[gpt-oss] tool parser supports for /chat/completions [1/n] (#22386)

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>
Co-authored-by: Simon Mo <simon.mo@hey.com>

tests/entrypoints/openai/test_serving_chat.py

@@ -1,13 +1,16 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+from __future__ import annotations
+
 import asyncio
 from contextlib import suppress
 from dataclasses import dataclass, field
-from typing import Any, Optional
+from typing import TYPE_CHECKING, Any, Optional
 from unittest.mock import MagicMock
 
 import pytest
+import pytest_asyncio
 
 from vllm.config import MultiModalConfig
 from vllm.engine.multiprocessing.client import MQLLMEngineClient
@@ -17,6 +20,164 @@ from vllm.entrypoints.openai.serving_models import (BaseModelPath,
                                                     OpenAIServingModels)
 from vllm.transformers_utils.tokenizer import get_tokenizer
 
+from ...utils import RemoteOpenAIServer
+
+if TYPE_CHECKING:
+    from openai import OpenAI
+
+GPT_OSS_MODEL_NAME = "openai/gpt-oss-20b"
+
+
+@pytest.fixture(scope="module")
+def monkeypatch_module():
+    from _pytest.monkeypatch import MonkeyPatch
+    mpatch = MonkeyPatch()
+    yield mpatch
+    mpatch.undo()
+
+
+@pytest.fixture(scope="module")
+def gptoss_server(monkeypatch_module: pytest.MonkeyPatch):
+    with monkeypatch_module.context() as m:
+        m.setenv("VLLM_ATTENTION_BACKEND", "TRITON_ATTN_VLLM_V1")
+        args = [
+            "--enforce-eager",
+            "--max-model-len",
+            "8192",
+            "--tool-call-parser",
+            "openai",
+            "--reasoning-parser",
+            "openai_gptoss",
+            "--enable-auto-tool-choice",
+        ]
+        with RemoteOpenAIServer(GPT_OSS_MODEL_NAME, args) as remote_server:
+            yield remote_server
+
+
+@pytest_asyncio.fixture
+async def gptoss_client(gptoss_server):
+    async with gptoss_server.get_async_client() as async_client:
+        yield async_client
+
+
+@pytest.mark.asyncio
+async def test_gpt_oss_chat_tool_call_streaming(gptoss_client: OpenAI):
+    tools = [{
+        "type": "function",
+        "function": {
+            "name": "get_current_weather",
+            "description": "Get the current weather in a given location",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "city": {
+                        "type": "string"
+                    },
+                    "state": {
+                        "type": "string"
+                    },
+                    "unit": {
+                        "type": "string",
+                        "enum": ["celsius", "fahrenheit"],
+                    },
+                },
+                "required": ["city", "state", "unit"],
+            },
+        },
+    }]
+
+    messages = [
+        {
+            "role": "user",
+            "content": "What is the weather in Dallas, TX?"
+        },
+    ]
+
+    stream = await gptoss_client.chat.completions.create(
+        model=GPT_OSS_MODEL_NAME, messages=messages, tools=tools, stream=True)
+
+    name = None
+    args_buf = ""
+    async for chunk in stream:
+        delta = chunk.choices[0].delta
+        if delta.tool_calls:
+            tc = delta.tool_calls[0]
+            if tc.function and tc.function.name:
+                name = tc.function.name
+            if tc.function and tc.function.arguments:
+                args_buf += tc.function.arguments
+
+    assert name is not None
+    assert len(args_buf) > 0
+
+
+@pytest.mark.asyncio
+async def test_gpt_oss_multi_turn_chat(gptoss_client: OpenAI):
+    tools = [{
+        "type": "function",
+        "function": {
+            "name": "get_current_weather",
+            "description": "Get the current weather in a given location",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "city": {
+                        "type": "string"
+                    },
+                    "state": {
+                        "type": "string"
+                    },
+                    "unit": {
+                        "type": "string",
+                        "enum": ["celsius", "fahrenheit"],
+                    },
+                },
+                "required": ["city", "state", "unit"],
+            },
+        },
+    }]
+
+    messages = [
+        {
+            "role": "system",
+            "content": "you are a helpful assistant"
+        },
+        {
+            "role": "user",
+            "content": "What is the weather in Dallas, TX?"
+        },
+    ]
+
+    first = await gptoss_client.chat.completions.create(
+        model=GPT_OSS_MODEL_NAME,
+        messages=messages,
+        tools=tools,
+        temperature=0.0,
+    )
+    first_msg = first.choices[0].message
+    assert first_msg.tool_calls is not None and len(first_msg.tool_calls) > 0
+    tc = first_msg.tool_calls[0]
+    assert tc.function is not None and tc.function.name == "get_current_weather"
+    args1 = tc.function.arguments
+    assert args1 is not None and len(args1) > 0
+
+    messages.append({"role": "assistant", "content": args1})
+    messages.append({
+        "role": "user",
+        "content": "Now convert to celsius and return JSON only"
+    })
+
+    second = await gptoss_client.chat.completions.create(
+        model=GPT_OSS_MODEL_NAME,
+        messages=messages,
+        tools=tools,
+        temperature=0.0,
+    )
+    second_msg = second.choices[0].message
+    assert (second_msg.content is not None and len(second_msg.content) > 0) or \
+        (second_msg.tool_calls is not None and len(second_msg.tool_calls) > 0)  # noqa: E501
+
+
 MODEL_NAME = "openai-community/gpt2"
 CHAT_TEMPLATE = "Dummy chat template for testing {}"
 BASE_MODEL_PATHS = [BaseModelPath(name=MODEL_NAME, model_path=MODEL_NAME)]

