Instruction file imported from kassiwin/checkout-legacy (.cursor/rules/architecture.mdc). Copyright stays with the author.

Architecture checkout-service

Target described in docs/ARCHITECTURE.md. Every proposal must comply with it.

Non-negotiable rules

• A single source of truth for price. Price calculation lives in src/domain/pricing.ts. No other file recomputes a total, VAT, a discount or shipping.
• Transport holds no business logic. src/server.ts parses, routes, serializes. Nothing else.
• The domain is pure. No I/O, no console.log, no clock, no Date.now() in src/domain/. These dependencies are injected.
• Typed errors. Never throw "message".

// ❌ BAD
if (!body.customerId) throw "customerId missing";

// ✅ GOOD
if (!body.customerId) throw new ValidationError("customerId missing");

• No any in a public signature. Model the cart, the customer, the quote.

Business rules to preserve

• Stacked discounts are capped at 50% of the subtotal.
• VAT applies after discount, on the amount excluding shipping.
• A quote and its corresponding order must produce the same total.