithGrammar(unittest.TestCase):
             ("structural_tag", '{"structures": [], "triggers": []}'),
         )
 
+    def test_falsy_structural_tag_still_resolves_a_key(self):
+        """The selection chain must cover every value the entry condition admits.
+        A falsy-but-set constraint used to match no branch and hit the key lookup
+        with nothing assigned.
+        """
+        mgr = self._make_mgr()
+        future = Future()
+        mgr.grammar_backend.get_cached_or_future_value.return_value = (future, False)
+
+        req = _make_req(structural_tag="")
+        result = mgr.process_req_with_grammar(req)
+
+        self.assertTrue(result)
+        self.assertEqual(req.grammar_key, ("structural_tag", ""))
+
     def test_cache_hit_returns_false(self):
         """Cache hit should NOT add to grammar queue."""
         mgr = self._make_mgr()
diff --git a/test/registered/unit/sampling/test_sampling_params.py b/test/registered/unit/sampling/test_sampling_params.py
index 6143c51da..4ea9712af 100644
--- a/test/registered/unit/sampling/test_sampling_params.py
+++ b/test/registered/unit/sampling/test_sampling_params.py
@@ -73,6 +73,16 @@ class TestSamplingParamsInit(CustomTestCase):
         sp = SamplingParams(stop_token_ids=[])
         self.assertIsNone(sp.stop_token_ids)
 
+    def test_empty_grammar_constraint_becomes_none(self):
+        """An empty grammar string means "unset", not "constrain to nothing".
+        Left as "" it reads as set to the is-not-None checks downstream while
+        the constraint selection skips it.
+        """
+        for field in ("json_schema", "regex", "ebnf", "structural_tag"):
+            with self.subTest(field=field):
+                sp = SamplingParams(**{field: ""})
+                self.assertIsNone(getattr(sp, field))
+
 
 class TestSamplingParamsVerify(CustomTestCase):