Reasoning Parser

SGLang supports parsing reasoning content out from “normal” content for reasoning models such as DeepSeek R1.

Supported Models & Parsers

Model	Reasoning tags	Parser	Notes
Apertus 2509 models	`<\|inner_prefix\|>` … `<\|inner_suffix\|>`	`apertus2509`	Supports `enable_thinking` parameter
DeepSeek‑R1 series	`<think>` … `</think>`	`deepseek-r1`	Supports all variants (R1, R1-0528, R1-Distill)
DeepSeek‑V3 series	`<think>` … `</think>`	`deepseek-v3`	Including DeepSeek‑V3.2. Supports `thinking` parameter
Standard Qwen3 models	`<think>` … `</think>`	`qwen3`	Supports `enable_thinking` parameter
Qwen3-Thinking models	`<think>` … `</think>`	`qwen3` or `qwen3-thinking`	Always generates thinking content
Kimi K2 Thinking	`◁think▷` … `◁/think▷`	`kimi_k2`	Uses special thinking delimiters. Also requires `--tool-call-parser kimi_k2` for tool use.
GPT OSS	`<\|channel\|>analysis<\|message\|>` … `<\|end\|>`	`gpt-oss`	N/A

Model-Specific Behaviors

Apertus 2509:

Uses <|inner_prefix|> and <|inner_suffix|> to delimit reasoning content. For agentic tool use, also specify --tool-call-parser apertus2509.

DeepSeek-R1 Family:

DeepSeek-R1: No <think> start tag, jumps directly to thinking content
DeepSeek-R1-0528: Generates both <think> start and </think> end tags
Both are handled by the same deepseek-r1 parser

DeepSeek-V3 Family:

DeepSeek-V3.1/V3.2: Hybrid model supporting both thinking and non-thinking modes, use the deepseek-v3 parser and thinking parameter (NOTE: not enable_thinking)

Qwen3 Family:

Standard Qwen3 (e.g., Qwen3-2507): Use qwen3 parser, supports enable_thinking in chat templates
Qwen3-Thinking (e.g., Qwen3-235B-A22B-Thinking-2507): Use qwen3 or qwen3-thinking parser, always thinks

Kimi K2:

Kimi K2 Thinking: Uses special ◁think▷ and ◁/think▷ tags. For agentic tool use, also specify --tool-call-parser kimi_k2.

GPT OSS:

GPT OSS: Uses special <|channel|>analysis<|message|> and <|end|> tags

Usage

Launching the Server

Specify the --reasoning-parser option.

Example

import requests
from openai import OpenAI
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 deepseek-ai/DeepSeek-R1-Distill-Qwen-7B --host 0.0.0.0 --reasoning-parser deepseek-r1 --log-level warning"
)

wait_for_server(f"http://localhost:{port}")

Note that --reasoning-parser defines the parser used to interpret responses.

OpenAI Compatible API

Using the OpenAI compatible API, the contract follows the DeepSeek API design established with the release of DeepSeek-R1:

reasoning_content: The content of the CoT.
content: The content of the final answer.

Example

# Initialize OpenAI-like client
client = OpenAI(api_key="None", base_url=f"http://0.0.0.0:{port}/v1")
model_name = client.models.list().data[0].id

messages = [
    {
        "role": "user",
        "content": "What is 1+3?",
    }
]

Non-Streaming Request

Example

response_non_stream = client.chat.completions.create(
    model=model_name,
    messages=messages,
    temperature=0.6,
    top_p=0.95,
    stream=False,  # Non-streaming
    extra_body={"separate_reasoning": True},
)
print_highlight("==== Reasoning ====")
print_highlight(response_non_stream.choices[0].message.reasoning_content)

print_highlight("==== Text ====")
print_highlight(response_non_stream.choices[0].message.content)

Streaming Request

Example

response_stream = client.chat.completions.create(
    model=model_name,
    messages=messages,
    temperature=0.6,
    top_p=0.95,
    stream=True,  # Non-streaming
    extra_body={"separate_reasoning": True},
)

reasoning_content = ""
content = ""
for chunk in response_stream:
    if chunk.choices[0].delta.content:
        content += chunk.choices[0].delta.content
    if chunk.choices[0].delta.reasoning_content:
        reasoning_content += chunk.choices[0].delta.reasoning_content

print_highlight("==== Reasoning ====")
print_highlight(reasoning_content)

print_highlight("==== Text ====")
print_highlight(content)

Optionally, you can buffer the reasoning content to the last reasoning chunk (or the first chunk after the reasoning content).

