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.