> For clean Markdown content of this page, append .md to this URL. For the complete documentation index, see https://buildwithfern.com/learn/llms.txt.

# API version

> Configure API version schemes using `info.version` and `x-fern-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](https://semver.org/) (`1.2.3`), [CalVer](https://calver.org/) (`2026-06-23`), or custom strings.

**`openapi.yml`**

```yaml title="openapi.yml"
openapi: 3.1.0
info:
  title: My API
  version: "2026-06-23"
paths: ...
```

The value carries through to the [downloadable OpenAPI spec](/learn/docs/developer-tools/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`:

**`openapi.yml`**

```yaml title="openapi.yml"
x-fern-version:
  header: X-API-Version
  default: "2.0.0"
  values:
    - "1.0.0"
    - "2.0.0"
    - "latest"
paths: ...
```

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

> Configure API version schemes using `info.version` and `x-fern-version`