Stripe

01What is the core product — moving money, or the API contract itself?

Both, but the contract is sacred: a payment-intents API that a million merchants integrated must stay backward compatible for years — versioning is a first-class feature.

02Who owns idempotency — us or the merchant?

The merchant supplies the key per request; the platform stores key → outcome and replays it — making MERCHANT retries safe is the product promise.

03One acquirer or many, and who chooses?

Many: routing picks per payment (cost, health, auth-rate history) and fails over on acquirer errors (the acquirer: the bank that accepts the card payment on the merchant’s behalf) — invisible to the merchant, visible in their auth rates.

04How do merchants learn what happened?

Webhooks: signed events, at-least-once with retries and backoff, per-merchant endpoints — plus the API to re-fetch truth when in doubt.

05Where does merchant money live between charge and payout?

In a per-merchant ledger: settled charges minus fees minus refunds accumulate, and payouts sweep the balance on schedule — every payout explainable to the cent.

06Is fraud screening per merchant or platform-wide?

Platform-wide models with per-merchant context: the network effect (seeing cards across merchants) is the moat a single merchant cannot replicate.

01What does API stability actually require?

Versioning per merchant account, additive-only changes within a version, and years-long deprecation windows — the integration a merchant wrote in 2020 still works.

02What isolates tenants from each other?

Per-merchant rate limits, quotas, and key scoping: a runaway integration throttles itself, never the platform — and keys are scoped so leaks are containable.

03An acquirer starts declining everything — how fast do we react?

Health-based routing shifts traffic in seconds; the merchant sees a dip in latency at worst, never a wall of declines.

04What is the webhook delivery contract?

At-least-once, retried with backoff for days, order NOT guaranteed (documented) — merchants must dedupe by event id and re-fetch objects for truth.

05Who audits this?

Everyone: merchant finance reconciles payouts, regulators audit flows — so ledgers are immutable, and every fee line traceable to a rule version.