Payment API & SDKs

Overview

The quality of your API is the first and most lasting impression most technical partners will have of your payments business. A confusing, inconsistent or poorly documented API creates support burden, slows partner onboarding and pushes integrators toward competitors with cleaner developer experiences. Pay Engineers designs and builds payment APIs and SDKs specifically to remove that friction.

We approach API design as a product discipline in its own right, not an afterthought generated from internal database models. Resource naming, error handling, pagination, idempotency and webhook design are all deliberately specified, documented in OpenAPI, and validated against real integration scenarios before a partner ever sees the documentation.

The output is not just an API that works, but one that developers can integrate correctly on the first attempt, because the more mistakes an API allows, the more support tickets and failed transactions you will see in production.

Who This Is For

• PSPs, gateways and processors whose current API is inconsistent, undocumented or difficult to integrate
• Fintechs preparing to open their platform to third-party developers or partners for the first time
• Businesses that have grown their API organically and now need to formalise it before scaling partner integrations
• Any payments business measuring high time-to-first-transaction or high integration support volume from partners

What You Get

• A complete OpenAPI specification covering every resource, parameter, error and response your API exposes
• Webhook signing so partners can verify authenticity of every event you send them, protecting against spoofed notifications
• Idempotency keys implemented correctly across mutating endpoints, so retried requests never create duplicate transactions
• SDK starter kits in the languages your partner base actually uses, built from and kept consistent with the OpenAPI specification

Technical Approach

We specify the API contract first in OpenAPI, treating it as the source of truth from which documentation, SDKs and, where appropriate, server-side validation are generated or derived. This keeps documentation and implementation from drifting apart over time, a common failure mode in payment APIs that grow quickly.

The reference implementation is typically built in Laravel, with TypeScript and PHP SDKs generated or hand-tuned from the specification and a Node SDK provided where your partner base skews toward JavaScript back ends. Idempotency is implemented using a dedicated key store with a defined retention window, and webhook signing uses HMAC signatures with timestamp validation to prevent replay attacks. Sandbox credentials and realistic test data are made available from the earliest stages of integration, not just before go-live.

Delivery Process

• Audit of your existing API, if one exists, and discovery of partner integration pain points
• API resource and contract design, captured directly in an OpenAPI specification reviewed with your engineering team
• Sprint-based build of endpoints, webhook signing and idempotency handling against the agreed specification
• SDK generation and manual refinement for your priority languages, tested against real integration flows
• Documentation publication and support through initial partner integrations to catch friction points early

Outcomes and Benefits

• Significantly faster partner integration times, often measured in days rather than weeks
• Fewer duplicate-transaction and webhook-related support incidents through correct idempotency and signing
• Documentation and SDKs that stay in sync with the actual API through the OpenAPI-first approach
• A developer experience that becomes a genuine competitive advantage when partners are choosing between providers