backend

Place the route and DTO validation in the API layer, orchestrate the flow in an application-layer use case, keep pricing logic in the pure domain BillingCalculator, and access data through a Prisma adapter behind a repository port. Add unit and integration tests before considering the task done.

Use decimal.js for all monetary math; never use Number or float for final amounts. Compute amount as quantity divided by unitSize times unitPrice, round totalAmount to two decimals, and store a price snapshot (unitPrice, unitSize, currency, amount) on each billing item.

Do not use it for frontend/src/ UI tasks (use the frontend role), prose documentation updates (techwriter role), running or diagnosing tests without code changes (testing role), or reviewing a finished diff (reviewer role).

No. Prisma models must not leak into domain or application types; a mapper converts them at the infrastructure boundary. Express routes also must never call Prisma directly for business operations.

Repeated usage with the same clientAccountId and sessionId must not create duplicate charges. Usage and the billing result are saved in a single transaction, and stored billing results are never recalculated with current prices.

Stop and consult the architect when a task requires changing a business rule such as the pricing formula, an error code, or the idempotency policy, or when it would change expected values of existing tests.