Setup TS Deep Modules

Setup TS Deep Modules

Make every package in this repo a deep module: a lot of behaviour behind a small interface. A package's public surface is its entry points — the files at the package root — and everything in its subfolders is hidden. This skill installs dependency-cruiser and the rules that make the entry points the only way in, then proves the rules bite.

For the vocabulary (deep module, interface, seam, depth), call the Skill tool with "codebase-design" — use its language throughout.

The shape this enforces

src/packages/
  <name>/
    index.ts        ← an entry point (public). Import this from outside.
    client.ts       ← another entry point. Packages may expose SEVERAL.
    lib/            ← implementation: hidden from outside, free to import each other.
    tests/          ← co-located tests + fixtures (a subfolder, so private).

The public surface is the package's root files — not one designated index.ts. By convention implementation lives in lib/ and tests in tests/, giving every package the same two-folder shape. The rule itself is general, though: anything in any subfolder is private, so you never extend the config to add a folder.

Four rules, all error:

1. Entry-point boundary — code outside a package (app code or another package) may import only that package's entry points (its root files), never anything in its subfolders.
2. Intra-package freedom — a package's own files import each other freely.
3. Tests through the entry points — files under <pkg>/tests/ may import any package's entry points and their own tests/ fixtures, but never any package's subfolder internals (not even their own). Integration tests across packages are fine; deep imports are not.
4. No cycles — no dependency cycles.

Entry points, not a barrel. Because the public surface is every root file, a package can expose several small entry points (index.ts, client.ts, server.ts) instead of funnelling everything through one giant index.ts. Barrel files that re-export a whole subtree are discouraged — keep entry points small and hide implementation in subfolders.

Layering (which packages may depend on which) is a different concern and is left as a commented stub in the config for this repo to fill in.

Steps

1. Detect the environment

• Package manager — pnpm-lock.yaml → pnpm, yarn.lock → yarn, bun.lockb → bun, else npm. Use it for every command below (pnpm/yarn/npm run/bunx).
• Packages root — if src/ exists use src/packages, else packages. Confirm the choice with the user if the repo already has a different obvious convention.
• Existing config — check for a .dependency-cruiser.* file. If one exists, do not overwrite it: merge the four rules and the options in, and tell the user what you added.

Done when: package manager, packages root, and existing-config status are all known.

2. Install dependency-cruiser

Install dependency-cruiser as a devDependency with the detected package manager.

Done when: dependency-cruiser is in devDependencies.

3. Write the config

Copy dependency-cruiser.config.cjs to the repo root as .dependency-cruiser.cjs. Set PACKAGES_ROOT to the root detected in step 1. The rules are path-depth based and extension-agnostic, so nothing else needs adapting.

Done when: .dependency-cruiser.cjs exists with the correct PACKAGES_ROOT, and the four forbidden rules are present.

4. Wire it into the checks

• Add a lint:boundaries script: depcruise <packages-root> (or depcruise src).
• Fold it into the repo's umbrella check command — the one that already runs typecheck (e.g. a check / ci / validate script). Do not touch tsconfig or add path aliases.
• If there is no umbrella script, add lint:boundaries and tell the user to include it in CI.

Done when: lint:boundaries exists and runs as part of the same command as typecheck.

5. Scaffold the example package

Create a committed <packages-root>/example/ as a copy-me template:

• index.ts — an entry point. Export one function that delegates to an internal file (so the package is visibly deep, not a pass-through).
• lib/impl.ts — an internal file in a subfolder, imported by index.ts, not reachable from outside.
• tests/example.test.ts — imports only ../index (an entry point), and asserts against the public function.

Tell the user this is a starter template to copy or delete.

Done when: the example package exists, exposes its behaviour through a root entry point, and hides impl in a subfolder.

6. Prove the rules bite

This is the completion criterion for the whole skill — a config that doesn't fail on a violation is worthless.

1. Run lint:boundaries. It must pass on the clean example.
2. Temporarily add a deep import to tests/example.test.ts (e.g. import { thing } from "../lib/impl"). Run lint:boundaries again — it must fail with tests-through-entrypoints.
3. Revert the deep import. Run once more — it must pass.

Done when: you have observed a pass, then a fail on the deep import, then a pass again. If step 2 does not fail, the rules are not wired correctly — fix before finishing.

7. Document the convention

Write a README.md in the packages folder (<packages-root>/README.md) — next to the packages it governs — covering: the src/packages/<name>/ layout (entry points at the root, lib/ for implementation, tests/ for tests), "import only through a package's entry points (its root files)", and how to run lint:boundaries. Discourage barrel files explicitly — expose several small entry points instead of re-exporting a whole subtree through one index. Keep it to the copy-me snippet plus the four rules in one paragraph each.

Then add a context pointer to it from the repo's agent-instructions file — CLAUDE.md if present, else AGENTS.md (create AGENTS.md if neither exists). One line is enough, e.g. Packages are deep modules — see [src/packages/README.md](./src/packages/README.md) before adding or importing one. This is what makes an agent discover the boundary rule instead of tripping over it.

Done when: <packages-root>/README.md exists and discourages barrels, and the repo's CLAUDE.md/AGENTS.md links to it.

Notes

• The config's $1 back-references (dependency-cruiser's group matching) are what let a package reach its own internals while outsiders can't — don't flatten them into separate per-package rules.
• Public vs private is decided by depth: a package's root files are entry points; anything in a subfolder is private. The conventional subfolders are lib/ (implementation) and tests/, but the rule doesn't hardcode them — any subfolder is private, so a new folder never needs a config change. Adding an entry point is just adding a root file — no barrel.
• Packages are flat: one tier of immediate children under the root. A package's internals may nest as deep as you like; a package may not contain another package.
• Use .cjs (not .js) so the config's module.exports works even in "type": "module" repos.