Add overridable hooks for custom chat serving implementations (#25807)

Co-authored-by: Cursor <cursoragent@cursor.com>

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

@@ -475,5 +475,20 @@ class TestValidationEdgeCases(unittest.TestCase):
         self.assertEqual(len(restored_request.messages), len(original_request.messages))
 
 
+class TestParsedResponseFieldsProtocol(unittest.TestCase):
+    """Test ParsedResponseFields protocol."""
+
+    def test_parsed_response_fields_protocol(self):
+        """ParsedResponseFields protocol works with isinstance."""
+        from sglang.srt.entrypoints.openai.protocol import ParsedResponseFields
+
+        class MockFields:
+            content = "hello"
+            tool_calls = None
+            reasoning_content = None
+
+        self.assertIsInstance(MockFields(), ParsedResponseFields)
+
+
 if __name__ == "__main__":
     unittest.main(verbosity=2)

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

@@ -1474,6 +1474,26 @@ class ServingChatTestCase(unittest.TestCase):
             result = self.chat._apply_conversation_template(req, is_multimodal=False)
         self.assertEqual(result.prompt, "BASE_PROMPT")
 
+    # ------------- hook method tests -------------
+    def test_encode_messages_returns_none_by_default(self):
+        """Default _encode_messages returns None (use standard encoding)."""
+        result = self.chat._encode_messages([], Mock(), False)
+        self.assertIsNone(result)
+
+    def test_decode_response_returns_text(self):
+        """Default _decode_response returns ret_item['text']."""
+        ret_item = {"text": "Hello world", "output_ids": [1, 2, 3]}
+        result = self.chat._decode_response(ret_item)
+        self.assertEqual(result, "Hello world")
+
+    def test_get_parsed_response_fields_passthrough(self):
+        """Default _get_parsed_response_fields passes through values."""
+        reasoning = "thinking..."
+        tool_calls = [{"name": "foo"}]
+        r, t = self.chat._get_parsed_response_fields(reasoning, tool_calls)
+        self.assertEqual(r, reasoning)
+        self.assertEqual(t, tool_calls)
+
 
 class TestProcessToolCallsWithRequiredToolChoice(unittest.TestCase):
     """Test _process_tool_calls with tool_choice='required' uses model-specific parser."""