Fintech
Fintech software where the money always balances
Taking a payment is the easy half. Agreeing on what happened, after a timeout and a retry, is the half that keeps finance teams awake.
Four things that decide whether a fintech build survives
4 things that decide this
- 01Your PCI scope gets decided by how you embed the payment form, not by which processor you chose. A hosted field and a direct-post form put you in different questionnaires.
- 02Keep your money in integer minor units, never a float. A float can't hold 0.10 exactly, and the drift shows up in a reconciliation nobody scheduled.
- 03A payment call that times out hasn't failed. It has an unknown outcome, and your only safe repair is an idempotency key you sent before the first attempt.
- 04Design your ledger before the screens. Money moves as entries that get reversed rather than edited, which is what keeps your totals and the processor's agreeing.
Fintech bugs do not crash. They disagree.
A normal web bug announces itself. A money bug does not. Your system says the customer paid. The processor says the charge was disputed. Both records look reasonable and neither can prove the other wrong.
The gap usually opens in an ordinary place. A webhook arrives twice. A network call times out after the charge succeeded. A refund lands before the payment it reverses. Each is a normal Tuesday for a payment processor, and each becomes a support ticket a month later.
So we design the ledger before the screens. Money moves as entries that never get edited, only reversed. That is duller to demo and it is the reason the numbers still tie out in year two.
Our own count
What we have actually shipped
7
Production builds with Stripe integrated, including Stripe Connect for marketplace payouts
3
Named AI-on-financial-data products: Trading CoPilot, IRS Escape Plan, PremiumAudit.io
The fintech work we take
Product engineering where payments, real-time data and AI meet. Not core migrations.
Payment integration that reconciles
You get hosted fields, webhook handling that tolerates duplicates, and a ledger that agrees with the processor. Stripe across seven of our builds, plus Stripe Connect where money is held between two strangers.
Double-entry ledgers
Append-only entries, reversals instead of edits, and balances derived rather than stored. When a regulator or an auditor asks you how a figure arose, your answer is a query.
Marketplace and escrow flows
We hold funds, release on a condition, and freeze on dispute. WorkMateAI does exactly this for Australian trades, with chargeback evidence generated from the record.
Real-time data pipelines
You get webhook ingestion, enrichment and alerting where late data is worse than no data. Trading CoPilot takes TradingView alerts and routes them through a human approval step.
AI on financial documents
We read 1040s, K-1s, 1099s and audit paperwork, then check the extraction rather than trust it. IRS Escape Plan and PremiumAudit.io both run this pattern.
Human approval gates
An agent proposes and a person confirms. In Trading CoPilot the trade waits on a yes from Telegram before the broker connection executes anything.
- Charge sentIdempotency key attached first.
- TimeoutNo response. Outcome unknown.
- RetrySame key, so no second charge.
- WebhookMay arrive twice, or out of order.
- Ledger entryWritten once, never edited.
- ReconciledYour total equals the processor's.
Every step here is ordinary. The failure is treating a timeout as a failure, retrying without a key, and charging a customer twice for one order.
One analytics script can change your compliance position
Most teams believe using Stripe puts them out of scope. That's true only for certain integration types. A hosted redirect or an iframe keeps you in the lightest questionnaire, but a direct-post form or a JavaScript SDK that touches the page can move you into a heavier one.
The 2025 revision made this sharper. Two script-control requirements got lifted out of the lightest questionnaire and replaced with a broader attestation about your site itself. Qualifying now means confirming your site isn't open to script attacks, which is a wider claim than protecting one page.
Practically, a marketing tag your other team adds to checkout can move you. We treat your checkout page as a controlled surface, with a written list of every script on it and a reason each one is there.
- Pick your integration model before the design, because it sets the scope.
- Never let a full card number reach your servers, your logs or your error tracker.
- Keep an inventory of your checkout scripts, and make adding one a decision.
Systems where money and documents have to agree
“Their attention to detail, quality of employees, and work ethic were outstanding.”
Nicolas de Quesada · CEO, Lexpair
Shipping fast against shipping something that balances
Money storage
The usual build
A decimal or float column, because it renders nicely.
How we build
Integer minor units, with the currency stored beside the amount.
Retries
The usual build
Retry on timeout and hope the customer notices the double charge.
How we build
An idempotency key generated before the first attempt, reused on every retry.
Webhooks
The usual build
Process on arrival, assume each event lands once.
How we build
Deduplicated by event id, processed out of order safely.
Corrections
The usual build
Update the row and lose the old value.
How we build
Write a reversing entry, so the history survives the fix.
PCI position
The usual build
Discovered during the annual questionnaire.
How we build
Chosen at design time, with the integration model picked deliberately.
What these builds run on
Application
- React
- TypeScript
- FastAPI
- Next.js
- Ruby on Rails
Data
- PostgreSQL
- Supabase
- Redis
- Row-level security
Payments
- Stripe
- Stripe Connect
- Webhook reconciliation
- PayPal
AI
- Document extraction
- Approval gates
- Evaluation suites
Questions fintech teams ask us first
01How much does it cost to build a fintech product?+
Your drivers are how many money movements you support, how many parties hold funds, and which licence or standard you must answer to. A single-currency subscription product is a different build from a marketplace holding money between strangers. Scoping calls cost nothing. Where we'd have to read an existing codebase to answer honestly, we run a paid two-week diagnostic and you keep the findings either way.
02Which SAQ type will our architecture put us in?+
Your integration choice sets it, not your processor. Fully hosted fields from a compliant provider point you toward the lightest questionnaire. A form that posts card details directly, or a script that can alter the payment page, points you to a heavier one. Decide this before you build the checkout, because changing it later means rebuilding it.
03Can you build our ledger if we already have a payment provider?+
Yes, and it's a common starting point. Your processor knows what it charged, but it doesn't know what you owe a seller after fees, refunds and a partial chargeback. That reconciliation layer is yours to own. We build it as double-entry, so every figure can be traced back to the events that produced it.
04Can you build the layer that sits around our core platform?+
Yes, and that's where most of the value sits. Your core holds the balances and answers to the regulator, but everything around it decides whether your product actually works. That means a sub-ledger that reconciles daily and payment flows that treat a timeout as unknown rather than failed. It also means onboarding that leaves evidence you can examine later. We build that layer for you, and we keep it portable.
05How do you keep an AI feature from inventing a number?+
Never let the model be the source of a figure. In our tax and audit work the model extracts and the system checks the arithmetic, so a wrong reading becomes a flagged discrepancy, not a confident total. Where an action moves money, a person approves it. Trading CoPilot waits for a yes before any trade reaches the broker.
Go deeper
- Professional and financial services hub →The industry hub this page belongs to.
- Hire fintech developers →Senior engineers who have shipped money-handling systems, interviewed by you first.
- Idempotency key →How a retried payment avoids a second charge.
- Payment gateway integration →Reconciliation, disputes and multi-currency edge cases.
- Banking software development →The same ledger and payment discipline, argued for teams building beside a core.
- Wealth management software →Portfolio data, reporting and advisor workflow that has to reconcile.
