diff --git a/tests/entrypoints/openai/test_response_api_with_harmony.py b/tests/entrypoints/openai/test_response_api_with_harmony.py
new file mode 100644
index 000000000000..1ca52599c519
--- /dev/null
+++ b/tests/entrypoints/openai/test_response_api_with_harmony.py
@@ -0,0 +1,624 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import json
+import time
+
+import pytest
+import pytest_asyncio
+import requests
+from openai import BadRequestError, NotFoundError, OpenAI
+
+from ...utils import RemoteOpenAIServer
+
+pytest.skip(allow_module_level=True, reason="gpt-oss can't run on CI yet.")
+
+MODEL_NAME = "openai/gpt-oss-20b"
+DTYPE = "bfloat16"
+
+
+@pytest.fixture(scope="module")
+def server():
+    args = ["--enforce-eager", "--tool-server", "demo"]
+
+    with RemoteOpenAIServer(MODEL_NAME, args) 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_basic_with_instructions(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input="What is 13 * 24?",
+        instructions="Respond in Korean.",
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_basic_with_reasoning_effort(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input="What is the capital of South Korea?",
+        reasoning={"effort": "low"},
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_chat(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input=[
+            {
+                "role": "system",
+                "content": "Respond in Korean."
+            },
+            {
+                "role": "user",
+                "content": "Hello!"
+            },
+            {
+                "role": "assistant",
+                "content": "Hello! How can I help you today?"
+            },
+            {
+                "role": "user",
+                "content": "What is 13 * 24? Explain your answer."
+            },
+        ],
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_chat_with_input_type(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input=[
+            {
+                "role": "user",
+                "content": [{
+                    "type": "input_text",
+                    "text": "What is 13*24?"
+                }],
+            },
+        ],
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_structured_output(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input=[
+            {
+                "role": "system",
+                "content": "Extract the event information."
+            },
+            {
+                "role": "user",
+                "content":
+                "Alice and Bob are going to a science fair on Friday.",
+            },
+        ],
+        text={
+            "format": {
+                "type": "json_schema",
+                "name": "calendar_event",
+                "schema": {
+                    "type": "object",
+                    "properties": {
+                        "name": {
+                            "type": "string"
+                        },
+                        "date": {
+                            "type": "string"
+                        },
+                        "participants": {
+                            "type": "array",
+                            "items": {
+                                "type": "string"
+                            }
+                        },
+                    },
+                    "required": ["name", "date", "participants"],
+                    "additionalProperties": False,
+                },
+                "description": "A calendar event.",
+                "strict": True,
+            }
+        },
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_structured_output_with_parse(client: OpenAI, model_name: str):
+    from pydantic import BaseModel
+
+    class CalendarEvent(BaseModel):
+        name: str
+        date: str
+        participants: list[str]
+
+    response = await client.responses.parse(
+        model=model_name,
+        input="Alice and Bob are going to a science fair on Friday",
+        instructions="Extract the event information",
+        text_format=CalendarEvent,
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_store(client: OpenAI, model_name: str):
+    for store in [True, False]:
+        response = await client.responses.create(
+            model=model_name,
+            input="What is 13 * 24?",
+            store=store,
+        )
+        assert response is not None
+
+        try:
+            _retrieved_response = await client.responses.retrieve(response.id)
+            is_not_found = False
+        except NotFoundError:
+            is_not_found = True
+
+        assert is_not_found == (not store)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_background(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input="What is 13 * 24?",
+        background=True,
+    )
+    assert response is not None
+
+    retries = 0
+    max_retries = 30
+    while retries < max_retries:
+        response = await client.responses.retrieve(response.id)
+        if response.status == "completed":
+            break
+        time.sleep(1)
+        retries += 1
+
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_background_cancel(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input="Write a long story about a cat.",
+        background=True,
+    )
+    assert response is not None
+    time.sleep(1)
+
+    cancelled_response = await client.responses.cancel(response.id)
+    assert cancelled_response is not None
