Module Depth Review

Module depth review

Find shallow modules. Propose deep ones.

A shallow module has an interface nearly as complex as its implementation — it forces callers to learn a lot for little behavioural payoff. A deep module hides substantial behaviour behind a small interface. This skill audits a region of the codebase against that lens and proposes consolidations.

This is an audit skill. It produces findings and proposals — never edits. Pair with audit-vs-fix-discipline for the output discipline.

When to use

Apply when the user asks about module boundaries, file organisation, or "is this over-modularised?". Concrete triggers:

• "Is this folder doing too much / too little?"
• "Should these be one module?"
• "Too many small files for one concept"
• "The interface is as complex as the implementation"
• "Review the architecture of X" (where X is a feature or subsystem, not the whole app)

Not the right skill for:

• Tactical complexity inside a function — use simplify
• Removing dead code or orphans — use cleanup
• Whole-app architecture decisions — use the architect agent
• Pre-merge multi-file review — use architect-reviewer

Vocabulary discipline

This skill uses precise terms. Do not substitute "component," "service," "API," "boundary," or "layer" — those words drift in meaning across teams and obscure the question being asked.

The full glossary lives in @language.md. The short version:

• Module — anything with an interface and an implementation. Scale-agnostic (function, class, package, slice).
• Interface — everything a caller must know to use the module correctly. Includes signature, invariants, ordering, error modes, perf characteristics. Wider than "API".
• Depth — leverage at the interface. Lots of behaviour per unit of interface a caller must learn = deep. Interface nearly as complex as implementation = shallow.
• Seam — where the interface lives. Distinct from what sits behind it.
• Adapter — a concrete thing that satisfies an interface at a seam.
• Leverage — what callers get from depth.
• Locality — what maintainers get from depth (change concentrates in one place).

Use these exact words in findings. Vocabulary drift is the failure mode.

Process

1. Frame the region

Confirm with the user what region is in scope: a folder, a subsystem, a feature. Don't audit the whole codebase. If unclear, ask.

2. Walk the region

Read the files in scope. For each module, ask the deletion test:

If I deleted this module and inlined its contents into the call sites, would complexity vanish (it was a pass-through) or reappear distributed across N callers (it was earning its keep)?

If complexity vanishes → it's a shallow module candidate for consolidation. If complexity reappears → it's earning its keep. Leave it.

Also flag:

• Concepts scattered across many small modules that mostly call each other
• Interfaces with as many methods/parameters as the implementation has lines
• Functions extracted purely for test coverage with no true locality benefit
• Tight coupling that leaks across a seam (a "boundary" the implementation routinely bypasses)
• Single-adapter ports — an interface with only one implementation is hypothetical, not a real seam

3. Categorise dependencies

For each shallow-module candidate, classify its dependencies — this determines the consolidation shape and testing strategy. See @dependency-categories.md for the four categories (in-process, local-substitutable, remote-owned, true-external) and the testing strategy each one implies.

Without this step, "just merge them" recommendations skip the question of how the deepened module gets tested.

4. Produce findings

Output template — markdown, no HTML reports:

## Module depth review: <region>

### Candidates for deepening

#### 1. <one-line title>

**Strength:** Strong | Worth exploring | Speculative
**Dependency category:** in-process | local-substitutable | remote-owned | true-external

**Files involved:**
- `path/a.ts`
- `path/b.ts`
- `path/c.ts`

**Current shape:**
Three shallow modules. `a.ts` exposes `parseFoo`, `b.ts` exposes `validateFoo`, `c.ts` exposes `persistFoo`. Every caller invokes all three in sequence; the only callers of each are the others plus one top-level entry point. Interfaces total 9 functions; implementations total ~80 lines.

**Deletion test result:**
Inlining each into the single top-level caller eliminates ~60 lines of glue and the 9-function surface area. Complexity doesn't reappear elsewhere — it was glue, not encapsulation.

**Proposed deep module:**
One module exposing `processFoo(input): Result`. Three private helpers inside; one public entry point.

**Wins:**
- **Leverage:** callers learn one function instead of three.
- **Locality:** changes to the parse/validate/persist sequence happen in one place.
- **Test surface:** one interface to assert against, not three.