tests/tool_use/test_openai_tool_parser.py

@@ -0,0 +1,147 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import json
+
+import pytest
+from openai_harmony import (Conversation, DeveloperContent,
+                            HarmonyEncodingName, Message, Role, SystemContent,
+                            load_harmony_encoding)
+
+from vllm.entrypoints.openai.protocol import FunctionCall, ToolCall
+from vllm.entrypoints.openai.tool_parsers import OpenAIToolParser
+from vllm.transformers_utils.tokenizer import get_tokenizer
+
+MODEL = "gpt2"
+
+
+@pytest.fixture(scope="module")
+def openai_tokenizer():
+    # The parser does not use the tokenizer, but the constructor requires it.
+    return get_tokenizer(MODEL)
+
+
+@pytest.fixture
+def openai_tool_parser(openai_tokenizer):
+    return OpenAIToolParser(openai_tokenizer)
+
+
+@pytest.fixture(scope="module")
+def harmony_encoding():
+    return load_harmony_encoding(HarmonyEncodingName.HARMONY_GPT_OSS)
+
+
+def assert_tool_calls(
+    actual_tool_calls: list[ToolCall],
+    expected_tool_calls: list[ToolCall],
+):
+    assert len(actual_tool_calls) == len(expected_tool_calls)
+
+    for actual_tool_call, expected_tool_call in zip(actual_tool_calls,
+                                                    expected_tool_calls):
+        assert isinstance(actual_tool_call.id, str)
+        assert len(actual_tool_call.id) > 16  # Default from protocol.py
+        assert actual_tool_call.type == "function"
+        assert actual_tool_call.function == expected_tool_call.function
+
+
+def test_extract_tool_calls_no_tools(openai_tool_parser, harmony_encoding):
+    convo = Conversation.from_messages([
+        Message.from_role_and_content(
+            Role.SYSTEM,
+            SystemContent.new(),
+        ),
+        Message.from_role_and_content(
+            Role.DEVELOPER,
+            DeveloperContent.new().with_instructions("Talk like a pirate!")),
+        Message.from_role_and_content(Role.USER, "Arrr, how be you?"),
+        Message.from_role_and_content(Role.ASSISTANT,
+                                      "This is a test").with_channel("final")
+    ])
+    token_ids = harmony_encoding.render_conversation_for_completion(
+        convo, Role.ASSISTANT)
+    extracted_info = openai_tool_parser.extract_tool_calls(
+        "",
+        request=None,
+        token_ids=token_ids,
+    )
+    assert not extracted_info.tools_called
+    assert extracted_info.tool_calls == []
+    assert extracted_info.content == "This is a test"
+
+
+def test_extract_tool_calls_single_tool(openai_tool_parser, harmony_encoding):
+    convo = Conversation.from_messages([
+        Message.from_role_and_content(Role.USER,
+                                      "What is the weather in Tokyo?"),
+        Message.from_role_and_content(
+            Role.ASSISTANT,
+            'User asks: "What is the weather in Tokyo?" We need to use get_current_weather tool.',  #  noqa: E501
+        ).with_channel("analysis"),
+        Message.from_role_and_content(
+            Role.ASSISTANT,
+            '{"location": "Tokyo"}').with_channel("commentary").with_recipient(
