Bolt 09 is a small Node.js library that provides helpers for encoding, decoding, and inspecting Lightning Network feature flags as defined in BOLT 09. It lets developers convert feature bits to human‑readable types, serialize them to hex or Bech32 words for inclusion in BOLT‑11 invoices, and parse those representations back into structured data. The package is aimed at Lightning‑protocol developers who need to work with feature flag data programmatically.