Open ASR Server

OpenAI-compatible ASR server with pluggable local transcription backends.

Install

Base install includes the API server and shared models/formatters:

uv tool install "open-asr-server"

Quick start by hardware

Pick one path:

# Apple Silicon (MLX backends bundle)
uv tool install --python 3.11 "open-asr-server[metal]"

# CPU only (cross-platform bundle)
uv tool install "open-asr-server[cpu]"

# NVIDIA CUDA (NeMo backend)
uv tool install "open-asr-server[nemo]"

Backend extras (advanced)

Install only the backend framework you want:

uv tool install "open-asr-server[parakeet-mlx]"
uv tool install "open-asr-server[whisper-mlx]"
uv tool install "open-asr-server[lightning-whisper-mlx]"
uv tool install "open-asr-server[kyutai-mlx]"
uv tool install "open-asr-server[faster-whisper]"
uv tool install "open-asr-server[whisper-cpp]"
uv tool install "open-asr-server[nemo]"

Bundle extras:

• metal: Parakeet MLX, Whisper MLX, Lightning Whisper MLX, Kyutai MLX
• cpu: faster-whisper + whisper.cpp
• cuda: CUDA dependency bundle for NeMo/Torch installs

Need help deciding what to run?

uv tool run open-asr-server doctor
uv tool run open-asr-server backends

# Auto-setup (no manual --python needed)
uv tool run open-asr-server setup --apply
uv tool run open-asr-server setup metal --apply
uv tool run open-asr-server setup nemo-parakeet --apply

# Machine-readable output
uv tool run open-asr-server doctor --json
uv tool run open-asr-server backends --json
uv tool run open-asr-server setup --json

Notes:

• Parakeet MLX, Whisper MLX, and Lightning Whisper MLX are currently pinned to Python 3.11.
• Kyutai MLX is currently pinned to Python 3.12.

MLX troubleshooting

If MLX extras fail on newer Python versions, use Python 3.11 for Parakeet/Whisper/Lightning:

uv run --python 3.11 --extra whisper-mlx -- open-asr-server serve --host 127.0.0.1 --port 8000

Or let the CLI choose the right Python automatically:

uv tool run open-asr-server setup metal --apply

For Kyutai MLX, use Python 3.12:

uv run --python 3.12 --extra kyutai-mlx -- open-asr-server serve --host 127.0.0.1 --port 8000

CUDA setup (NeMo)

NeMo requires a CUDA-enabled PyTorch build. Use the PyTorch install selector to find the right index URL for your CUDA version:

https://pytorch.org/get-started/locally/

Example (CUDA 12.1):

uv pip install torch --index-url https://download.pytorch.org/whl/cu121
uv pip install "open-asr-server[nemo]"

Alternative (auto-detect CUDA with uv):

uv pip install torch --torch-backend=auto
uv pip install "open-asr-server[nemo]"

Repo-based installs can optionally use tool.uv.sources to route torch downloads to a CUDA index automatically when the nemo extra is enabled (see pyproject.toml). This only applies when working from the repo (not a PyPI install).

Install the CUDA-enabled torch build before the nemo extra to avoid pulling in a CPU-only torch dependency.

NeMo expects mono audio; the backend uses ffmpeg to downmix or convert inputs to 16kHz mono WAV when needed. Ensure ffmpeg is available in your environment.

If you see CUDA graph capture errors from NeMo decoding, set OPEN_ASR_NEMO_DISABLE_CUDA_GRAPHS=1 (default behavior disables CUDA graphs).

If NeMo fails with a Weights only load failed checkpoint error, the backend retries once with torch.load(weights_only=False) when OPEN_ASR_NEMO_WEIGHTS_ONLY_FALLBACK=1 (enabled by default). Set this to 0 to disable fallback.

Tip: CUDA backends are often easiest to run in Docker with the NVIDIA Container Toolkit; we do not ship a container image yet, but this keeps CUDA deps isolated.