+                "functions.get_current_weather").with_content_type("json"),
+    ])
+    token_ids = harmony_encoding.render_conversation_for_completion(
+        convo, Role.ASSISTANT)
+
+    extracted_info = openai_tool_parser.extract_tool_calls(
+        "",
+        request=None,
+        token_ids=token_ids,
+    )
+    assert extracted_info.tools_called
+    expected_tool_calls = [
+        ToolCall(function=FunctionCall(
+            name="get_current_weather",
+            arguments=json.dumps({"location": "Tokyo"}),
+        ))
+    ]
+    assert_tool_calls(extracted_info.tool_calls, expected_tool_calls)
+    assert extracted_info.content is None
+
+
+def test_extract_tool_calls_multiple_tools(
+    openai_tool_parser,
+    harmony_encoding,
+):
+    convo = Conversation.from_messages([
+        Message.from_role_and_content(
+            Role.USER, "What is the weather in Tokyo based on where I'm at?"),
+        Message.from_role_and_content(
+            Role.ASSISTANT,
+            'User asks: "What is the weather in Tokyo?" based on their location. We need to use get_current_weather tool and get_user_location tool.',  #  noqa: E501
+        ).with_channel("analysis"),
+        Message.from_role_and_content(
+            Role.ASSISTANT,
+            '{"location": "Tokyo"}').with_channel("commentary").with_recipient(
+                "functions.get_current_weather").with_content_type("json"),
+        Message.from_role_and_content(
+            Role.ASSISTANT,
+            '{"location": "Tokyo"}').with_channel("commentary").with_recipient(
+                "functions.get_user_location").with_content_type("json"),
+    ])
+    token_ids = harmony_encoding.render_conversation_for_completion(
+        convo,
+        Role.ASSISTANT,
+    )
+
+    extracted_info = openai_tool_parser.extract_tool_calls(
+        "",
+        request=None,
+        token_ids=token_ids,
+    )
+    assert extracted_info.tools_called
+    expected_tool_calls = [
+        ToolCall(function=FunctionCall(
+            name="get_current_weather",
+            arguments=json.dumps({"location": "Tokyo"}),
+        )),
+        ToolCall(function=FunctionCall(
+            name="get_user_location",
+            arguments=json.dumps({"location": "Tokyo"}),
+        ))
+    ]
+    assert_tool_calls(extracted_info.tool_calls, expected_tool_calls)
+    assert extracted_info.content is None

vllm/entrypoints/harmony_utils.py

@@ -1,5 +1,8 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from __future__ import annotations
+
 import datetime
 import json
 from collections.abc import Iterable, Sequence
@@ -18,7 +21,8 @@ from openai_harmony import (Author, Conversation, DeveloperContent,
                             Role, StreamableParser, SystemContent, TextContent,
                             ToolDescription, load_harmony_encoding)
 
-from vllm.entrypoints.openai.protocol import ResponseInputOutputItem
+from vllm.entrypoints.openai.protocol import (ChatCompletionToolsParam,
+                                              ResponseInputOutputItem)
 from vllm.utils import random_uuid
 
