lmdeploy/docs/en/llm/api_server_reasoning.md

Reasoning Outputs

For models that support reasoning capabilities, such as DeepSeek R1, LMDeploy can parse reasoning outputs on the server side and expose them via reasoning_content.

Examples

DeepSeek R1

We can start DeepSeek R1's api_server like other models, but we need to specify the --reasoning-parser argument.

lmdeploy serve api_server deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B --reasoning-parser deepseek-r1

Then, we can call the service's functionality from the client:

from openai import OpenAI

openai_api_key = "Your API key"
openai_api_base = "http://0.0.0.0:23333/v1"

client = OpenAI(
    api_key=openai_api_key,
    base_url=openai_api_base,
)

models = client.models.list()
model = models.data[0].id

messages = [{"role": "user", "content": "9.11 and 9.8, which is greater?"}]
response = client.chat.completions.create(model=model, messages=messages, stream=True)
for stream_response in response:
    print('reasoning content: ',stream_response.choices[0].delta.reasoning_content)
    print('content: ', stream_response.choices[0].delta.content)

response = client.chat.completions.create(model=model, messages=messages, stream=False)
reasoning_content = response.choices[0].message.reasoning_content
content = response.choices[0].message.content

print("reasoning_content:", reasoning_content)
print("content:", content)

Custom parser

Built-in reasoning parser names include:

• qwen-qwq
• qwen3
• intern-s1
• deepseek-r1
• deepseek-v3
• gpt-oss

Notes

• deepseek-v3: starts in reasoning mode only when enable_thinking=True. When enable_thinking is None (default), output is usually plain content without a reasoning segment.
• gpt-oss: parses OpenAI Harmony channels:
  • final -> content
  • analysis -> reasoning_content
  • commentary with functions.* recipient -> tool_calls

Add a custom parser

Add a parser class under lmdeploy/serve/openai/reasoning_parser/ and register it with ReasoningParserManager.

from lmdeploy.serve.openai.reasoning_parser import (
    ReasoningParser, ReasoningParserManager
)

@ReasoningParserManager.register_module(["example"])
class ExampleParser(ReasoningParser):
    def __init__(self, tokenizer: object, **kwargs):
        super().__init__(tokenizer, **kwargs)

    def get_reasoning_open_tag(self) -> str | None:
        return "<think>"

    def get_reasoning_close_tag(self) -> str | None:
        return "</think>"

    def starts_in_reasoning_mode(self) -> bool:
        return True

Then start the service with:

lmdeploy serve api_server $model_path --reasoning-parser example