Invariant is an open-source CLI engine that continuously tests Stripe & Razorpay webhooks against real-world edge cases (duplicate deliveries, out-of-order refunds, tampered HMAC signatures) and mathematically proves database state post-conditions in under 10 seconds. - 🔒 100% Offline & Private (localhost) - ⚡ Zero Telemetry & Instant Local Execution - 🛡️ Works with Node.js, Python, Go, Java, Ruby Try it: npx @yavona/invariant init