Piper is a fast, local neural text-to-speech engine that uses VITS models exported to ONNX for inference. The repository provides a comprehensive application stack: a Python package (`piper-tts`) with a clean API for voice synthesis, a CLI tool, an HTTP server with a web interface (including viseme animation), and a C/C++ shared library (`libpiper`) for embedding in other applications. It also includes a full training pipeline using PyTorch Lightning, allowing users to fine-tune or train new voices from scratch, with support for multi-speaker models, custom phonemes, and VAD silence trimming. The architecture is modular: phonemization is handled by language-specific backends (espeak-ng, Chinese pinyin/g2pW, Hebrew Nakdimon, Japanese OpenJTalk, Arabic tashkeel), and the core inference engine uses ONNX Runtime for efficient, cross-platform execution. The codebase is well-structured with clear separation of concerns, extensive testing, and CI/CD pipelines that build wheels for all major platforms. Although primarily an application, the training code (VITS model implementations, dataset caching, monotonic alignment Cython) exhibits data/research characteristics, making it a hybrid project. The repository is actively maintained, with a responsive issue tracker and regular updates, though the last commit date appears anomalous.