VieNeu-TTS is an advanced on-device Vietnamese text-to-speech library with instant voice cloning, supporting bilingual (Vietnamese/English) synthesis. The system provides a Python SDK (`vieneu`) with multiple backends: v3 Turbo (default, 48kHz, ONNX or PyTorch), v3 Nano (lightweight 24kHz, ONNX only), and legacy v1/v2 (deprecated). It includes a Gradio web UI (`vieneu-web`), a FastAPI streaming server, and a browser-based HTML/JS client. The core architecture is a factory pattern (`Vieneu(mode=...)`) that returns the appropriate engine, all inheriting from a shared base class. Inference can run entirely torch-free on CPU via ONNX Runtime, with optional GPU acceleration (CUDA) for batched generation and lower latency. Feature highlights include instant voice cloning from a 3β8 second reference clip using a speaker encoder (ONNX) and denoiser, inline emotion cues (`[cΖ°α»i]`, `[thα» dΓ i]`), frame-level streaming, SRT subtitle-to-speech, and LoRA fine-tuning for custom voices (requires ~6GB GPU). The system is deployed via Docker with CPU and GPU profiles, and has CI/CD pipelines for testing and PyPI publishing. The codebase is well-structured with clear separation of concerns, though it carries some legacy weight from older model versions and a few deprecated parameters. Other notable components include a sliding-window repetition penalty, a babble guard heuristic for short utterances, and a voice persistence system for user-defined presets.