API version

Fern supports two complementary ways to express the version of your API. info.version records a single version string that flows through to your generated docs and downloadable spec. x-fern-version defines a header-based version scheme for APIs that dispatch behavior based on a version header (similar to Stripe’s Stripe-Version).

info.version

Fern preserves the info.version field from your OpenAPI specification. Any format is supported, including SemVer (1.2.3), CalVer (2026-06-23), or custom strings.

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openapi: 3.1.0
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info:
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  title: My API
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  version: "2026-06-23"
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paths: ...

The value carries through to the downloadable OpenAPI spec served from your docs site and to Markdown output on API Reference pages.

x-fern-version

To define a header-based API version scheme, use x-fern-version:

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x-fern-version:
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  header: X-API-Version
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  default: "2.0.0"
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  values:
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    - "1.0.0"
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    - "2.0.0"
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    - "latest"
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paths: ...

This generates a version selector dropdown in the API Reference and attaches the chosen version as a request header on each call.