sglang/docs/basic_usage/openai_api_completions.ipynb

OpenAI APIs - Completions

SGLang provides OpenAI-compatible APIs to enable a smooth transition from OpenAI services to self-hosted local models. A complete reference for the API is available in the OpenAI API Reference.

This tutorial covers the following popular APIs:

• chat/completions
• completions

Check out other tutorials to learn about vision APIs for vision-language models and embedding APIs for embedding models.

Launch A Server

Launch the server in your terminal and wait for it to initialize.

In [ ]:

from sglang.test.doc_patch import launch_server_cmd
from sglang.utils import wait_for_server, print_highlight, terminate_process

server_process, port = launch_server_cmd(
    "python3 -m sglang.launch_server --model-path qwen/qwen2.5-0.5b-instruct --host 0.0.0.0 --log-level warning"
)

wait_for_server(f"http://localhost:{port}")
print(f"Server started on http://localhost:{port}")

Chat Completions

Usage

The server fully implements the OpenAI API. It will automatically apply the chat template specified in the Hugging Face tokenizer, if one is available. You can also specify a custom chat template with --chat-template when launching the server.

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import openai

client = openai.Client(base_url=f"http://127.0.0.1:{port}/v1", api_key="None")

response = client.chat.completions.create(
    model="qwen/qwen2.5-0.5b-instruct",
    messages=[
        {"role": "user", "content": "List 3 countries and their capitals."},
    ],
    temperature=0,
    max_tokens=64,
)

print_highlight(f"Response: {response}")

Model Thinking/Reasoning Support

Some models support internal reasoning or thinking processes that can be exposed in the API response. SGLang provides unified support for various reasoning models through the chat_template_kwargs parameter and compatible reasoning parsers.

Supported Models and Configuration

Model Family	Chat Template Parameter	Reasoning Parser	Notes
DeepSeek-R1 (R1, R1-0528, R1-Distill)	enable_thinking	--reasoning-parser deepseek-r1	Standard reasoning models
DeepSeek-V3.1	thinking	--reasoning-parser deepseek-v3	Hybrid model (thinking/non-thinking modes)
Qwen3 (standard)	enable_thinking	--reasoning-parser qwen3	Hybrid model (thinking/non-thinking modes)
Qwen3-Thinking	N/A (always enabled)	--reasoning-parser qwen3-thinking	Always generates reasoning
Kimi	N/A (always enabled)	--reasoning-parser kimi	Kimi thinking models
Gpt-Oss	N/A (always enabled)	--reasoning-parser gpt-oss	Gpt-Oss thinking models

Basic Usage

To enable reasoning output, you need to:

1. Launch the server with the appropriate reasoning parser
2. Set the model-specific parameter in chat_template_kwargs
3. Optionally use separate_reasoning: False to not get reasoning content separately (default to True)

Note for Qwen3-Thinking models: These models always generate thinking content and do not support the enable_thinking parameter. Use --reasoning-parser qwen3-thinking or --reasoning-parser qwen3 to parse the thinking content.

Example: Qwen3 Models

# Launch server:
# python3 -m sglang.launch_server --model Qwen/Qwen3-4B --reasoning-parser qwen3

from openai import OpenAI

client = OpenAI(
    api_key="EMPTY",
    base_url=f"http://127.0.0.1:30000/v1",
)

model = "Qwen/Qwen3-4B"
messages = [{"role": "user", "content": "How many r's are in 'strawberry'?"}]

response = client.chat.completions.create(
    model=model,
    messages=messages,
    extra_body={
        "chat_template_kwargs": {"enable_thinking": True},
        "separate_reasoning": True
    }
)

print("Reasoning:", response.choices[0].message.reasoning_content)
print("-"*100)
print("Answer:", response.choices[0].message.content)

ExampleOutput:

Reasoning: Okay, so the user is asking how many 'r's are in the word 'strawberry'. Let me think. First, I need to make sure I have the word spelled correctly. Strawberry... S-T-R-A-W-B-E-R-R-Y. Wait, is that right? Let me break it down.

Starting with 'strawberry', let's write out the letters one by one. S, T, R, A, W, B, E, R, R, Y. Hmm, wait, that's 10 letters. Let me check again. S (1), T (2), R (3), A (4), W (5), B (6), E (7), R (8), R (9), Y (10). So the letters are S-T-R-A-W-B-E-R-R-Y. 
...
Therefore, the answer should be three R's in 'strawberry'. But I need to make sure I'm not counting any other letters as R. Let me check again. S, T, R, A, W, B, E, R, R, Y. No other R's. So three in total. Yeah, that seems right.

----------------------------------------------------------------------------------------------------
Answer: The word "strawberry" contains **three** letters 'r'. Here's the breakdown:

1. **S-T-R-A-W-B-E-R-R-Y**  
   - The **third letter** is 'R'.  
   - The **eighth and ninth letters** are also 'R's.  

Thus, the total count is **3**.  

**Answer:** 3.

Note: Setting "enable_thinking": False (or omitting it) will result in reasoning_content being None. Qwen3-Thinking models always generate reasoning content and don't support the enable_thinking parameter.

