Fern
Takes an OpenAPI spec and generates hand-crafted-quality SDKs across TypeScript, Python, Go, Java, Ruby, C#, and PHP, plus integrated documentation sites. SDKs and docs stay in sync from a single source of truth. Used by Cohere, ElevenLabs, and Merge.
Redocly
Combines Redoc (25k+ GitHub stars) with a powerful CLI for linting, validating, and bundling OpenAPI specs. Supports OpenAPI 3.x, Swagger 2.0, AsyncAPI, and Arazzo. Enterprise offering adds developer portals and auto-generated code samples across teams.