Agent instructions — rules met at the moment they apply

The instruction file is the context an agent carries into every session: AGENTS.md, CLAUDE.md, or both. Below it is called the entry file, because it is where every session enters and what routes the rest. This skill is how to write one for an app repository and keep it working as the product grows. It was learned on SaaS repositories built with coding agents on Next.js + Supabase + Clerk + Netlify — a starter repository and the products built from it — and on a content repository run day to day by agents, whose instructions had grown to hundreds of thousands of words of doctrine. The method is stack-independent; the examples are from those repositories, named by kind.

A rule written down is not yet a rule an agent follows: it must be met at the moment it applies. Everything below follows from that sentence:

1. The entry file routes; the depth lives where the moment sends you. A rule the agent is not looking at when it acts does not exist for that act.
2. A rule whose compliance depends on the agent remembering it at the decisive moment becomes a code path — a test, a script, a required argument — because prose loses that contest.
3. The file is maintained, not accumulated: a pass at every release, and a fix at every mistake, with the receipt.

The shape: a chain, not a manual

entry file (loaded every session)   — routes; holds only what every session needs
 └─ router rows: "when you are…"    — the rule that is easy to miss, and what to open
     └─ operative files (docs/, skills) — the procedure, the depth, the gotchas
         └─ evidence                — receipts, tables, postmortems, case studies