diff --git a/python/sglang/srt/constrained/base_grammar_backend.py b/python/sglang/srt/constrained/base_grammar_backend.py
index dda3fab4f7b3..18cb9ebe4af7 100644
--- a/python/sglang/srt/constrained/base_grammar_backend.py
+++ b/python/sglang/srt/constrained/base_grammar_backend.py
@@ -112,6 +112,9 @@ def jump_and_retokenize(
         """
         raise NotImplementedError()
 
+    def accept_token_length(self) -> int:
+        raise NotImplementedError()
+
 
 INVALID_GRAMMAR_OBJ = BaseGrammarObject()
 
diff --git a/python/sglang/srt/constrained/reasoner_grammar_backend.py b/python/sglang/srt/constrained/reasoner_grammar_backend.py
index 57fd55a3bf98..1614fcadcb74 100644
--- a/python/sglang/srt/constrained/reasoner_grammar_backend.py
+++ b/python/sglang/srt/constrained/reasoner_grammar_backend.py
@@ -25,11 +25,16 @@
 
 
 class ReasonerGrammarObject(BaseGrammarObject):
-    def __init__(self, grammar: BaseGrammarObject, think_end_id):
+    def __init__(
+        self,
+        grammar: BaseGrammarObject,
+        think_end_id,
+        is_in_reasoning=True,
+    ):
         super().__init__()
         self.grammar = grammar
         self.think_end_id = think_end_id
-        self.is_in_reasoning = True
+        self.is_in_reasoning = is_in_reasoning
 
     def accept_token(self, token: int):
         if token == self.think_end_id:
@@ -38,12 +43,27 @@ def accept_token(self, token: int):
         if not self.is_in_reasoning and token != self.think_end_id:
             self.grammar.accept_token(token)
 
+    def rollback(self, k: int):
+        if self.accept_token_length() > 0:
+            return self.grammar.rollback(k)
+        else:
+            # for spec decoding.
+            self.is_in_reasoning = True
+
+    def is_terminated(self):
+        return self.grammar.is_terminated()
+
     def allocate_vocab_mask(
         self, vocab_size: int, batch_size: int, device
     ) -> torch.Tensor:
         return self.grammar.allocate_vocab_mask(vocab_size, batch_size, device)
 
     def fill_vocab_mask(self, vocab_mask: torch.Tensor, idx: int) -> None:
+        # in reasoning, do not mask, otherwise there will be no token to sample
+        if self.is_in_reasoning:
+            vocab_mask[idx].fill_(-1)
+            return
+
         if not self.is_in_reasoning:
             self.grammar.fill_vocab_mask(vocab_mask, idx)
 
@@ -55,7 +75,11 @@ def apply_vocab_mask(self):
         return self.grammar.apply_vocab_mask
 
     def copy(self) -> BaseGrammarObject:
-        return ReasonerGrammarObject(self.grammar.copy(), self.think_end_id)
+        return ReasonerGrammarObject(
+            self.grammar.copy(),
+            self.think_end_id,
+            self.is_in_reasoning,
+        )
 
     @property
     def finished(self):
@@ -78,6 +102,9 @@ def jump_and_retokenize(
             old_output_ids, new_output_ids, next_state
         )
 
+    def accept_token_length(self) -> int:
+        return self.grammar.accept_token_length()
+
 
 class ReasonerGrammarBackend(BaseGrammarBackend):
     def __init__(self, grammar_backend: BaseGrammarBackend, think_end_id):
diff --git a/python/sglang/srt/constrained/xgrammar_backend.py b/python/sglang/srt/constrained/xgrammar_backend.py
index 58ea764d6220..b0c62978acfd 100644
--- a/python/sglang/srt/constrained/xgrammar_backend.py
+++ b/python/sglang/srt/constrained/xgrammar_backend.py
@@ -158,6 +158,9 @@ def jump_and_retokenize(
         for i in range(k, len(new_output_ids)):
             assert self.matcher.accept_token(new_output_ids[i])
 
+    def accept_token_length(self) -> int:
+        return len(self.accepted_tokens)
+
     def __repr__(self):
         return f"XGrammarGrammar({self.key_string=}, {self.accepted_tokens=}, {self.current_token=})"