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Fern

Turn one API definition into type-safe SDKs, beautiful docs, and MCP servers.

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About

Fern is a developer-tools platform that takes a single API definition and generates the artifacts developers expect from a serious API: type-safe, idiomatic client SDKs in TypeScript, Python, Java, Go, Ruby, PHP, C#, Swift, and Rust; a hosted documentation site with interactive reference and API playground; and now MCP servers so AI agents can call your API too. It accepts OpenAPI, AsyncAPI, Protobuf, and its own Fern Definition format as input. An 'Ask Fern' AI search assistant, trained on your docs and SDKs, answers developer questions on the docs site.

Fern is open-core: the CLI and generators are Apache-2.0 and free, while hosted docs and SDK publishing run on Hobby, Team, and Enterprise plans. The CLI had nearly 4,000 stars and commits within hours of this writing, so maintenance is clearly active. The honest caveat is lock-in shape: the generated SDKs are yours, but the prettiest parts of the experience (hosted docs, Ask Fern) live on Fern's platform, so evaluate what happens to your docs site if you ever leave.

What it can do

  • Type-safe SDK generation in 9 languages from one API definition
  • Hosted docs with interactive reference and API playground
  • Ask Fern: AI assistant trained on your docs, APIs, and SDKs
  • MCP server generation for AI agent access
  • Open-source CLI and generators (Apache-2.0); registry publishing to npm, PyPI, Maven

Setup

  1. Install the CLI: npm install -g fern-api (Node 18+).
  2. Initialize from your spec: fern init --openapi ./openapi.yml.
  3. Configure generators in fern/generators.yml for the SDK languages you want.
  4. Run fern generate and find SDKs under /generated/sdks/; publish docs and SDKs through the Fern dashboard.

Prerequisites

Node.js 18+; an OpenAPI/AsyncAPI/Protobuf spec or Fern Definition; a Fern account for hosted docs.

Use cases

apidocumentationsdk-generationdeveloper-toolsopen-source

Works with

Trust score — 8/10

/10 trustMetrics-checked

Scored 8/10 (metrics tier, metrics-v1). Evidence: 3,815 GitHub stars (GitHub API), Apache-2.0, pushed 2026-10-16; README documents the SDK/docs/AI-search surface and the init-to-generate flow; buildwithfern.com verified live. Strengths: excellent maintenance cadence, open-core licensing, broad language coverage. Docked for: (1) hosted docs and AI search are platform-locked; (2) crowded space (Speakeasy, Stainless, readme). License: Apache-2.0 (CLI/generators); hosted platform proprietary.

★ 3,815 GitHub stars

How the trust score is built

Full methodology →
Adoption · stars/downloads, log-scaled vs peers
7.0/10
Maintenance · recency, releases, issue load
9.0/10
Openness · license + docs
8.0/10
Manual review · sanity read + red-flag check
8.0/10
  • · 3,815 GitHub stars (GitHub API), pushed 2026-10-16
  • · README documents SDK/docs/AI-search surface and init-to-generate flow
  • · buildwithfern.com verified live; open-core model (CLI free, hosted docs on paid plans)

Sources

Every fact above was checked against these sources. Where a claim comes from the provider and couldn't be independently verified, the text says so.