 REASONING_EFFORT = {
@@ -63,13 +67,29 @@ def get_system_message(
     return sys_msg
 
 
-def get_developer_message(instructions: Optional[str] = None,
+def create_tool_definition(tool: Union[ChatCompletionToolsParam, Tool]):
-                          tools: Optional[list[Tool]] = None) -> Message:
+    if isinstance(tool, ChatCompletionToolsParam):
+        return ToolDescription.new(
+            name=tool.function.name,
+            description=tool.function.description,
+            parameters=tool.function.parameters,
+        )
+    return ToolDescription.new(
+        name=tool.name,
+        description=tool.description,
+        parameters=tool.parameters,
+    )
+
+
+def get_developer_message(
+    instructions: Optional[str] = None,
+    tools: Optional[list[Union[Tool, ChatCompletionToolsParam]]] = None,
+) -> Message:
     dev_msg_content = DeveloperContent.new()
     if instructions is not None:
         dev_msg_content = dev_msg_content.with_instructions(instructions)
     if tools is not None:
-        function_tools = []
+        function_tools: list[Union[Tool, ChatCompletionToolsParam]] = []
         for tool in tools:
             if tool.type in ("web_search_preview", "code_interpreter"):
                 # These are built-in tools that are added to the system message.
@@ -80,11 +100,7 @@ def get_developer_message(instructions: Optional[str] = None,
                 raise ValueError(f"tool type {tool.type} not supported")
         if function_tools:
             function_tool_descriptions = [
-                ToolDescription.new(
+                create_tool_definition(tool) for tool in function_tools
-                    name=tool.name,
-                    description=tool.description,
-                    parameters=tool.parameters,
-                ) for tool in function_tools
             ]
             dev_msg_content = dev_msg_content.with_function_tools(
                 function_tool_descriptions)
@@ -148,16 +164,46 @@ def parse_response_input(
     return msg
 
 
-def parse_chat_input(chat_msg) -> Message:
+def parse_chat_input(chat_msg) -> list[Message]:
-    role = chat_msg["role"]
+    if not isinstance(chat_msg, dict):
-    content = chat_msg["content"]
+        # Handle Pydantic models
+        chat_msg = chat_msg.model_dump(exclude_none=True)
+
+    role = chat_msg.get("role")
+
+    # Assistant message with tool calls
+    tool_calls = chat_msg.get("tool_calls")
+    if role == "assistant" and tool_calls:
+        msgs: list[Message] = []
+        for call in tool_calls:
+            func = call.get("function", {})
+            name = func.get("name", "")
+            arguments = func.get("arguments", "") or ""
+            msg = Message.from_role_and_content(Role.ASSISTANT, arguments)
+            msg = msg.with_channel("commentary")
+            msg = msg.with_recipient(f"functions.{name}")
+            msg = msg.with_content_type("json")
+            msgs.append(msg)
+        return msgs
+
+    # Tool role message (tool output)
+    if role == "tool":
+        name = chat_msg.get("name", "")
+        content = chat_msg.get("content", "") or ""
+        msg = Message.from_author_and_content(
+            Author.new(Role.TOOL, f"functions.{name}"),
+            content).with_channel("commentary")
+        return [msg]
+
+    # Default: user/assistant/system messages with content
+    content = chat_msg.get("content", "")
     if isinstance(content, str):
         contents = [TextContent(text=content)]
     else:
         # TODO: Support refusal.
         contents = [TextContent(text=c.get("text", "")) for c in content]
     msg = Message.from_role_and_contents(role, contents)
-    return msg
+    return [msg]
 
 
 def render_for_completion(messages: list[Message]) -> list[int]:

vllm/entrypoints/openai/serving_chat.py

@@ -6,7 +6,7 @@ import json
 import time
 from collections.abc import AsyncGenerator, AsyncIterator
 from collections.abc import Sequence as GenericSequence
-from typing import Callable, Final, Optional, Union
+from typing import TYPE_CHECKING, Callable, Final, Optional, Union
 
