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.

Fern publishes 2 APIs on the APIs.io network: Ask API and Website Sources API. Tagged areas include API Lifecycle, SDK Generation, Client Library, API Documentation, and Developer Tools.

Fern’s developer surface includes authentication, documentation, engineering blog, and 7 more developer resources.

🌐 Visit website 📡 Source on GitHub

21.5/100 emerging ▬ flat Agent 20/100 agent aware Full breakdown ↓
scored 2026-09-14 · rubric v0.22.0

AccessFreemiumSelf serve⚡ Free to try

1 APIs

API Lifecycle SDK Generation Client Library API Documentation Developer Tools OpenAPI CLI Open-Source Developer Experience

Kin Score

Kin Score How this is scored →
scored 2026-09-14 · rubric v0.22.0

Composite quality — 21.5/100 · emerging

Contract Quality 0.0 / 19

Developer Ergonomics 4.8 / 15

Access Clarity 5.9 / 15

Operational Transparency 3.3 / 10

Contract Governance 0.0 / 9

Discoverability 4.4 / 8

Regulatory Posture 1.7 / 15

Create-or-Update Ergonomics 0.0 / 10

Agent readiness — 20/100 · agent aware

Machine-Readable Contract 18 / 18

Agentic Access Contract 10 / 10

Documented Reversibility 0 / 6

MCP Server 0 / 12

Machine-Readable Auth 10 / 10

Idempotency 0 / 9

Stable Error Semantics 0 / 8

Request/Response Examples 0 / 7

Rate-Limit Signaling 7 / 7

Typed Event Surface 0 / 6

Agent Skills 0 / 5

Well-Known Catalog 0 / 4

Consent & Bot Identity 0 / 3

A2A Agent Card 0 / 8

Dry-Run / Simulate Mode 0 / 4

Delegated User Identity 0 / 6

Protected Resource Metadata 0 / 5

Registration Without a Human 0 / 6

Agentic Commerce Well-Known Document 0 / 5

Regulatory Posture applies to this provider. Its tags matched the Education & Research regime, so Regulatory Posture carries 15 points of the composite. If this regime is wrong for your business, say so on your provider repo — the applicability map is public and we will correct it.

Create-or-Update Ergonomics applies to this provider. This API accepts writes, so it carries 10 points of the composite. It is scored from the published contracts themselves: whether a caller can create-or-update in one call, whether the write accepts a key the caller already holds, and whether the response says which branch ran. Without that, every write needs a search-and-branch in front of it, and the first time that check is skipped a duplicate record is created. Scored against the observed mean rather than raw — a provider at the catalog average is unchanged by this facet, not penalised by it.

The six quality facets above are damped to 75 points between them, because the conditional facet above carries the other 25. That is why each facet's contribution is shown against a damped maximum: raising a quality facet moves the composite by 75% of its nominal weight, not 100%. The full arithmetic is at apis.io/rating/.

Improve this rating by publishing the missing artifacts — every area above can be raised, and the full rubric is at apis.io/rating/. Every facet and dimension name above is a link: it opens that measurement's own page — what it means, the exact checks that feed it, how the whole catalog distributes on it, and the providers at the top of it. This rating is computed from github.com/api-evangelist/fern-api: open an issue to ask a question, or submit a pull request to add artifacts. Submit an artifact on GitHub — free → Manage your own listing — the Influence plan, $499/mo →

APIs 5

Individual APIs this provider publishes, each with its own machine-readable definition.

Fern SDK Generation

Capability area (CLI/platform, not a hosted REST API) that generates idiomatic, type-safe client SDKs in TypeScript, Python, Go, Java, C#, PHP, Ruby, Swift, and Rust from OpenAP...

Fern API Documentation

Capability area (CLI/platform) that builds a hosted documentation website with a generated API reference, interactive API playground, changelogs, versioning, keyword search, and...

Fern Definition and CLI

Capability area covering Fern's open-source (Apache-2.0) command-line interface and the proprietary Fern Definition API-description format. Core commands include `fern init`, `f...

Fern Ask API

Query indexed documentation for AI-generated, grounded answers.

Fern Website Sources API

Index documentation websites and check indexing job status.

