deepen — Claude Code skill: what it does, how to install

---
name: deepen
description: |
  Optional design-improvement pass for when you have spare usage to drain. Finds
  the shallowest modules in the code the spec touches, researches a deeper
  design, and proposes refactors that shrink interfaces and hide decisions —
  behavior held constant, tests green before and after. Proposes §I/§V/§T edits,
  never silent rewrites. Triggers when the user says "deepen this", "improve the
  design", "this module feels shallow", "pull complexity down", "use spare
  budget on the codebase", or invokes /ck:deepen. Leans on the codebase-design
  skill's deep-module vocabulary when present.
---

# deepen — make modules deep

**Behavior is sacred: tests green before AND after. Every change shrinks an interface or hides a decision — deepen, don't churn.**

A **deep module** hides a lot behind a small interface; a **shallow** one's
interface costs as much to use as writing the code yourself. Complexity =
dependencies + obscurity, and it compounds. Deepen spends spare usage paying
that down *before* it becomes a §B. Run it when the build is green & you have
budget to drain — not under deadline.

## WHEN TO DEEPEN

- Build is green, tests pass, & you have token budget spare.
- A module's interface feels as complex as its implementation (shallow smell).
- The same change keeps touching many files (change amplification).
- User explicitly asks to improve design quality.

⊥ run mid-feature or under pressure. Deepen is the deliberate pass, not the reflex.

## FIVE STEPS

### 1. PICK THE SHALLOW
Scan the modules the spec touches. Rank by shallowness — interface surface vs
work done. Pick the **one** worst offender. Tells:
- Pass-through method that only forwards to one other (shallow layer).
- Caller must set 5 flags right to use it (config leakage).
- Same abstraction repeated at two layers (no information hiding).
- A `?` or §B that traces back to a confusing interface.

One module per pass. Deepening is surgical, ⊥ a codebase sweep.

### 2. DIAGNOSE
Name the design defect in caveman, citing file:line:
> src/auth/token.go: 6-arg ctor leaks rotation policy to every caller. shallow.
Complexity is real only if it shows: change amplification, high cognitive load,
or an unknown-unknown (caller must know a hidden fact to call it right).

### 3. RESEARCH THE DEEPENING
What does a deeper version look like? Pull a known pattern (hand to **research**
for the external case → §R) or derive from the codebase's own better modules.
Moves that deepen:
- **Pull complexity down** — hide the hard part inside, give callers the simple path.
- **Define errors out of existence** — design the interface so the edge can't occur.
- **Information hiding** — one decision, one module; callers don't learn it.
- **General-purpose interface** over a pile of special-case methods.

### 4. PROPOSE
Draft the change as spec edits, not a silent rewrite:
- New/simpler §I shape for the module.
- §V that locks the deepened invariant so a future build can't re-shallow it.
- §T refactor row(s), each citing the §V/§I it serves.
Hand to **spec** to write. Show the before/after interface so the user sees the shrink.

### 5. VERIFY BEHAVIOR HELD
Refactor ≠ rewrite. Full suite green before you start AND after. A deepening that
changes behavior is a feature in disguise — stop, route through `/spec` + `/build`.
New interface gets a test proving the old callers still work.

## WHEN TO STOP

Done when the chosen module's interface is strictly smaller, its hidden decision
no longer leaks, tests are green, and §I/§V record the new shape. One module
deepened beats five churned. Budget left → pick the next shallowest, fresh pass.

## BOUNDARIES

- ⊥ change behavior. Green before, green after. Pure structure.
- ⊥ write SPEC.md. Propose §I/§V/§T; spec writes.
- ⊥ deepen more than one module per pass.
- ⊥ run under deadline or mid-feature. This is the spare-budget pass.
- ⊥ add abstraction for single-use code. A deep module earns its hiding; a speculative one is just more surface.