OpenAPI is the standard machine-readable description of a REST API (endpoints, parameters, schemas, errors) from which typed client libraries, documentation and API tool collections can be generated.
An OpenAPI file, in YAML or JSON, describes every endpoint precisely enough for tools to work from it: generate a client in TypeScript, Python or Java, import into Postman or Insomnia, validate requests, render reference docs. For a small API it replaces official SDKs entirely, and it stays current with the API by construction.
It is also the most useful artefact an AI coding assistant can read to integrate an API correctly.
At SMSMeteor: the OpenAPI 3.1 description is served at /docs/openapi.yaml and versioned with the API; generation recipes are on the SDKs page.