**Diagram:**

```mermaid
graph LR
    subgraph Before
        A[parseFoo] --> B[validateFoo] --> C[persistFoo]
    end
    subgraph After
        P[processFoo]
    end

2.

Shallow modules that are actually earning their keep

(Optional section — name modules that looked shallow but passed the deletion test. Useful when the user expected them flagged.)

Top recommendation

Of the N candidates above, #X is the strongest because <reason rooted in leverage/locality>. The others are worth exploring once #X lands.

Want me to dig deeper into any candidate? Specify by number. Or want me to draft the consolidation as a fix (separate task)?

If no candidates found, say so. Empty findings are valid — don't manufacture shallow modules.

## Calibration

A clean review is a valid outcome. The codebase is allowed to be well-shaped. Inventing candidates to seem thorough is the failure mode this skill exists to prevent.

**Confidence gate: ≥80% sure the consolidation would improve the codebase, or downgrade to "Speculative" or drop entirely.**

**Pre-finding checks:**

1. Have I actually read the call sites of each module in the candidate?
2. Can I name the leverage and locality wins concretely, not in the abstract?
3. Does the deletion test point clearly at "complexity vanishes" or am I guessing?
4. Have I categorised the dependencies, so the consolidation has a testing strategy?

If any answer is "no" → drop the candidate or move it to Suggested follow-ups.

**Do NOT flag:**

- Modules that look small but pass the deletion test (complexity would reappear)
- Single-file utilities that genuinely have no related siblings to merge with
- "Could be deeper" without naming what behaviour would move behind the interface
- Folder organisation preferences ("these should be in `lib/` not `utils/`")
- Anything you can't tie to a concrete leverage or locality win

## Seam discipline

When proposing a deep module that crosses a network or external-service boundary:

- **One adapter is a hypothetical seam. Two adapters is a real one.** Don't propose a port unless at least two adapter implementations are justified (typically a production adapter and a test adapter, or two production targets).
- **Internal seams stay internal.** A deep module can have private seams used by its own tests. Don't expose them at the public interface just because tests use them.
- **The interface is the test surface.** If you'd want to test past the proposed interface, the module is shaped wrong — adjust the proposal.

## Optional: design it twice

For high-impact candidates (Strong strength, large surface area, or contentious shape), explicitly explore alternative designs before recommending one. Approaches:

- **Minimise the interface** — what's the smallest possible API that still does the job?
- **Maximise flexibility** — what if every caller wanted to vary one knob?
- **Optimise the common case** — what's the shape if 90% of calls take one path?
- **Port-and-adapter** — if a network/external dep is involved, what does the production-vs-test adapter split look like?

Present 2-3 designs, contrast them on depth/locality/seam placement, then make a recommendation. Don't always do this — for clear-cut cases it's overhead.

## Red flags — STOP and re-anchor

- About to recommend consolidating modules without checking the call sites
- About to say "this is a god module" without applying the deletion test (god modules can also be deep — the question is whether the interface is small)
- About to introduce a port with one adapter (that's not a seam, that's indirection)
- About to use the words "component", "service", "API", or "boundary" instead of the precise term
- About to bundle the audit and a fix into one response
- About to manufacture a candidate because the report looked empty

**Each of these means: stop. Re-read @language.md or @dependency-categories.md before continuing.**

## Per-project override

Before reviewing, check the project root for `REVIEW.md` (general review policy) or `ARCHITECTURE.md` (project-specific architectural conventions). If present, they override this skill's defaults. Common overrides:

- "This codebase deliberately uses many small modules — flag only depth issues at the package level."
- "Ports and adapters are not used here; propose direct dependencies instead."
- "Don't propose consolidations across these specific folders (they're owned by different teams)."

## When NOT to use

- The user wants tactical complexity reduction inside one function — use `simplify`.
- The user wants to remove dead code or orphans — use `cleanup`.
- The user wants whole-app architectural decisions (which database, which framework) — use the `architect` agent.
- The user has already authorised fixes (`audit and consolidate the X folder`) — the audit discipline doesn't apply, but the lens and vocabulary still do.