diff --git a/web/src/app/api/sync/route.ts b/web/src/app/api/sync/route.ts
index 95ff08c..b0b35f9 100644
--- a/web/src/app/api/sync/route.ts
+++ b/web/src/app/api/sync/route.ts
@@ -202,32 +202,44 @@ async function processOperation(op: SyncOperation, userId: string) {
 
         // If the incoming list is an inbox, check for an existing inbox and merge
         if (incomingIsInbox) {
-          const [existingInbox] = await db
-            .select()
-            .from(slLists)
-            .where(and(eq(slLists.userId, userId), eq(slLists.isInbox, true), isNull(slLists.deletedAt)));
+          await db.transaction(async (tx) => {
+            const [existingInbox] = await tx
+              .select()
+              .from(slLists)
+              .where(and(eq(slLists.userId, userId), eq(slLists.isInbox, true), isNull(slLists.deletedAt)));
 
-          if (existingInbox && existingInbox.id !== entityId) {
-            // Reassign all tasks from the old inbox to the new one
-            await db.update(slTasks)
-              .set({ listId: entityId, updatedAt: now })
-              .where(and(eq(slTasks.listId, existingInbox.id), eq(slTasks.userId, userId)));
-            // Soft-delete the old inbox
-            await db.update(slLists)
-              .set({ deletedAt: now, updatedAt: now })
-              .where(eq(slLists.id, existingInbox.id));
-          }
+            if (existingInbox && existingInbox.id !== entityId) {
+              // Reassign all tasks (including subtasks) from the old inbox to the new one
+              await tx.update(slTasks)
+                .set({ listId: entityId, updatedAt: now })
+                .where(and(eq(slTasks.listId, existingInbox.id), eq(slTasks.userId, userId)));
+              // Soft-delete the old inbox
+              await tx.update(slLists)
+                .set({ deletedAt: now, updatedAt: now })
+                .where(eq(slLists.id, existingInbox.id));
+            }
+
+            await tx.insert(slLists).values({
+              id: entityId,
+              userId,
+              name: d.name as string || 'Untitled',
+              color: d.color as string | undefined,
+              icon: d.icon as string | undefined,
+              position: d.position as number | undefined,
+              isInbox: incomingIsInbox,
+            }).onConflictDoNothing();
+          });
+        } else {
+          await db.insert(slLists).values({
+            id: entityId,
+            userId,
+            name: d.name as string || 'Untitled',
+            color: d.color as string | undefined,
+            icon: d.icon as string | undefined,
+            position: d.position as number | undefined,
+            isInbox: incomingIsInbox,
+          }).onConflictDoNothing();
         }
-
-        await db.insert(slLists).values({
-          id: entityId,
-          userId,
-          name: d.name as string || 'Untitled',
-          color: d.color as string | undefined,
-          icon: d.icon as string | undefined,
-          position: d.position as number | undefined,
-          isInbox: incomingIsInbox,
-        }).onConflictDoNothing();
       } else if (action === 'update') {
         await verifyOwnership(slLists, entityId, userId);
         await db.update(slLists)
diff --git a/web/src/db/seed.ts b/web/src/db/seed.ts
index 4f48b0d..0632530 100644
--- a/web/src/db/seed.ts
+++ b/web/src/db/seed.ts
@@ -18,12 +18,10 @@ async function seed() {
     .from(slLists)
     .where(and(eq(slLists.userId, userId), eq(slLists.isInbox, true)));
 
-  // Use the same fixed inbox ID as the mobile app to avoid duplicates during sync
-  const INBOX_ID = '00000000-0000-0000-0000-000000000001';
-
   if (existing.length === 0) {
+    // Let the DB generate a random UUID — the sync endpoint handles
+    // inbox deduplication when mobile pushes its fixed-ID inbox.
     await db.insert(slLists).values({
-      id: INBOX_ID,
       userId,
       name: 'Inbox',
       isInbox: true,