All Guides, Organized by Category
Every reference guide Luhnly publishes, grouped by what you're actually trying to test.
Payment Processors
Official test cards and sandbox behavior for the five processors developers integrate with most.
Test Scenarios
The specific failure conditions your checkout flow needs to handle — declines, insufficient funds, expired cards, and verification mismatches.
Declined Transactions
Generic declines, fraud blocks, and do-not-honor responses
Insufficient Funds
The single most common real-world decline reason
Expired Cards
Testing date-validation logic specifically
CVV / AVS Mismatch
Verification-check failures, independent of the card number itself
3D Secure
Why Luhn validity alone can't test SCA authentication
Concepts
The underlying mechanics — how card numbers are actually structured and validated, and how to handle test data responsibly.
Card Brands
Network-specific formatting rules that trip up validators built around one network's assumptions.