Docs from a single source of truth
Generate interactive documentation websites from an OpenAPI spec, with docs-as-code workflows, version control, pull requests, and AI-assisted authoring.
Fern
BeanspruchenFern turns an OpenAPI spec into docs, SDKs, and a branded CLI for API teams, with AI search, llms.txt, and MCP server.
Overview
Fern is a developer platform for turning an API specification into documentation, SDKs, and a CLI from one source of truth. The site positions it for teams that want docs, client libraries, and terminal workflows kept in sync as the API changes.
The product supports modern API protocols including REST, WebSockets, and GraphQL, and the pricing page also lists support for Webhooks, Server-sent events, Protobuf, AsyncAPI, gRPC, OpenAPI Overlays, Arazzo, and OpenRPC. Fern’s docs offering adds search, AI chat, llms.txt, and an MCP server so both developers and agents can work from the same content.
Features
Multi-language SDK generation
Ship SDKs in nine languages with idiomatic APIs, strong typing, retries, pagination, idempotency helpers, multipart upload support, and tests.
CLI generation and publishing
Generate a branded CLI from your spec, distribute it through package managers, and keep commands aligned with SDKs and docs.
Agent-facing documentation features
Add AI search, built-in chat grounded in your docs, llms.txt, and an MCP server so agents can read and query documentation in real time.
Enterprise deployment and governance
Use enterprise controls such as visitor authentication, RBAC, SSO, self-hosting, translated content, and Postman collection generation from the same source.
Use Cases
Publish API docs from one spec
Create a documentation site that stays aligned with your API changes, supports version switching, and can be maintained with Git-based review workflows.
Distribute SDKs to external developers
Ship client libraries that match your API’s behavior across languages and include built-in helpers for pagination, retries, and idempotency.
Generate an API CLI
Offer a branded terminal interface for developers and agents who prefer command-line workflows, with JSON output for automation.
Make documentation usable by agents
Support AI tools and internal assistants by exposing docs through llms.txt, AI chat, and an MCP server that can query your API and documentation.
Deploy docs for larger organizations
Roll out enterprise documentation with authentication, RBAC, localization, and self-hosting when security or compliance requirements are stricter.
Pros and Cons
Pros
- One spec can generate docs, SDKs, and a CLI, which helps keep surfaces aligned.
- Docs support modern workflows like docs-as-code, AI search, AI chat, llms.txt, and MCP access.
- SDKs are designed for language idioms and include practical helpers such as retries, pagination, and idempotency.
- The CLI is packaged for common distribution channels including Homebrew, npm, pnpm, Scoop, and signed binaries on GitHub Releases.
- Enterprise options include security controls, localization, self-hosting, and support channels for larger teams.
Cons
- Enterprise capabilities such as visitor authentication, RBAC, SSO, self-hosting, translated content, and dedicated support sit on higher-tier plans.
- The source does not provide published pricing for the Enterprise plan.
FAQ
What does Fern generate from an API spec?
Fern starts from an OpenAPI spec and can generate docs, SDKs, and a CLI from the same source of truth. The docs product also supports search, llms.txt, an MCP server, and AI chat grounded in the documentation.
Does Fern offer a free or trial plan?
Yes. The pricing page includes a free Hobby plan, a Team plan with a 14-day trial, and a custom-priced Enterprise plan.
Can Fern be self-hosted?
Fern supports managed cloud hosting or self-hosting for stricter security, networking, and compliance requirements.
Does Fern support access control and localization?
Fern supports visitor authentication with JWT and SSO, role-based access control, translated content, and enterprise authentication options on higher-tier plans.
What kinds of APIs and outputs does Fern support?
Fern publishes SDKs in nine languages and supports API protocols including REST, Webhooks, WebSockets, Server-sent events, Protobuf, GraphQL, and gRPC.
Quick Facts
- Category
- Developer Tool
- Primary source
- OpenAPI spec
- Outputs
- Docs, SDKs, CLI
- Supported protocol families
- REST, WebSockets, GraphQL, Webhooks, SSE, Protobuf, gRPC
- Pricing shape
- Free Hobby plan, paid Team plan, custom Enterprise pricing
- Source domain
- buildwithfern.com
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