Example

response_stream = client.chat.completions.create(
    model=model_name,
    messages=messages,
    temperature=0.6,
    top_p=0.95,
    stream=True,  # Non-streaming
    extra_body={"separate_reasoning": True, "stream_reasoning": False},
)

reasoning_content = ""
content = ""
for chunk in response_stream:
    if chunk.choices[0].delta.content:
        content += chunk.choices[0].delta.content
    if chunk.choices[0].delta.reasoning_content:
        reasoning_content += chunk.choices[0].delta.reasoning_content

print_highlight("==== Reasoning ====")
print_highlight(reasoning_content)

print_highlight("==== Text ====")
print_highlight(content)

The reasoning separation is enable by default when specify . To disable it, set the separate_reasoning option to False in request.

Example

response_non_stream = client.chat.completions.create(
    model=model_name,
    messages=messages,
    temperature=0.6,
    top_p=0.95,
    stream=False,  # Non-streaming
    extra_body={"separate_reasoning": False},
)

print_highlight("==== Original Output ====")
print_highlight(response_non_stream.choices[0].message.content)

SGLang Native API

Example

from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("deepseek-ai/DeepSeek-R1-Distill-Qwen-7B")
input = tokenizer.apply_chat_template(
    messages, tokenize=False, add_generation_prompt=True, return_dict=False
)

gen_url = f"http://localhost:{port}/generate"
gen_data = {
    "text": input,
    "sampling_params": {
        "skip_special_tokens": False,
        "max_new_tokens": 1024,
        "temperature": 0.6,
        "top_p": 0.95,
    },
}
gen_response = requests.post(gen_url, json=gen_data).json()["text"]

print_highlight("==== Original Output ====")
print_highlight(gen_response)

parse_url = f"http://localhost:{port}/separate_reasoning"
separate_reasoning_data = {
    "text": gen_response,
    "reasoning_parser": "deepseek-r1",
}
separate_reasoning_response_json = requests.post(
    parse_url, json=separate_reasoning_data
).json()
print_highlight("==== Reasoning ====")
print_highlight(separate_reasoning_response_json["reasoning_text"])
print_highlight("==== Text ====")
print_highlight(separate_reasoning_response_json["text"])

Example

terminate_process(server_process)

Offline Engine API

Example

import sglang as sgl
from sglang.srt.parser.reasoning_parser import ReasoningParser
from sglang.utils import print_highlight

llm = sgl.Engine(model_path="deepseek-ai/DeepSeek-R1-Distill-Qwen-7B")
tokenizer = AutoTokenizer.from_pretrained("deepseek-ai/DeepSeek-R1-Distill-Qwen-7B")
input = tokenizer.apply_chat_template(
    messages, tokenize=False, add_generation_prompt=True, return_dict=False
)
sampling_params = {
    "max_new_tokens": 1024,
    "skip_special_tokens": False,
    "temperature": 0.6,
    "top_p": 0.95,
}
result = llm.generate(prompt=input, sampling_params=sampling_params)

generated_text = result["text"]  # Assume there is only one prompt

print_highlight("==== Original Output ====")
print_highlight(generated_text)

parser = ReasoningParser("deepseek-r1")
reasoning_text, text = parser.parse_non_stream(generated_text)
print_highlight("==== Reasoning ====")
print_highlight(reasoning_text)
print_highlight("==== Text ====")
print_highlight(text)

Example

llm.shutdown()

Supporting New Reasoning Model Schemas

For future reasoning models, you can implement the reasoning parser as a subclass of BaseReasoningFormatDetector in python/sglang/srt/reasoning_parser.py and specify the reasoning parser for new reasoning model schemas accordingly.

Basic Usage

Advanced Features

Supported Models

Developer Guide

References

Supported Models & Parsers

Model-Specific Behaviors

Usage

Launching the Server

OpenAI Compatible API

Non-Streaming Request

Streaming Request

SGLang Native API

Offline Engine API

Supporting New Reasoning Model Schemas

​Supported Models & Parsers

​Model-Specific Behaviors

​Usage

​Launching the Server

​OpenAI Compatible API

​Non-Streaming Request

​Streaming Request

​SGLang Native API

​Offline Engine API

​Supporting New Reasoning Model Schemas

Supported Models & Parsers

Model-Specific Behaviors

Usage

Launching the Server

OpenAI Compatible API

Non-Streaming Request

Streaming Request

SGLang Native API

Offline Engine API

Supporting New Reasoning Model Schemas