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_stateful_multi_turn(client: OpenAI, model_name: str):
+    response1 = await client.responses.create(
+        model=model_name,
+        input="What is 13 * 24?",
+    )
+    assert response1 is not None
+    assert response1.status == "completed"
+
+    response2 = await client.responses.create(
+        model=model_name,
+        input="What if I increase both numbers by 1?",
+        previous_response_id=response1.id,
+    )
+    assert response2 is not None
+    assert response2.status == "completed"
+
+    response3 = await client.responses.create(
+        model=model_name,
+        input="Divide the result by 2.",
+        previous_response_id=response2.id,
+    )
+    assert response3 is not None
+    assert response3.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_streaming(client: OpenAI, model_name: str):
+    prompts = [
+        "tell me a story about a cat in 20 words",
+        "What is 13 * 24? Use python to calculate the result.",
+        "When did Jensen found NVIDIA? Search it and answer the year only.",
+    ]
+
+    for prompt in prompts:
+        response = await client.responses.create(
+            model=model_name,
+            input=prompt,
+            reasoning={"effort": "low"},
+            tools=[
+                {
+                    "type": "web_search_preview"
+                },
+                {
+                    "type": "code_interpreter",
+                    "container": {
+                        "type": "auto"
+                    }
+                },
+            ],
+            stream=True,
+        )
+
+        events = []
+        current_event_mode = None
+        async for event in response:
+            if current_event_mode != event.type:
+                current_event_mode = event.type
+                print(f"\n[{event.type}] ", end="", flush=True)
+
+            if "text.delta" in event.type:
+                print(event.delta, end="", flush=True)
+            elif "reasoning_text.delta" in event.type:
+                print(f"{event.delta}", end="", flush=True)
+            elif "response.code_interpreter_call_code.done" in event.type:
+                print(f"Code: {event.code}", end="", flush=True)
+            elif ("response.output_item.added" in event.type
+                  and event.item.type == "web_search_call"):
+                print(f"Web search: {event.item.action}", end="", flush=True)
+            events.append(event)
+
+        assert len(events) > 0
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_web_search(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input="Who is the president of South Korea as of now?",
+        tools=[{
+            "type": "web_search_preview"
+        }],
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_code_interpreter(client: OpenAI, model_name: str):
+    response = await client.responses.create(
+        model=model_name,
+        input="Multiply 64548*15151 using builtin python interpreter.",
+        tools=[{
+            "type": "code_interpreter",
+            "container": {
+                "type": "auto"
+            }
+        }],
+    )
+    assert response is not None
+    assert response.status == "completed"
+
+
+def get_weather(latitude, longitude):
+    response = requests.get(
+        f"https://api.open-meteo.com/v1/forecast?latitude={latitude}&longitude={longitude}&current=temperature_2m,wind_speed_10m&hourly=temperature_2m,relative_humidity_2m,wind_speed_10m"  # noqa
+    )
+    data = response.json()
+    return data["current"]["temperature_2m"]
+
+
+def get_place_to_travel():
+    return "Paris"
+
+
+def call_function(name, args):
+    if name == "get_weather":
+        return get_weather(**args)
+    elif name == "get_place_to_travel":
+        return get_place_to_travel()
+    else:
+        raise ValueError(f"Unknown function: {name}")
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_function_calling(client: OpenAI, model_name: str):
+    tools = [{
+        "type": "function",
+        "name": "get_weather",
+        "description":
+        "Get current temperature for provided coordinates in celsius.",  # noqa
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "latitude": {
+                    "type": "number"
+                },
+                "longitude": {
+                    "type": "number"
+                },
+            },
+            "required": ["latitude", "longitude"],
+            "additionalProperties": False,
+        },
+        "strict": True,
+    }]
+
+    response = await client.responses.create(
+        model=model_name,
+        input="What's the weather like in Paris today?",
+        tools=tools,
+    )
+    assert response is not None
+    assert response.status == "completed"
+    assert len(response.output) == 2
+    assert response.output[0].type == "reasoning"
+    assert response.output[1].type == "function_call"
+
+    tool_call = response.output[1]
+    name = tool_call.name
+    args = json.loads(tool_call.arguments)
+
+    result = call_function(name, args)
+
+    response_2 = await client.responses.create(
+        model=model_name,
+        input=[{
+            "type": "function_call_output",
+            "call_id": tool_call.call_id,
+            "output": str(result),
+        }],
+        tools=tools,
+        previous_response_id=response.id,
+    )
+    assert response_2 is not None
+    assert response_2.status == "completed"
+    assert response_2.output_text is not None
+
+    # NOTE: chain-of-thought should be removed.
