[RL][TITO] Preserve whitespace in reasoning parser outputs (#24251)

test/registered/unit/entrypoints/openai/test_serving_chat.py

@@ -1228,6 +1228,37 @@ class ServingChatTestCase(unittest.TestCase):
                 req.reasoning_effort = effort
                 self.assertEqual(chat._get_reasoning_from_request(req), expected)
 
+    def test_non_stream_reasoning_response_preserves_payload_whitespace(self):
+        self.chat.reasoning_parser = "qwen3"
+        self.template_manager.force_reasoning = False
+
+        req = ChatCompletionRequest(
+            model="x",
+            messages=[{"role": "user", "content": "Hi?"}],
+            stream=False,
+            separate_reasoning=True,
+        )
+        ret = [
+            {
+                "text": "<think>\nLet me think\n</think>\n\nThe answer is 42.\n",
+                "meta_info": {
+                    "id": "chatcmpl-test",
+                    "prompt_tokens": 5,
+                    "completion_tokens": 8,
+                    "cached_tokens": 0,
+                    "finish_reason": {"type": "stop", "matched": None},
+                    "weight_version": "test",
+                },
+                "index": 0,
+            }
+        ]
+
+        response = self.chat._build_chat_response(req, ret, created=123)
+
+        message = response.choices[0].message
+        self.assertEqual(message.reasoning_content, "\nLet me think\n")
+        self.assertEqual(message.content, "\n\nThe answer is 42.\n")
+
     # ------------- reasoning config tests -------------
     def test_get_reasoning_from_request_default_true_toggle(self):
         self.tm.server_args.reasoning_parser = "qwen3"

test/registered/unit/parser/test_reasoning_parser.py

@@ -861,6 +861,37 @@ class TestReasoningParser(CustomTestCase):
         self.assertEqual(reasoning, "Let me think")
         self.assertEqual(normal, "The answer is 42.")
 
+    def test_parse_non_stream_preserves_payload_whitespace(self):
+        """Non-streaming parsing must not rewrite text inside or after reasoning."""
+        parser = ReasoningParser("qwen3")
+        reasoning, normal = parser.parse_non_stream(
+            "<think>\nLet me think\n</think>\n\nThe answer is 42.\n"
+        )
+        self.assertEqual(reasoning, "\nLet me think\n")
+        self.assertEqual(normal, "\n\nThe answer is 42.\n")
+
+    def test_parse_non_stream_strips_repeated_leading_start_tokens(self):
+        """Repeated leading start tokens are markers, not reasoning payload."""
+        parser = ReasoningParser("qwen3")
+        reasoning, normal = parser.parse_non_stream(
+            "<think><think>Let me think</think>The answer is 42."
+        )
+        self.assertEqual(reasoning, "Let me think")
+        self.assertEqual(normal, "The answer is 42.")
+
+    def test_parse_stream_chunk_preserves_payload_whitespace(self):
+        """Streaming parsing preserves the same generated payload whitespace."""
+        parser = ReasoningParser("qwen3")
+        reasoning, normal = parser.parse_stream_chunk("<think>")
+        self.assertEqual(reasoning, "")
+        self.assertEqual(normal, "")
+
+        reasoning, normal = parser.parse_stream_chunk(
+            "\nLet me think\n</think>\n\nThe answer is 42.\n"
+        )
+        self.assertEqual(reasoning, "\nLet me think\n")
+        self.assertEqual(normal, "\n\nThe answer is 42.\n")
+
     def test_parse_stream_chunk(self):
         """Test streaming chunk parsing."""
         parser = ReasoningParser("qwen3")