diff --git a/python/sglang/srt/constrained/reasoner_grammar_backend.py b/python/sglang/srt/constrained/reasoner_grammar_backend.py
index 57d0f65d5..6ffb157fe 100644
--- a/python/sglang/srt/constrained/reasoner_grammar_backend.py
+++ b/python/sglang/srt/constrained/reasoner_grammar_backend.py
@@ -78,6 +78,7 @@ class ReasonerGrammarObject(BaseGrammarObject):
     def maybe_init_reasoning(self, reasoning: bool):
         if reasoning:
             self.tokens_in_think = 0
+            self.tokens_after_end = -1
         else:
             self.tokens_in_think = -1
             self.tokens_after_end = 0