What is Fern?
Fern is a developer tool that turns your API specification into production-ready SDKs and beautiful, branded documentation—all automatically. Instead of spending months building and maintaining client libraries in multiple languages, you start with an OpenAPI (or other spec) file and let Fern generate clean, idiomatic code that feels native to each programming language.
Whether you're a startup or enterprise, Fern helps you deliver a top-tier developer experience without hiring a dedicated SDK team. The generated SDKs include strong typing, inline documentation, intuitive error handling, and support for advanced features like OAuth 2.0, auto-pagination, and retries—so your users can integrate faster and with fewer mistakes.
What are the features of Fern?
- Multi-Language SDK Generation: Automatically create SDKs in TypeScript, Python, Go, Java, C#, PHP, Ruby—with Swift and Rust coming soon.
- Branded Interactive Documentation: Generate SEO-optimized, customizable API docs that match your company’s look and feel.
- Idiomatic & Maintainable Code: Every SDK follows language-specific best practices, so developers feel right at home.
- Advanced API Features Built-In: Supports OAuth 2.0, server-sent events, multipart uploads, idempotency, polymorphism, and auto-pagination out of the box.
- Docs as Code Workflow: Manage documentation through Git, preview changes per pull request, and version releases seamlessly.
- AI & Agent Ready: Includes
/llms.txtsupport and AI search so LLMs and developer agents can understand your API. - Automated Publishing: Fern publishes SDKs directly to npm, PyPI, Maven, and other package registries with semantic versioning.
- Customization Without Overwrite Risk: Add custom logic to generated SDKs—your changes won’t be lost on regeneration.
What are the use cases of Fern?
- A fintech company needs reliable, type-safe SDKs in 5+ languages to support global developer adoption—without expanding their engineering team.
- A SaaS platform wants interactive, branded API docs that let users try endpoints instantly with their own API keys.
- An engineering team adopts API-first development: they update their OpenAPI spec, run
fern generate, and keep docs, SDKs, and backend perfectly in sync. - A developer relations team reduces onboarding friction by embedding real SDK code snippets directly in documentation.
- Companies preparing for enterprise sales need SOC 2-compliant, secure docs with role-based access and SSO support.
- Startups launching new APIs want to ship high-quality SDKs on day one to impress early adopters and investors.
How to use Fern?
- Start by uploading your OpenAPI, AsyncAPI, gRPC, or Fern Definition spec to Fern.
- Select the programming languages you want SDKs for (e.g., Python, TypeScript, Go).
- Customize your documentation theme using CSS, JavaScript, or React components to match your brand.
- Use the Fern CLI in your CI/CD pipeline to auto-generate and publish SDKs on every API change.
- Extend generated SDKs with utility functions—your custom code stays safe during re-generation.
- Publish packages to public or private registries with one command; Fern handles versioning automatically.
