Fern

Fern is a developer-tools platform that turns a single API specification into idiomatic client SDKs, beautiful API documentation, and MCP servers. Given OpenAPI, AsyncAPI, gRPC/Protobuf, or Fern's own Fern Definition as input, Fern generates type-safe SDKs in TypeScript, Python, Go, Java, C#, PHP, Ruby, Swift, and Rust, publishes them to registries like npm, PyPI, and Maven, and builds a hosted docs site with an interactive API reference, API playground, and an AI-powered "Ask Fern" search. The primary interface is the open-source `fern` CLI (Apache-2.0, github.com/fern-api/fern) driving a hosted platform - the fern-api catalog documents Fern's SDK-generation, documentation-generation, and Fern Definition capability surfaces as capability areas, plus the one hosted public REST API Fern exposes to customers, the Ask Fern API. Fern is open-core: the CLI and generators are open source, and Docs and SDKs are sold on Hobby (free), Team, and Enterprise plans.

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API LifecycleSDK GenerationClient LibraryAPI DocumentationDeveloper ToolsOpenAPICLIOpen SourceDeveloper Experience

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opencollection: 1.0.0
info:
  name: Fern Ask Fern API
  version: '1.0'
request:
  auth:
    type: bearer
    token: '{{bearerToken}}'
items:
- info:
    name: Website Sources
    type: folder
  items:
  - info:
      name: Index a documentation website
      type: http
    http:
      method: POST
      url: https://fai.buildwithfern.com/sources/website/:domain/index
      params:
      - name: domain
        value: docs.example.com
        type: path
        description: The documentation domain to index.
      body:
        type: json
        data: '{"base_url": "https://docs.example.com"}'
    docs: Start an indexing job that crawls and indexes a documentation website. Returns a job_id. Modeled endpoint - reconcile
      against the live reference.
  - info:
      name: Get website indexing status
      type: http
    http:
      method: GET
      url: https://fai.buildwithfern.com/sources/website/:domain/status
      params:
      - name: domain
        value: docs.example.com
        type: path
        description: The documentation domain being indexed.
      - name: job_id
        value: ''
        type: query
        description: The job ID returned from the index endpoint.
    docs: Return the status of a website indexing job, including pages_indexed and pages_failed. Confirmed endpoint.
- info:
    name: Ask
    type: folder
  items:
  - info:
      name: Ask a question
      type: http
    http:
      method: GET
      url: https://fai.buildwithfern.com/ask
      params:
      - name: q
        value: ''
        type: query
        description: URI-encoded natural-language question.
    docs: Submit a natural-language question and receive an AI-generated answer grounded in the indexed documentation. Modeled
      endpoint shape - reconcile against the live reference.

Fern

APIs

Fern SDK Generation

Fern API Documentation

Fern Definition and CLI

Ask Fern API

Collections

Fern Ask Fern API

Pricing Plans

Fern Api Plans Pricing

Rate Limits

Fern Api Rate Limits

FinOps

Fern Api Finops

Resources

Sources