Your agent can't test the discount without a database

The pricing rule lives inside the database handler, three lines of math between two queries, so its only test needs a Postgres container, migrations and seed data. Nobody runs it before pushing, not you, not the agent. Pull the rule out: a core that knows nothing about databases, talking through ports like CartRepository. Postgres plugs in from outside; in tests an in-memory adapter does, and a thousand runs take seconds. Rules in the middle. Machines at the edges.

Comment below: Can you test your core without a database?

Brakes: a series on the checks that let an AI coding agent go fast without shipping bugs. Every best practice shortens the time between writing a bug and finding it. Brakes are why race cars go fast. All code is real TypeScript on Node 24, typechecked and tested before it went on screen.