OpenAI Compatible Server

vLLM provides an HTTP server that implements OpenAI's Completions and Chat API.

You can start the server using Python, or using Docker:

python -m vllm.entrypoints.openai.api_server --model meta-llama/Llama-2-7b-hf --dtype float32 --api-key token-abc123

To call the server, you can use the official OpenAI Python client library, or any other HTTP client.

from openai import OpenAI
client = OpenAI(
    base_url="http://localhost:8000/v1",
    api_key="token-abc123",
)

completion = client.chat.completions.create(
  model="meta-llama/Llama-2-7b-hf",
  messages=[
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": "Hello!"}
  ]
)

print(completion.choices[0].message)

API Reference

Please see the OpenAI API Reference for more information on the API. We support all parameters except:

Chat: tools, and tool_choice.
Completions: suffix.

Extra Parameters

vLLM supports a set of parameters that are not part of the OpenAI API. In order to use them, you can pass them as extra parameters in the OpenAI client. Or directly merge them into the JSON payload if you are using HTTP call directly.

completion = client.chat.completions.create(
  model="meta-llama/Llama-2-7b-hf",
  messages=[
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": "Classify this sentiment: vLLM is wonderful!"}
  ],
  extra_body={
    "guided_choice": ["positive", "negative"]
  }
)

Extra Parameters for Chat API

The following sampling parameters (click through to see documentation) are supported.

:language: python
:start-after: begin-chat-completion-sampling-params
:end-before: end-chat-completion-sampling-params

The following extra parameters are supported:

:language: python
:start-after: begin-chat-completion-extra-params
:end-before: end-chat-completion-extra-params

Extra Parameters for Completions API

The following sampling parameters (click through to see documentation) are supported.

:language: python
:start-after: begin-completion-sampling-params
:end-before: end-completion-sampling-params

The following extra parameters are supported:

:language: python
:start-after: begin-completion-extra-params
:end-before: end-completion-extra-params

Chat Template

In order for the language model to support chat protocol, vLLM requires the model to include a chat template in its tokenizer configuration. The chat template is a Jinja2 template that specifies how are roles, messages, and other chat-specific tokens are encoded in the input.

An example chat template for meta-llama/Llama-2-7b-chat-hf can be found here

Some models do not provide a chat template even though they are instruction/chat fine-tuned. For those model, you can manually specify their chat template in the --chat-template parameter with the file path to the chat template, or the template in string form. Without a chat template, the server will not be able to process chat and all chat requests will error.

python -m vllm.entrypoints.openai.api_server \
  --model ... \
  --chat-template ./path-to-chat-template.jinja

vLLM community provides a set of chat templates for popular models. You can find them in the examples directory here

Command line arguments for the server

:module: vllm.entrypoints.openai.cli_args
:func: make_arg_parser
:prog: vllm-openai-server

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