Example Docker workflow (Linux):

docker run --rm -it --gpus all \
  -v "$(pwd)":/workspace -w /workspace \
  nvidia/cuda:12.1.0-runtime-ubuntu22.04 \
  bash -lc "\
    apt-get update && apt-get install -y curl python3 python3-venv && \
    curl -LsSf https://astral.sh/uv/install.sh | sh && \
    export PATH=\"$HOME/.local/bin:$PATH\" && \
    uv pip install torch --index-url https://download.pytorch.org/whl/cu121 && \
    uv pip install '.[nemo]' && \
    python scripts/smoke_nemo_parakeet.py samples/jfk_0_5.flac\
  "

Dockerfile alternative (dev-first split):

docker build -f Dockerfile.nemo.base \
  --build-arg TORCH_INDEX_URL=https://download.pytorch.org/whl/cu121 \
  -t open-asr-nemo-base:torch2.5.1-cu121 .
docker build -f Dockerfile.nemo --build-arg BASE_IMAGE=open-asr-nemo-base:torch2.5.1-cu121 -t open-asr-nemo-dev:torch2.5.1-cu121 .
docker run --rm -it --gpus all -v "$(pwd)":/workspace -w /workspace open-asr-nemo-dev:torch2.5.1-cu121

docker build -f Dockerfile.nemo.base \
  --build-arg CUDA_BASE_IMAGE=pytorch/pytorch:2.5.1-cuda11.8-cudnn8-runtime \
  --build-arg TORCH_INDEX_URL=https://download.pytorch.org/whl/cu118 \
  -t open-asr-nemo-base:torch2.5.1-cu118 .
docker build -f Dockerfile.nemo \
  --build-arg BASE_IMAGE=open-asr-nemo-base:torch2.5.1-cu118 \
  -t open-asr-nemo-dev:torch2.5.1-cu118 .
docker run --rm -it --gpus all -v "$(pwd)":/workspace -w /workspace open-asr-nemo-dev:torch2.5.1-cu118

Makefile helpers:

make nemo-base
make nemo-dev
make nemo-run
make nemo-base CUDA_BASE_IMAGE=pytorch/pytorch:2.5.1-cuda11.8-cudnn8-runtime BASE_TAG=torch2.5.1-cu118 TORCH_INDEX_URL=https://download.pytorch.org/whl/cu118
make nemo-dev BASE_TAG=torch2.5.1-cu118 DEV_TAG=torch2.5.1-cu118

Docker smoke script:

scripts/smoke_nemo_parakeet_docker.sh
INFO=1 scripts/smoke_nemo_parakeet_docker.sh
HF_CACHE_DIR=/path/to/hf-cache scripts/smoke_nemo_parakeet_docker.sh

Run

Install at least one backend extra before running (the default model uses Parakeet MLX):

uv tool install --python 3.11 "open-asr-server[parakeet-mlx]"

Then start the server:

uv tool run open-asr-server serve --host 127.0.0.1 --port 8000

Environment variables:

• OPEN_ASR_SERVER_DEFAULT_MODEL: default model ID for requests
• OPEN_ASR_DEFAULT_BACKEND: preferred backend when model patterns overlap
• OPEN_ASR_SERVER_PRELOAD: comma-separated models to preload at startup
• OPEN_ASR_SERVER_API_KEY: optional shared secret for requests
• OPEN_ASR_SERVER_ALLOWED_MODELS: comma-separated allowed model IDs or patterns
• OPEN_ASR_SERVER_MAX_UPLOAD_BYTES: max upload size in bytes (default: 26214400)
• OPEN_ASR_SERVER_RATE_LIMIT_PER_MINUTE: optional per-client request limit (off by default)
• OPEN_ASR_SERVER_TRANSCRIBE_TIMEOUT_SECONDS: optional transcription timeout (off by default)
• OPEN_ASR_SERVER_TRANSCRIBE_WORKERS: optional thread pool size for transcriptions
• OPEN_ASR_SERVER_MODEL_IDLE_SECONDS: unload models after idle timeout (off by default)
• OPEN_ASR_SERVER_MODEL_EVICT_INTERVAL_SECONDS: idle eviction sweep interval (default: 60)
• OPEN_ASR_SERVER_EVICT_PRELOADED_MODELS: allow preloaded models to be evicted (default: false)
• OPEN_ASR_SERVER_MODEL_DIR: override the Hugging Face cache location for this server
• OPEN_ASR_SERVER_HF_TOKEN: optional Hugging Face token for gated/private models

Models default to the Hugging Face cache unless a local path is provided. Use OPEN_ASR_SERVER_MODEL_DIR if you want a dedicated cache without changing your global HF environment. Use OPEN_ASR_SERVER_HF_TOKEN to authenticate downloads without setting global HF environment variables.

