Your API is your product. Build it like money moves through it — because it does.
We build production APIs for fintech startups — payments, ledger, KYC, disbursements, and webhook infrastructure. Idempotent, auditable, and documented well enough that your enterprise customers actually integrate. Fixed scope, fixed price.
Your MVP API was stitched together to land the first design partner. Now bigger customers want rate limits, signed webhooks, idempotency keys, versioning, sandbox environments, and docs that don't look like a Notion dump. You don't have any of that yet.
In fintech, the API is the product. Your customers' engineers are evaluating you on whether the webhooks are signed, whether POST /transfers is idempotent, whether you version cleanly, whether the sandbox actually mirrors production, and whether they can read your docs without Slacking you. If any of those answers is "not yet," they downgrade you mentally from "platform" to "vendor" — and the deal gets smaller.
Most early fintech APIs ship in a rush to close the first design partner. That's fine. The rush creates specific technical debt that's lethal at the next stage: no idempotency, so retries create duplicate transfers. Unsigned webhooks, so customers can't trust them and you end up doing polling patches. No request versioning, so the first breaking change you need to make costs you a weekend of support tickets from every integrator. One shared database across tenants, no separation between sandbox and production, and a reconciliation job that's one engineer's memory away from being unrunnable.
Then the compliance side. Every movement of money needs an audit trail — who initiated, who approved, the request payload, the response, the hash. You probably have pieces of this in application logs, none of it in a real ledger. The first time an enterprise customer asks for a full transaction audit, or your banking partner audits you, the gaps surface. In fintech, those gaps aren't "tech debt" — they're existential risk.
A fintech API that's ready for real developer integrations — complete with sandbox, SDKs, versioning, auth keys, webhook signatures, and the reliability contract a risk-averse customer expects.
Idempotent endpoints with keyed retry semantics
Every money-moving request supports `Idempotency-Key`; retries return the original response, not a duplicate transfer. Industry-standard, customer-expected.
Signed, versioned webhooks with retry and delivery guarantees
HMAC signatures, configurable retry with backoff, a replay endpoint for missed events, and a delivery log customers can inspect.
Double-entry ledger for the money surface
Proper immutable event store + derived balances, so every dollar is traceable to an entry. Handles holds, captures, reversals, and adjustments correctly.
KYC/KYB integration and sanction screening
Persona, Alloy, Sardine, or Middesk wired into onboarding, with document storage, review queues, and audit-ready decisioning.
Authentication and key management
Scoped API keys, OAuth where appropriate, per-environment sandboxes, rotation and revocation flows. IP allowlists when customers ask.
Real documentation and a working sandbox
OpenAPI schema auto-generated from the implementation, SDK stubs (TypeScript and Python minimum), and a sandbox environment that mirrors prod, seeded with test data. Built on Next.js API routes or a dedicated Node service, Postgres for the ledger (with Convex for the real-time product layer where it fits), and the standard observability stack — structured logs, traces, and metrics so your on-call actually sees what's happening.
One honest number to start.
Fixed-scope, fixed-price. The number below is the starting point — final scope is built from your brief.
A fintech API that's ready for real developer integrations — complete with sandbox, SDKs, versioning, auth keys, webhook signatures, and the reliability contract a risk-averse customer expects.
Three steps, every time.
The same repeatable engagement on every project. No surprises, no mystery, no billable ambiguity.
Brief & discovery.
We send you questions, then get on a call. Output: a written scope with every step, feature, and integration listed.
Build & ship.
Fixed schedule, weekly reviews. No scope creep unless you change the scope — and if you do, we reprice it transparently.
Warranty & retainer.
30-day warranty on every launch. Most clients stay on a monthly retainer for ongoing features and maintenance.
Why Fixed-Price Matters Here
Fintech founders manage two parallel risks: technical and regulatory. The last thing you need is a hourly-billed build stretching into quarters while your banking partner or regulator is waiting on controls you told them would be in place. Fixed scope means we commit to the idempotency semantics, the webhook signing, the audit logging, and the documentation — before you sign the contract. If it's in the spec, it ships, on date.
Related engagements.
Questions, answered.
Postgres with a strictly append-only event table plus derived balance views. We've considered specialized ledger databases (TigerBeetle, Fragment) and use them when scale justifies it, but 99% of early-stage fintechs are better served by a well-designed Postgres schema that any engineer can reason about, audit, and operate.
Yes. Unit, Column, Synctera, Increase, Stripe Treasury, and most BaaS providers have similar integration patterns — authenticated REST, webhooks, sandbox environments. We wire into the partner's API, normalize the data into your ledger, and build the reconciliation jobs.
By avoiding it where possible. For card payments we route through Stripe Elements or the equivalent so raw card data never touches your infrastructure (SAQ A scope). For rare cases where you have to touch a PAN — usually card issuing — we use tokenization vaults (Basis Theory, VGS) to keep scope controlled.
The engineering side, yes — access controls, logging, encryption, environment separation, backup and restore — is built to SOC 2 controls by default. The audit itself is a separate process with Vanta, Drata, or Secureframe. We work alongside those tools regularly.
A production API with idempotency, webhooks, ledger, auth, and docs typically runs $50k–$120k depending on scope and integrations. Greenfield builds with KYC, BaaS, and a user-facing product scope higher. Every engagement is fixed-price.
Tell Ryel about your project.
Describe what you’re building and what outcome you need. You’ll have a written, fixed-price scope within the week.