Fern

Fern is a developer-tools platform that turns a single API specification (OpenAPI, AsyncAPI, gRPC/Protobuf, or Fern's own Fern Definition format) into idiomatic client SDKs, a hosted documentation site, and MCP servers. The fern CLI is open source under Apache-2.0 — npm install -g fern-api, run fern init --openapi, then fern generate — and drives a hosted dashboard for publishing. Generated docs include an interactive API reference, an API playground, and 'Ask Fern', an AI assistant trained on your docs, APIs, and SDKs. SDKs publish to npm, PyPI, Maven, and other registries across TypeScript, Python, Go, Java, C#, PHP, Ruby, Swift, and Rust.

The honest caveat: the genuinely free part is the CLI and generators; hosted docs and managed SDK publishing sit on paid Team/Enterprise plans (Hobby is free but limited). Fern's own Definition format is powerful but proprietary — teams standardized on plain OpenAPI get slightly less of the magic. For pure SDK generation without the docs platform, Speakeasy's open-sourced generator may be the leaner choice.

Install with npm install -g fern-api (Node 18+). Run fern init --openapi ./openapi.yml, configure generators in generators.yml, then fern generate. Connect to the hosted dashboard to publish docs.