Use a computer

For full performance and fluidity, please open Pay Engineers on a desktop or laptop. On mobile, the experience is limited — especially authenticated sections and advanced tools after login.

Pay Engineers Pay Engineers
Log in
Language
Home
Custom processor Bespoke payment infrastructure Configurator Design your processor setup Stripe workspace Connected Stripe operations Compliance Authorizations & licensing
All services Engineering & consulting offers Order a processor Start a custom build File a claim Complaints & claims desk
Processor guides Architecture & best practices API docs (opens in a new tab) Developer documentation Blog Product & engineering notes
About Who we are Contact Talk to our team
Create an account

Payment Infrastructure

Payment API & SDKs

Developer-first integration layer

Presentation

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

Technologies

OpenAPI Laravel TypeScript PHP SDK Node SDK

FAQ

We start by mapping the real integration scenarios your partners and merchants need to complete, then design resource models, error responses, pagination and webhook events around those scenarios rather than generating an API mechanically from your database schema. Every design decision is documented and reviewed with your engineering team before implementation begins, and we validate the design against a small pilot integration to catch usability issues early. The result is specified formally in OpenAPI, which becomes both the source of truth for documentation and the basis for generating client SDKs. This product-discipline approach is what separates an API developers enjoy using from one that generates constant support tickets.
SDKs are generated and hand-refined for the languages your integrator base actually uses, commonly including PHP, Node.js, Python, Java and mobile platforms, and we prioritise based on your partner and merchant technology mix identified during discovery. Each SDK includes idiomatic error handling, automatic retries for transient failures, and built-in signature verification for webhooks, so integrators are not left to implement security-critical logic themselves. We maintain a changelog and versioning policy for SDKs so breaking changes are clearly communicated rather than silently shipped. Sandbox credentials and example applications are provided alongside each SDK to shorten the time from first API call to working integration.
Every state-changing endpoint supports idempotency keys, so a retried request due to a network timeout or client-side retry logic cannot result in a duplicate charge, transfer or other financial action. This is one of the most consequential design decisions in a payments API, since retry storms and timeouts are routine in production, not edge cases. We document the idempotency window and behaviour explicitly, and provide test scenarios in the sandbox so integrators can verify their own retry logic behaves correctly before going live. This significantly reduces reconciliation disputes caused by duplicate processing.
Webhooks are signed using a shared secret or asymmetric signature scheme so receivers can verify authenticity before acting on a payload, and we implement retry with exponential backoff plus a dead-letter mechanism so temporary outages on the receiving end do not result in silently lost events. Event payloads are versioned, and we document a clear deprecation policy for event schema changes so integrators are not broken by unannounced updates. Sandbox environments allow integrators to trigger test events on demand, which speeds up their own webhook handler development considerably. This reliability engineering is usually the difference between an integration that works in a demo and one that survives production.
We provide a defined post-launch support period covering bug fixes, documentation clarifications and rapid triage of integrator-reported issues, since early integration problems are disproportionately damaging to partner trust. Beyond that period, an ongoing retainer covers new SDK language additions, API version upgrades and continued documentation maintenance as your product evolves. We can also support your developer relations function directly, for example reviewing integrator code or joining onboarding calls for strategic partners. The OpenAPI specification and documentation are handed over in full regardless of whether you continue an ongoing engagement.

Similar services

Custom Payment Processor

We design and build independent payment processors that give fintechs, banks and platforms full ownership of authorisation, routing, settlement and risk logic. The engagement spans scheme connectivity, merchant lifecycle management, fee configuration and compliance hooks, engineered around your corridors and volumes. You end up with a processor you can extend, audit and scale on your own roadmap, not a black box you rent from someone else.

Payment Gateway

A production-grade payment gateway connecting your checkout to acquirers, card schemes and local payment methods, engineered around strong customer authentication and tokenised credentials. We handle 3-D Secure and SCA orchestration, smart transaction routing and webhook-driven integration so your checkout converts more customers while staying compliant with PSD2 and scheme mandates.

Merchant Portal

A branded merchant portal giving your merchants self-service visibility into onboarding status, transactions, statements, disputes and payout schedules. Role-based access control and full audit logging let you extend access safely across finance, support and management users within each merchant account. The portal reduces support ticket volume while giving merchants the transparency modern payment businesses expect.