Add x-fern-default extension documentation

Author: fern-api[bot]

fern/products/api-def/api-def.yml

@@ -49,6 +49,8 @@ navigation:
             path: ./openapi/extensions/availability.mdx
           - page: Base path
             path: ./openapi/extensions/base-path.mdx
+          - page: Default values
+            path: ./openapi/extensions/default.mdx
           - page: Enum descriptions, names, and availability
             path: ./openapi/extensions/enums.mdx
             slug: enum-descriptions-and-names

fern/products/api-def/openapi/extensions/default.mdx

@@ -0,0 +1,78 @@
+---
+title: Default values
+headline: Default values (OpenAPI)
+description: Use `x-fern-default` to set client-side default values for path, header, and query parameters in generated SDKs.
+---
+
+
+The `x-fern-default` extension lets you specify a client-side default value for a path, header, or query parameter. When present, the generated SDK makes the parameter optional and automatically sends the default value if the caller omits it.
+
+This is useful for pinning an API version header or a region path parameter while still allowing callers to override the value.
+
+<Note>
+  `x-fern-default` supports `string` and `boolean` values. Other types (such as numbers) are ignored.
+</Note>
+
+## Path parameters
+
+Use `x-fern-default` on a path parameter to provide a default that the SDK sends automatically. This makes a normally required path parameter optional in the generated SDK.
+
+```yaml {8} title="openapi.yml"
+paths:
+  /regions/{region}/resources:
+    get:
+      operationId: list_resources
+      parameters:
+        - name: region
+          in: path
+          required: true
+          x-fern-default: "us-east-1"
+          schema:
+            type: string
+```
+
+## Headers
+
+Use `x-fern-default` on a header parameter to pin a default value, such as an API version:
+
+```yaml {8} title="openapi.yml"
+paths:
+  /users:
+    get:
+      operationId: list_users
+      parameters:
+        - name: X-API-Version
+          in: header
+          x-fern-default: "2024-02-08"
+          schema:
+            type: string
+```
+
+## Query parameters
+
+Use `x-fern-default` on a query parameter to set a default that the SDK sends when the caller omits it:
+
+```yaml {8} title="openapi.yml"
+paths:
+  /search:
+    get:
+      operationId: search
+      parameters:
+        - name: sort
+          in: query
+          x-fern-default: "relevance"
+          schema:
+            type: string
+```
+
+## Supported languages
+
+`x-fern-default` is supported in the following SDK generators:
+
+- TypeScript
+- Python
+- Java
+- Go
+- C#
+- Ruby
+- PHP

fern/products/api-def/openapi/extensions/overview.md

@@ -20,6 +20,7 @@ The table below shows all available extensions and links to detailed documentati
 | [`x-fern-bearer`](/api-definitions/openapi/authentication#bearer-security-scheme) | Customize bearer authentication parameter names and environment variables |
 | [`x-fern-availability`](./availability) | Mark availability status (beta, generally-available, deprecated) |
 | [`x-fern-base-path`](./base-path) | Set base path prepended to all endpoints |
+| [`x-fern-default`](./default-values) | Set client-side default values for path, header, and query parameters |
 | [`x-displayName`](./tag-display-names) | Specify how tag names display in your API Reference |
 | [`x-fern-enum`](./enum-descriptions-and-names) | Add descriptions, custom names, and deprecation status to enum values |
 | [`x-fern-examples`](./request-response-examples) | Associate request and response examples |