Finance Expert

Finance Expert

Expert guidance for financial systems, FinTech applications, banking platforms, payment processing, and financial technology development.

Core Concepts

Financial Systems

• Core banking systems
• Payment processing
• Trading platforms
• Risk management
• Regulatory compliance (PCI-DSS, SOX, Basel III)
• Financial reporting

FinTech Stack

• Payment gateways (Stripe, PayPal, Square)
• Banking APIs (Plaid, Yodlee)
• Blockchain/crypto
• Open Banking APIs
• Mobile banking
• Digital wallets

Key Challenges

• Security and fraud prevention
• Real-time processing
• High availability (99.999%)
• Regulatory compliance
• Data privacy
• Transaction accuracy

Payment Processing

These examples move real money. Read Money movement guardrails before running any of them. Credentials come from the environment or a secrets manager, never from source.

## Payment gateway integration (Stripe)
import os
from decimal import Decimal, ROUND_HALF_UP

import stripe

## Never hardcode a key. Load it from the environment or a secrets manager, and
## fail closed if it is absent rather than falling back to a default.
stripe.api_key = os.environ["STRIPE_API_KEY"]
WEBHOOK_SECRET = os.environ["STRIPE_WEBHOOK_SECRET"]


def to_minor_units(amount: Decimal) -> int:
    """Convert a decimal amount to integer minor units (cents).

    int(amount * 100) truncates: Decimal("0.615") would silently become 61
    instead of 62. Money must round explicitly, half-up, and only from Decimal
    - never from float.
    """
    if not isinstance(amount, Decimal):
        raise TypeError("monetary amounts must be Decimal, not %s" % type(amount).__name__)
    if amount < 0:
        raise ValueError("amount must not be negative")
    return int((amount * 100).quantize(Decimal("1"), rounding=ROUND_HALF_UP))


class PaymentService:
    def create_payment_intent(
        self, amount: Decimal, order_id: str, idempotency_key: str, currency: str = "usd"
    ):
        """Create a payment intent.

        The idempotency key is supplied by the caller and derived from the
        order, so a retried request cannot charge the customer twice. Stripe
        replays the original response instead of creating a second intent.
        """
        return stripe.PaymentIntent.create(
            amount=to_minor_units(amount),
            currency=currency,
            payment_method_types=["card"],
            metadata={"order_id": order_id},
            idempotency_key=idempotency_key,
        )

    def process_refund(
        self, payment_intent_id: str, idempotency_key: str, amount: Decimal | None = None
    ):
        """Process a full or partial refund (idempotent, like the charge)."""
        return stripe.Refund.create(
            payment_intent=payment_intent_id,
            amount=to_minor_units(amount) if amount is not None else None,
            idempotency_key=idempotency_key,
        )

    def handle_webhook(self, payload: bytes, signature: str):
        """Handle a Stripe webhook event.

        Signature verification is authentication: a failure must be rejected,
        never treated as a malformed payload. Returning 2xx on an unverified
        event tells Stripe the forged event was accepted.
        """
        try:
            event = stripe.Webhook.construct_event(payload, signature, WEBHOOK_SECRET)
        except stripe.error.SignatureVerificationError:
            # Forged or replayed event - fail closed, log, do not process.
            return {"status": "rejected"}, 400
        except ValueError:
            return {"status": "invalid_payload"}, 400

        # Webhooks are delivered at least once: deduplicate on event.id before
        # acting, or the same payment is booked twice.
        if self.already_processed(event.id):
            return {"status": "duplicate"}, 200

        if event.type == "payment_intent.succeeded":
            self.handle_successful_payment(event.data.object)
        elif event.type == "payment_intent.payment_failed":
            self.handle_failed_payment(event.data.object)

        self.mark_processed(event.id)
        return {"status": "success"}, 200

Banking Integration

## Open Banking API integration (Plaid)
import os

from plaid import Client
from plaid.errors import PlaidError

class BankingService:
    def __init__(self):
        # Credentials from the environment; sandbox unless the deployment
        # explicitly opts into production.
        self.client = Client(
            client_id=os.environ["PLAID_CLIENT_ID"],
            secret=os.environ["PLAID_SECRET"],
            environment=os.environ.get("PLAID_ENV", "sandbox"),
        )

    def create_link_token(self, user_id: str):
        """Create link token for Plaid Link"""
        response = self.client.LinkToken.create({
            "user": {"client_user_id": user_id},
            "client_name": "My App",
            "products": ["auth", "transactions"],
            "country_codes": ["US"],
            "language": "en"
        })
        return response["link_token"]

    def exchange_public_token(self, public_token: str):
        """Exchange a public token for an access token.