Example: DeepSeek-V3 Models

DeepSeek-V3 models support thinking mode through the thinking parameter:

# Launch server:
# python3 -m sglang.launch_server --model deepseek-ai/DeepSeek-V3.1 --tp 8  --reasoning-parser deepseek-v3

from openai import OpenAI

client = OpenAI(
    api_key="EMPTY",
    base_url=f"http://127.0.0.1:30000/v1",
)

model = "deepseek-ai/DeepSeek-V3.1"
messages = [{"role": "user", "content": "How many r's are in 'strawberry'?"}]

response = client.chat.completions.create(
    model=model,
    messages=messages,
    extra_body={
        "chat_template_kwargs": {"thinking": True},
        "separate_reasoning": True
    }
)

print("Reasoning:", response.choices[0].message.reasoning_content)
print("-"*100)
print("Answer:", response.choices[0].message.content)

Example Output:

Reasoning: First, the question is: "How many r's are in 'strawberry'?"

I need to count the number of times the letter 'r' appears in the word "strawberry".

Let me write out the word: S-T-R-A-W-B-E-R-R-Y.

Now, I'll go through each letter and count the 'r's.
...
So, I have three 'r's in "strawberry".

I should double-check. The word is spelled S-T-R-A-W-B-E-R-R-Y. The letters are at positions: 3, 8, and 9 are 'r's. Yes, that's correct.

Therefore, the answer should be 3.
----------------------------------------------------------------------------------------------------
Answer: The word "strawberry" contains **3** instances of the letter "r". Here's a breakdown for clarity:

- The word is spelled: S-T-R-A-W-B-E-R-R-Y
- The "r" appears at the 3rd, 8th, and 9th positions.

Note: DeepSeek-V3 models use the thinking parameter (not enable_thinking) to control reasoning output.

Parameters

The chat completions API accepts OpenAI Chat Completions API's parameters. Refer to OpenAI Chat Completions API for more details.

SGLang extends the standard API with the extra_body parameter, allowing for additional customization. One key option within extra_body is chat_template_kwargs, which can be used to pass arguments to the chat template processor.

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response = client.chat.completions.create(
    model="qwen/qwen2.5-0.5b-instruct",
    messages=[
        {
            "role": "system",
            "content": "You are a knowledgeable historian who provides concise responses.",
        },
        {"role": "user", "content": "Tell me about ancient Rome"},
        {
            "role": "assistant",
            "content": "Ancient Rome was a civilization centered in Italy.",
        },
        {"role": "user", "content": "What were their major achievements?"},
    ],
    temperature=0.3,  # Lower temperature for more focused responses
    max_tokens=128,  # Reasonable length for a concise response
    top_p=0.95,  # Slightly higher for better fluency
    presence_penalty=0.2,  # Mild penalty to avoid repetition
    frequency_penalty=0.2,  # Mild penalty for more natural language
    n=1,  # Single response is usually more stable
    seed=42,  # Keep for reproducibility
)

print_highlight(response.choices[0].message.content)

Streaming mode is also supported.

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stream = client.chat.completions.create(
    model="qwen/qwen2.5-0.5b-instruct",
    messages=[{"role": "user", "content": "Say this is a test"}],
    stream=True,
)
for chunk in stream:
    if chunk.choices[0].delta.content is not None:
        print(chunk.choices[0].delta.content, end="")

Completions

Usage

Completions API is similar to Chat Completions API, but without the messages parameter or chat templates.

In [ ]:

response = client.completions.create(
    model="qwen/qwen2.5-0.5b-instruct",
    prompt="List 3 countries and their capitals.",
    temperature=0,
    max_tokens=64,
    n=1,
    stop=None,
)

print_highlight(f"Response: {response}")

Parameters

The completions API accepts OpenAI Completions API's parameters. Refer to OpenAI Completions API for more details.

Here is an example of a detailed completions request:

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response = client.completions.create(
    model="qwen/qwen2.5-0.5b-instruct",
    prompt="Write a short story about a space explorer.",
    temperature=0.7,  # Moderate temperature for creative writing
    max_tokens=150,  # Longer response for a story
    top_p=0.9,  # Balanced diversity in word choice
    stop=["\n\n", "THE END"],  # Multiple stop sequences
    presence_penalty=0.3,  # Encourage novel elements
    frequency_penalty=0.3,  # Reduce repetitive phrases
    n=1,  # Generate one completion
    seed=123,  # For reproducible results
)

print_highlight(f"Response: {response}")

Structured Outputs (JSON, Regex, EBNF)

For OpenAI compatible structured outputs API, refer to Structured Outputs for more details.

Using LoRA Adapters

SGLang supports LoRA (Low-Rank Adaptation) adapters with OpenAI-compatible APIs. You can specify which adapter to use directly in the model parameter using the base-model:adapter-name syntax.

Server Setup:

python -m sglang.launch_server \
    --model-path qwen/qwen2.5-0.5b-instruct \
    --enable-lora \
    --lora-paths adapter_a=/path/to/adapter_a adapter_b=/path/to/adapter_b

For more details on LoRA serving configuration, see the LoRA documentation.

API Call:

(Recommended) Use the model:adapter syntax to specify which adapter to use:

response = client.chat.completions.create(
    model="qwen/qwen2.5-0.5b-instruct:adapter_a",  # ← base-model:adapter-name
    messages=[{"role": "user", "content": "Convert to SQL: show all users"}],
    max_tokens=50,
)

Backward Compatible: Using extra_body

The old extra_body method is still supported for backward compatibility:

# Backward compatible method
response = client.chat.completions.create(
    model="qwen/qwen2.5-0.5b-instruct",
    messages=[{"role": "user", "content": "Convert to SQL: show all users"}],
    extra_body={"lora_path": "adapter_a"},  # ← old method
    max_tokens=50,
)

Note: When both model:adapter and extra_body["lora_path"] are specified, the model:adapter syntax takes precedence.

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terminate_process(server_process)