The mock API and the real one answer at the same path

The backend team says the orders endpoint lands in two sprints. You need it today.

You write orders/[id].ts in the exec instance named exec-test and return the JSON you agreed on. Your React app points at it. When the real handler is ready, it lands in exec-1 at the same path, and you change one hostname. The payload shape is already battle-tested because the UI has been eating it for two weeks.

Need the mock to flake like production will? A pre.ts beside it returns a 503 every fifth request and skips the main script. No mock-server library, no MSW fixtures, no second process to keep running.

The contract ships before the backend, at the URL the backend will have.