anthropic/claude-opus-5

```sql -- Atomic: check and deduct in one statement UPDATE accounts SET balance = balance - :cost WHERE user_id = :user_id AND balance >= :cost; -- 0 rows affected => insufficient funds, reject ``` Or, if you need to read other fields first, lock the row inside a transaction: ```sql BEGIN; SELECT balance FROM accounts WHERE user_id = :user_id FOR UPDATE; -- app-side checks UPDATE accounts SET balance = balance - :cost WHERE user_id = :user_id; COMMIT; ``` **What breaks:** both requests read the same balance before either writes, so both pass the check and the second write overwrites the first — the user gets charged once but spends twice.