Generative Models

vLLM provides first-class support for generative models, which covers most of LLMs.

In vLLM, generative models implement the VllmModelForTextGeneration interface. Based on the final hidden states of the input, these models output log probabilities of the tokens to generate, which are then passed through Sampler to obtain the final text.

For generative models, the only supported --task option is "generate". Usually, this is automatically inferred so you don’t have to specify it.

Offline Inference

The LLM class provides various methods for offline inference. See Engine Arguments for a list of options when initializing the model.

LLM.generate

The generate method is available to all generative models in vLLM. It is similar to its counterpart in HF Transformers, except that tokenization and detokenization are also performed automatically.

from vllm import LLM

llm = LLM(model="facebook/opt-125m")
outputs = llm.generate("Hello, my name is")

for output in outputs:
    prompt = output.prompt
    generated_text = output.outputs[0].text
    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

You can optionally control the language generation by passing SamplingParams. For example, you can use greedy sampling by setting temperature=0:

from vllm import LLM, SamplingParams

llm = LLM(model="facebook/opt-125m")
params = SamplingParams(temperature=0)
outputs = llm.generate("Hello, my name is", params)

for output in outputs:
    prompt = output.prompt
    generated_text = output.outputs[0].text
    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

Important

By default, vLLM will use sampling parameters recommended by model creator by applying the generation_config.json from the huggingface model repository if it exists. In most cases, this will provide you with the best results by default if SamplingParams is not specified.

However, if vLLM’s default sampling parameters are preferred, please pass generation_config="vllm" when creating the LLM instance.

A code example can be found here: examples/offline_inference/basic/basic.py

LLM.beam_search

The beam_search method implements beam search on top of generate. For example, to search using 5 beams and output at most 50 tokens:

from vllm import LLM
from vllm.sampling_params import BeamSearchParams

llm = LLM(model="facebook/opt-125m")
params = BeamSearchParams(beam_width=5, max_tokens=50)
outputs = llm.beam_search([{"prompt": "Hello, my name is "}], params)

for output in outputs:
    generated_text = output.sequences[0].text
    print(f"Generated text: {generated_text!r}")

LLM.chat

The chat method implements chat functionality on top of generate. In particular, it accepts input similar to OpenAI Chat Completions API and automatically applies the model’s chat template to format the prompt.

Important

In general, only instruction-tuned models have a chat template. Base models may perform poorly as they are not trained to respond to the chat conversation.

from vllm import LLM

llm = LLM(model="meta-llama/Meta-Llama-3-8B-Instruct")
conversation = [
    {
        "role": "system",
        "content": "You are a helpful assistant"
    },
    {
        "role": "user",
        "content": "Hello"
    },
    {
        "role": "assistant",
        "content": "Hello! How can I assist you today?"
    },
    {
        "role": "user",
        "content": "Write an essay about the importance of higher education.",
    },
]
outputs = llm.chat(conversation)

for output in outputs:
    prompt = output.prompt
    generated_text = output.outputs[0].text
    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

A code example can be found here: examples/offline_inference/basic/chat.py

If the model doesn’t have a chat template or you want to specify another one, you can explicitly pass a chat template:

from vllm.entrypoints.chat_utils import load_chat_template

# You can find a list of existing chat templates under `examples/`
custom_template = load_chat_template(chat_template="<path_to_template>")
print("Loaded chat template:", custom_template)

outputs = llm.chat(conversation, chat_template=custom_template)

Online Serving

Our OpenAI-Compatible Server provides endpoints that correspond to the offline APIs:

• Completions API is similar to LLM.generate but only accepts text.

• Chat API is similar to LLM.chat, accepting both text and multi-modal inputs for models with a chat template.