The bug

Every ort.InferenceSession in the project is created with providers=["CPUExecutionProvider"] hardcoded, in four places:

• src/livekit/wakeword/models/feature_extractor.py:42-44 (Mel)
• src/livekit/wakeword/models/feature_extractor.py:101-103 (Embedding)
• src/livekit/wakeword/inference/model.py:87-89 (classifier)
• src/livekit/wakeword/eval/evaluate.py:197 (eval)

Even if a user installs onnxruntime-gpu on a GPU pod, CUDA is never selected. The Swift package already supports pluggable execution providers (ExecutionProvider.coreML), so this gap is Python-specific.

Impact

Feature extraction during augment is the most ORT-heavy stage — roughly 17 model calls per 2 s clip (1 mel + 16 sliding embedding windows). On an RTX 3090 pod this runs on CPU at ~3 clips/sec. A production config (~150k augmented clips across 6 splits × 2 rounds) takes ~14 hours of CPU time; with CUDA active, the same workload finished in ~13 minutes on the same pod.

The bug also affects WakeWordModel (the listener) and run_eval, both of which silently stay on CPU even when the host has onnxruntime-gpu installed.

The fix

New helper src/livekit/wakeword/_ort_providers.py with a single function:

def get_providers() -> list[str]:
    # 1. LIVEKIT_WAKEWORD_ORT_PROVIDERS env var (comma-separated) wins if set
    # 2. Otherwise intersect ort.get_available_providers() with
    #    (\"CUDAExecutionProvider\", \"CPUExecutionProvider\") — CUDA first, CPU fallback
    # 3. If neither is available, fall through to whatever ORT reports

All four call sites now use it. Behaviour on CPU-only installs is unchanged (plain onnxruntime reports only CPUExecutionProvider).

The env var escape hatch covers:

• Forcing CPU on GPU hosts for reproducibility
• Opting into CoreML / DirectML / ROCm / TensorRT providers without plumbing a config field

Design decisions:

• Helper, not duplicated call-site logic — four copies of provider selection would drift.
• Env var, not config field or constructor arg — a config field only helps YAML-driven training, not library consumers of WakeWordModel; a constructor arg would thread through multiple public APIs. Env var works everywhere with zero surface change.
• Default preference is CUDA / CPU only — CoreML / DirectML / ROCm / TensorRT are exotic enough to warrant explicit opt-in via the env var.
• No warning when CPU is selected on a GPU host — can't cheaply detect "GPU hardware present" from Python without importing torch; users who suspect slowness can check the INFO log for the selected provider list.

Packaging

Adds an optional gpu extra in pyproject.toml:

gpu = [\"onnxruntime-gpu>=1.17\"]

README gets a new "GPU acceleration" subsection documenting the switch:

pip uninstall -y onnxruntime
pip install livekit-wakeword[train,eval,export,gpu]

The uninstall step is required because onnxruntime and onnxruntime-gpu share the Python module name — pip cannot keep them side-by-side. The README also points at ONNX Runtime's GPU compatibility matrix since the onnxruntime-gpu wheel bundles specific CUDA toolkit versions.

What's in this PR

Out of scope (deliberately — these are follow-ups)

• Cross-clip batching in run_extraction: features.py processes one clip at a time. Even on GPU this leaves throughput on the table; a batched path would give another large speedup. Worth its own PR — touches the extraction loop, not provider selection.
• Splitting augment into augment + extract CLI commands: right now run_augmentation deletes all *_rN.wav files at the top, so if feature extraction fails the user re-runs and loses the augmented clips. A --no-clean flag and/or a separate extract command is a UX concern, not a provider concern.

Verification

uv run ruff check src/livekit/wakeword/_ort_providers.py <touched files>  # clean
uv run ruff format --check <touched files>                                 # clean
uv run mypy src/livekit/wakeword/                                          # same 1 pre-existing error as main
uv run pytest tests/                                                       # 72 passed (60 existing + 12 new)

Reproduction of the original bug:

livekit-wakeword generate configs/prod.yaml
livekit-wakeword augment configs/prod.yaml
# Feature extraction sub-stage runs at ~3 cps on a GPU pod. nvidia-smi shows 0% util.

After this PR, with livekit-wakeword[train,eval,export,gpu] installed (and onnxruntime uninstalled), the same workload saturates the GPU — measured ~200 cps on RTX 3090.