Fintech Infrastructure

API Design for Payment Platforms: Lessons from a Decade of Integrations

Payomatix EngineeringJune 22, 20257 min read

What Lasts

Resource-oriented endpoints (`/payments`, `/refunds`, `/disputes`) age better than RPC-style.

Idempotency keys on every write.

Expandable resources (`?expand[]=customer`) avoid N+1.

Cursor-based pagination scales beyond 10K results.

Versioned by date, with long deprecation runways.

Returning amounts as floats (always integers in minor units).

Letting timestamps default to local timezone.

Mixing snake_case and camelCase.

Conflating HTTP status with business outcome (a `200` with `"status": "failed"` is fine; a `500` for a declined card is not).

Built on these lessons from day one. Predictable, idempotent, well-versioned, with an SDK in every major language.

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