Rembg is a production-grade tool for removing image backgrounds, usable as a Python library, CLI, HTTP server, or Docker container. It wraps multiple ONNX-based segmentation models (U2Net, BiRefNet, SAM, BRIA-RMBG, etc.) and provides several cutout strategies: naive, alpha matting, color decontamination, and ViTMatte refinement. The architecture is modular: a session abstraction handles model download and inference, while the `remove()` function in `rembg/bg.py` orchestrates preprocessing, mask prediction, and post-processing. The CLI offers subcommands for single files (`i`), batch processing (`p`), HTTP serving (`s`), binary stream input (`b`), model download (`d`), and legacy migration (`m`). The HTTP server uses FastAPI with a Gradio UI and includes security measures like DNS rebinding protection. The codebase is well-tested, with synthetic fixtures for color decontamination and integration tests using perceptual hashing. It demonstrates strong engineering practices: idempotent migration, session caching, and careful handling of edge cases like oversized uploads and failed alpha matting convergence.