        The returned access token is a long-lived credential granting read
        access to the user's bank accounts. Store it encrypted, scoped to the
        user, never in logs, and revoke it when the user disconnects.
        """
        response = self.client.Item.public_token.exchange(public_token)
        return {
            "access_token": response["access_token"],
            "item_id": response["item_id"]
        }

    def get_accounts(self, access_token: str):
        """Get user's bank accounts"""
        response = self.client.Accounts.get(access_token)
        return response["accounts"]

    def get_transactions(self, access_token: str, start_date: str, end_date: str):
        """Get transactions for date range"""
        response = self.client.Transactions.get(
            access_token,
            start_date,
            end_date
        )
        return response["transactions"]

Financial Calculations

from decimal import Decimal, ROUND_HALF_UP
from datetime import datetime, timedelta

class FinancialCalculator:
    @staticmethod
    def calculate_interest(principal: Decimal, rate: Decimal, periods: int) -> Decimal:
        """Calculate compound interest"""
        return principal * ((1 + rate) ** periods - 1)

    @staticmethod
    def calculate_loan_payment(principal: Decimal, annual_rate: Decimal, months: int) -> Decimal:
        """Calculate monthly loan payment (amortization)"""
        monthly_rate = annual_rate / 12
        payment = principal * (monthly_rate * (1 + monthly_rate) ** months) / \
                  ((1 + monthly_rate) ** months - 1)
        return payment.quantize(Decimal('0.01'), rounding=ROUND_HALF_UP)

    @staticmethod
    def calculate_npv(cash_flows: list[Decimal], discount_rate: Decimal) -> Decimal:
        """Calculate Net Present Value"""
        npv = Decimal('0')
        for i, cf in enumerate(cash_flows):
            npv += cf / ((1 + discount_rate) ** i)
        return npv.quantize(Decimal('0.01'), rounding=ROUND_HALF_UP)

    @staticmethod
    def calculate_roi(gain: Decimal, cost: Decimal) -> Decimal:
        """Calculate Return on Investment"""
        return ((gain - cost) / cost * 100).quantize(Decimal('0.01'))

Fraud Detection

from sklearn.ensemble import RandomForestClassifier
import pandas as pd

class FraudDetectionService:
    def __init__(self):
        self.model = RandomForestClassifier()

    def extract_features(self, transaction: dict) -> dict:
        """Extract features for fraud detection"""
        return {
            "amount": transaction["amount"],
            "hour_of_day": transaction["timestamp"].hour,
            "day_of_week": transaction["timestamp"].weekday(),
            "merchant_category": transaction["merchant_category"],
            "is_international": transaction["is_international"],
            "card_present": transaction["card_present"],
            "transaction_velocity_1h": self.get_velocity(transaction, hours=1),
            "transaction_velocity_24h": self.get_velocity(transaction, hours=24)
        }

    def predict_fraud(self, transaction: dict) -> dict:
        """Predict if transaction is fraudulent"""
        features = self.extract_features(transaction)
        fraud_probability = self.model.predict_proba([features])[0][1]

        return {
            "is_fraud": fraud_probability > 0.8,
            "fraud_score": fraud_probability,
            "risk_level": self.get_risk_level(fraud_probability)
        }

    def get_risk_level(self, score: float) -> str:
        if score > 0.9:
            return "CRITICAL"
        elif score > 0.7:
            return "HIGH"
        elif score > 0.5:
            return "MEDIUM"
        else:
            return "LOW"

Regulatory Compliance

## PCI-DSS Compliance
class PCICompliantPaymentHandler:
    def process_payment(self, card_data: dict):
        # Never store full card number, CVV, or PIN
        # Tokenize card data immediately
        token = self.tokenize_card(card_data)

