| name | setup-ts-deep-modules |
| description | Wires dependency-cruiser into a TypeScript repo so each package is a deep module — implementation hidden in subfolders, reachable only through entry-point files. Use when the user wants deep-module enforcement or dependency-cruiser setup. |
| disable-model-invocation | true |
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), run the /mattpocock:codebase-design skill — 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:
- 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.
- Intra-package freedom — a package's own files import each other freely.
- 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.
- 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.
- Run
lint:boundaries. It must pass on the clean example.
- 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.
- 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.
