keiyaku

https://github.com/onyxblade/keiyaku

Generates Ruby clients from OpenAPI documents that carry only what is specific to one API — an operation table and the schema fields. Transport, parameter serialization, casting and error mapping live in a shared runtime, so a nineteen-operation client is about seventy lines. A schema becomes a frozen value type with RBS emitted beside it, so terse Ruby costs nothing in tooling. Constructs it cannot translate faithfully are refused at generation time rather than emitted as plausible guesses.