Test harness — so a green gate means something

A suite that passes against the wrong server, the wrong database, a fixture on the wrong branch or a developer's .env reports green and proves nothing. This skill is how the harnesses are arranged so that each one makes something real, how the gate is run and reported, and the rules — each from a bug, with its receipt — that keep a green run honest.

The rules come from two kinds of product, named here only by kind. Most were learned on web apps built on Next.js (App Router), Supabase Postgres and Clerk, tested with Vitest and Playwright (a live Q&A product, a monitoring product and others). The two-halves harnesses and several rules from bugs — marked the remote-control product where they appear — were learned on a relay service with a web client and an editor extension with desktop installers that drives a CLI agent. Each states its principle first and the mechanism second: on another stack, keep the principle and swap the mechanism. The product's own instructions come first — where they name its scripts, folders or coverage file, follow them.

The one question: what does each harness fake?

A bug survives exactly where every harness fakes the same side. Coverage of surfaces is not coverage of journeys. Before adding a suite, write down what it makes real; if everything else already fakes the same side, it mostly re-covers ground.