API docs + SDK generation from a single OpenAPI spec. Generates production SDKs in 9 languages alongside documentation. Acquired by Postman (Jan 2026). Used by Square, Auth0, Adobe, Twilio, ElevenLabs.
Where it wins
Only tool that generates both docs AND production SDKs (9 languages) from a single spec
Acquired by Postman — access to 500K+ companies (98% of Fortune 500)
Enterprise customers: Square, Auth0, Adobe, Twilio, ElevenLabs
Very active development — daily pushes, 4.37.8 released Mar 2026
Even competitor (Speakeasy) acknowledges technical strengths
Where to be skeptical
Small OSS footprint — 3.6K stars
Postman acquisition is a distribution bet that hasn't played out yet
If you're not generating SDKs, Fern's unique value prop doesn't apply
369 open issues — high issue count
Editorial verdict
The most strategically significant move in API docs in 2026. Only tool generating both docs AND production SDKs from a single spec. Postman acquisition (Jan 2026) gives access to 500K+ companies. 3.6K stars, 89K npm/week. Daily pushes post-acquisition confirm continued development.
Source
Related
Swagger UI
95The original interactive API documentation renderer. 28.7K stars, 603K npm/week combined. Actively maintained with weekly releases (v5.32.1, Mar 2026). OpenAPI 3.1.2 support. The universal baseline.
Docusaurus
92Meta's open-source docs framework. Largest community (64K stars, 765K npm/week), built-in versioned docs, massive plugin ecosystem. The incumbent safe default for large OSS projects.
MkDocs Material
92Python ecosystem's default docs framework. 26.3K stars, 4.2M PyPI weekly downloads (highest of ANY docs tool). Now in maintenance mode — creator building Zensical as successor with 4-5x faster builds.
Astro Starlight
91Astro-based docs framework shipping zero client-side JS by default. Go-compiled dev server, multi-framework component support (React, Svelte, Vue, Solid). The DX leader for content-first documentation.
Public evidence
Fern reaches Postman's 500K+ company user base. Used by Square, Auth0, Adobe, Twilio, ElevenLabs. The only tool generating both docs AND production SDKs from a single spec.
Fern has real authentication in docs, auto-populated examples, and environment switching. Even a competitor acknowledges strengths.
4.37.8 released Mar 19, 2026. fern-api npm: 89K/week. Product/brand/roadmap continuing under Postman umbrella.
Raw GitHub source
Constrained peek so you can sanity-check the source material without leaving the site.
Fern is a platform that transforms your API definitions into production-ready SDKs and beautiful documentation in minutes.
With Fern, you can offer your users:
Fern supports leading API specifications including OpenAPI (REST, Webhooks), AsyncAPI (WebSockets), Protobuf (gRPC), and OpenRPC.
<div align="center"> <a href="/fern/images/overview-dark.png" target="_blank"> <picture> <source srcset="/fern/images/overview-dark.png" media="(prefers-color-scheme: dark)"> <source srcset="/fern/images/overview-light.png" media="(prefers-color-scheme: light)"> <img src="https://raw.githubusercontent.com/fern-api/fern/main/fern/images/overview-light.png" width="700" alt="Overview Diagram"> </picture> </a> </div>The Fern platform is available via a command line interface (CLI) and requires Node 18+. To install it, run:
npm install -g fern-api
Initialize Fern with your OpenAPI spec:
fern init --openapi ./path/to/openapi.yml
# or
fern init --openapi https://link.buildwithfern.com/plantstore-openapi
Your directory should look like the following:
fern/
├─ fern.config.json
├─ generators.yml # generators you're using
└─ openapi/
└─ openapi.json # your openapi document
Finally, to invoke the generator, run:
fern generate
🎉 Once the command completes, you'll see your SDK in /generated/sdks/typescript.
Fern can also build and host a documentation website with an auto-generated API reference. Write additional pages in markdown and have them versioned with git. Search, SEO, dark mode, and popular components are provided out-of-the-box. Plus, you can customize the colors, font, logo, and domain name.
Check out docs built with Fern:
Get started here.
Generators are processes that take your API Definition as input and output artifacts (SDKs, Server boilerplate, etc.). To add a generator, run fern add <generator id>.
| Generator ID | Latest Version | Changelog |
|---|
Fern's spec generators can output an OpenAPI spec.
Note: The OpenAPI spec generator is primarily intended for Fern Definition users. This prevents lock-in so that one can always export to OpenAPI.
| Generator ID | Latest Version | Changelog |
|---|
Here's a quick look at the most popular CLI commands. View the documentation for all CLI commands.
fern init: adds a new starter API to your repository.
fern check: validate your API definition and Fern configuration.
fern generate: run the generators specified in generators.yml in the cloud.
fern generate --local: run the generators specified in generators.yml in docker locally.
fern add <generator>: include a new generator in your generators.yml. For example, fern add fern-python-sdk.
Fern is inspired by internal tooling built to enhance the developer experience. We stand on the shoulders of giants. While teams were responsible for building the following tools, we want to give a shout out to Mark Elliot (creator of Conjure at Palantir), Michael Dowling (creator of Smithy at AWS), and Ian McCrystal (creator of Stripe Docs).
Join our Slack! We are here to answer questions and help you get the most out of Fern.
We welcome community contributions. For guidelines, refer to our CONTRIBUTING.md. To contribute to our documentation, refer to our docs repo.
This repository uses DevBox for reproducible development environments. DevBox provides cross-platform support (Mac, Linux, Windows via WSL) with exact version pinning based on Nix.
To get started:
# Install DevBox (https://www.jetify.com/devbox/docs/installing_devbox/)
curl -fsSL https://get.jetify.com/devbox | bash