[responsesAPI] parse reasoning item input (#28248)

Signed-off-by: Andrew Xia <axia@fb.com>
Co-authored-by: Andrew Xia <axia@fb.com>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

examples/online_serving/openai_responses_client.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Set up this example by starting a vLLM OpenAI-compatible server.
+Reasoning models can be used through the Responses API as seen here
+https://platform.openai.com/docs/api-reference/responses
+For example:
+vllm serve Qwen/Qwen3-8B --reasoning-parser qwen3
+
+"""
+
+from openai import OpenAI
+
+input_messages = [{"role": "user", "content": "What model are you?"}]
+
+
+def main():
+    base_url = "http://localhost:8000/v1"
+    client = OpenAI(base_url=base_url, api_key="empty")
+    model = "Qwen/Qwen3-8B"  # get_first_model(client)
+    response = client.responses.create(
+        model=model,
+        input=input_messages,
+    )
+
+    for message in response.output:
+        if message.type == "reasoning":
+            # append reasoning message
+            input_messages.append(message)
+
+    response_2 = client.responses.create(
+        model=model,
+        input=input_messages,
+    )
+    print(response_2.output_text)
+    # I am Qwen, a large language model developed by Alibaba Cloud.
+    # I am designed to assist with a wide range of tasks, including
+    # answering questions, creating content, coding, and engaging in
+    # conversations. I can help with various topics and provide
+    # information or support in multiple languages. How can I assist you today?
+
+
+if __name__ == "__main__":
+    main()

tests/entrypoints/openai/test_response_api_simple.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+
+import pytest
+import pytest_asyncio
+from openai import OpenAI
+
+from ...utils import RemoteOpenAIServer
+
+MODEL_NAME = "Qwen/Qwen3-8B"
+
+
+@pytest.fixture(scope="module")
+def server():
+    args = ["--reasoning-parser", "qwen3", "--max_model_len", "5000"]
+    env_dict = dict(
+        VLLM_ENABLE_RESPONSES_API_STORE="1",
+        # uncomment for tool calling
+        # PYTHON_EXECUTION_BACKEND="dangerously_use_uv",
+    )
+
+    with RemoteOpenAIServer(MODEL_NAME, args, env_dict=env_dict) as remote_server:
+        yield remote_server
+
+
+@pytest_asyncio.fixture
+async def client(server):
+    async with server.get_async_client() as async_client:
+        yield async_client
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_basic(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input="What is 13 * 24?",
+    )
+    assert response is not None
+    print("response: ", response)
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_reasoning_item(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input=[
+            {"type": "message", "content": "Hello.", "role": "user"},
+            {
+                "type": "reasoning",
+                "id": "lol",
+                "content": [
+                    {
+                        "type": "reasoning_text",
+                        "text": "We need to respond: greeting.",
+                    }
+                ],
+                "summary": [],
+            },
+        ],
+        temperature=0.0,
+    )
+    assert response is not None
+    assert response.status == "completed"
+    # make sure we get a reasoning and text output
+    assert response.output[0].type == "reasoning"
+    assert response.output[1].type == "message"
+    assert type(response.output[1].content[0].text) is str

tests/entrypoints/openai/test_response_api_with_harmony.py

@@ -35,7 +35,7 @@ GET_WEATHER_SCHEMA = {
 
 @pytest.fixture(scope="module")
 def server():
-    args = ["--enforce-eager", "--tool-server", "demo"]
+    args = ["--enforce-eager", "--tool-server", "demo", "--max_model_len", "5000"]
     env_dict = dict(
         VLLM_ENABLE_RESPONSES_API_STORE="1",
         PYTHON_EXECUTION_BACKEND="dangerously_use_uv",
@@ -550,6 +550,31 @@ def call_function(name, args):
         raise ValueError(f"Unknown function: {name}")
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_reasoning_item(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input=[
+            {"type": "message", "content": "Hello.", "role": "user"},
+            {
+                "type": "reasoning",
+                "id": "lol",
+                "content": [
+                    {
+                        "type": "reasoning_text",
+                        "text": "We need to respond: greeting.",
+                    }
+                ],
+                "summary": [],
+            },
+        ],
+        temperature=0.0,
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize("model_name", [MODEL_NAME])
 async def test_function_calling(client: OpenAI, model_name: str):

tests/entrypoints/test_responses_utils.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import pytest
+from openai.types.responses.response_reasoning_item import (
+    Content,
+    ResponseReasoningItem,
+    Summary,
+)
+
 from vllm.entrypoints.responses_utils import (
+    construct_chat_message_with_tool_call,
     convert_tool_responses_to_completions_format,
 )
 
@@ -28,3 +36,53 @@ class TestResponsesUtils:
         result = convert_tool_responses_to_completions_format(input_tool)
 
         assert result == {"type": "function", "function": input_tool}
+
+    def test_construct_chat_message_with_tool_call(self):
+        item = ResponseReasoningItem(
+            id="lol",
+            summary=[],
+            type="reasoning",
+            content=[
+                Content(
+                    text="Leroy Jenkins",
+                    type="reasoning_text",
+                )
+            ],
+            encrypted_content=None,
+            status=None,
+        )
+        formatted_item = construct_chat_message_with_tool_call(item)
+        assert formatted_item["role"] == "assistant"
+        assert formatted_item["reasoning"] == "Leroy Jenkins"
+
+        item = ResponseReasoningItem(
+            id="lol",
+            summary=[
+                Summary(
+                    text='Hmm, the user has just started with a simple "Hello,"',
+                    type="summary_text",
+                )
+            ],
+            type="reasoning",
+            content=None,
+            encrypted_content=None,
+            status=None,
+        )
+
+        formatted_item = construct_chat_message_with_tool_call(item)
+        assert formatted_item["role"] == "assistant"
+        assert (
+            formatted_item["reasoning"]
+            == 'Hmm, the user has just started with a simple "Hello,"'
+        )
+
+        item = ResponseReasoningItem(
+            id="lol",
+            summary=[],
+            type="reasoning",
+            content=None,
+            encrypted_content="TOP_SECRET_MESSAGE",
+            status=None,
+        )
+        with pytest.raises(ValueError):
+            construct_chat_message_with_tool_call(item)

vllm/entrypoints/responses_utils.py

@@ -10,6 +10,7 @@ from openai.types.chat.chat_completion_message_tool_call_param import (
     Function as FunctionCallTool,
 )
 from openai.types.responses import ResponseFunctionToolCall
+from openai.types.responses.response_reasoning_item import ResponseReasoningItem
 from openai.types.responses.tool import Tool
 
 from vllm import envs
@@ -37,6 +38,18 @@ def construct_chat_message_with_tool_call(
                 )
             ],
         )
+    elif isinstance(item, ResponseReasoningItem):
+        reasoning_content = ""
+        if item.encrypted_content:
+            raise ValueError("Encrypted content is not supported.")
+        if len(item.summary) == 1:
+            reasoning_content = item.summary[0].text
+        elif item.content and len(item.content) == 1:
+            reasoning_content = item.content[0].text
+        return {
+            "role": "assistant",
+            "reasoning": reasoning_content,
+        }
     elif item.get("type") == "function_call_output":
         # Append the function call output as a tool message.
         return ChatCompletionToolMessageParam(