docs: add documentation for x-fern-sdk-variables extension

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

@@ -58,6 +58,8 @@ navigation:
           - page: SDK method names
             path: ./openapi-pages/extensions/method-names.mdx
             slug: method-names
+          - page: SDK variables
+            path: ./openapi-pages/extensions/sdk-variables.mdx
           - page: Tag display names
             path: ./openapi-pages/extensions/tag-display-name.mdx
           - page: Parameter names

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

@@ -26,6 +26,7 @@ The table below shows all available extensions and links to detailed documentati
 | [`x-fern-ignore`](./ignoring-elements) | Skip reading specific endpoints or schemas |
 | [`x-fern-sdk-method-name`](./method-names) | Customize SDK method names |
 | [`x-fern-sdk-group-name`](./method-names) | Organize methods into SDK groups |
+| [`x-fern-sdk-variables`](./sdk-variables) | Set common path parameters across all requests |
 | [`x-fern-parameter-name`](./parameter-names) | Customize parameter variable names |
 | [`x-fern-property-name`](./property-names) | Customize object property variable names |
 | [`x-fern-type-name`](./schema-names) | Override auto-generated names for inline schemas |

fern/products/api-def/openapi-pages/extensions/sdk-variables.mdx

@@ -0,0 +1,78 @@
+---
+title: SDK variables
+description: Use `x-fern-sdk-variables` to set common path parameters across all requests
+---
+
+The `x-fern-sdk-variables` extension allows you to define variables that are set once during SDK client initialization and automatically used in path parameters across all endpoint calls. This is useful for common parameters like tenant IDs, organization IDs, or environment identifiers that appear in many endpoint paths.
+
+<Note>
+  SDK variables are supported in TypeScript (v2.6.3+), Python (v4.24.0+), and Java (v3.6.3+). Only string types are supported.
+</Note>
+
+## Configuration
+
+Define variables at the document level using `x-fern-sdk-variables`, then mark path parameters as variables using `x-fern-sdk-variable`:
+
+```yaml {1-7,17,23} title="openapi.yml"
+x-fern-sdk-variables:
+  gardenId:
+    type: string
+    description: The unique identifier for your garden
+  zoneId:
+    type: string
+    description: The zone within the garden
+
+paths:
+  /gardens/{gardenId}/zones/{zoneId}/plants:
+    get:
+      operationId: list_plants_in_zone
+      parameters:
+        - name: gardenId
+          in: path
+          required: true
+          x-fern-sdk-variable: gardenId
+          schema:
+            type: string
+        - name: zoneId
+          in: path
+          required: true
+          x-fern-sdk-variable: zoneId
+          schema:
+            type: string
+```
+
+## SDK usage
+
+Variables become required constructor parameters instead of being passed to individual method calls:
+
+<CodeBlocks>
+```typescript {2-3}
+const client = new PlantClient({
+  gardenId: "garden_123",
+  zoneId: "zone_456",
+  apiKey: "your-api-key"
+});
+
+const plants = await client.listPlantsInZone();
+```
+
+```python {2-3}
+client = PlantClient(
+    garden_id="garden_123",
+    zone_id="zone_456",
+    api_key="your-api-key"
+)
+
+plants = client.list_plants_in_zone()
+```
+
+```java {2-3}
+PlantClient client = PlantClient.builder()
+    .gardenId("garden_123")
+    .zoneId("zone_456")
+    .apiKey("your-api-key")
+    .build();
+
+List<Plant> plants = client.listPlantsInZone();
+```
+</CodeBlocks>