        # Store only last 4 digits and token
        payment_record = {
            "token": token,
            "last_4": card_data["number"][-4:],
            "exp_month": card_data["exp_month"],
            "exp_year": card_data["exp_year"]
        }

        return self.process_with_token(token)

    def tokenize_card(self, card_data: dict) -> str:
        # Use payment gateway tokenization
        return stripe.Token.create(card=card_data)["id"]

## KYC/AML Compliance
class ComplianceService:
    def verify_customer(self, customer_data: dict) -> dict:
        """Perform KYC verification"""
        # Identity verification
        identity_verified = self.verify_identity(customer_data)

        # Sanctions screening
        sanctions_clear = self.screen_sanctions(customer_data)

        # Risk assessment
        risk_level = self.assess_risk(customer_data)

        return {
            "verified": identity_verified and sanctions_clear,
            "risk_level": risk_level,
            "requires_manual_review": risk_level == "HIGH"
        }

Money movement guardrails

This skill documents code that charges cards, issues refunds, and reads bank accounts. Treat every such call as an irreversible side effect and gate it accordingly. A Snyk audit classifies this capability as W009 (direct money access); these are the controls that make it acceptable.

Credentials

• Load keys from a secrets manager or the environment; never inline, never in version control, never in logs or error messages.
• Use restricted keys with the narrowest scope the operation needs. A service that only creates charges must not hold a key that can issue payouts.
• Rotate on a schedule and on any suspected exposure; keep test and live keys in separate accounts so a misconfiguration cannot reach production funds.

Default to test mode

• Point at Stripe test keys and the Plaid sandbox unless the deployment has explicitly opted into production. Make the production switch a deliberate, reviewed configuration change, not a default.
• Fail closed when the environment is ambiguous rather than guessing.

Authorisation before execution

• Require an explicit human approval step for any operation that moves money, including refunds. An agent must not be able to complete a charge on its own initiative.
• Enforce per-transaction and per-window amount limits, and reject anything above them rather than clamping silently.
• Check authorisation per object, not per route: verify the caller owns the order, the payment intent, and the bank item being acted on.

Correctness under retry

• Every mutating call carries a caller-supplied idempotency key derived from the business action, so a retry cannot double-charge.
• Webhooks are delivered at least once: deduplicate on event id before acting.
• Verify webhook signatures and reject failures with 4xx. An unverified event must never reach business logic.

Money arithmetic

• Represent amounts as Decimal or integer minor units, never float.
• Round explicitly (ROUND_HALF_UP) when converting to minor units; truncation loses fractions of a cent and the loss compounds.
• Store the currency alongside every amount and refuse cross-currency arithmetic.

Audit and detection

• Log every financial operation with actor, amount, currency, idempotency key, and outcome, to append-only storage. Never log card data, access tokens, or full account numbers.
• Alert on refund spikes, repeated failures, and limit rejections; a log that triggers nothing protects nothing.
• Reconcile against the provider's records on a schedule; do not treat your own database as the source of truth for money that moved.

Best Practices

Security

• Never log sensitive financial data (PAN, CVV)
• Use tokenization for card storage
• Implement strong encryption (AES-256)
• Use TLS 1.2+ for all communications
• Implement rate limiting and fraud detection
• Regular security audits

Data Handling

• Use Decimal type for money (never float)
• Store amounts in smallest currency unit (cents)
• Implement idempotency for all transactions
• Maintain complete audit trails
• Handle timezone conversions properly

Transaction Processing

• Implement two-phase commits
• Use database transactions (ACID)
• Handle network failures gracefully
• Implement retry logic with exponential backoff
• Support transaction reversals and refunds

Anti-Patterns

❌ Using float for money calculations ❌ Storing credit card data unencrypted ❌ No transaction logging/audit trail ❌ Synchronous payment processing ❌ No idempotency in payment APIs ❌ Ignoring PCI-DSS compliance ❌ No fraud detection

Resources

• PCI-DSS: https://www.pcisecuritystandards.org/
• Stripe API: https://stripe.com/docs/api
• Plaid: https://plaid.com/docs/
• Open Banking: https://www.openbanking.org.uk/