Speech-to-Text (Transcription/Translation) Support

This document walks you through the steps to add support for speech-to-text (ASR) models to vLLM’s transcription and translation APIs by implementing SupportsTranscription. Please refer to the supported models for further guidance.

Update the base vLLM model

It is assumed you have already implemented your model in vLLM according to the basic model guide. Extend your model with the SupportsTranscription interface and implement the following class attributes and methods.

`supported_languages` and `supports_transcription_only`

Declare supported languages and capabilities:

The supported_languages mapping is validated at init time.
Set supports_transcription_only=True if the model should not serve text generation (eg Whisper).

from typing import ClassVar, Mapping, Literal
import numpy as np
import torch
from torch import nn

from vllm.config import ModelConfig, SpeechToTextConfig
from vllm.inputs import PromptType
from vllm.model_executor.models.interfaces import SupportsTranscription

class YourASRModel(nn.Module, SupportsTranscription):
    # Map of ISO 639-1 language codes to language names
    supported_languages: ClassVar[Mapping[str, str]] = {
        "en": "English",
        "it": "Italian",
        # ... add more as needed
    }

    # If your model only supports audio-conditioned generation
    # (no text-only generation), enable this flag.
    supports_transcription_only: ClassVar[bool] = True

Provide an ASR configuration via get_speech_to_text_config.

This is for controlling general behavior of the API when serving your model:

class YourASRModel(nn.Module, SupportsTranscription):
    ...

    @classmethod
    def get_speech_to_text_config(
        cls,
        model_config: ModelConfig,
        task_type: Literal["transcribe", "translate"],
    ) -> SpeechToTextConfig:
        return SpeechToTextConfig(
            sample_rate=16_000,
            max_audio_clip_s=30,
            # Set to None to disable server-side chunking if your
            # model/processor handles it already
            min_energy_split_window_size=None,
        )

See Audio preprocessing and chunking for what each field controls.

Implement the prompt construction via get_generation_prompt. The server builds a SpeechToTextParams object that bundles the resampled waveform, task parameters, and request-specific options. Your model receives this single object and returns a valid PromptType. There are two common patterns:

Multimodal LLM with audio embeddings (e.g., Voxtral, Gemma3n)

Return a dict containing multi_modal_data with the audio, and either a prompt string or prompt_token_ids:

from vllm.config.speech_to_text import SpeechToTextParams

class YourASRModel(nn.Module, SupportsTranscription):
    ...

    @classmethod
    def get_generation_prompt(
        cls,
        stt_params: SpeechToTextParams,
    ) -> PromptType:
        audio = stt_params.audio
        stt_config = stt_params.stt_config
        task_type = stt_params.task_type

        task_word = "Transcribe" if task_type == "transcribe" else "Translate"
        prompt = (
            "<start_of_turn>user\n"
            f"{task_word} this audio: <audio_soft_token>"
            "<end_of_turn>\n<start_of_turn>model\n"
        )

        return {
            "multi_modal_data": {"audio": (audio, stt_config.sample_rate)},
            "prompt": prompt,
        }

For further clarification on multi modal inputs, please refer to Multi-Modal Inputs.

Encoder–decoder audio-only (e.g., Whisper)

Return a dict with separate encoder_prompt and decoder_prompt entries:

from vllm.config.speech_to_text import SpeechToTextParams

class YourASRModel(nn.Module, SupportsTranscription):
    ...

    @classmethod
    def get_generation_prompt(
        cls,
        stt_params: SpeechToTextParams,
    ) -> PromptType:
        audio = stt_params.audio
        stt_config = stt_params.stt_config
        language = stt_params.language
        task_type = stt_params.task_type
        request_prompt = stt_params.request_prompt

        if language is None:
            raise ValueError("Language must be specified")

        prompt = {
            "encoder_prompt": {
                "prompt": "",
                "multi_modal_data": {
                    "audio": (audio, stt_config.sample_rate),
                },
            },
            "decoder_prompt": (
                (f"<|prev|>{request_prompt}" if request_prompt else "")
                + f"<|startoftranscript|><|{language}|>"
                + f"<|{task_type}|><|notimestamps|>"
            ),
        }
        return cast(PromptType, prompt)

`validate_language` (optional)

Language validation via validate_language

If your model requires a language and you want a default, override this method (see Whisper):

@classmethod
def validate_language(cls, language: str | None) -> str | None:
    if language is None:
        logger.warning(
            "Defaulting to language='en'. If you wish to transcribe "
            "audio in a different language, pass the `language` field "
            "in the TranscriptionRequest."
        )
        language = "en"
    return super().validate_language(language)

`get_num_audio_tokens` (optional)

Token accounting for streaming via get_num_audio_tokens

Provide a fast duration→token estimate to improve streaming usage statistics:

class YourASRModel(nn.Module, SupportsTranscription):
    ...

