This is the canonical implementation of OpenAI's Harmony response format, the structured prompt format used by the gpt-oss open-weight model series. The repository provides a high-performance Rust core for rendering conversations into token sequences and parsing model outputs back into structured messages. It supports multi-channel reasoning (analysis, commentary, final), tool/function calling with JSON Schema-defined parameters, structured outputs, and streaming incremental parsing. The library is packaged with first-class Python bindings via PyO3 and a JavaScript/WASM build for browser use. A Next.js demo visualizes the token-level rendering. The codebase demonstrates strong engineering: a custom BPE tokenizer (tiktoken) implementation in Rust, a streaming parser that gracefully handles partial UTF-8 sequences, and a JSON Schema-to-TypeScript converter for tool definitions. Tests are comprehensive with parity between Rust and Python test suites.