Improve Codebase Architecture

/improve-codebase-architecture

Explore a codebase like an AI would, surface architectural friction, and propose module-deepening refactors as GitHub issue RFCs.

Structured execution spec: protocol.yon. Read it for the canonical rules and step sequence; this file is explanation. The two must stay in sync — if you edit one, update the other and refresh the @STAMP date.

A deep module has a small interface hiding a large implementation. Deep modules are more testable, more AI-navigable, and let you test at the boundary instead of inside. (John Ousterhout, A Philosophy of Software Design)

The deepening test: would merging these modules produce a smaller combined interface than they have separately? If no, it's not a deepening opportunity — it's just reorganization.

Reference — Dependency Categories

Classify each candidate's dependencies; the category determines the entire testing strategy.

1. In-process — pure computation, in-memory state, no I/O. Always deepenable; merge and test directly.
2. Local-substitutable — dependencies with local test stand-ins (PGLite for Postgres, memfs for filesystem). Deepenable if the stand-in exists; test the merged module with the stand-in running in the suite.
3. Remote but owned (Ports & Adapters) — your own services across a network boundary. Define a port at the boundary; the deep module owns the logic, transport is injected. Tests use an in-memory adapter; production uses the real one.
4. True external (Mock) — third-party services you don't control (Stripe, Twilio). Mock at the boundary; the deepened module takes the external dependency as an injected port, tests provide a mock.

Reference — Testing Strategy

Replace, don't layer. Write new tests at the deepened module's interface boundary. Old unit tests on the now-merged shallow modules become redundant — delete them. Tests assert on observable outcomes through the public interface, not internal state, so they survive internal refactors.

Step 1 — Explore the codebase

Navigate organically. Note where you experience friction: where understanding one concept requires bouncing between many small files; where modules are so shallow the interface is nearly as complex as the implementation; where pure functions were extracted just for testability while the real bugs hide in how they're called; where tightly-coupled modules create integration risk at the seams; which parts are untested or hard to test. The friction you encounter IS the signal — don't follow rigid heuristics.

If you find no strong candidates, say so directly. A thin adapter over a stable external API is correct design, not a problem. Don't manufacture candidates — tell the user what you found and why the codebase resists the pattern.

Step 2 — Present candidates

⚠️ Deepening test: before listing a candidate, verify merging produces a smaller interface than the modules have separately. If not, it's reorganization — skip it.

For each genuine candidate show: Cluster (modules/concepts involved), Why they're coupled (shared types, call patterns, co-owned concept), Dependency category (one of four, one-sentence rationale), Test delta (specific test files that would be deleted, and the new boundary tests that replace them). Then ask: "Which of these would you like to explore?"

Step 3 — User picks a candidate

Step 4 — Frame the problem space

Write a user-facing explanation of the chosen candidate: the constraints any new interface must satisfy, the dependencies it must handle (with category), and a rough illustrative code sketch to make the constraints concrete (a grounding device, not a proposal). Show this, then immediately proceed to Step 5 — the user reads while designs are worked.

Step 5 — Design multiple interfaces

Produce 3–4 radically different interface designs, each under a different constraint:

• A — Minimal: 1–3 entry points max. Ruthlessly hide everything.
• B — Flexible: support many use cases and extension points; maximize what callers can customize.
• C — Caller-optimized: make the most common call site trivial; ergonomics over generality.
• D — Ports & Adapters (if applicable): design around injected adapters for cross-boundary dependencies.

Each design produces: (1) interface signature, (2) usage example, (3) what complexity it hides, (4) dependency strategy per category, (5) trade-offs.

⚠️ Don't mistake extraction for deepening. If a design has the same total interface surface as the original modules combined, it's a rename. The interface must get smaller.

If sub-agents are available (Claude Code / Cowork): spawn 3–4 in parallel with the brief template below. Do not share prior designs between agents — isolation is the point. If not (Claude.ai): work each constraint sequentially, treating each design as complete and committed before the next — contamination defeats the purpose.

## Design Brief

### Context
Codebase: [repo/language]
Candidate: [module cluster name]

### Files involved
[file path] — [one-line summary]

### Coupling description
[Why these modules are tightly coupled — shared types, call patterns, co-owned concepts]

### Dependency category
[In-process / Local-substitutable / Remote but owned / True external] — Rationale: [one sentence]

### What the deepened module should hide
[Internal state, wiring, error handling, retry logic, etc.]

### What it must expose
[The minimum surface callers actually need]

### Your design constraint
[One of: Minimal / Flexible / Caller-optimized / Ports & Adapters]

### Output format required
1. Interface signature  2. Usage example  3. What complexity it hides  4. Dependency strategy  5. Trade-offs

After all designs are presented, compare them in prose and give an opinionated recommendation — which is strongest and why, or a hybrid. The user wants a strong read, not a menu.

Step 6 — User picks an interface (or accepts the recommendation)

Step 7 — Create GitHub issue

Draft the RFC using the template below. Render the complete draft as a markdown block in the conversation, then ask: "Should I create this issue now?" Wait for explicit confirmation before running gh issue create. If gh is unavailable, print the final draft for manual paste.

## Problem
- Which modules are shallow and tightly coupled
- What integration risk exists at the seams
- Why this makes the codebase harder to navigate and maintain

## Proposed Interface
- Interface signature (types, methods, params)
- Usage example showing how callers use it
- What complexity it hides internally

## Dependency Strategy
[Category name] — [one sentence on how deps are handled]
- Production: [real adapter / merged module / mock boundary]
- Tests: [stand-in / in-memory adapter / mock implementation]

## Testing Strategy
- New boundary tests to write: [behaviors to verify at the interface — inputs and expected outcomes]
- Old tests to delete: [specific test file names and why they become redundant]
- Test environment needs: [stand-ins, adapters, or fixtures required]

## Implementation Recommendations
[Durable architectural principles, not file-path instructions.]
- What the module should own (responsibilities)
- What it should hide (implementation details)
- What it should expose (the interface contract)
- How callers should migrate to the new interface

⚠️ "Old tests to delete" must name specific test files. If you cannot, return to Step 2 and find them — a vague entry leaves dead test weight behind.

⚠️ "Implementation Recommendations" must be file-path-agnostic. Write principles a future engineer can apply even after the codebase is reorganized.

Human output. This skill's handler-facing output obeys the human-output contract (human-output/SKILL.md).

Self-improvement. On completion, run the Self-Improvement Protocol (self-improve/SKILL.md): if this run surfaced a concrete, blocking-or-recurring weakness in this skill, propose a specific fix for the handler to approve. Conservative — silent otherwise. Never auto-apply.