Fern
SDK and documentation generator from OpenAPI or Fern's own IDL. Fern generates idiomatic, production-quality SDKs in TypeScript, Python, Java, Go, Ruby, and C# from your API definition. Unlike generic code generators, Fern's output follows language idioms (dataclasses in Python, interfaces in TypeScript) rather than generic scaffolding. Used by Anthropic, ElevenLabs, Cohere, and others for their official SDKs. Also generates interactive API reference documentation.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
MIT open source core for auditability. Generated SDKs follow security best practices for each language. API tokens stored locally via CLI auth. No SOC2 status confirmed publicly.
⚡ Reliability
Best When
You're building a public API and want to offer official, high-quality SDKs in multiple languages without dedicating engineering resources to SDK maintenance.
Avoid When
You need a quick one-off client, your API lacks an OpenAPI spec, or you have very specific SDK customization needs that override Fern's opinionated generation.
Use Cases
- • Generate production-quality TypeScript and Python SDKs from OpenAPI spec that developers actually want to use — not boilerplate codegen output
- • Keep SDKs in sync with API changes automatically by running Fern in CI — no manual SDK maintenance for API-first companies
- • Generate interactive API documentation with code samples in multiple languages from a single Fern definition
- • Create SDKs for agent tool integrations with proper typing, auto-complete, and error handling that agents can use reliably
- • Publish generated SDKs to npm, PyPI, and Maven automatically via Fern's GitHub Actions integration
Not For
- • One-off API clients for internal use — Fern's setup cost is high for one-time integrations; use openapi-generator or hand-write for simple cases
- • APIs without OpenAPI specs — Fern requires a well-defined API contract; undocumented or informal APIs need spec writing first
- • Real-time or streaming-heavy APIs — Fern's SDK generation works best for request/response REST patterns
Interface
Authentication
Fern CLI uses API token for accessing Fern Cloud features (publishing, docs hosting). Local SDK generation with fern generate works without auth. Token managed via fern login CLI command.
Pricing
Fern CLI is MIT open source. Local SDK generation is free. Fern Cloud (hosted documentation, automated npm/PyPI publishing, GitHub sync) requires paid subscription. Many companies use Fern CLI locally and self-manage publishing.
Agent Metadata
Known Gotchas
- ⚠ Fern's IDL (fern definition language) is more expressive than OpenAPI for certain patterns (discriminated unions, errors) but requires learning Fern's YAML format; OpenAPI import is the simpler starting point
- ⚠ Generated SDK versions must be manually bumped or configured for automatic versioning — Fern doesn't auto-increment semver based on breaking changes
- ⚠ Fern's TypeScript output uses ES modules by default — consuming projects must have 'type': 'module' in package.json or use specific import patterns
- ⚠ Python SDK generation requires Python 3.8+ and uses pydantic v2 in recent versions — older codebases on pydantic v1 need version pinning
- ⚠ Fern's pagination handling requires API to follow specific pagination patterns (cursor, offset) — non-standard pagination needs custom configuration
- ⚠ OpenAPI specs with $ref to external files may require bundling before Fern can process them — use openapi-bundler or Redocly CLI first
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for Fern.
Scores are editorial opinions as of 2026-03-06.