Use OPEN_ASR_SERVER_TRANSCRIBE_TIMEOUT_SECONDS to bound long transcriptions. If you set OPEN_ASR_SERVER_TRANSCRIBE_WORKERS, transcriptions run in a background thread pool instead of the event loop.

Admin model management (requires API key if configured):

curl -X POST http://127.0.0.1:8000/v1/admin/models/unload \
  -H "Content-Type: application/json" \
  -d '{"model":"nvidia/parakeet-tdt-0.6b-v3"}'

curl -X POST http://127.0.0.1:8000/v1/admin/models/unload-all \
  -H "Content-Type: application/json" \
  -d '{"include_pinned":true}'

curl http://127.0.0.1:8000/v1/admin/models/status

Sample audio

Two short clips are included in samples/ for quick smoke tests:

• samples/jfk_0_5.flac
• samples/jfk_5_10.flac

They are derived from tests/jfk.flac in the OpenAI Whisper repo (MIT); the original JFK speech is public domain.

uv run --extra parakeet-mlx scripts/smoke_parakeet.py samples/jfk_0_5.flac
uv run --python 3.11 --extra whisper-mlx scripts/smoke_whisper.py samples/jfk_0_5.flac
uv run --python 3.11 --extra lightning-whisper-mlx scripts/smoke_lightning.py samples/jfk_0_5.flac
uv run --extra whisper-cpp scripts/smoke_whisper_cpp.py samples/jfk_0_5.flac
uv run --python 3.12 --extra kyutai-mlx scripts/smoke_kyutai_mlx.py samples/jfk_0_5.flac
uv run --extra nemo scripts/smoke_nemo_parakeet.py samples/jfk_0_5.flac

Backend options

Backends are selected by model ID patterns. Use backend:model when you need an explicit backend.

Metal (Apple Silicon)

• Parakeet MLX: mlx-community/parakeet-tdt-0.6b-v3 (default) or parakeet-*
• MLX Whisper: whisper-large-v3-turbo or mlx-community/whisper-large-v3-turbo
• Lightning Whisper MLX: lightning-whisper-distil-large-v3
• Kyutai STT MLX: kyutai/stt-*-mlx

CPU

• Faster-Whisper: openai/whisper-* and distil-whisper/*
• whisper.cpp: tiny*, base*, small*, medium*, large*

CUDA

• NeMo Parakeet: nvidia/parakeet*

API compatibility

The server implements:

• POST /v1/audio/transcriptions
• GET /v1/models
• GET /v1/models/metadata

Load-time backend failures return structured detail payloads with retry hints:

{
  "detail": {
    "type": "backend_load_error",
    "code": "model_load_oom",
    "message": "Failed to load ...",
    "backend": "nemo-parakeet",
    "model": "nvidia/parakeet-tdt-0.6b-v3",
    "retryable": true
  }
}

Current load error codes include weights_only_incompat, model_load_oom, and backend_busy.

/v1/models/metadata includes install hints for known backends (install_extra, install_bundle, install_python, install_command) so automation can guide setup without hardcoded backend mappings.

Example:

curl -s -X POST "http://127.0.0.1:8000/v1/audio/transcriptions" \
  -F "file=@audio.wav" \
  -F "model=whisper-large-v3-turbo"

Security

This server is designed for trusted networks. If you expose it publicly, enable OPEN_ASR_SERVER_API_KEY and front it with a reverse proxy that provides TLS and rate limiting. OPEN_ASR_SERVER_RATE_LIMIT_PER_MINUTE offers a simple in-process limiter for single-instance use, but it is not a substitute for production-grade rate limiting.

API key headers:

• Authorization: Bearer <token>
• X-API-Key: <token>

Use OPEN_ASR_SERVER_ALLOWED_MODELS to limit which model IDs can be loaded and prevent unbounded downloads. Avoid logging request bodies or filenames if those may contain sensitive data, and review reverse-proxy access logs for any retention concerns.

Release

Follow the PR-first release flow in CONTRIBUTING.md.

At a high level:

1. Prepare release changes on a release-x.y.z branch.
2. Open and merge a PR to main.
3. Tag the merged commit and create a GitHub release.