Sorbet's sidecar type-signature file format. RBI files look like normal Ruby but contain only method/constant declarations with `sig` signatures — no implementations. Used to (a) add types for untyped gems, (b) describe methods produced by metaprogramming, and (c) keep signatures out of hot implementation files (Erwin Kroon considers an RBI redefinition of `self` a clean workaround for `T.bind(self, …)`). Tapioca is the tool of choice for generating RBIs automatically.