diff --git a/backend/billing/domain/stripe/StripeBillingService.ts b/backend/billing/domain/stripe/StripeBillingService.ts
index 8a8bda4b..c08240a9 100644
--- a/backend/billing/domain/stripe/StripeBillingService.ts
+++ b/backend/billing/domain/stripe/StripeBillingService.ts
@@ -64,6 +64,8 @@ export interface StripeCustomerInput {
 export interface StripeSubscriptionItemInput {
   readonly priceId: string;
   readonly quantity?: number;
+  /** Existing Stripe subscription item to update instead of adding a new item. */
+  readonly subscriptionItemId?: string;
 }
 
 /**
@@ -270,7 +272,11 @@ export class StripeBillingService {
   ): Promise<any> {
     const params: StripeParams = { expand: ['latest_invoice.payment_intent'] };
     if (input.items) params.items = input.items.map((item) => this.itemParams(item));
-    if (input.prorationBehavior) params.proration_behavior = input.prorationBehavior;
+    if (input.prorationBehavior) {
+      params.proration_behavior = input.prorationBehavior;
+    } else if (input.items) {
+      params.proration_behavior = 'create_prorations';
+    }
     if (input.coupon !== undefined) params.coupon = input.coupon;
     if (input.cancelAtPeriodEnd !== undefined) {
       params.cancel_at_period_end = input.cancelAtPeriodEnd;
@@ -503,6 +509,7 @@ export class StripeBillingService {
 
   private itemParams(item: StripeSubscriptionItemInput): Record<string, unknown> {
     const params: Record<string, unknown> = { price: item.priceId };
+    if (item.subscriptionItemId) params.id = item.subscriptionItemId;
     if (item.quantity !== undefined) params.quantity = item.quantity;
     return params;
   }
diff --git a/backend/billing/tests/stripeBilling.test.ts b/backend/billing/tests/stripeBilling.test.ts
index c568a9f6..a99c7b71 100644
--- a/backend/billing/tests/stripeBilling.test.ts
+++ b/backend/billing/tests/stripeBilling.test.ts
@@ -444,6 +444,42 @@ describe('StripeBillingService', () => {
       expect(calls[0].init.headers['Idempotency-Key']).toBe('subtrackr:subscription:update:sub_1');
     });
 
+    it.each([1, 5])('prorates a quantity change to %i seats on the existing item', async (quantity) => {
+      const invoice = { id: 'in_proration', total: 1200 };
+      const { service, calls } = createService([json({ id: 'sub_1', latest_invoice: invoice })]);
+
+      const updated = await service.updateSubscription('sub_1', {
+        items: [{ priceId: 'price_seat', subscriptionItemId: 'si_seat', quantity }],
+      });
+
+      expect(calls[0].params.get('items[0][id]')).toBe('si_seat');
+      expect(calls[0].params.get('items[0][price]')).toBe('price_seat');
+      expect(calls[0].params.get('items[0][quantity]')).toBe(String(quantity));
+      expect(calls[0].params.get('proration_behavior')).toBe('create_prorations');
+      expect(calls[0].params.getAll('expand[]')).toContain('latest_invoice.payment_intent');
+      expect(updated.latest_invoice).toEqual(invoice);
+    });
+
+    it('preserves an explicit proration behavior for item changes', async () => {
+      const { service, calls } = createService([json({ id: 'sub_1' })]);
+
+      await service.updateSubscription('sub_1', {
+        items: [{ priceId: 'price_seat', subscriptionItemId: 'si_seat', quantity: 0 }],
+        prorationBehavior: 'none',
+      });
+
+      expect(calls[0].params.get('proration_behavior')).toBe('none');
+      expect(calls[0].params.get('items[0][quantity]')).toBe('0');
+    });
+
+    it('does not set proration for metadata-only updates', async () => {
+      const { service, calls } = createService([json({ id: 'sub_1' })]);
+
+      await service.updateSubscription('sub_1', { metadata: { source: 'admin' } });
+
+      expect(calls[0].params.has('proration_behavior')).toBe(false);
+    });
+
     it('ends a trial early with trial_end=now', async () => {
       const { service, calls } = createService([json({ id: 'sub_1' })]);