--- a/app.py
+++ b/app.py
 def withdraw(conn, user_id: int, amount: int):
     row = conn.execute("SELECT balance FROM accounts WHERE id = ?", (user_id,)).fetchone()
-    # single atomic guarded update
-    cur = conn.execute("UPDATE accounts SET balance = balance - ? WHERE id = ? AND balance >= ?", (amount, user_id, amount))
-    if cur.rowcount == 0:
-        raise ValueError("insufficient funds")
-    conn.commit()
-    return
     if row[0] < amount: