GPT-OSS - SGLang Documentation

1.Model Introduction

GPT-OSS is an advanced large language model developed by OpenAI designed for power reasoning, agentic tasks, and versatile developer use cases. It has versions with two model sizes.

gpt-oss-120b — for production, general purpose, high reasoning use cases that fit into a single 80GB GPU (like NVIDIA H100 80GB or AMD MI300X 192GB) (117B parameters with 5.1B active parameters)
gpt-oss-20b — for lower latency, and local or specialized use cases (21B parameters with 3.6B active parameters)

GPT-OSS introduces several groundbreaking innovations:

Configurable reasoning effort: Easily adjust the reasoning effort (low, medium, high) based on your specific use case and latency needs.
Full chain-of-thought: Gain complete access to the model’s reasoning process, facilitating easier debugging and increased trust in outputs. It’s not intended to be shown to end users.
Fine-tunable: Fully customize models to your specific use case through parameter fine-tuning.
Agentic capabilities: Use the models’ native capabilities for function calling, web browsing, Python code execution, and Structured Outputs.
MXFP4 quantization: The models were post-trained with MXFP4 quantization of the MoE weights, making gpt-oss-120b run on a single 80GB GPU (like NVIDIA H100 80GB or AMD MI300X 192GB) and the gpt-oss-20b model run within 16GB of memory. All evals were performed with the same MXFP4 quantization.

2.SGLang Installation

SGLang offers multiple installation methods. You can choose the most suitable installation method based on your hardware platform and requirements. Please refer to the official SGLang installation guide for installation instructions.

3.Model Deployment

This section provides deployment configurations optimized for different hardware platforms and use cases.

3.1 Basic Configuration

The GPT-OSS series comes in two sizes. Recommended starting configurations vary depending on hardware. Interactive Command Generator: Use the configuration selector below to automatically generate the appropriate deployment command for your hardware platform, model size, quantization method, and thinking capabilities.

3.2 Configuration Tips

For more detailed configuration tips, please refer to GPS-OSS Usage.

4.Model Invocation

4.1 Basic Usage

For basic API usage and request examples, please refer to:

SGLang Basic Usage Guide

4.2 Advanced Usage

4.2.1 Reasoning Parser

GPT-OSS supports reasoning mode. Enable the reasoning parser during deployment to separate the thinking and content sections:

Command

python -m sglang.launch_server \
  --model openai/gpt-oss-120b \
  --reasoning-parser gpt-oss \
  --tp 8

Example

from openai import OpenAI

client = OpenAI(
    base_url="http://localhost:8000/v1",
    api_key="EMPTY"
)

# Enable streaming to see the thinking process in real-time
response = client.chat.completions.create(
    model="openai/gpt-oss-120b",
    messages=[
        {"role": "user", "content": "Solve this problem step by step: What is 15% of 240?"}
    ],
    temperature=0.7,
    max_tokens=2048,
    stream=True
)

# Process the stream
has_thinking = False
has_answer = False
thinking_started = False

for chunk in response:
    if chunk.choices and len(chunk.choices) > 0:
        delta = chunk.choices[0].delta

        # Print thinking process
        if hasattr(delta, 'reasoning_content') and delta.reasoning_content:
            if not thinking_started:
                print("=============== Thinking =================", flush=True)
                thinking_started = True
            has_thinking = True
            print(delta.reasoning_content, end="", flush=True)

        # Print answer content
        if delta.content:
            # Close thinking section and add content header
            if has_thinking and not has_answer:
                print("\n=============== Content =================", flush=True)
                has_answer = True
            print(delta.content, end="", flush=True)

print()

Output Example:

Output

=============== Thinking =================
The user asks: "Solve this problem step by step: What is 15% of 240?" So we need to provide step-by-step solution. Compute 15% of 240: 0.15 * 240 = 36. Provide steps: convert percent to decimal, multiply, maybe use fraction. Provide answer.
=============== Content =================
**Step‑by‑step solution**

1. **Understand what “percent” means**
   “15 %” means 15 out of every 100 parts, i.e. the fraction \(\displaystyle \frac{15}{100}\).

2. **Convert the percent to a decimal (or fraction)**
   \[
   \frac{15}{100}=0.15
   \]

3. **Set up the multiplication**
   To find 15 % of 240 we multiply 240 by the decimal 0.15:
   \[
   240 \times 0.15
   \]

4. **Do the multiplication**
   One convenient way is to break it into two easier parts:
   \[
   240 \times 0.15 = 240 \times \left(\frac{15}{100}\right)
                = \frac{240 \times 15}{100}
   \]

   - First compute \(240 \times 15\):
     \[
     240 \times 15 = 240 \times (10 + 5) = 2400 + 1200 = 3600
     \]

   - Then divide by 100:
     \[
     \frac{3600}{100} = 36
     \]

5. **Write the result**
   \[
   15\% \text{ of } 240 = 36
   \]

---

**Answer:** \(36\)

4.2.2 Tool Calling

GPT-OSS supports tool calling capabilities. Enable the tool call parser: Python Example (without Thinking Process): Start sglang server:

Command

python -m sglang.launch_server \
  --model openai/gpt-oss-120b \
  --tool-call-parser gpt-oss \
  --tp 8

Example

from openai import OpenAI

client = OpenAI(
    base_url="http://localhost:8000/v1",
    api_key="EMPTY"
)

# Define available tools
tools = [
    {
        "type": "function",
        "function": {
            "name": "get_weather",
            "description": "Get the current weather for a location",
            "parameters": {
                "type": "object",
                "properties": {
                    "location": {
                        "type": "string",
                        "description": "The city name"
                    },
                    "unit": {
                        "type": "string",
                        "enum": ["celsius", "fahrenheit"],
                        "description": "Temperature unit"
                    }
                },
                "required": ["location"]
            }
        }
    }
]

# Make request with streaming to see thinking process
response = client.chat.completions.create(
    model="openai/gpt-oss-120b",
    messages=[
        {"role": "user", "content": "What's the weather in Beijing?"}
    ],
    tools=tools,
    temperature=0.7,
    stream=True
)

# Process streaming response
thinking_started = False
has_thinking = False

for chunk in response:
    if chunk.choices and len(chunk.choices) > 0:
        delta = chunk.choices[0].delta

        # Print thinking process
        if hasattr(delta, 'reasoning_content') and delta.reasoning_content:
            if not thinking_started:
                print("=============== Thinking =================", flush=True)
                thinking_started = True
            has_thinking = True
            print(delta.reasoning_content, end="", flush=True)

        # Print tool calls
        if hasattr(delta, 'tool_calls') and delta.tool_calls:
            # Close thinking section if needed
            if has_thinking and thinking_started:
                print("\n=============== Content =================", flush=True)
                thinking_started = False

            for tool_call in delta.tool_calls:
                if tool_call.function:
                    print(f"🔧 Tool Call: {tool_call.function.name}")
                    print(f"   Arguments: {tool_call.function.arguments}")

        # Print content
        if delta.content:
            print(delta.content, end="", flush=True)

print()

Output Example:

Output

🔧 Tool Call: get_weather
   Arguments: {"location": "Beijing", "unit": "celsius"}

Python Example (with Thinking Process): Start sglang server:

Command

python -m sglang.launch_server \
  --model openai/gpt-oss-120b \
  --reasoning-parser gpt-oss \
  --tool-call-parser gpt-oss \
  --tp 8

Example

from openai import OpenAI

client = OpenAI(
    base_url="http://localhost:8000/v1",
    api_key="EMPTY"
)

# Define available tools
tools = [
    {
        "type": "function",
        "function": {
            "name": "get_weather",
            "description": "Get the current weather for a location",
            "parameters": {
                "type": "object",
                "properties": {
                    "location": {
                        "type": "string",
                        "description": "The city name"
                    },
                    "unit": {
                        "type": "string",
                        "enum": ["celsius", "fahrenheit"],
                        "description": "Temperature unit"
                    }
                },
                "required": ["location"]
            }
        }
    }
]

# Make request with streaming to see thinking process
response = client.chat.completions.create(
    model="openai/gpt-oss-120b",
    messages=[
        {"role": "user", "content": "What's the weather in Beijing?"}
    ],
    tools=tools,
    temperature=0.7,
    stream=True
)

# Process streaming response
thinking_started = False
has_thinking = False

for chunk in response:
    if chunk.choices and len(chunk.choices) > 0:
        delta = chunk.choices[0].delta

        # Print thinking process
        if hasattr(delta, 'reasoning_content') and delta.reasoning_content:
            if not thinking_started:
                print("=============== Thinking =================", flush=True)
                thinking_started = True
            has_thinking = True
            print(delta.reasoning_content, end="", flush=True)

        # Print tool calls
        if hasattr(delta, 'tool_calls') and delta.tool_calls:
            # Close thinking section if needed
            if has_thinking and thinking_started:
                print("\n=============== Content =================", flush=True)
                thinking_started = False

            for tool_call in delta.tool_calls:
                if tool_call.function:
                    print(f"🔧 Tool Call: {tool_call.function.name}")
                    print(f"   Arguments: {tool_call.function.arguments}")

        # Print content
        if delta.content:
            print(delta.content, end="", flush=True)

print()

Output Example:

Output

=============== Thinking =================
User asks: "What's the weather in Beijing?" We need to get current weather. Use function get_weather with location "Beijing". No unit specified; default? Probably use default (maybe Celsius). We can specify unit as "celsius". We'll call function.
=============== Content =================
🔧 Tool Call: get_weather
   Arguments: {"location": "Beijing", "unit": "celsius"}

Note:

The reasoning parser shows how the model decides to use a tool
Tool calls are clearly marked with the function name and arguments
You can then execute the function and send the result back to continue the conversation

Handling Tool Call Results:

Example

# After getting the tool call, execute the function
def get_weather(location, unit="celsius"):
    # Your actual weather API call here
    return f"The weather in {location} is 22°{unit[0].upper()} and sunny."

# Send tool result back to the model
messages = [
    {"role": "user", "content": "What's the weather in Beijing?"},
    {
        "role": "assistant",
        "content": None,
        "tool_calls": [{
            "id": "call_123",
            "type": "function",
            "function": {
                "name": "get_weather",
                "arguments": '{"location": "Beijing", "unit": "celsius"}'
            }
        }]
    },
    {
        "role": "tool",
        "tool_call_id": "call_123",
        "content": get_weather("Beijing", "celsius")
    }
]

final_response = client.chat.completions.create(
    model="openai/gpt-oss-120b",
    messages=messages,
    temperature=0.7
)

print(final_response.choices[0].message.content)
# Output: "The current weather in Beijing is 22 °C and sunny. Let me know if you’d like a forecast for the next few days or any other details!"

5.Benchmark

5.1 Speed Benchmark

Hardware: NVIDIA B200 GPU (8x)
Tensor Parallelism: 8
Model: openai/gpt-oss-120b
sglang version: 0.5.6

We use SGLang’s built-in benchmarking tool to conduct performance evaluation on the ShareGPT_Vicuna_unfiltered dataset. This dataset contains real conversation data and can better reflect performance in actual use scenarios.

5.1.1 Latency-Sensitive Benchmark

Server Command:

Output

python -m sglang.launch_server \
  --model openai/gpt-oss-120b \
  --tp 8

Test Command:

Command

python3 -m sglang.bench_serving \
  --backend sglang \
  --num-prompt 100 \
  --max-concurrency 1

Test Results:

