Gemini 3.1 Pro Preview vs GPT-5.3-Codexfor coding

**Key:** A client-generated UUID passed in the `Idempotency-Key` HTTP header. 

**Storage:** A transactional database (e.g., PostgreSQL) or distributed cache (e.g., Redis) storing the key, request payload hash, execution status (pending/completed), and the final HTTP response.

**Concurrent Duplicate:** Handled via atomic operations (e.g., Redis `SETNX` or a SQL `UNIQUE` constraint). The first request atomically inserts the key and marks it "pending". If a concurrent duplicate arrives, the atomic insert fails. The server detects the "pending" s…

Use a client-supplied **Idempotency-Key** (UUID) on `POST /payments`, scoped by `merchant_id` (or customer+endpoint).

Store it in a durable `payment_idempotency` table:

- `(merchant_id, idempotency_key)` **UNIQUE**
- `request_hash` (amount/currency/source fingerprint)
- `status` (`processing|succeeded|failed`)
- `payment_id`
- full `http_status` + `response_body`
- timestamps/TTL

Flow:

1. Start DB transaction.
2. `INSERT ... ON CONFLICT DO NOTHING` a row with `processing`.
3. If insert succeeds, this request “owns” execution; call processor…