From 35bc22f23ce3fcd9c57f318836ec102f7c85647a Mon Sep 17 00:00:00 2001
From: Jialin Ouyang <Jialin.Ouyang@gmail.com>
Date: Mon, 13 Oct 2025 16:31:35 -0700
Subject: [PATCH] [ResponseAPI] Further polish message serialization and unit
 tests (#26728)

Signed-off-by: Jialin Ouyang <Jialin.Ouyang@gmail.com>
---
 tests/entrypoints/openai/test_protocol.py     | 36 +++++++++++++++++++
 .../openai/test_response_api_with_harmony.py  | 31 ----------------
 vllm/entrypoints/openai/protocol.py           |  2 +-
 3 files changed, 37 insertions(+), 32 deletions(-)
 create mode 100644 tests/entrypoints/openai/test_protocol.py

diff --git a/tests/entrypoints/openai/test_protocol.py b/tests/entrypoints/openai/test_protocol.py
new file mode 100644
index 0000000000000..e9b1cfb58b502
--- /dev/null
+++ b/tests/entrypoints/openai/test_protocol.py
@@ -0,0 +1,36 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from openai_harmony import (
+    Message,
+)
+
+from vllm.entrypoints.openai.protocol import serialize_message, serialize_messages
+
+
+def test_serialize_message() -> None:
+    dict_value = {"a": 1, "b": "2"}
+    assert serialize_message(dict_value) == dict_value
+
+    msg_value = {
+        "role": "assistant",
+        "name": None,
+        "content": [{"type": "text", "text": "Test 1"}],
+        "channel": "analysis",
+    }
+    msg = Message.from_dict(msg_value)
+    assert serialize_message(msg) == msg_value
+
+
+def test_serialize_messages() -> None:
+    assert serialize_messages(None) is None
+    assert serialize_messages([]) is None
+
+    dict_value = {"a": 3, "b": "4"}
+    msg_value = {
+        "role": "assistant",
+        "name": None,
+        "content": [{"type": "text", "text": "Test 2"}],
+        "channel": "analysis",
+    }
+    msg = Message.from_dict(msg_value)
+    assert serialize_messages([msg, dict_value]) == [msg_value, dict_value]
diff --git a/tests/entrypoints/openai/test_response_api_with_harmony.py b/tests/entrypoints/openai/test_response_api_with_harmony.py
index 0720c8aa51219..57d88f84d2519 100644
--- a/tests/entrypoints/openai/test_response_api_with_harmony.py
+++ b/tests/entrypoints/openai/test_response_api_with_harmony.py
@@ -12,8 +12,6 @@ from openai_harmony import (
     Message,
 )
 
-from vllm.entrypoints.openai.protocol import serialize_message, serialize_messages
-
 from ...utils import RemoteOpenAIServer
 
 MODEL_NAME = "openai/gpt-oss-20b"
@@ -760,32 +758,3 @@ async def test_output_messages_enabled(client: OpenAI, model_name: str, server):
     assert response.status == "completed"
     assert len(response.input_messages) > 0
     assert len(response.output_messages) > 0
-
-
-def test_serialize_message() -> None:
-    dict_value = {"a": 1, "b": "2"}
-    assert serialize_message(dict_value) == dict_value
-
-    msg_value = {
-        "role": "assistant",
-        "name": None,
-        "content": [{"type": "text", "text": "Test 1"}],
-        "channel": "analysis",
-    }
-    msg = Message.from_dict(msg_value)
-    assert serialize_message(msg) == msg_value
-
-
-def test_serialize_messages() -> None:
-    assert serialize_messages(None) is None
-    assert serialize_messages([]) is None
-
-    dict_value = {"a": 3, "b": "4"}
-    msg_value = {
-        "role": "assistant",
-        "name": None,
-        "content": [{"type": "text", "text": "Test 2"}],
-        "channel": "analysis",
-    }
-    msg = Message.from_dict(msg_value)
-    assert serialize_messages([msg, dict_value]) == [msg_value, dict_value]
diff --git a/vllm/entrypoints/openai/protocol.py b/vllm/entrypoints/openai/protocol.py
index de5cf80105a58..1f2c40e703834 100644
--- a/vllm/entrypoints/openai/protocol.py
+++ b/vllm/entrypoints/openai/protocol.py
@@ -2110,7 +2110,7 @@ def serialize_message(msg):
     """
     if isinstance(msg, dict):
         return msg
-    elif hasattr(msg, "__dict__"):
+    elif hasattr(msg, "to_dict"):
         return msg.to_dict()
     else:
         # fallback to pyandic dump