diff --git a/vllm/v1/structured_output/__init__.py b/vllm/v1/structured_output/__init__.py
index c701ab1d35a5..07b422814e13 100644
--- a/vllm/v1/structured_output/__init__.py
+++ b/vllm/v1/structured_output/__init__.py
@@ -149,31 +149,37 @@ class StructuredOutputManager:
         # NOTE: This outer loop can likely be parallelized to improve
         # performance of bitmask generation for large batches.
         for req_id, _ in ordered_seq:
-            request = requests[req_id].structured_output_request
-            if TYPE_CHECKING:
-                assert request is not None
-                assert request.grammar is not None
+            request = requests[req_id]
+            structured_output_request = request.structured_output_request
 
-            apply_bitmask = (
-                request.reasoning_ended if self.reasoner is not None else True
-            )  # noqa: E501
+            if TYPE_CHECKING:
+                assert structured_output_request is not None
+                assert structured_output_request.grammar is not None
+            apply_bitmask: bool = True
+            if self.reasoner is not None:
+                if structured_output_request.reasoning_ended is None:
+                    structured_output_request.reasoning_ended = \
+                        self.reasoner.is_reasoning_end(request.prompt_token_ids)
+                apply_bitmask = structured_output_request.reasoning_ended
 
             state_advancements = 0
             req_tokens = scheduled_spec_decode_tokens.get(req_id, []) + [None]
             for i, token in enumerate(req_tokens):
-                if apply_bitmask and not request.grammar.is_terminated():
-                    request.grammar.fill_bitmask(bitmask_tensor,
-                                                 cumulative_index)
+                if apply_bitmask and not \
+                    structured_output_request.grammar.is_terminated():
+                    structured_output_request.grammar.fill_bitmask(
+                        bitmask_tensor, cumulative_index)
                     if token is not None:
                         # In order to generate the correct bitmask for each
                         # position in the speculative sequence, we advance
                         # the FSM state for each speculative token and rollback
                         # to restore the previous state when we are finished.
-                        assert request.grammar.accept_tokens(req_id, [token])
+                        assert structured_output_request.grammar.accept_tokens(
+                            req_id, [token])
                         state_advancements += 1
                 cumulative_index += 1
             if state_advancements > 0:
-                request.grammar.rollback(state_advancements)
+                structured_output_request.grammar.rollback(state_advancements)
 
         if cumulative_index < bitmask_tensor.shape[0]:
             bitmask_tensor = bitmask_tensor[:cumulative_index]
diff --git a/vllm/v1/structured_output/request.py b/vllm/v1/structured_output/request.py
index c16320b9e74c..9a7e30d41aaa 100644
--- a/vllm/v1/structured_output/request.py
+++ b/vllm/v1/structured_output/request.py
@@ -20,7 +20,7 @@ class StructuredOutputRequest:
     sampling_params: SamplingParams
     _grammar: Optional[Union[Future[StructuredOutputGrammar],
                              StructuredOutputGrammar]] = None
-    reasoning_ended: bool = False
+    reasoning_ended: Optional[bool] = None
 
     def _check_grammar_completion(self) -> bool:
         # NOTE: We have to lazy import to gate circular imports