fern: Generate API SDKs and Documentation

Overview

Fern is a platform and CLI for transforming API definitions into generated SDKs and developer documentation. It aims to reduce the work of maintaining language-specific client libraries and API references separately from the API specification.

It supports OpenAPI, AsyncAPI, Protobuf, and OpenRPC inputs. Fern may fit teams that want a specification-driven workflow for publishing SDKs and docs; generated artifacts and hosted documentation are part of a broader platform, so teams should review its generation and hosting workflows against their needs.

Key Features

• Generate type-safe SDKs for languages including TypeScript, Python, Java, Go, Ruby, PHP, C#, Swift, and Rust.
• Accept API definitions in OpenAPI, AsyncAPI, Protobuf, and OpenRPC formats.
• Provide a CLI for initializing projects, checking definitions, and running generation.
• Generate API documentation and references, with support for additional Markdown pages.
• Offer SDK, CLI, and OpenAPI spec generators.
• Support an interactive documentation UI with search, versioning, dark mode, and customization options.
• Include an AI Search feature described as answering developer questions using documentation, APIs, and SDKs.

Use Cases

• API platform teams can generate and distribute client SDKs for customers across multiple programming languages.
• Developer relations teams can keep API references connected to API definitions while adding hand-written guides in Markdown.
• Teams maintaining REST, WebSocket, gRPC, or JSON-RPC APIs can use Fern to generate SDKs or documentation from supported specifications.
• Organizations that already maintain OpenAPI definitions can evaluate Fern as a way to introduce SDK generation without replacing those definitions.

Project Facts

• Language: TypeScript
• License: Apache-2.0
• Stars: 3.8k
• Forks: 358
• Topics: api, docs-as-code, docs-generator, openapi, sdk-generator
• Archived: false

Getting Started

Install the CLI with Node 18 or later:

npm install -g fern-api

Initialize from an OpenAPI file with fern init --openapi ./path/to/openapi.yml, then run fern generate. See the README for configuration and additional commands.

Considerations

• The CLI requires Node 18 or later.
• Fern's generators cover multiple languages, but each generator is a separate component with its own versions and changelog. Check the relevant generator's support and release details before adopting it.
• The README distinguishes local Docker-based generation for legacy generator groups from remote generation for SDK Config targets, so workflows may differ depending on configuration.
• The repository describes hosted documentation and AI Search as platform capabilities; evaluate those offerings separately if you need them, rather than assuming they are provided solely by the CLI.