Open Collections 4

Open, tool-agnostic API collections (OpenAPI-derived and Bruno).

API Collection

OPEN COLLECTION

Fern Fern Ask API

OPEN COLLECTION

Fern Fern Ask Website Sources API

OPEN COLLECTION

Fern Ask Fern API

OPEN COLLECTION

Pricing Plans 1

Published pricing tiers and plan structures.

Fern Api Plans Pricing

4 plans

PLANS

Rate Limits 1

Documented rate limits and quota policies.

Fern Api Rate Limits

4 limits

RATE LIMITS

FinOps 1

Cost, billing, and metering signals for API financial operations.

Fern Api Finops

FINOPS

Security Posture 1

Authentication, domain security, vulnerability disclosure, and trust-center signals.

Fern Api Authentication

http · 1 scheme

SECURITY

Agentic Access 1

Recommended x-agentic-access execution contracts for AI agents.

Fern Api Agentic Access

3 operations · 1 acting

AGENTIC

Resources

Documentation 1

Reference material describing how the API behaves

Documentation

Documentation

Agent Surfaces 1

MCP servers, agent skills, and machine-readable catalogs

AgenticAccess

AgenticAccess

Build 1

SDKs, sample code, and the tooling you integrate with

GitHubOrganization

GitHubOrganization

Access & Security 1

Authentication, authorization, and security posture

Authentication

Authentication

Operate 1

Status, limits, changes, and where to get help

RateLimits

RateLimits

Commercial 2

Pricing, plans, and the legal terms of use

Plans

Plans

FinOps

FinOps

Company 3

The organization behind the API

LinkedIn

Website

Website

Blog

Blog

Source (apis.yml)

