Fern

Fern turns an API specification into three deliverables: a documentation website, client SDKs, and a CLI for interacting with the API from the terminal. You point it at a spec — OpenAPI, GraphQL, AsyncAPI, or gRPC — and run fern generate to produce the output. The docs site, SDKs, and CLI all stay in sync because they come from the same source of truth.

The documentation product supports customization through config files: a docs.yml defines navigation and page structure, theme.css controls colors, fonts, and radii, and content is written in MDX. On the AI side, Fern's docs ship with search, an MCP server, llms.txt, and an "Ask Fern" AI search feature, so both developers and coding agents (Claude, Gemini, Copilot, ChatGPT, Cursor are shown on the site) can consume the docs. The SDKs are described as type-safe clients in every major language, designed by language experts.

Fern is SOC 2 compliant according to its trust center, and the company offers a free tier with sign-up through its dashboard, plus a demo booking option. The homepage cites customers including ElevenLabs, Frame, and Unleash, with claims like 20M+ docs pages served per year and a 99% reduction in time to first API call for Frame.

Best for

• API-first companies that want docs, SDKs, and a CLI generated from a single OpenAPI, GraphQL, AsyncAPI, or gRPC spec
• Teams whose documentation needs to be readable by AI agents and coding assistants, not just humans
• Engineering teams that want type-safe client libraries in multiple languages without maintaining them by hand
• Companies that want to customize docs branding and navigation through config files rather than a GUI
• Teams that need a terminal CLI for their API alongside the docs

Frequently asked questions

Which API specifications does Fern support?

The homepage lists OpenAPI, GraphQL, AsyncAPI, and gRPC as supported protocols.

Is Fern free to use?

The site offers a "Start for free" sign-up and has a pricing page, but the homepage doesn't detail what the free tier includes or what paid plans cost.

How do I customize the documentation site?

Fern uses configuration files: docs.yml for navigation and page structure, theme.css for styling like colors and fonts, and MDX files for page content.

Does Fern work with AI tools?

Yes. The docs include an MCP server, llms.txt, AI search, and an "Ask Fern" feature, and the site shows integrations with Claude, Gemini, Copilot, ChatGPT, and Cursor.

What SDK languages does Fern generate?

The site says SDKs are type-safe clients "in every major language" but doesn't enumerate the specific languages on the homepage.

Is Fern secure?

Fern's trust center states it is SOC 2 compliant.

Alternatives

• Mintlify — a good fit if you want polished documentation out of the box without the SDK and CLI generation.
• KnowledgeOwl — better if you need public and private knowledge bases for support content rather than API reference docs.
• Helpjuice — suited to internal knowledge management and team collaboration rather than spec-driven API documentation.