Fern

Platform that generates SDKs and API documentation from your API specifications. Accepts OpenAPI, AsyncAPI, OpenRPC, gRPC, or the native Fern Definition format as input and outputs idiomatic client libraries for multiple languages plus interactive docs, keeping both derived from a single source of truth.

Trusted by Square, Merge, ElevenLabs, Cohere, LaunchDarkly, Intercom, and Candid Health. Fern stands out by producing language-idiomatic SDKs with strong typing, inlined docs, and built-in handling for OAuth, pagination, retries, and server-sent events. Compared to generic generators like OpenAPI Generator, customers report better developer adoption and smoother migrations.

Key capabilities:

• Multi-language SDK generation for TypeScript, Python, Go, Java, Ruby, C#, and PHP, with Swift and Rust in development
• Docs as code with git-based versioning, preview deployments, and SEO optimization
• Built-in support for OAuth 2.0, server-sent events, auto-pagination, multipart uploads, and idempotency headers
• CI/CD integration via Fern CLI to generate and publish packages to npm, PyPI, Maven, and similar registries on each release
• Optional AI search assistant trained on your docs, WebSocket documentation via AsyncAPI, and llms.txt for agent discovery

API-first teams adopt Fern when they want SDKs and docs to stay in sync without manual maintenance. Developer relations teams use it to scale support across many languages, and product companies migrate from hand-written or template-generated client libraries for cleaner, type-safe codebases.