What it is

Fern is an Apache-2.0 TypeScript project in the API documentation and SDK generation ecosystem. It takes API definitions as input and produces client SDKs and developer documentation. The project centers on the fern-api CLI, published on npm, and supports OpenAPI, AsyncAPI, Protobuf, and OpenRPC.

It solves the manual work required to keep API contracts, client libraries, and documentation aligned. A team points Fern at an OpenAPI document and runs a generator, producing typed SDK output and a documentation site from the same source definition, which reduces drift between the API and developer guidance.

Key capabilities

• Generates type-safe SDKs for TypeScript, Python, Java, Go, Ruby, PHP, and C# from API definitions.
• Accepts OpenAPI for REST and Webhooks, plus AsyncAPI for WebSockets, Protobuf for gRPC, and OpenRPC.
• Builds a hosted developer documentation website with an auto-generated API reference and markdown pages.
• Provides search, SEO, dark mode, and customizable branding for colors, font, logo, and domain name.
• Adds generators as processes that take API definitions and output artifacts such as SDKs and server boilerplate.
• Runs through a CLI that requires Node 18 or newer and stores configuration in fern/fern.config.json and generators.yml.
• Exposes generated output locally, such as a TypeScript SDK in /generated/sdks/typescript.

Who uses it and how

• API teams initialize a Fern project from a local OpenAPI file or remote OpenAPI URL, then run fern generate to produce SDKs.
• Documentation teams add markdown pages alongside generated references and version those pages through git.
• Teams can publish branded docs sites for public or partner developers, as shown by elevenlabs.io/docs, launchdarkly.com/docs, and docs.hume.ai.
• Engineering teams add language-specific generators from the CLI to create client libraries for languages their users consume.

Getting started

Install the CLI globally with npm install -g fern-api, then initialize a project from a local or remote OpenAPI document using fern init --openapi ./path/to/openapi.yml or a URL. After adding generators, run fern generate to produce SDK output under /generated/sdks/typescript.

When to use it — and when not to

Use Fern when your goal is to turn existing API specifications into SDKs and docs from a single CLI workflow, especially in a Node.js or TypeScript toolchain. The README describes a hosted documentation option and dashboard signup, but it does not state a fully self-hosted deployment model or list required services such as a database, storage backend, or SMTP server, so teams needing an entirely on-premises stack should verify that before adopting it. The registry facts show 343 open issues and no listed contributor count, which means prospective users should check maintenance activity and backlog before depending on it for production SDK generation.

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npm install -g fern-api

fern init --openapi ./path/to/openapi.yml
# or
fern init --openapi https://link.buildwithfern.com/plantstore-openapi

fern/
├─ fern.config.json
├─ generators.yml # generators you're using
└─ openapi/
  └─ openapi.json # your openapi document

fern generate