 import jinja2
 import partial_json_parser
@@ -489,6 +489,8 @@ class OpenAIServingChat(OpenAIServing):
                 get_streamable_parser_for_assistant()
                 for _ in range(num_choices)
             ]
+            harmony_tools_streamed = [False] * num_choices
+        tools_streamed = [False] * num_choices
 
         if isinstance(request.tool_choice, ChatCompletionNamedToolChoiceParam):
             tool_choice_function_name = request.tool_choice.function.name
@@ -662,13 +664,11 @@ class OpenAIServingChat(OpenAIServing):
 
                     if self.use_harmony:
                         harmony_parser = harmony_parsers[i]
+                        prev_recipient = harmony_parser.current_recipient
                         for token_id in output.token_ids:
                             harmony_parser.process(token_id)
-                        is_reasoning = \
+                        cur_channel = harmony_parser.current_channel
-                            harmony_parser.current_channel == "analysis"
+                        cur_recipient = harmony_parser.current_recipient
-                        if not request.include_reasoning and is_reasoning:
-                            # Skip the reasoning content.
-                            continue
                         delta_text = harmony_parser.last_content_delta or ""
                     else:
                         delta_text = output.text
@@ -681,8 +681,7 @@ class OpenAIServingChat(OpenAIServing):
                     delta_message: Optional[DeltaMessage]
 
                     # just update previous_texts and previous_token_ids
-                    if ((tool_choice_auto or self.reasoning_parser)
+                    if tool_choice_auto or self.reasoning_parser:
-                            and not self.use_harmony):
                         assert previous_texts is not None
                         assert all_previous_token_ids is not None
                         previous_text = previous_texts[i]
@@ -696,11 +695,54 @@ class OpenAIServingChat(OpenAIServing):
                             current_token_ids = as_list(output.token_ids)
 
                     if self.use_harmony:
-                        if is_reasoning:
+                        if cur_channel == "final":
-                            delta_message = DeltaMessage(
-                                reasoning_content=delta_text)
-                        else:
                             delta_message = DeltaMessage(content=delta_text)
+                        elif cur_channel == "analysis":
+                            if request.include_reasoning:
+                                delta_message = DeltaMessage(
+                                    reasoning_content=delta_text)
+                            else:
+                                delta_message = None
+                        elif (cur_channel == "commentary" and cur_recipient
+                              and cur_recipient.startswith("functions.")):
+                            # Count completed tool calls to determine index
+                            base_index = 0
+                            for msg in harmony_parser.messages:
+                                if (msg.channel == "commentary"
+                                        and msg.recipient
+                                        and msg.recipient.startswith(
+                                            "functions.")):
+                                    base_index += 1
+
+                            if prev_recipient != cur_recipient:
+                                tool_name = cur_recipient.split(
+                                    "functions.", 1)[1]
+                                delta_message = DeltaMessage(tool_calls=[
+                                    DeltaToolCall(
+                                        id=make_tool_call_id(),
+                                        type="function",
+                                        function=DeltaFunctionCall(
+                                            name=tool_name,
+                                            arguments="",
+                                        ),
+                                        index=base_index,
+                                    )
+                                ])
+                            elif delta_text:
+                                delta_message = DeltaMessage(tool_calls=[
+                                    DeltaToolCall(
+                                        index=base_index,
+                                        function=DeltaFunctionCall(
+                                            arguments=delta_text),
+                                    )
+                                ])
+                            else:
+                                delta_message = None
+
+                            if delta_message is not None:
+                                harmony_tools_streamed[i] = True
+                        else:
+                            delta_message = None
                     # handle streaming deltas for tools with named tool_choice
                     elif tool_choice_function_name:
                         if (self.reasoning_parser and not reasoning_end_arr[i]
@@ -758,6 +800,7 @@ class OpenAIServingChat(OpenAIServing):
                             delta_message = DeltaMessage(tool_calls=[
                                 delta_tool_call,
                             ])
+                            tools_streamed[i] = True
 
                     elif request.tool_choice == "required":
                         assert previous_texts is not None
@@ -783,6 +826,7 @@ class OpenAIServingChat(OpenAIServing):
                         if (delta_message and delta_message.tool_calls and
                                 delta_message.tool_calls[0].id is not None):
                             history_tool_call_cnt += 1
+                            tools_streamed[i] = True
 
