diff --git a/server/scripts/size-baseline.json b/server/scripts/size-baseline.json
index 68ded4957..81a14c753 100644
--- a/server/scripts/size-baseline.json
+++ b/server/scripts/size-baseline.json
@@ -17,7 +17,7 @@
   "src/nest/places/places.service.ts": 2020,
   "src/nest/plugins/plugin-runtime.service.ts": 1463,
   "src/nest/plugins/runtime/plugin-sdk.ts": 1094,
-  "src/nest/reservations/reservations.service.ts": 1248,
+  "src/nest/reservations/reservations.service.ts": 1247,
   "src/nest/trips/trips.service.ts": 1013,
   "src/nest/vacay/vacay.service.ts": 1440
 }
diff --git a/server/src/nest/reservations/reservations.service.ts b/server/src/nest/reservations/reservations.service.ts
index 124722509..7cd214eb8 100644
--- a/server/src/nest/reservations/reservations.service.ts
+++ b/server/src/nest/reservations/reservations.service.ts
@@ -1192,9 +1192,8 @@ export class ReservationsService {
         const updated = await this.budget.updateBudgetItem(existing.id, tripId, { name: itemName, category, total_price: entry.total_price });
         this.sendCost(tripId, { event: 'budget:updated', payload: { item: updated } }, socketId, collect);
       } else {
-        const item = await this.budget.createBudgetItem(tripId, { name: itemName, category, total_price: entry.total_price });
-        await this.budgetItemsRepo.setReservationId(item.id, id);
-        item.reservation_id = Number(id);
+        // The link travels on the insert, so a cost is never written without its booking.
+        const item = await this.budget.linkBudgetItemToReservation(tripId, Number(id), { name: itemName, category, total_price: entry.total_price! });
         this.sendCost(tripId, { event: 'budget:created', payload: { item } }, socketId, collect);
       }
     } catch (err) {
diff --git a/server/tests/unit/nest/reservations.service.test.ts b/server/tests/unit/nest/reservations.service.test.ts
index 621f67e76..ace4a4687 100644
--- a/server/tests/unit/nest/reservations.service.test.ts
+++ b/server/tests/unit/nest/reservations.service.test.ts
@@ -695,16 +695,16 @@ describe('ReservationsService (DI-native, real SQL)', () => {
       expect(broadcast).toHaveBeenCalledWith(String(trip.id), 'budget:updated', { item: { id: item.id } }, 'sock');
     });
 
-    it('creates + links a new item when none exists, using the current title fallback', async () => {
+    it('creates a new item linked on its insert when none exists, using the current title fallback', async () => {
       const { trip } = ownerTrip();
       const res = createReservation(testDb, trip.id);
-      const created = createBudgetItem(testDb, trip.id); // the row the mocked create "returns"
-      budget.createBudgetItem.mockReturnValue({ id: created.id });
+      budget.linkBudgetItemToReservation.mockReturnValue({ id: 31, reservation_id: res.id });
       await svc.syncBudgetOnUpdate(String(trip.id), String(res.id), '', undefined, 'Old title', 'flight', { total_price: 120 }, 'sock');
-      expect(budget.createBudgetItem).toHaveBeenCalledWith(String(trip.id), { name: 'Old title', category: 'flight', total_price: 120 });
-      // The service back-links the created item to the reservation itself.
-      expect(testDb.prepare('SELECT reservation_id FROM budget_items WHERE id = ?').get(created.id)).toEqual({ reservation_id: res.id });
-      expect(broadcast).toHaveBeenCalledWith(String(trip.id), 'budget:created', { item: { id: created.id, reservation_id: res.id } }, 'sock');
+      // One statement carries the link: no unlinked insert followed by a separate
+      // back-link that a swallowed failure inside updateWithCost could leave half done.
+      expect(budget.linkBudgetItemToReservation).toHaveBeenCalledWith(String(trip.id), res.id, { name: 'Old title', category: 'flight', total_price: 120 });
+      expect(budget.createBudgetItem).not.toHaveBeenCalled();
+      expect(broadcast).toHaveBeenCalledWith(String(trip.id), 'budget:created', { item: { id: 31, reservation_id: res.id } }, 'sock');
     });
 
     it('RESV-TX-001: updateWithCost rolls the booking edit back when its cost write fails', async () => {