[Frontend] OpenAI Responses API supports Tool/Function calling

examples/online_serving/openai_responses_client_with_tools.py

@@ -0,0 +1,81 @@
+# 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 with tool call
+options enabled.
+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/Qwen3-1.7B --reasoning-parser qwen3 \
+   --guided-decoding-backend xgrammar \
+      --enable-auto-tool-choice --tool-call-parser hermes
+"""
+
+import json
+
+from openai import OpenAI
+
+
+def get_weather(latitude: float, longitude: float) -> str:
+    """
+    Mock function to simulate getting weather data.
+    In a real application, this would call an external weather API.
+    """
+    return f"Current temperature at ({latitude}, {longitude}) is 20°C."
+
+
+tools = [
+    {
+        "type": "function",
+        "name": "get_weather",
+        "description": "Get current temperature for provided coordinates in celsius.",
+        "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?"}
+]
+
+
+def main():
+    base_url = "http://0.0.0.0:8000/v1"
+    model = "Qwen/Qwen3-1.7B"
+    client = OpenAI(base_url=base_url, api_key="empty")
+    response = client.responses.create(
+        model=model, input=input_messages, tools=tools, tool_choice="required"
+    )
+    tool_call = response.output[0]
+    args = json.loads(tool_call.arguments)
+
+    result = get_weather(args["latitude"], args["longitude"])
+
+    input_messages.append(tool_call)  # 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 = client.responses.create(
+        model=model,
+        input=input_messages,
+        tools=tools,
+    )
+    print(response_2.output_text)
+
+
+if __name__ == "__main__":
+    main()

tests/v1/entrypoints/openai/responses/conftest.py

@@ -15,8 +15,13 @@ def default_server_args():
         "--max-model-len",
         "8192",
         "--enforce-eager",  # For faster startup.
+        "--enable-auto-tool-choice",
+        "--guided-decoding-backend",
+        "xgrammar",
+        "--tool-call-parser",
+        "hermes",
         "--reasoning-parser",
-        "deepseek_r1",
+        "qwen3",
     ]
 
 

tests/v1/entrypoints/openai/responses/test_function_call.py

@@ -0,0 +1,196 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import json
+
+import openai  # use the official client for correctness check
+import pytest
+
+MODEL_NAME = "Qwen/Qwen3-0.6B"
+tools = [
+    {
+        "type": "function",
+        "name": "get_current_weather",
+        "description": "Get the current weather in a given location",
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "city": {
+                    "type": "string",
+                    "description":
+                    "The city to find the weather for, e.g. 'Vienna'",
+                    "default": "Vienna",
+                },
+                "country": {
+                    "type":
+                    "string",
+                    "description":
+                    "The country that the city is in, e.g. 'Austria'",
+                },
+                "unit": {
+                    "type": "string",
+                    "description": "The unit to fetch the temperature in",
+                    "enum": ["celsius", "fahrenheit"],
+                },
+                "options": {
+                    "$ref": "#/$defs/WeatherOptions",
+                    "description": "Optional parameters for weather query",
+                },
+            },
+            "required": ["country", "unit"],
+            "$defs": {
+                "WeatherOptions": {
+                    "title": "WeatherOptions",
+                    "type": "object",
+                    "additionalProperties": False,
+                    "properties": {
+                        "unit": {
+                            "type": "string",
+                            "enum": ["celsius", "fahrenheit"],
+                            "default": "celsius",
+                            "description": "Temperature unit",
+                            "title": "Temperature Unit",
+                        },
+                        "include_forecast": {
+                            "type": "boolean",
+                            "default": False,
+                            "description":
+                            "Whether to include a 24-hour forecast",
+                            "title": "Include Forecast",
+                        },
+                        "language": {
+                            "type": "string",
+                            "default": "zh-CN",
+                            "description": "Language of the response",
+                            "title": "Language",
+                            "enum": ["zh-CN", "en-US", "ja-JP"],
+                        },
+                    },
+                },
+            },
+        },
+    },
+    {
+        "type": "function",
+        "name": "get_forecast",
+        "description": "Get the weather forecast for a given location",
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "city": {
+                    "type": "string",
+                    "description":
+                    "The city to get the forecast for, e.g. 'Vienna'",
+                    "default": "Vienna",
+                },
+                "country": {
+                    "type":
+                    "string",
+                    "description":
+                    "The country that the city is in, e.g. 'Austria'",
+                },
+                "days": {
+                    "type": "integer",
+                    "description":
+                    "Number of days to get the forecast for (1-7)",
+                },
+                "unit": {
+                    "type": "string",
+                    "description": "The unit to fetch the temperature in",
+                    "enum": ["celsius", "fahrenheit"],
+                },
+            },
+            "required": ["country", "days", "unit"],
+        },
+    },
+]
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+@pytest.mark.parametrize("tool_choice", ["auto", "required"])
+async def test_function_tool_use(client: openai.AsyncOpenAI, model_name: str,
+                                 tool_choice: str):
+    prompt = [{
+            "role": "user",
+            "content":
+            "Can you tell me what the current weather is in Berlin and the "\
+            "forecast for the next 5 days, in fahrenheit?",
+        },]
+    response = await client.responses.create(
+        model=model_name,
+        input=prompt,
+        tools=tools,
+        tool_choice=tool_choice,
+    )
+
+    assert len(response.output) >= 1
+    tool_call = response.output[0]
+
+    assert tool_call.type == "function_call"
+    assert json.loads(tool_call.arguments) is not None
+
+
+@pytest.mark.asyncio
+async def test_named_tool_use(client: openai.AsyncOpenAI):
+
+    def get_weather(latitude: float, longitude: float) -> str:
+        """
+        Mock function to simulate getting weather data.
+        In a real application, this would call an external weather API.
+        """
+        return f"Current temperature at ({latitude}, {longitude}) is 20°C."
+
+    tools = [{
+        "type": "function",
+        "name": "get_weather",
+        "description":
+        "Get current temperature for provided coordinates in celsius.",
+        "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,
+                                             tool_choice={
+                                                 "type": "function",
+                                                 "name": "get_weather"
+                                             })
+    assert len(response.output) == 1
+    tool_call = response.output[0]
+    assert tool_call.type == "function_call"
+    assert tool_call.name == "get_weather"
+    args = json.loads(tool_call.arguments)
+    assert args["latitude"] is not None
+    assert args["longitude"] is not None
+    # call the tool
+    result = get_weather(args["latitude"], args["longitude"])
+    input_messages.append(tool_call)  # 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)
+    })
+    # create a new response with the tool call result
+    response_2 = await client.responses.create(model=MODEL_NAME,
+                                               input=input_messages)
+    # check the output
+    assert len(response_2.output_text) > 0

vllm/entrypoints/openai/api_server.py

@@ -1659,6 +1659,8 @@ async def init_app_state(
         chat_template_content_format=args.chat_template_content_format,
         return_tokens_as_token_ids=args.return_tokens_as_token_ids,
         enable_auto_tools=args.enable_auto_tool_choice,
+        exclude_tools_when_tool_choice_none=args.
+        exclude_tools_when_tool_choice_none,
         tool_parser=args.tool_call_parser,
         tool_server=tool_server,
         reasoning_parser=args.reasoning_parser,