                         # update the previous values for the next iteration
                         previous_texts[i] = current_text
@@ -859,6 +903,8 @@ class OpenAIServingChat(OpenAIServing):
                                     current_token_ids=current_token_ids,
                                     delta_token_ids=delta_token_ids,
                                     request=request))
+                            if delta_message and delta_message.tool_calls:
+                                tools_streamed[i] = True
                     # when only tool calls
                     elif tool_choice_auto:
                         assert tool_parser is not None
@@ -871,6 +917,8 @@ class OpenAIServingChat(OpenAIServing):
                                 current_token_ids=current_token_ids,
                                 delta_token_ids=output.token_ids,
                                 request=request))
+                        if delta_message and delta_message.tool_calls:
+                            tools_streamed[i] = True
 
                     # when only reasoning
                     elif self.reasoning_parser:
@@ -907,7 +955,10 @@ class OpenAIServingChat(OpenAIServing):
                     # wasn't ready to send a token, then
                     #   get the next token without streaming a chunk
                     if delta_message is None:
-                        continue
+                        if output.finish_reason is None:
+                            continue
+                        else:
+                            delta_message = DeltaMessage()
 
                     # Log streaming delta if output logging is enabled
                     if self.enable_log_outputs and self.request_logger:
@@ -993,12 +1044,18 @@ class OpenAIServingChat(OpenAIServing):
                             ])
 
                         # Send the finish response for each request.n only once
+                        if auto_tools_called or tools_streamed[i] or (
+                                self.use_harmony
+                                and harmony_tools_streamed[i]):
+                            finish_reason_ = "tool_calls"
+                        else:
+                            finish_reason_ = output.finish_reason \
+                                if output.finish_reason else "stop"
                         choice_data = ChatCompletionResponseStreamChoice(
                             index=i,
                             delta=delta_message,
                             logprobs=logprobs,
-                            finish_reason=output.finish_reason
+                            finish_reason=finish_reason_,
-                            if not auto_tools_called else "tool_calls",
                             stop_reason=output.stop_reason,
                             token_ids=(as_list(output.token_ids)
                                        if request.return_token_ids else None))
@@ -1131,31 +1188,32 @@ class OpenAIServingChat(OpenAIServing):
                 logprobs = None
 
             if self.use_harmony:
-                reasoning_content, final_content, is_tool_call = (
+                if TYPE_CHECKING:
-                    parse_chat_output(token_ids))
+                    assert self.tool_parser is not None
-                if not request.include_reasoning:
+                tool_parser = self.tool_parser(tokenizer)
-                    reasoning_content = None
+                # NOTE: We use token_ids for openai tool parser
-
+                tool_call_info = tool_parser.extract_tool_calls(
-                if is_tool_call:
+                    "",
-                    # TODO(woosuk): Implement tool call for gpt-oss.
+                    request=request,
-                    # For now, only Responses API supports tool call for
+                    token_ids=token_ids,  # type: ignore
-                    # gpt-oss.
+                )
-                    raise NotImplementedError(
+                reasoning_content, content = None, tool_call_info.content
-                        "Tool call in Chat Completion API is not supported "
+                if request.include_reasoning:
-                        "for gpt-oss yet. Please use Responses API instead.")
+                    reasoning_content, content, _ = parse_chat_output(
-                else:
+                        token_ids)
-                    # Normal message
+                message = ChatMessage(
-                    message = ChatMessage(
+                    role=role,
-                        role=role,
+                    reasoning_content=reasoning_content,
-                        reasoning_content=reasoning_content,
+                    content=content,
-                        content=final_content,
+                    tool_calls=tool_call_info.tool_calls,
-                    )
+                )
 
                 choice_data = ChatCompletionResponseChoice(
                     index=output.index,
                     message=message,
                     logprobs=logprobs,
-                    finish_reason="tool_calls" if is_tool_call else
+                    finish_reason="tool_calls"
+                    if tool_call_info.tools_called else
                     output.finish_reason if output.finish_reason else "stop",
                     stop_reason=output.stop_reason,
                 )
@@ -1504,12 +1562,12 @@ class OpenAIServingChat(OpenAIServing):
         messages.append(sys_msg)
 