+    response_3 = await client.responses.create(
+        model=model_name,
+        input="What's the weather like in Paris today?",
+        tools=tools,
+        previous_response_id=response_2.id,
+    )
+    assert response_3 is not None
+    assert response_3.status == "completed"
+    assert response_3.output_text is not None
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_function_calling_multi_turn(client: OpenAI, model_name: str):
+    tools = [
+        {
+            "type": "function",
+            "name": "get_place_to_travel",
+            "description": "Get a random place to travel",
+            "parameters": {
+                "type": "object",
+                "properties": {},
+                "required": [],
+                "additionalProperties": False,
+            },
+            "strict": True,
+        },
+        {
+            "type": "function",
+            "name": "get_weather",
+            "description":
+            "Get current temperature for provided coordinates in celsius.",  # noqa
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "latitude": {
+                        "type": "number"
+                    },
+                    "longitude": {
+                        "type": "number"
+                    },
+                },
+                "required": ["latitude", "longitude"],
+                "additionalProperties": False,
+            },
+            "strict": True,
+        },
+    ]
+
+    response = await client.responses.create(
+        model=model_name,
+        input=
+        "Help me plan a trip to a random place. And tell me the weather there.",
+        tools=tools,
+    )
+    assert response is not None
+    assert response.status == "completed"
+    assert len(response.output) == 2
+    assert response.output[0].type == "reasoning"
+    assert response.output[1].type == "function_call"
+
+    tool_call = response.output[1]
+    name = tool_call.name
+    args = json.loads(tool_call.arguments)
+
+    result = call_function(name, args)
+
+    response_2 = await client.responses.create(
+        model=model_name,
+        input=[{
+            "type": "function_call_output",
+            "call_id": tool_call.call_id,
+            "output": str(result),
+        }],
+        tools=tools,
+        previous_response_id=response.id,
+    )
+    assert response_2 is not None
+    assert response_2.status == "completed"
+    assert len(response_2.output) == 2
+    assert response_2.output[0].type == "reasoning"
+    assert response_2.output[1].type == "function_call"
+
+    tool_call = response_2.output[1]
+    name = tool_call.name
+    args = json.loads(tool_call.arguments)
+
+    result = call_function(name, args)
+
+    response_3 = await client.responses.create(
+        model=model_name,
+        input=[{
+            "type": "function_call_output",
+            "call_id": tool_call.call_id,
+            "output": str(result),
+        }],
+        tools=tools,
+        previous_response_id=response_2.id,
+    )
+    assert response_3 is not None
+    assert response_3.status == "completed"
+    assert response_3.output_text is not None
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_function_calling_required(client: OpenAI, model_name: str):
+    tools = [{
+        "type": "function",
+        "name": "get_weather",
+        "description":
+        "Get current temperature for provided coordinates in celsius.",  # noqa
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "latitude": {
+                    "type": "number"
+                },
+                "longitude": {
+                    "type": "number"
+                },
+            },
+            "required": ["latitude", "longitude"],
+            "additionalProperties": False,
+        },
+        "strict": True,
+    }]
+
+    with pytest.raises(BadRequestError):
+        await client.responses.create(
+            model=model_name,
+            input="What's the weather like in Paris today?",
+            tools=tools,
+            tool_choice="required",
+        )
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_function_calling_full_history(client: OpenAI, model_name: str):
+    tools = [{
+        "type": "function",
+        "name": "get_weather",
+        "description":
+        "Get current temperature for provided coordinates in celsius.",  # noqa
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "latitude": {
+                    "type": "number"
+                },
+                "longitude": {
+                    "type": "number"
+                },
+            },
+            "required": ["latitude", "longitude"],
+            "additionalProperties": False,
+        },
+        "strict": True,
+    }]
+
+    input_messages = [{
+        "role": "user",
+        "content": "What's the weather like in Paris today?"
+    }]
+
+    response = await client.responses.create(
+        model=model_name,
+        input=input_messages,
+        tools=tools,
+    )
+
+    assert response is not None
+    assert response.status == "completed"
+
+    tool_call = response.output[-1]
+    name = tool_call.name
+    args = json.loads(tool_call.arguments)
+
+    result = call_function(name, args)
+
+    input_messages.extend(
+        response.output)  # append model's function call message
+    input_messages.append(
+        {  # append result message
+            "type": "function_call_output",
+            "call_id": tool_call.call_id,
+            "output": str(result),
+        }
+    )
+
+    response_2 = await client.responses.create(
+        model=model_name,
+        input=input_messages,
+        tools=tools,
+    )
+    assert response_2 is not None
+    assert response_2.status == "completed"
+    assert response_2.output_text is not None