diff --git a/database/music_database.py b/database/music_database.py
index 120846be..a2c9df7c 100644
--- a/database/music_database.py
+++ b/database/music_database.py
@@ -11649,9 +11649,22 @@ class MusicDatabase:
         return cursor.rowcount > 0
 
     def reorder_hifi_instances(self, urls: List[str]) -> bool:
-        """Update priorities based on the given URL order."""
+        """Update priorities based on the given URL order.
+        Returns False if any URL does not exist in the database.
+        """
+        if not urls:
+            return True
         conn = self._get_connection()
         cursor = conn.cursor()
+        placeholders = ",".join("?" for _ in urls)
+        cursor.execute(
+            f"SELECT url FROM hifi_instances WHERE url IN ({placeholders})",
+            urls
+        )
+        existing = {row["url"] for row in cursor.fetchall()}
+        missing = [u for u in urls if u not in existing]
+        if missing:
+            return False
         for i, url in enumerate(urls):
             cursor.execute("UPDATE hifi_instances SET priority = ? WHERE url = ?", (i, url))
         conn.commit()
diff --git a/tests/test_hifi_instance_methods.py b/tests/test_hifi_instance_methods.py
index c41db12b..d300591c 100644
--- a/tests/test_hifi_instance_methods.py
+++ b/tests/test_hifi_instance_methods.py
@@ -274,10 +274,10 @@ def test_reorder_hifi_instances_returns_true_on_empty_list(db):
     assert db.reorder_hifi_instances([]) is True
 
 
-def test_reorder_hifi_instances_returns_true_even_with_unknown_urls(db):
-    """UPDATE that matches 0 rows is not an error."""
+def test_reorder_hifi_instances_returns_false_with_unknown_urls(db):
+    """Reorder should fail when any URL doesn't exist."""
     _seed(db, instances=[("http://a.com", 0, 1)])
-    assert db.reorder_hifi_instances(["http://a.com", "http://phantom.com"]) is True
+    assert db.reorder_hifi_instances(["http://a.com", "http://phantom.com"]) is False
 
 
 # ── seed_hifi_instances ───────────────────────────────────────────────────
diff --git a/web_server.py b/web_server.py
index 2d4b8eb4..b732a46d 100644
--- a/web_server.py
+++ b/web_server.py
@@ -24782,7 +24782,8 @@ def hifi_reorder_instances():
             return jsonify({'success': False, 'error': 'URL list is required'}), 400
         from database.music_database import get_database
         db = get_database()
-        db.reorder_hifi_instances(urls)
+        if not db.reorder_hifi_instances(urls):
+            return jsonify({'success': False, 'error': 'One or more URLs not found'}), 400
         # Reload the client
         if soulseek_client and hasattr(soulseek_client, 'hifi') and soulseek_client.hifi:
             soulseek_client.hifi.reload_instances()