diff --git a/server/src/internal/billing/v2/providers/stripe/utils/checkoutSessions/buildCheckoutSessionParams.ts b/server/src/internal/billing/v2/providers/stripe/utils/checkoutSessions/buildCheckoutSessionParams.ts
index 2b70eed7c..79e711c50 100644
--- a/server/src/internal/billing/v2/providers/stripe/utils/checkoutSessions/buildCheckoutSessionParams.ts
+++ b/server/src/internal/billing/v2/providers/stripe/utils/checkoutSessions/buildCheckoutSessionParams.ts
@@ -1,6 +1,31 @@
 import type Stripe from "stripe";
 import { buildCheckoutSessionMetadata } from "./buildCheckoutSessionMetadata";
 
+/**
+ * Deep-merges subscription_data so user-provided fields (e.g. metadata)
+ * are preserved alongside Autumn-internal fields (e.g. trial_end).
+ */
+const mergeSubscriptionData = ({
+	paramsSubscriptionData,
+	userSubscriptionData,
+}: {
+	paramsSubscriptionData?: Stripe.Checkout.SessionCreateParams.SubscriptionData;
+	userSubscriptionData?: Stripe.Checkout.SessionCreateParams.SubscriptionData;
+}): Stripe.Checkout.SessionCreateParams.SubscriptionData | undefined => {
+	if (!paramsSubscriptionData && !userSubscriptionData) {
+		return undefined;
+	}
+
+	return {
+		...(userSubscriptionData ?? {}),
+		...(paramsSubscriptionData ?? {}),
+		metadata: {
+			...(userSubscriptionData?.metadata ?? {}),
+			...(paramsSubscriptionData?.metadata ?? {}),
+		},
+	};
+};
+
 export const buildCheckoutSessionParams = ({
 	params,
 	checkoutSessionParams,
@@ -44,5 +69,13 @@ export const buildCheckoutSessionParams = ({
 			checkoutSessionMetadata: checkoutSessionParams?.metadata,
 			autumnMetadataId,
 		}),
+		subscription_data: mergeSubscriptionData({
+			paramsSubscriptionData: params.subscription_data as
+				| Stripe.Checkout.SessionCreateParams.SubscriptionData
+				| undefined,
+			userSubscriptionData: checkoutSessionParams?.subscription_data as
+				| Stripe.Checkout.SessionCreateParams.SubscriptionData
+				| undefined,
+		}),
 	};
 };
diff --git a/server/tests/integration/billing/attach/attach-misc.test.ts b/server/tests/integration/billing/attach/attach-misc.test.ts
new file mode 100644
index 000000000..73603d6a8
--- /dev/null
+++ b/server/tests/integration/billing/attach/attach-misc.test.ts
@@ -0,0 +1,122 @@
+/**
+ * Attach Miscellaneous Tests
+ *
+ * Tests for checkout_session_params passthrough to Stripe checkout sessions.
+ * Verifies that user-provided subscription_data.metadata and session-level
+ * metadata are correctly merged with Autumn's internal params.
+ */
+
+import { expect, test } from "bun:test";
+import type { ApiCustomerV3, AttachParamsV1Input } from "@autumn/shared";
+import { expectCustomerFeatureCorrect } from "@tests/integration/billing/utils/expectCustomerFeatureCorrect";
+import { expectCustomerInvoiceCorrect } from "@tests/integration/billing/utils/expectCustomerInvoiceCorrect";
+import { expectProductActive } from "@tests/integration/billing/utils/expectCustomerProductCorrect";
+import { TestFeature } from "@tests/setup/v2Features";
+import { completeStripeCheckoutFormV2 as completeStripeCheckoutForm } from "@tests/utils/browserPool/completeStripeCheckoutFormV2";
+import { items } from "@tests/utils/fixtures/items";
+import { products } from "@tests/utils/fixtures/products";
+import { timeout } from "@tests/utils/genUtils";
+import { initScenario, s } from "@tests/utils/testInitUtils/initScenario";
+import chalk from "chalk";
+import { CusService } from "@/internal/customers/CusService";
+
+// ═══════════════════════════════════════════════════════════════════════════════
+// TEST 1: subscription_data.metadata passthrough via Stripe checkout
+// ═══════════════════════════════════════════════════════════════════════════════
+
+/**
+ * Scenario:
+ * - Customer with no payment method (forces Stripe checkout)
+ * - Attach with checkout_session_params containing subscription_data.metadata
+ *
+ * Expected Result:
+ * - Stripe subscription created after checkout has the user-provided metadata
+ * - Autumn's internal metadata (autumn_metadata_id) is also present on the session
+ */
+test.concurrent(`${chalk.yellowBright("checkout_session_params: subscription_data.metadata passthrough")}`, async () => {
+	const customerId = "attach-misc-sub-data-metadata";
+
+	const messagesItem = items.monthlyMessages({ includedUsage: 100 });
+	const pro = products.pro({
+		id: "pro-sub-metadata",
+		items: [messagesItem],
+	});
+
+	const { autumnV1, autumnV2_1, ctx } = await initScenario({
+		customerId,
+		setup: [
+			s.customer({ testClock: true }), // No payment method → Stripe checkout
+			s.products({ list: [pro] }),
+		],
+		actions: [],
+	});
+
+	// 1. Attach with checkout_session_params including subscription_data.metadata
+	const result = await autumnV2_1.billing.attach<AttachParamsV1Input>({
+		customer_id: customerId,
+		plan_id: pro.id,
+		checkout_session_params: {
+			subscription_data: {
+				metadata: {
+					user_id: "test-user-123",
+					custom_field: "custom-value",
+				},
+			},
+		},
+	});
+
+	// Should return a Stripe checkout URL
+	expect(result.payment_url).toBeDefined();
+	expect(result.payment_url).toContain("checkout.stripe.com");
+
+	// 2. Complete checkout
+	await completeStripeCheckoutForm({ url: result.payment_url });
+	await timeout(12000);
+
+	// 3. Verify product is attached
+	const customer = await autumnV1.customers.get<ApiCustomerV3>(customerId);
+
+	await expectProductActive({
+		customer,
+		productId: pro.id,
+	});
+
+	expectCustomerFeatureCorrect({
+		customer,
+		featureId: TestFeature.Messages,
+		includedUsage: 100,
+		balance: 100,
+		usage: 0,
+	});
+
+	await expectCustomerInvoiceCorrect({
+		customer,
+		count: 1,
+		latestTotal: 20,
+	});
+
+	// 4. Verify Stripe subscription has user-provided metadata
+	const fullCustomer = await CusService.getFull({
+		ctx,
+		idOrInternalId: customerId,
+	});
+
+	const stripeCustomerId = fullCustomer.processor?.id;
+	expect(stripeCustomerId).toBeDefined();
+
+	const subs = await ctx.stripeCli.subscriptions.list({
+		customer: stripeCustomerId!,
+		status: "all",
+	});
+
+	expect(subs.data.length).toBeGreaterThan(0);
+
+	const subscription = subs.data.find(
+		(sub) => sub.status === "active" || sub.status === "trialing",
+	);
+	expect(subscription).toBeDefined();
+
+	// Verify user-provided metadata is on the subscription
+	expect(subscription!.metadata.user_id).toBe("test-user-123");
+	expect(subscription!.metadata.custom_field).toBe("custom-value");
+});