Fern
Generates API documentation sites, SDKs in multiple languages and a CLI from one API specification.
Fern is a developer tooling platform that takes an API definition, such as an OpenAPI file, and generates a documentation website, client SDKs in several languages and a command-line interface from it. Fern adds AI features to the docs, including an assistant that answers from the content, an MCP server and an automatically produced llms.txt file.
Best for API companies that want documentation, SDKs and a CLI kept in sync from a single specification and readable by AI agents.
- Listed in
- Developer APIAI Code Assistant
- Platforms
- Web, MCP
- Pricing model
- Freemium
- Free plan
- Yes
- API
- Yes
- Made by
- Fern, a Postman company
- Added
Last verified: . Information taken from buildwithfern.com.
What is Fern?
Fern is a platform for API publishers that produces three outputs from one definition: a documentation site, client libraries and a terminal tool. It accepts OpenAPI, AsyncAPI, gRPC and GraphQL inputs and covers REST, WebSocket and other protocols. Documentation is managed as code in Git, reviewed through pull requests and themed with configuration files. Several AI functions are built in: a chat assistant called Ask Fern that answers from the published docs, an authoring agent that drafts updates and opens GitHub pull requests, AI search, Markdown output with a generated llms.txt for language models, and an MCP server that exposes docs and the API to AI tools. For larger organisations there is role-based access with SAML or OIDC, multi-repository sites, localisation and self-hosting. Postman collections can be generated from the same source. Fern is now a Postman company and offers a free plan plus custom Enterprise terms.
Key Features
Documentation Sites
Builds a branded docs website with guides, API reference, changelog and an interactive API explorer.
SDK Generation
Produces client libraries in multiple programming languages from the API specification.
CLI Generation
Creates a command-line tool so developers and agents can call the API from a terminal.
Ask Fern and AI Search
Answers reader questions in the docs using the published content as the source.
Agent Access
Ships an MCP server and an llms.txt file so AI tools can read the docs and query the API.
Access Control
Restricts pages by role, team or plan through SAML or OIDC sign-on.
Use Cases
- Launching a Public API
- Publish reference docs and SDKs together when an API goes live.
- Keeping SDKs Current
- Regenerate every language library after a single spec change.
- Consolidating Docs
- Pull pages from several repositories into a single site where readers switch between products and versions.
- Migrating from Another Docs Tool
- Move an existing documentation site with help from Fern's team.
- Preparing Docs for AI Agents
- Make documentation machine-readable for coding assistants.
Things to know
- Free plan limits
- The Free documentation plan covers a single site and caps the team at 10 people, content at 1,000 pages and AI usage at 250 credits monthly.
- Enterprise features are gated
- SSO, SCIM, multiple repositories, custom React components and self-hosting belong to the Enterprise plan.
- Compliance
- The site states SOC 2 Type II compliance and annual penetration tests.
- SDK pricing not captured
- Only the documentation plans were readable when checked; pricing for SDKs is shown on a separate tab.
FAQs
What is Fern?
Fern is a platform that generates API documentation, SDKs and a CLI from an API specification.
What is Fern used for?
Fern is used by API teams to publish documentation sites and maintain client libraries in several languages without writing each by hand.
Is Fern free?
Yes, Fern lists a Free plan for documentation described as free forever, with limits such as 10 team members and 1,000 pages. An Enterprise plan has custom pricing.
Which API specifications does Fern support?
Fern supports OpenAPI, AsyncAPI, gRPC, GraphQL and OpenRPC definitions, according to its site.
Does Fern have an MCP server?
Yes. Fern provides an MCP server that gives AI tools live access to a company's documentation and API.
Can Fern be self-hosted?
Yes. Fern says self-hosting is available for organisations with stricter security or compliance needs, as part of its Enterprise offering.
Who makes Fern?
Fern describes itself as a Postman company that was founded in Brooklyn, New York.
Who it's for
- API product teams
- Developer experience engineers
- Technical writers
- SDK maintainers
- Enterprises
Comments
No comments yet. If you have used Fern, tell other readers how it went.
Alternatives to Fern
| Tool | Free plan | Starting price | API |
|---|---|---|---|
 TheneoAPI documentation and developer portal platform with AI-generated docs, Ask AI search, changelogs and an MCP server. |
Yes | — | Yes |
 Kapa.aiIngestion and retrieval platform that turns company docs, tickets and wikis into AI agents and a search API. |
No | — | Yes |
 AtomDomain marketplace and registrar with AI tools for generating, testing and contesting business names. |
— | — | Yes |
 BannerbearAPI that generates images, PDFs, animations and videos automatically from reusable design templates. |
— | $49/mo | Yes |
 BigjpgAI image enlarger that upscales artwork and photos up to 16x while reducing noise and jagged edges. |
Yes | $6 | Yes |
Listed in the same categories as Fern. A starting price appears only where we checked it on the maker’s pricing page; a dash means not checked or not published. All Developer API tools