aid: fern-api
url: https://raw.githubusercontent.com/api-evangelist/fern-api/refs/heads/main/apis.yml
name: Fern
kind: company
description: '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.'
deliveryModel:
  model: unknown
  open_source: unknown
  commercial: true
  callable_host: true
  label: Delivery model not determined — needs a product licence on record
  confidence: low
  source:
  - openapi
  - pricing
  - repository-unlicensed
  generated: '2026-08-28'
  method: derived
accessModel:
  pricing: freemium
  onboarding: self-serve
  trial: false
  try_now: true
  public: false
  label: Freemium · Self-serve signup
  confidence: medium
  source:
  - plans
  - authentication
  generated: '2026-09-03'
  method: derived
image: https://kinlane-images.s3.amazonaws.com/shared/apis-json/icons/fern-api.png
tags:
- API Lifecycle
- SDK Generation
- Client Library
- API Documentation
- Developer Tools
- OpenAPI
- CLI
- Open-Source
- Developer Experience
tags_raw:
- API Lifecycle
- SDK Generation
- Client Library
- API Documentation
- Developer Tools
- OpenAPI
- CLI
- Open Source
- Developer Experience
created: '2026-07-11'
modified: '2026-07-11'
specificationVersion: '0.23'
apis:
- aid: fern-api:fern-api-sdk-generation
  name: Fern SDK Generation
  tags:
  - SDK Generation
  - Client Library
  - Developer Tools
  - API Lifecycle
  image: https://kinlane-images.s3.amazonaws.com/shared/apis-json/apis-json-logo.jpg
  humanURL: https://buildwithfern.com/learn/sdks/overview
  properties:
  - url: https://buildwithfern.com/learn/sdks/overview
    type: Documentation
  - url: https://buildwithfern.com/learn/cli-api-reference/cli-reference/commands
    type: APIReference
  - url: https://github.com/fern-api/fern
    type: SourceCode
  description: Capability area (CLI/platform, not a hosted REST API) that generates idiomatic, type-safe client SDKs in TypeScript,
    Python, Go, Java, C#, PHP, Ruby, Swift, and Rust from OpenAPI, AsyncAPI, gRPC, or Fern Definition input. Driven by `fern
    generate`; SDKs are published to GitHub and registries (npm, PyPI, Maven). Endpoints are modeled from the CLI surface
    rather than a public REST API.
- aid: fern-api:fern-api-documentation
  name: Fern API Documentation
  tags:
  - API Documentation
  - Developer Portal
  - Docs as Code
  - Developer Experience
  image: https://kinlane-images.s3.amazonaws.com/shared/apis-json/apis-json-logo.jpg
  humanURL: https://buildwithfern.com/learn/docs/getting-started/overview
  properties:
  - url: https://buildwithfern.com/learn/docs/getting-started/overview
    type: Documentation
  - url: https://buildwithfern.com/learn/docs/api-references/generate-api-ref
    type: APIReference
  - url: https://github.com/fern-api/fern
    type: SourceCode
  description: Capability area (CLI/platform) that builds a hosted documentation website with a generated API reference, interactive
    API playground, changelogs, versioning, keyword search, and docs-as-code (Markdown + git) authoring. Driven by `fern generate
    --docs` and `fern docs dev`. Endpoints are modeled from the CLI and platform surface rather than a public REST API.
- aid: fern-api:fern-api-definition-cli
  name: Fern Definition and CLI
  tags:
  - CLI
  - API Design
  - API Lifecycle
  - Developer Tools
  image: https://kinlane-images.s3.amazonaws.com/shared/apis-json/apis-json-logo.jpg
  humanURL: https://buildwithfern.com/learn/cli-api-reference/cli-reference/commands
  properties:
  - url: https://buildwithfern.com/learn/cli-api-reference/cli-reference/commands
    type: Documentation
  - url: https://buildwithfern.com/learn/api-definitions/openapi/authentication
    type: APIReference
  - url: https://github.com/fern-api/fern
    type: SourceCode
  description: Capability area covering Fern's open-source (Apache-2.0) command-line interface and the proprietary Fern Definition
    API-description format. Core commands include `fern init`, `fern check`, `fern generate`, `fern login`, `fern token`,
    `fern export`, and `fern docs dev`. Endpoints are modeled from the CLI surface, not a hosted REST API.
- aid: fern-api:fern-api-ask-api
  name: Fern Ask API
  description: Query indexed documentation for AI-generated, grounded answers.
  humanURL: https://buildwithfern.com/learn/sdks/overview
  baseURL: https://fai.buildwithfern.com
  tags:
  - Ask
  properties:
  - type: OpenAPI
    url: openapi/fern-api-ask-api-openapi.yml
  - type: Documentation
    url: https://buildwithfern.com/learn/docs/ai-features/ask-fern/api-reference/overview
  - type: APIReference
    url: https://buildwithfern.com/learn/docs/ai-features/ask-fern/api-reference/website/get-website-status
  - type: PostmanCollection
    url: collections/fern-api.postman_collection.json
  - type: OpenCollection
    url: collections/fern-api.opencollection.json
- aid: fern-api:fern-api-website-sources-api
  name: Fern Website Sources API
  description: Index documentation websites and check indexing job status.
  humanURL: https://buildwithfern.com/learn/sdks/overview
  baseURL: https://fai.buildwithfern.com
  tags:
  - Website Sources
  properties:
  - type: OpenAPI
    url: openapi/fern-api-website-sources-api-openapi.yml
  - type: Documentation
    url: https://buildwithfern.com/learn/docs/ai-features/ask-fern/api-reference/overview
  - type: APIReference
    url: https://buildwithfern.com/learn/docs/ai-features/ask-fern/api-reference/website/get-website-status
  - type: PostmanCollection
    url: collections/fern-api.postman_collection.json
  - type: OpenCollection
    url: collections/fern-api.opencollection.json
common:
- type: AgenticAccess
  url: agentic-access/fern-api-agentic-access.yml
- type: Authentication
  url: authentication/fern-api-authentication.yml
- type: GitHubOrganization
  url: https://github.com/fern-api
- type: LinkedIn
  url: https://www.linkedin.com/company/buildwithfern
- type: Website
  url: https://buildwithfern.com
- type: Documentation
  url: https://buildwithfern.com/learn/home
- type: Plans
  url: plans/fern-api-plans-pricing.yml
- type: RateLimits
  url: rate-limits/fern-api-rate-limits.yml
- type: FinOps
  url: finops/fern-api-finops.yml
- url: https://buildwithfern.com/blog
  type: Blog
maintainers:
- FN: Kin Lane
  email: kin@apievangelist.com

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