Fintech APIs and backend Digital product Security Anonymous case

Public API for a regional fintech

We designed and built the public API for partners and merchants to integrate the fintech's products, with robust authentication, rate limiting, observability and living documentation.

LATAM Fintech ·

22% Volume via API in 6 months Out of total transactional volume.
5 days Partner onboarding From 4 weeks to 5 days with SDKs and docs portal.
38 Integrated partners 6 months after public launch.

Problem

The fintech had clients asking for programmatic integrations, but only internal endpoints existed without a stable contract, public docs or a security model fit for third parties.

Intervention

We defined the API contract with OpenAPI, implemented OAuth 2.0 with scopes and mTLS authentication for high-value cases, added per-client rate limiting, per-endpoint metrics and a documentation portal generated from the OpenAPI spec with executable examples. We ran a closed beta with 5 partners before the general launch.

Outcome

6 months after public launch, 38 partners are integrated and process more than 22% of total volume. Generated documentation and SDKs in 3 languages reduced partner onboarding time from 4 weeks to 5 days.

Stack

.NET 8 ASP.NET Core Azure SQL Server OpenAPI Azure API Management Application Insights

API as a product

A public API is not an exposed endpoint. It’s a product with onboarding, documentation, support, versioning and a clear contract with its consumers. That was the conversation we had with the client from week one.

Security without unnecessary friction

We designed a two-tier authentication model: OAuth 2.0 with scopes for standard integrations and mTLS for high-value flows (large transactions, sensitive data). That lets small partners get started fast without sacrificing security for critical cases.

Closed beta to reduce risk

Before opening the API to the world, we ran 8 weeks of closed beta with 5 partners of different profiles. That uncovered real problems (inconsistent pagination, unclear errors, broken examples) that we fixed before public launch.

The API feels like a product, not an exposed endpoint. That's what we hoped for but didn't know how to articulate.
VP of Product VP Product · Anonymous client

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