[Bug]: While serving GPT-OSS, Streaming function calls output only reasoning_text, without function tool call

[gsu2017]

Your current environment

The output of python collect_env.py

==============================
      Python Environment
==============================
Python version               : 3.12.11 (main, Jun  4 2025, 08:56:18) [GCC 11.4.0] (64-bit runtime)
Python platform              : Linux-3.10.0-1160.92.1.el7.x86_64-x86_64-with-glibc2.35

==============================
       CUDA / GPU Info
==============================
Is CUDA available            : True
CUDA runtime version         : 12.8.93
CUDA_MODULE_LOADING set to   : LAZY
GPU models and configuration :
GPU 0: NVIDIA L20
GPU 1: NVIDIA L20
GPU 2: NVIDIA L20
GPU 3: NVIDIA L20
GPU 4: NVIDIA L20
GPU 5: NVIDIA L20
GPU 6: NVIDIA L20
GPU 7: NVIDIA L20

Nvidia driver version        : 550.90.07
cuDNN version                : Could not collect
HIP runtime version          : N/A
MIOpen runtime version       : N/A
Is XNNPACK available         : True

==============================
         vLLM Info
==============================
ROCM Version                 : Could not collect
Neuron SDK Version           : N/A
vLLM Version                 : 0.10.1.1
vLLM Build Flags:
  CUDA Archs: Not Set; ROCm: Disabled; Neuron: Disabled
GPU Topology:
  	GPU0	GPU1	GPU2	GPU3	GPU4	GPU5	GPU6	GPU7	CPU Affinity	NUMA Affinity	GPU NUMA ID
GPU0	 X 	PIX	NODE	NODE	SYS	SYS	SYS	SYS	0-31,64-95	0		N/A
GPU1	PIX	 X 	NODE	NODE	SYS	SYS	SYS	SYS	0-31,64-95	0		N/A
GPU2	NODE	NODE	 X 	PIX	SYS	SYS	SYS	SYS	0-31,64-95	0		N/A
GPU3	NODE	NODE	PIX	 X 	SYS	SYS	SYS	SYS	0-31,64-95	0		N/A
GPU4	SYS	SYS	SYS	SYS	 X 	PIX	NODE	NODE	32-63,96-127	1		N/A
GPU5	SYS	SYS	SYS	SYS	PIX	 X 	NODE	NODE	32-63,96-127	1		N/A
GPU6	SYS	SYS	SYS	SYS	NODE	NODE	 X 	PIX	32-63,96-127	1		N/A
GPU7	SYS	SYS	SYS	SYS	NODE	NODE	PIX	 X 	32-63,96-127	1		N/A

🐛 Describe the bug

after serving gpt-oss-120b using vllm, I tried streaming function call examples according to openai cookbook streaming function call.

• if set stream=True in CLIENT.responses.create(...), the output event contains reasoning_text, but function tool call is not included. like this,

ResponseCreatedEvent(response=Response(id='resp_f76035824b624b3da83ce3cb6eefdf8f', created_at=1756783814.0, error=None, incomplete_details=None, instructions=None, metadata=None, model='gpt-oss-120b', object='response', output=[], parallel_tool_calls=True, temperature=1.0, tool_choice='auto', tools=[FunctionTool(name='get_weather', parameters={'type': 'object', 'properties': {'location': {'type': 'string', 'description': 'City and country e.g. Bogotá, Colombia'}}, 'required': ['location'], 'additionalProperties': False}, strict=None, type='function', description='Get current temperature for a given location.')], top_p=1.0, background=False, max_output_tokens=130933, max_tool_calls=None, previous_response_id=None, prompt=None, prompt_cache_key=None, reasoning=None, safety_identifier=None, service_tier='auto', status='in_progress', text=None, top_logprobs=0, truncation='disabled', usage=None, user=None), sequence_number=0, type='response.created')
ResponseInProgressEvent(response=Response(id='resp_f76035824b624b3da83ce3cb6eefdf8f', created_at=1756783814.0, error=None, incomplete_details=None, instructions=None, metadata=None, model='gpt-oss-120b', object='response', output=[], parallel_tool_calls=True, temperature=1.0, tool_choice='auto', tools=[FunctionTool(name='get_weather', parameters={'type': 'object', 'properties': {'location': {'type': 'string', 'description': 'City and country e.g. Bogotá, Colombia'}}, 'required': ['location'], 'additionalProperties': False}, strict=None, type='function', description='Get current temperature for a given location.')], top_p=1.0, background=False, max_output_tokens=130933, max_tool_calls=None, previous_response_id=None, prompt=None, prompt_cache_key=None, reasoning=None, safety_identifier=None, service_tier='auto', status='in_progress', text=None, top_logprobs=0, truncation='disabled', usage=None, user=None), sequence_number=1, type='response.in_progress')
ResponseOutputItemDoneEvent(item=ResponseReasoningItem(id='', summary=[], type='reasoning', content=[Content(text='The user asks: "What\'s the weather like in Paris today?" Need to fetch weather via function get_weather with location "Paris, France". Use function.', type='reasoning_text')], encrypted_content=None, status='completed'), output_index=1, sequence_number=36, type='response.output_item.done')

• elif set stream=False in CLIENT.responses.create(...), the model can output both reasoning_text (ResponseReasoningItem) and function tool call (ResponseFunctionToolCall). like this,

Response(id='resp_d51ea05de80048f39ce97fa56f88d9c4', created_at=1756783924.0, error=None, incomplete_details=None, instructions=None, metadata=None, model='gpt-oss-120b', object='response', output=[ResponseReasoningItem(id='rs_88edd5e0ff8143068093e9eb2bd3fdf1', summary=[], type='reasoning', content=[Content(text='We need to get weather. Use function get_weather with location "Paris, France".', type='reasoning_text')], encrypted_content=None, status=None), ResponseFunctionToolCall(arguments='{\n  "location": "Paris, France"\n}', call_id='call_bc1a92fab0b44fcf8874ec261e5b06f2', name='get_weather', type='function_call', id='ft_bc1a92fab0b44fcf8874ec261e5b06f2', status=None)], parallel_tool_calls=True, temperature=1.0, tool_choice='auto', tools=[FunctionTool(name='get_weather', parameters={'type': 'object', 'properties': {'location': {'type': 'string', 'description': 'City and country e.g. Bogotá, Colombia'}}, 'required': ['location'], 'additionalProperties': False}, strict=None, type='function', description='Get current temperature for a given location.')], top_p=1.0, background=False, max_output_tokens=130933, max_tool_calls=None, previous_response_id=None, prompt=None, prompt_cache_key=None, reasoning=None, safety_identifier=None, service_tier='auto', status='completed', text=None, top_logprobs=0, truncation='disabled', usage=ResponseUsage(input_tokens=0, input_tokens_details=InputTokensDetails(cached_tokens=0), output_tokens=0, output_tokens_details=OutputTokensDetails(reasoning_tokens=0), total_tokens=0), user=None)

here is the python code

from openai import OpenAI

client = OpenAI(
      base_url='',
      api_key=''
)

tools = [{
    "type": "function",
    "name": "get_weather",
    "description": "Get current temperature for a given location.",
    "parameters": {
        "type": "object",
        "properties": {
            "location": {
                "type": "string",
                "description": "City and country e.g. Bogotá, Colombia"
            }
        },
        "required": [
            "location"
        ],
        "additionalProperties": False
    }
}]

# when stream=True
stream = client.responses.create(
    model="gpt-oss-120b",
    input=[{"role": "user", "content": "What's the weather like in Paris today?"}],
    tools=tools,
    stream=True
)

for event in stream:
    print(event)

# when stream=False
responses = CLIENT.responses.create(
    model=MODEL_NAME,
    input=[{"role": "user", "content": "What's the weather like in Paris today?"}],
    tools=tools,
)
print(responses)

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[chaunceyjiang]

streaming function call

Not implemented yet, I’m working on it. #23222

[yeqcharlotte]

#24466