toon-format/toon

Combines YAML-style indentation with CSV-like tabular layouts for uniform arrays, achieving ~40% token savings over JSON while maintaining lossless round-trip conversion. The format uses explicit array-length declarations `[N]` and field headers `{fields}` to provide LLMs with clear schema boundaries, improving both parsing accuracy (74% vs JSON's 70%) and reliability. Spec-driven with implementations across TypeScript, Python, Go, Rust, and .NET, designed as a translation layer between JSON-based application logic and token-efficient LLM input.

23,207 stars and 1,595,658 monthly downloads. Used by 20 other packages. Actively maintained with 7 commits in the last 30 days. Available on npm.