Document checkout_session_params subscription_data merge behavior (#998)

Generated-By: mintlify-agent

Co-authored-by: mintlify[bot] <109931778+mintlify[bot]@users.noreply.github.com>

apps/docs/api-reference-generator/billing/attach.mdx

@@ -50,4 +50,25 @@ const response = await autumn.billing.attach({
 });
 ```
 
+```typescript Pass metadata to Stripe subscription
+const response = await autumn.billing.attach({
+  customerId: "cus_123",
+  planId: "pro_plan",
+  checkoutSessionParams: {
+    subscriptionData: {
+      metadata: {
+        userId: "internal-user-id",
+        source: "upgrade-flow",
+      },
+    },
+  },
+});
+```
+
 </CodeGroup>
+
+### Stripe checkout session params
+
+Use `checkoutSessionParams` to pass additional data to the Stripe checkout session. Values you provide are deep-merged with Autumn's internal parameters, so your fields are preserved alongside ones Autumn sets automatically (like `trial_end` or internal metadata).
+
+This is useful for attaching custom metadata to the Stripe subscription created during checkout — for example, linking subscriptions to internal user IDs or tracking the source of the purchase.

apps/docs/mintlify/changelog/changelog.mdx

@@ -51,6 +51,9 @@ description: "Some new things we've shipped at Autumn HQ"
       - New Express, Elysia, and Web Standard adapters for `autumnHandler` - [#985](https://github.com/useautumn/autumn/pull/985)
       - Improved API reference field descriptions for preview endpoints - [#985](https://github.com/useautumn/autumn/pull/985)
     </Accordion>
+    <Accordion title="Bug fixes">
+      - `checkout_session_params.subscription_data` (including `metadata`) is now properly deep-merged with Autumn's internal parameters instead of being overwritten - [#996](https://github.com/useautumn/autumn/pull/996)
+    </Accordion>
   </AccordionGroup>
 </Update>