diff --git a/vllm/reasoning/abs_reasoning_parsers.py b/vllm/reasoning/abs_reasoning_parsers.py
index 4a04292be009e..5c6ac7dad9930 100644
--- a/vllm/reasoning/abs_reasoning_parsers.py
+++ b/vllm/reasoning/abs_reasoning_parsers.py
@@ -121,7 +121,7 @@ class ReasoningParser:
         self,
         original_tag: str | None,
         tool_server: ToolServer | None,
-    ) -> str:
+    ) -> str | None:
         """
         Instance method that is implemented for preparing the structured tag
         Otherwise, None is returned
diff --git a/vllm/reasoning/gptoss_reasoning_parser.py b/vllm/reasoning/gptoss_reasoning_parser.py
index 0c1b54d0bd359..fa45b12856c7d 100644
--- a/vllm/reasoning/gptoss_reasoning_parser.py
+++ b/vllm/reasoning/gptoss_reasoning_parser.py
@@ -145,7 +145,7 @@ class GptOssReasoningParser(ReasoningParser):
     # This function prepares the structural tag to format reasoning output
     def prepare_structured_tag(
         self, original_tag: str | None, tool_server: ToolServer | None
-    ) -> str:
+    ) -> str | None:
         if original_tag is None:
             if tool_server is None:
                 return json.dumps(no_func_reaonsing_tag)