Surface shallow modules as deepening candidates, then grill the chosen one interactively — depth/seam paradigm. Use fo.

Kit Context

Before starting this skill, ensure session boot is complete:

Read CODEBASE_MAP.md for project understanding
Read CLAUDE.project.md if it exists for project-specific rules
Read agent_docs/architecture-language.md — required. This skill uses its vocabulary exactly.
Read tasks/decisions.md if it exists — don't re-litigate ADR-resolved decisions.

If agent_docs/architecture-language.md hasn't been read in this session, read it now before proceeding.

When to Use

Invoke with /deepening-review when:

The user wants to improve architecture for testability or AI-navigability
A module cluster feels tightly coupled and hard to reason about
Tests exist but bugs keep escaping — suggesting tests sit past the wrong seam
Onboarding to a codebase and noticing pass-through modules that hide nothing
After /architecture-review flagged structural issues without suggesting concrete refactors

When NOT to Use

For SOLID violations / module health metrics → use /architecture-review
For specific code smells (long function, feature envy, etc.) → use /code-quality-audit then /refactoring-guide
For unused code → use /dead-code-audit
For projects under ~5 source files — depth analysis needs surface area

Paradigm vs `/architecture-review`

Lens	This skill (`deepening-review`)	`architecture-review`
Vocabulary	Module / Interface / Depth / Seam / Adapter / Leverage / Locality	SOLID, layers, coupling, cohesion
Output mode	Interactive — candidates, grilling loop	Report — structured assessment
Question	"Where can a shallow module become deep?"	"Where are SOLID principles violated?"
Test angle	The interface is the test surface — replace tests, don't layer	Inject dependencies, abstract behind interfaces

Run them together for breadth + surgical depth. They don't overlap — they speak different languages on purpose.

Hard Rules

Use architecture-language.md vocabulary when classifying architecture. Don't reach for "service," "component," "API," or "boundary" as substitutes for module, interface, or seam in your reasoning. (Descriptive uses — "Stripe is a third-party service," "across a network boundary" — are fine; what matters is not classifying your own parts that way.)
Don't propose interfaces in step 2 (Present Candidates). Only candidates. Interfaces come in step 3 (Grilling) or /interface-design.
Don't flag ADR-resolved decisions as candidates unless the friction is severe enough to warrant reopening the ADR. Mark such candidates explicitly.
Apply the deletion test to every shallow-looking module before listing it.
One adapter = hypothetical seam. Don't propose ports without two real adapters.

Process

Phase 1: Explore

Read existing context first:

CODEBASE_MAP.md — module map and conventions
tasks/decisions.md — past architectural ADRs (don't re-litigate)
Any project-specific domain glossary (e.g. agent_docs/project/domain.md if it exists)

If domain context files don't exist, proceed silently — don't flag their absence or insist on creating them.

Then walk the codebase. Use the Agent tool with subagent_type=Explore for organic exploration — don't follow rigid heuristics. Note where you experience friction:

Where does understanding one concept require bouncing between many small modules?
Where are modules shallow — interface nearly as complex as the implementation?
Where have pure functions been extracted just for testability, while the real bugs hide in how they're called (no locality)?
Where do tightly-coupled modules leak across their seams?
Which parts of the codebase are untested, or hard to test through their current interface?

Apply the deletion test to anything you suspect is shallow. A "yes, deletion concentrates complexity meaningfully" is the signal.

Phase 2: Present Candidates

Present a numbered list of deepening opportunities. For each candidate:

### Candidate N: [Short name using domain language]

**Files**: `path/to/a.ts`, `path/to/b.ts`, `path/to/c.ts`

**Problem**: [Where the shallowness shows. What complexity leaks across the seams now.]

**Solution**: [Plain English: what would be merged, where the new seam goes, what sits behind it.]

**Benefits (locality)**: [What concentrates if we deepen — bug fixes, change surface, knowledge.]

**Benefits (leverage)**: [What callers stop having to know.]

**Test impact**: [What tests survive, what gets deleted, what gets written at the new interface.]

**Dependency category**: [in-process / local-substitutable / remote-but-owned / true-external — see references/dependency-categories.md]

If a candidate contradicts an existing ADR, mark it: "Contradicts ADR-NNN — but worth reopening because [load-bearing reason]." Skip theoretical contradictions.

End the list with: "Which candidate would you like to explore? (or 'all' / 'none')"

Do not propose interfaces in this phase. Naming a deepened module is fine; specifying its method signatures is not.

Phase 3: Grilling Loop

Once the user picks a candidate, drop into an interactive grilling conversation. Walk the design tree with them.

Detail in references/grilling-protocol.md. Key points:

Surface constraints first (what must hold true)
Map dependencies to a category (see references/dependency-categories.md)
Explore what sits behind the seam
Identify which existing tests survive, which get deleted, which get written
Side effects happen inline — update domain glossary if a new term emerges; offer ADR if user rejects with a load-bearing reason; invoke /interface-design if the user wants to explore alternative interfaces.

Phase 4: Hand-off

When the grilling settles on a direction:

Summarize the agreed-upon shape (deepened module, seam location, adapter strategy, test strategy)
Write a refactoring task to tasks/todo.md using the kit's standard plan template (see agent_docs/workflow.md)
If the agreed shape requires interface design → suggest /interface-design next
If the user rejected the candidate with a reason future explorations should respect → write an ADR entry to tasks/decisions.md

Do not start implementing. Deepening is a multi-file refactor; it goes through Plan First → confirmation → execution per the kit's workflow.

Output Format

# Deepening Review

## Summary
[1-2 sentences: how many candidates, what kind of friction dominates.]

## Candidates

### Candidate 1: [name]
[Files / Problem / Solution / Benefits (locality) / Benefits (leverage) / Test impact / Dependency category]

### Candidate 2: [name]
[...]

## Out of scope (logged for later)
- [Issues noticed but not pursued — to tasks/todo.md → ## Not Now]


**Which candidate would you like to explore? (or 'all' / 'none')**

After user picks → switch to grilling format (free-form Q&A, see references/grilling-protocol.md).

Common Rationalizations

Rationalization	Reality
"It's clean — every module does one thing"	Shallow modules each doing one thing produce diffusion of complexity. The user has to bounce between N tiny files to understand one operation. Locality lost.
"We extracted these for testability"	The pure-function extraction is fine, but if the bugs hide in how the pure functions are composed — your test suite tests past the wrong seam. The interface is the test surface.
"It's a port-and-adapters architecture"	Check: how many adapters? One adapter means a hypothetical seam — just indirection. Two means a real seam.
"We can't deepen, the modules are owned by different teams"	Then the seam is correctly placed at a team boundary. Don't deepen across teams; do deepen within.
"Refactoring is too risky"	Deepening that removes a shallow layer is one of the safest refactors — fewer abstractions to maintain. The risky one is deepening the wrong module by guessing where leverage lives. The grilling loop exists to prevent that.
"The architecture is already documented"	Documentation describes; the deletion test asks if the documentation is load-bearing. A module no one would miss isn't deep just because it's named.

Notes

This skill speaks a deliberately narrow vocabulary. If you find yourself reaching for "service," "boundary," or "component" — re-read agent_docs/architecture-language.md.
Inspired by John Ousterhout (deep modules), Michael Feathers (seams), and the improve-codebase-architecture skill by Matt Pocock.
Pairs well with /interface-design for the chosen candidate's interface, and /refactoring-guide for the tactical execution sequence.

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