vllm/docs/features/custom_arguments.md

Custom Arguments

You can use vLLM custom arguments to pass in arguments which are not part of the vLLM SamplingParams and REST API specifications. Adding or removing a vLLM custom argument does not require recompiling vLLM, since the custom arguments are passed in as a dictionary.

Custom arguments can be useful if, for example, you want to use a custom logits processor without modifying the vLLM source code.

Offline Custom Arguments

Custom arguments passed to SamplingParams.extra_args as a dict will be visible to any code which has access to SamplingParams:

SamplingParams(extra_args={"your_custom_arg_name": 67})

This allows arguments which are not already part of SamplingParams to be passed into LLM as part of a request.

Online Custom Arguments

The vLLM REST API allows custom arguments to be passed to the vLLM server via vllm_xargs. The example below integrates custom arguments into a vLLM REST API request:

curl http://localhost:8000/v1/completions \
    -H "Content-Type: application/json" \
    -d '{
        "model": "Qwen/Qwen2.5-1.5B-Instruct",
        ...
        "vllm_xargs": {"your_custom_arg": 67}
    }'

Furthermore, OpenAI SDK users can access vllm_xargs via the extra_body argument:

batch = await client.completions.create(
    model="Qwen/Qwen2.5-1.5B-Instruct",
    ...,
    extra_body={
        "vllm_xargs": {
            "your_custom_arg": 67
        }
    }
)

!!! note vllm_xargs is assigned to SamplingParams.extra_args under the hood, so code which uses SamplingParams.extra_args is compatible with both offline and online scenarios.