    @classmethod
    def get_num_audio_tokens(
        cls,
        audio_duration_s: float,
        stt_config: SpeechToTextConfig,
        model_config: ModelConfig,
    ) -> int | None:
        # Return None if unknown; otherwise return an estimate.
        return int(audio_duration_s * stt_config.sample_rate // 320)  # example

Audio preprocessing and chunking

The API server takes care of basic audio I/O and optional chunking before building prompts:

Resampling: Input audio is resampled to SpeechToTextConfig.sample_rate using AudioResampler.
Chunking: If SpeechToTextConfig.allow_audio_chunking is True and the duration exceeds max_audio_clip_s, the server splits the audio into chunks and generates a prompt per chunk. There is no overlap between chunks, overlap_chunk_second controls the size of the search window used to find the split point.
Energy-aware splitting: When min_energy_split_window_size is set, the server finds low-energy regions to minimize cutting within words.

Relevant server logic:

# vllm/entrypoints/openai/speech_to_text.py
async def _preprocess_speech_to_text(...):
    language = self.model_cls.validate_language(request.language)
    ...
    y, sr = load_audio(bytes_, sr=self.asr_config.sample_rate)
    duration = get_audio_duration(y=y, sr=sr)
    do_split_audio = (self.asr_config.allow_audio_chunking
                    and duration > self.asr_config.max_audio_clip_s)
    chunks = [y] if not do_split_audio else self._split_audio(y, int(sr))
    prompts = []
    for chunk in chunks:
        stt_params = request.build_stt_params(
            audio=chunk,
            stt_config=self.asr_config,
            model_config=self.model_config,
            task_type=self.task_type,
        )
        prompt = self.model_cls.get_generation_prompt(stt_params)
        prompts.append(prompt)
    return prompts, duration

Exposing tasks automatically

vLLM automatically advertises transcription support if your model implements the interface:

if supports_transcription(model):
    if model.supports_transcription_only:
        return ["transcription"]
    supported_tasks.append("transcription")

When enabled, the server initializes the transcription and translation handlers:

state.openai_serving_transcription = OpenAIServingTranscription(...) if "transcription" in supported_tasks else None
state.openai_serving_translation = OpenAIServingTranslation(...) if "transcription" in supported_tasks else None

No extra registration is required beyond having your model class available via the model registry and implementing SupportsTranscription.

Examples in-tree

Whisper encoder–decoder (audio-only): vllm/model_executor/models/whisper.py
Voxtral decoder-only (audio embeddings + LLM): vllm/model_executor/models/voxtral.py. Make sure to have installed mistral-common[audio].
Gemma3n decoder-only with fixed instruction prompt: vllm/model_executor/models/gemma3n_mm.py
Qwen3-Omni multimodal with audio embeddings: vllm/model_executor/models/qwen3_omni_moe_thinker.py

Test with the API

Once your model implements SupportsTranscription, you can test the endpoints (API mimics OpenAI):

Transcription (ASR):

curl -s -X POST \
  -H "Authorization: Bearer $VLLM_API_KEY" \
  -H "Content-Type: multipart/form-data" \
  -F "file=@/path/to/audio.wav" \
  -F "model=$MODEL_ID" \
  http://localhost:8000/v1/audio/transcriptions

Translation (source → English unless otherwise supported):

curl -s -X POST \
  -H "Authorization: Bearer $VLLM_API_KEY" \
  -H "Content-Type: multipart/form-data" \
  -F "file=@/path/to/audio.wav" \
  -F "model=$MODEL_ID" \
  http://localhost:8000/v1/audio/translations

Or check out more examples in examples/speech_to_text.

Note

If your model handles chunking internally (e.g., via its processor or encoder), set min_energy_split_window_size=None in the returned SpeechToTextConfig to disable server-side chunking.
Implementing get_num_audio_tokens improves accuracy of streaming usage metrics (prompt_tokens) without an extra forward pass.
For multilingual behavior, keep supported_languages aligned with actual model capabilities.

Speech-to-Text (Transcription/Translation) Support

Speech-to-Text (Transcription/Translation) Support

Update the base vLLM model

supported_languages and supports_transcription_only

supported_languages and supports_transcription_only

get_speech_to_text_config()

Multimodal LLM with audio embeddings (e.g., Voxtral, Gemma3n)

get_generation_prompt()

Encoder–decoder audio-only (e.g., Whisper)

get_generation_prompt()

validate_language (optional)

validate_language()

get_num_audio_tokens (optional)

get_num_audio_tokens()

Audio preprocessing and chunking

_preprocess_speech_to_text()

Exposing tasks automatically

Examples in-tree

Test with the API

`supported_languages` and `supports_transcription_only`

`validate_language` (optional)

`get_num_audio_tokens` (optional)