fintech-engineer

Provides expert guidance on building financial technology systems with proper accounting principles, regulatory compliance, and high-precision calculations. Specializes in ledger design, payment processing architectures, and financial data integrity.

When to Use

• Designing double-entry ledger systems or accounting databases

• Implementing high-precision financial calculations (avoiding floating-point errors)

• Building payment processing pipelines

• Ensuring PCI-DSS or SOX compliance

• Integrating with banking APIs (Plaid, Stripe, etc.)

• Handling currency conversions and multi-currency systems

• Implementing audit trails for financial transactions

• Designing reconciliation systems

Quick Start

Invoke this skill when:

• Building ledger or accounting systems

• Implementing financial calculations requiring precision

• Designing payment processing architectures

• Ensuring regulatory compliance (PCI, SOX, PSD2)

• Integrating banking or payment APIs

Do NOT invoke when:

• General database design without financial context → use /database-administrator

• API integration without financial specifics → use /api-designer

• Generic security hardening → use /security-engineer

• ML-based fraud detection models → use /ml-engineer

Decision Framework

Financial Calculation Needed?
├── Yes: Currency/Money
│   └── Use decimal types (never float)
│   └── Store amounts in smallest unit (cents)
├── Yes: Interest/Rates
│   └── Use arbitrary precision libraries
│   └── Document rounding rules explicitly
└── Ledger Design?
    ├── Simple: Single-entry (tracking only)
    └── Auditable: Double-entry (debits = credits)

Core Workflows

1. Double-Entry Ledger Implementation

• Define chart of accounts (assets, liabilities, equity, revenue, expenses)

• Create journal entry table with debit/credit columns

• Implement balance validation (sum of debits = sum of credits)

• Add audit trail with immutable transaction logs

• Build reconciliation queries

2. Payment Processing Pipeline

• Validate payment request and idempotency key

• Create pending transaction record

• Call payment processor with retry logic

• Handle webhook for async confirmation

• Update ledger entries atomically

• Generate receipt and audit log

3. Precision Calculation Setup

• Choose appropriate numeric type (DECIMAL, NUMERIC, BigDecimal)

• Define scale (decimal places) based on currency

• Implement rounding rules per jurisdiction

• Create calculation helper functions

• Add validation for overflow/underflow

Best Practices

• Store monetary values as integers in smallest unit (cents, paise)

• Use DECIMAL/NUMERIC database types, never FLOAT

• Implement idempotency for all financial operations

• Maintain immutable audit logs for every transaction

• Use database transactions for multi-table updates

• Document rounding rules and apply consistently

Anti-Patterns

| Using floats for money | Precision errors accumulate | Use decimal types or integer cents

| Mutable transaction records | Audit trail destroyed | Append-only logs, soft deletes

| Missing idempotency | Duplicate charges possible | Idempotency keys on all mutations

| Single-entry for auditable systems | Cannot reconcile or audit | Double-entry with balanced journals

| Hardcoded tax rates | Compliance failures | Configuration-driven, versioned rules