config: template-detected parsers go to the engine's control-plane overlay

`init_tokenizer_manager` wrote the chat-template-detected `reasoning_parser` /
`tool_call_parser` onto the published `ServerArgs`, after
`TokenizerManager.__init__` had already projected the config bags — so the
namespace readers and the resolved-config readback disagreed with the instance,
and a second `Engine` in the same process would inherit the first one's
detection through the shared bags.

Detection is per-engine control-plane state, which the manager already models:
`record_config_updates` records it, the readback endpoints overlay it, and
`config_value` reports what is in effect. `OpenAIServingChat` — the only reader
of these two fields in the tokenizer process — follows the overlay. The
architecture pass (`resolve_auto_parsers`, before the schedulers fork) is
unchanged: the scheduler resolves its own bags from the instance it receives.

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

@@ -74,6 +74,7 @@ class _MockTokenizerManager:
         self.model_path = self.server_args.model_path
         # The manager tracks the served name itself; a weight update rewrites it.
         self.served_model_name = "test-model"
+        self._config_updates = []
 
         # Mock hf_config for _resolve_chat_encoding_spec check
         mock_hf_config = Mock()
@@ -109,6 +110,13 @@ class _MockTokenizerManager:
         self.generate_request = Mock(return_value=_mock_generate())
         self.create_abort_task = Mock()
 
+    def config_value(self, name: str):
+        """The manager's overlay accessor: no control-plane update recorded."""
+        for _source, fields in reversed(self._config_updates):
+            if name in fields:
+                return fields[name]
+        return getattr(self.server_args, name)
+
 
 class _MockTemplateManager:
     """Minimal mock for TemplateManager."""
@@ -148,6 +156,35 @@ class ServingChatTestCase(unittest.TestCase):
         self.fastapi_request = Mock(spec=Request)
         self.fastapi_request.headers = {}
 
+    def test_parsers_follow_the_control_plane_overlay(self):
+        """Template detection records the parsers on the manager, not on its
+        ServerArgs — the instance keeps what the launcher passed."""
+        self.tm.server_args.tool_call_parser = "auto"
+        self.tm.server_args.reasoning_parser = "auto"
+        self.tm._config_updates.append(
+            (
+                "template-detection",
+                {"tool_call_parser": "qwen25", "reasoning_parser": None},
+            )
+        )
+
+        chat = OpenAIServingChat(self.tm, self.template_manager)
+
+        self.assertEqual(chat.tool_call_parser, "qwen25")
+        self.assertIsNone(chat.reasoning_parser)
+        self.assertEqual(self.tm.server_args.tool_call_parser, "auto")
+
+    def test_the_xgrammar_gate_follows_the_overlay(self):
+        """A detected `reasoning_parser` must gate xgrammar, not the seed's "auto"."""
+        self.tm.server_args.reasoning_parser = "auto"
+        self.tm._config_updates.append(
+            ("template-detection", {"reasoning_parser": "qwen3"})
+        )
+        chat = OpenAIServingChat(self.tm, self.template_manager)
+        self.assertEqual(chat.reasoning_parser, "qwen3")
+        # the gate reads the same value the parser was built from
+        self.assertIsNotNone(chat.reasoning_parser)
+
     def test_text_only_model_rejects_media_before_generation(self):
         media_parts = {
             "image_url": {

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

@@ -52,6 +52,7 @@ class MockTokenizerManager:
             tool_call_parser=None,
             incremental_streaming_output=False,
         )
+        self._config_updates = []
         self.tokenizer = Mock()
         self.tokenizer.encode.return_value = [1, 2, 3]
         self.tokenizer.chat_template = None
@@ -60,6 +61,13 @@ class MockTokenizerManager:
         self.generate_request = Mock()
         self.create_abort_task = Mock()
 
+    def config_value(self, name: str):
+        """The manager's overlay accessor: no control-plane update recorded."""
+        for _source, fields in reversed(self._config_updates):
+            if name in fields:
+                return fields[name]
+        return getattr(self.server_args, name)
+
 
 class MockTemplateManager:
     def __init__(self):