diff --git a/web/src/app/api/sync/route.ts b/web/src/app/api/sync/route.ts
index aebdf44..95ff08c 100644
--- a/web/src/app/api/sync/route.ts
+++ b/web/src/app/api/sync/route.ts
@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { db } from '@/db/client';
 import { slLists, slTasks, slTags, slTaskTags } from '@/db/schema';
-import { eq, and, gte } from 'drizzle-orm';
+import { eq, and, gte, isNull } from 'drizzle-orm';
 import { requireAuth, parseBody } from '@/lib/api';
 import { rateLimit } from '@/lib/rateLimit';
 import { syncPushSchema, type SyncOperation } from '@/lib/validators';
@@ -197,14 +197,36 @@ async function processOperation(op: SyncOperation, userId: string) {
   switch (entityType) {
     case 'list': {
       if (action === 'create') {
+        const d = (data as Record<string, unknown>) || {};
+        const incomingIsInbox = d.isInbox as boolean | undefined;
+
+        // 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)));
+
+          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));
+          }
+        }
+
         await db.insert(slLists).values({
           id: entityId,
           userId,
-          name: (data as Record<string, unknown>)?.name as string || 'Untitled',
-          color: (data as Record<string, unknown>)?.color as string | undefined,
-          icon: (data as Record<string, unknown>)?.icon as string | undefined,
-          position: (data as Record<string, unknown>)?.position as number | undefined,
-          isInbox: (data as Record<string, unknown>)?.isInbox as boolean | undefined,
+          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);
diff --git a/web/src/db/seed.ts b/web/src/db/seed.ts
index 4072b06..4f48b0d 100644
--- a/web/src/db/seed.ts
+++ b/web/src/db/seed.ts
@@ -18,8 +18,12 @@ 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) {
     await db.insert(slLists).values({
+      id: INBOX_ID,
       userId,
       name: 'Inbox',
       isInbox: true,