Most API design advice assumes a forgiving environment: retries are cheap, data has a single source of truth, and a slow response is only an inconvenience. Financial infrastructure assumes none of that. Card networks give you three seconds to auth or decline. Settlement takes days but fees post immediately. And if your ledger is off by a few cents, you might not find out until it's a legal problem.

During this talk, Fawziyah draws on her years building payments infrastructure to share four principles that make APIs more resilient under those conditions:

1. Continuous reconciliation
2. Edge case first design
3. Latency under pressure
4. Idempotency at scale

All four will make your APIs better whether or not they're moving money.