Output

============ Serving Benchmark Result ============
Backend:                                 sglang
Traffic request rate:                    inf
Max request concurrency:                 1
Successful requests:                     100
Benchmark duration (s):                  52.35
Total input tokens:                      33178
Total input text tokens:                 33178
Total input vision tokens:               0
Total generated tokens:                  21251
Total generated tokens (retokenized):    20868
Request throughput (req/s):              1.91
Input token throughput (tok/s):          633.76
Output token throughput (tok/s):         405.93
Peak output token throughput (tok/s):    433.00
Peak concurrent requests:                8
Total token throughput (tok/s):          1039.69
Concurrency:                             1.00
----------------End-to-End Latency----------------
Mean E2E Latency (ms):                   523.30
Median E2E Latency (ms):                 389.91
---------------Time to First Token----------------
Mean TTFT (ms):                          33.71
Median TTFT (ms):                        31.79
P99 TTFT (ms):                           108.98
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          2.31
Median TPOT (ms):                        2.31
P99 TPOT (ms):                           2.39
---------------Inter-Token Latency----------------
Mean ITL (ms):                           2.31
Median ITL (ms):                         2.31
P95 ITL (ms):                            2.35
P99 ITL (ms):                            2.38
Max ITL (ms):                            3.54
==================================================

5.1.2 Throughput-Sensitive Benchmark

Server Command:

Output

python -m sglang.launch_server \
  --model openai/gpt-oss-120b \
  --tp 8

Test Command:

Command

python3 -m sglang.bench_serving \
  --backend sglang \
  --num-prompt 1000 \
  --max-concurrency 100

Test Results:

Output

============ Serving Benchmark Result ============
Backend:                                 sglang
Traffic request rate:                    inf
Max request concurrency:                 100
Successful requests:                     1000
Benchmark duration (s):                  24.76
Total input tokens:                      297156
Total input text tokens:                 297156
Total input vision tokens:               0
Total generated tokens:                  192432
Total generated tokens (retokenized):    187145
Request throughput (req/s):              40.39
Input token throughput (tok/s):          12003.57
Output token throughput (tok/s):         7773.26
Peak output token throughput (tok/s):    13780.00
Peak concurrent requests:                156
Total token throughput (tok/s):          19776.83
Concurrency:                             89.23
----------------End-to-End Latency----------------
Mean E2E Latency (ms):                   2208.97
Median E2E Latency (ms):                 1591.11
---------------Time to First Token----------------
Mean TTFT (ms):                          102.94
Median TTFT (ms):                        31.53
P99 TTFT (ms):                           674.32
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          14.31
Median TPOT (ms):                        11.00
P99 TPOT (ms):                           91.28
---------------Inter-Token Latency----------------
Mean ITL (ms):                           11.00
Median ITL (ms):                         5.75
P95 ITL (ms):                            25.35
P99 ITL (ms):                            43.18
Max ITL (ms):                            621.42
==================================================

5.2 Accuracy Benchmark

5.2.1 GSM8K Benchmark

Benchmark Command:

Command

python3 -m sglang.test.few_shot_gsm8k --num-questions 200 --port 8000

Results:

GPT-OSS-120b

Output

Accuracy: 0.880
Invalid: 0.005
Latency: 5.262 s
Output throughput: 12143.675 token/s

GPT-OSS-20b

Output

Accuracy: 0.535
Invalid: 0.165
Latency: 4.157 s
Output throughput: 19589.165 token/s

Cookbook

​1.Model Introduction

​2.SGLang Installation

​3.Model Deployment

​3.1 Basic Configuration

​3.2 Configuration Tips

​4.Model Invocation

​4.1 Basic Usage

​4.2 Advanced Usage

​4.2.1 Reasoning Parser

​4.2.2 Tool Calling

​5.Benchmark

​5.1 Speed Benchmark

​5.1.1 Latency-Sensitive Benchmark

​5.1.2 Throughput-Sensitive Benchmark

​5.2 Accuracy Benchmark

​5.2.1 GSM8K Benchmark

1.Model Introduction

2.SGLang Installation

3.Model Deployment

3.1 Basic Configuration

3.2 Configuration Tips

4.Model Invocation

4.1 Basic Usage

4.2 Advanced Usage

4.2.1 Reasoning Parser

4.2.2 Tool Calling

5.Benchmark

5.1 Speed Benchmark

5.1.1 Latency-Sensitive Benchmark

5.1.2 Throughput-Sensitive Benchmark

5.2 Accuracy Benchmark

5.2.1 GSM8K Benchmark