Fern

Product Introduction

1. Fern is an API development platform that automatically generates software development kits (SDKs) in multiple programming languages and interactive API documentation from an API specification. It supports specifications like OpenAPI, AsyncAPI, OpenRPC, gRPC, or its own Fern Definition format.
2. The core value of Fern lies in streamlining API-first development by eliminating manual coding of SDKs and documentation, ensuring consistency across client libraries and developer resources while reducing engineering overhead.

Main Features

1. Fern generates idiomatic SDKs in TypeScript, Python, Go, Java, Ruby, C#, PHP, and soon Swift and Rust, with strong typing, inlined documentation, and language-specific error handling. SDKs include built-in support for OAuth 2.0, server-sent events (SSE), auto-pagination, retries with exponential backoff, and multipart form uploads.
2. Fern produces interactive API documentation with SEO optimization, versioning, preview deployments, and RBAC, including embedded API explorers for testing WebSocket and REST endpoints. Documentation supports federated authentication, API key injection, and custom branding via CSS/JavaScript or React components.
3. The platform integrates with CI/CD pipelines to auto-generate and publish SDKs to package registries (npm, PyPI, Maven) using semantic versioning. It includes vulnerability scanning for dependencies and offers enterprise-grade SOC 2 Type II compliance for security-sensitive workflows.

Problems Solved

1. Fern addresses the inefficiency of manually maintaining SDKs and documentation across multiple languages, which often leads to inconsistencies, outdated resources, and high engineering costs. It eliminates repetitive coding tasks and reduces human error in client library implementations.
2. The product targets API-first companies, developer experience teams, and enterprises requiring scalable SDK distribution and polished documentation. It serves organizations transitioning to API-driven architectures or expanding language support for their developer ecosystems.
3. Typical use cases include launching multi-language SDKs for public APIs, maintaining versioned documentation with real-time previews, and automating compliance workflows for regulated industries like fintech or healthcare.

Unique Advantages

1. Unlike generic SDK generators (e.g., OpenAPI Generator), Fern produces SDKs with native language idioms and advanced features like polymorphism handling, automatic idempotency headers, and MIME type-aware file uploads. Its documentation platform natively supports WebSocket testing and AI-powered search.
2. Fern introduces the Fern Definition format, which simplifies API specification with resource-oriented endpoint grouping, cross-file type imports, and explicit support for real-time APIs. The platform also provides AI chat assistants trained on documentation and automatic llms.txt generation for AI agent compatibility.
3. Competitive advantages include enterprise-grade customization services for SDKs and documentation, dedicated support for OpenAPI spec refinement, and partnerships for co-marketing SDK launches. Fern guarantees SLA-backed dependency vulnerability resolution and offers white-glove migration from legacy tools.

Frequently Asked Questions (FAQ)

1. What API specification formats does Fern support? Fern accepts OpenAPI, AsyncAPI, OpenRPC, gRPC, and its proprietary Fern Definition format, which enables advanced features like resource grouping and real-time API modeling. Users can migrate existing OpenAPI specs with Fern's conversion tools and validation support.
2. How does Fern ensure SDK quality across languages? Each SDK is designed by language experts to follow idiomatic conventions, with generated code including type-safe interfaces, inline examples, and preconfigured authentication flows. Fern maintains test suites for all SDKs and provides upgrade guides for backward-compatible changes.
3. Can I customize the generated documentation's design? Yes, Fern allows full CSS/JavaScript theming, custom React component integration, and template overrides for documentation layouts. Enterprise customers receive dedicated design support to match brand guidelines while retaining SEO and accessibility optimizations.
4. How does Fern handle API changes in SDK generation? The Fern CLI integrates with CI pipelines to regenerate SDKs on spec updates, preserving custom code in designated "extends" folders. Versioning follows semantic rules, and deprecated endpoints are automatically flagged in documentation.
5. Is Fern suitable for internal/private APIs? Yes, Fern supports RBAC for documentation access, SSO integration, and private package registry publishing. Enterprise plans include on-premises deployment options and audit logs for compliance with internal security policies.