For AI agents: a documentation index is available at the root level at /llms.txt and /llms-full.txt. Append /llms.txt to any URL for a page-level index, or .md for the markdown version of any page.
Book a demoLog inStart for free
  • Overview
    • What is an API definition?
    • Project structure
      • Overview
      • Overlays
      • Overrides
      • Authentication
      • Servers
      • Sync your specification
        • Overview
        • API version
        • Audiences
        • Availability
        • Base path
        • Default values
        • Enum descriptions, names, and availability
        • Request + response examples
        • API Explorer control
        • Global headers
        • Ignoring elements
        • SDK method names
        • SDK variables
        • Tag display names
        • Parameter names
        • Property names
        • Idempotency
        • Pagination
        • Retry behavior
        • Schema names
        • Server names and URL templating
      • generators.yml reference
Checking status...
SOC2Soc 2 Type II
© 2026 Fern • Birch Solutions, Inc., a Postman company

Documentation

SDKsDocsAsk FernCLI Reference

API Definitions

OpenAPIAsyncAPIOpenRPCgRPC

Resources

BlogSupportPricing

Company

Brand KitPrivacy PolicyTerms of Service
LogoLogo
Book a demoLog inStart for free
OpenAPIExtensions

API version

||View as Markdown|
Was this page helpful?
Edit this page
Previous

Extensions overview

Next

Use audiences to filter your API

You can define your API version scheme, such as a X-API-Version header. The supported versions and default value are specified like so:

openapi.yml
1x-fern-version:
2 header: X-API-Version
3 default: "2.0.0"
4 values:
5 - "1.0.0"
6 - "2.0.0"
7 - "latest"
8paths: ...