         # Add developer message.
-        dev_msg = get_developer_message()
+        dev_msg = get_developer_message(tools=request.tools)
         messages.append(dev_msg)
 
         # Add user message.
         for chat_msg in request.messages:
-            messages.append(parse_chat_input(chat_msg))
+            messages.extend(parse_chat_input(chat_msg))
 
         # Render prompt token ids.
         prompt_token_ids = render_for_completion(messages)

vllm/entrypoints/openai/tool_parsers/__init__.py

@@ -16,6 +16,7 @@ from .llama4_pythonic_tool_parser import Llama4PythonicToolParser
 from .llama_tool_parser import Llama3JsonToolParser
 from .minimax_tool_parser import MinimaxToolParser
 from .mistral_tool_parser import MistralToolParser
+from .openai_tool_parser import OpenAIToolParser
 from .phi4mini_tool_parser import Phi4MiniJsonToolParser
 from .pythonic_tool_parser import PythonicToolParser
 from .qwen3coder_tool_parser import Qwen3CoderToolParser
@@ -46,4 +47,5 @@ __all__ = [
     "Qwen3CoderToolParser",
     "SeedOssToolParser",
     "Step3ToolParser",
+    "OpenAIToolParser",
 ]

vllm/entrypoints/openai/tool_parsers/openai_tool_parser.py

@@ -0,0 +1,73 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from __future__ import annotations
+
+from collections.abc import Sequence
+from typing import TYPE_CHECKING
+
+from vllm.entrypoints.harmony_utils import parse_output_into_messages
+from vllm.entrypoints.openai.protocol import (ChatCompletionRequest,
+                                              DeltaMessage,
+                                              ExtractedToolCallInformation,
+                                              FunctionCall, ToolCall)
+from vllm.entrypoints.openai.tool_parsers.abstract_tool_parser import (
+    ToolParser, ToolParserManager)
+
+if TYPE_CHECKING:
+    from vllm.transformers_utils.tokenizer import AnyTokenizer
+
+
+@ToolParserManager.register_module("openai")
+class OpenAIToolParser(ToolParser):
+
+    def __init__(self, tokenizer: AnyTokenizer):
+        super().__init__(tokenizer)
+
+    def extract_tool_calls(
+        self,
+        model_output: str,
+        request: ChatCompletionRequest,
+        token_ids: Sequence[int] | None = None,
+    ) -> ExtractedToolCallInformation:
+        if token_ids is None:
+            raise NotImplementedError(
+                "OpenAIToolParser requires token IDs and does not support text-based extraction."  # noqa: E501
+            )
+
+        parser = parse_output_into_messages(token_ids)
+        tool_calls = []
+        final_content = None
+
+        if len(parser.messages) > 0:
+            for msg in parser.messages:
+                if msg.recipient and msg.recipient.startswith("functions."):
+                    tool_calls.append(
+                        ToolCall(
+                            type="function",
+                            function=FunctionCall(
+                                name=msg.recipient.split("functions.")[1],
+                                arguments=msg.content[0].text,
+                            ),
+                        ))
+                elif msg.channel == "final":
+                    final_content = msg.content[0].text
+
+        return ExtractedToolCallInformation(
+            tools_called=len(tool_calls) > 0,
+            tool_calls=tool_calls,
+            content=final_content,
+        )
+
+    def extract_tool_calls_streaming(
+        self,
+        previous_text: str,
+        current_text: str,
+        delta_text: str,
+        previous_token_ids: Sequence[int],
+        current_token_ids: Sequence[int],
+        delta_token_ids: Sequence[int],
+        request: ChatCompletionRequest,
+    ) -> DeltaMessage | None:
+        raise NotImplementedError(
+            "Not being used, manual parsing in serving_chat.py"  # noqa: E501
+        )

