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docs: add note explaining why x-fern-default is used instead of plain default #5332

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Swimburger merged 1 commit into from
May 3, 2026
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docs: add note explaining why x-fern-default is used instead of plain default #5332

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2 changes: 2 additions & 0 deletions fern/products/api-def/openapi/extensions/default.mdx
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Expand Up @@ -8,6 +8,8 @@ The `x-fern-default` extension lets you specify a client-side default value for

This is useful for pinning an API version header or a region path parameter while still allowing callers to override the value. `x-fern-default` is supported across all SDK languages.

<Note>Fern uses `x-fern-default` instead of the standard OpenAPI `default` property because they have different semantics. The OpenAPI `default` on a schema describes the value the **server** assumes when the parameter is omitted — it's a documentation hint and doesn't affect client behavior. `x-fern-default` instructs the generated SDK to **actively send** the value in every request when the caller doesn't provide one, ensuring the parameter is always present on the wire.</Note>

## Path parameters

In the example below, the SDK sends `us-east-1` for `region` when the caller doesn't specify one.
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