vllm/model_executor/models/config.py

@@ -256,7 +256,7 @@ class GptOssForCausalLMConfig(VerifyAndUpdateConfig):
     def verify_and_update_config(vllm_config: "VllmConfig") -> None:
         decoding_config = vllm_config.decoding_config
         if decoding_config.reasoning_backend == "":
-            decoding_config.reasoning_backend = "GptOss"
+            decoding_config.reasoning_backend = "openai_gptoss"
 
         # Increase the max capture size from 512 to 1024 for performance.
         # NOTE(woosuk): This will increase the number of CUDA graphs

vllm/reasoning/gptoss_reasoning_parser.py

@@ -6,6 +6,7 @@ from typing import Optional, Union
 
 from transformers import PreTrainedTokenizerBase
 
+from vllm.entrypoints.harmony_utils import parse_chat_output
 from vllm.entrypoints.openai.protocol import (ChatCompletionRequest,
                                               DeltaMessage)
 from vllm.logger import init_logger
@@ -14,7 +15,7 @@ from vllm.reasoning import ReasoningParser, ReasoningParserManager
 logger = init_logger(__name__)
 
 
-@ReasoningParserManager.register_module("GptOss")
+@ReasoningParserManager.register_module("openai_gptoss")
 class GptOssReasoningParser(ReasoningParser):
     """
     Reasoning parser for GptOss model.
@@ -39,9 +40,10 @@ class GptOssReasoningParser(ReasoningParser):
         return False
 
     def extract_content_ids(self, input_ids: list[int]) -> list[int]:
-        raise RuntimeError(
+        _, content, _ = parse_chat_output(input_ids)
-            "GptOss model uses harmony to extract reasoning content. This "
+        if content is None:
-            "function should not be called.")
+            return []
+        return self.model_tokenizer.encode(content)
 
     def extract_reasoning_content_streaming(
         self,
@@ -52,13 +54,34 @@ class GptOssReasoningParser(ReasoningParser):
         current_token_ids: Sequence[int],
         delta_token_ids: Sequence[int],
     ) -> Union[DeltaMessage, None]:
-        raise RuntimeError(
+        prev_reasoning, prev_content, _ = parse_chat_output(
-            "GptOss model uses harmony to extract reasoning content. This "
+            list(previous_token_ids))
-            "function should not be called.")
+        cur_reasoning, cur_content, _ = parse_chat_output(
+            list(current_token_ids))
+        reasoning_delta = None
+        content_delta = None
+        if cur_reasoning is not None:
+            prev_r = prev_reasoning or ""
+            if cur_reasoning.startswith(prev_r):
+                reasoning_delta = cur_reasoning[len(prev_r):] or None
+            else:
+                reasoning_delta = cur_reasoning
+        if cur_content is not None:
+            prev_c = prev_content or ""
+            if cur_content.startswith(prev_c):
+                content_delta = cur_content[len(prev_c):] or None
+            else:
+                content_delta = cur_content
+        if reasoning_delta is None and content_delta is None:
+            return None
+        return DeltaMessage(reasoning_content=reasoning_delta,
+                            content=content_delta)
 
     def extract_reasoning_content(
-            self, model_output: str, request: ChatCompletionRequest
+        self,
+        model_output: str,
+        request: ChatCompletionRequest,
     ) -> tuple[Optional[str], Optional[str]]:
-        raise RuntimeError(
+        raise NotImplementedError(
-            "GptOss model uses harmony to extract reasoning content. This "
+            "gpt-oss has a special branch for parsing reasoning in non-streaming mode. This method shouldn't be used."  # noqa: E501
-            "function should not be called.")
+        )