Improve Codebase Architecture

---
name: improve-codebase-architecture
description: Scan a codebase for deepening opportunities, present them as a visual HTML report, then grill through whichever one you pick.
disable-model-invocation: true
version: 1.2.1
source: mattpocock/skills@1.2.3 (improve-codebase-architecture)
---

# Improve Codebase Architecture

Surface architectural friction and propose **deepening opportunities** — refactors that turn shallow modules into deep ones. The aim is testability and AI-navigability.

Built on a shared design vocabulary: run `/codebase-design` first and use its terms and principles exactly in every suggestion.
Where the project keeps a domain glossary (an `AGENTS.md` glossary section or `CONCEPTS.md`), its terms give names to good seams.

## Process

### 1. Explore

**Scope before you scan — YAGNI.** Deepening a module pays off by making future changes to it easier, so put extra weight on the parts of the codebase that have recently changed. Decide *where* to look before you look:

- If the user named a direction — a module, a subsystem, a pain point — take it, and skip the inference below.
- Otherwise, walk back a good stretch of the commit history (`git log --oneline`) to find the codebase's hot spots — the files and areas that keep coming up — and let those paths pull your attention first. If the changes are scattered with no clear hot spot, widen the net.

Read the project's domain glossary first, then — for whichever of the two stores the loop config enables — search `docs/adr/` for standing decisions and `docs/solutions/` for learnings and recorded rejections in the area: decisions already settled there should not be re-litigated.
The loop config is `docs/agents/cantrips-loop.md`; when that doc is absent, both stores are off.

Then spawn a sub-agent to walk the codebase, in the background where the harness supports it (Claude Code: do not use `run_in_background: false`). Don't follow rigid heuristics — explore organically and 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, but 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: would deleting it concentrate complexity, or just move it? A "yes, concentrates" is the signal you want.

### 2. Present candidates as an HTML report

Write a self-contained HTML file to the OS temp directory so nothing lands in the repo. Resolve the temp dir from `$TMPDIR`, falling back to `/tmp` (or `%TEMP%` on Windows), and write to `<tmpdir>/architecture-review-<timestamp>.html` so each run gets a fresh file. Open it for the user — `xdg-open <path>` on Linux, `open <path>` on macOS, `start <path>` on Windows — and tell them the absolute path.

The report uses **Tailwind via CDN** for layout and styling, and **Mermaid via CDN** for diagrams where a graph/flow/sequence reliably communicates the structure. Mix Mermaid with hand-crafted CSS/SVG visuals — use Mermaid when relationships are graph-shaped (call graphs, dependencies, sequences), and hand-built divs/SVG when you want something more editorial (mass diagrams, cross-sections, collapse animations). Each candidate gets a **before/after visualisation**. Be visual.

For each candidate, render a card with:

- **Files** — which files/modules are involved
- **Problem** — why the current architecture is causing friction
- **Solution** — plain English description of what would change
- **Benefits** — explained in terms of locality and leverage, and how tests would improve
- **Before / After diagram** — side-by-side, custom-drawn, illustrating the shallowness and the deepening
- **Recommendation strength** — one of `Strong`, `Worth exploring`, `Speculative`, rendered as a badge

End the report with a **Top recommendation** section: which candidate you'd tackle first and why.

**Use the project's domain glossary for the domain, and the `/codebase-design` vocabulary for the architecture.** If the glossary defines "Order," talk about "the Order intake module" — not "the FooBarHandler," and not "the Order service."

**Settled-decision conflicts**: if a candidate contradicts a decision recorded in an enabled store — a `docs/adr/` record or a `docs/solutions/` rejection — only surface it when the friction is real enough to warrant revisiting the decision. Mark it clearly in the card (e.g. a warning callout: _"contradicts a recorded rejection — but worth reopening because…"_).

See [HTML-REPORT.md](HTML-REPORT.md) for the full HTML scaffold, diagram patterns, and styling guidance.

Do NOT propose interfaces yet. After the file is written, ask the user: "Which of these would you like to explore?"

### 3. Grilling loop

Once the user picks a candidate, run the `/grilling` skill to walk the decision tree with them — constraints, dependencies, the shape of the deepened module, what sits behind the seam, what tests survive.

Side effects happen inline as decisions crystallize — capture the durable ones through `/compound`:

- **A deepened module named after a term missing from the domain glossary, or a fuzzy term sharpened during the conversation** — a glossary candidate.
- **The user rejects a candidate with a load-bearing reason** — capture it so future architecture reviews don't re-suggest the same thing.
  Skip ephemeral reasons ("not worth it right now") and self-evident ones.

The interview hands back here rather than closing itself, so this step owns the close.
Close with a flow pointer (read [flow-pointers.md](../writing-for-agents/flow-pointers.md) for the format), picking by what the interview settled: `/codebase-design` when alternative interfaces for the deepened module are still worth exploring — its design-it-twice parallel sub-agent pattern surfaces the options; `/spec` (user-invoked) when the deepening is a change worth a written contract; `/implement` (user-invoked) when it is small enough to go straight in.
Where the interview ended in a rejection, the capture above is the outcome — say so and offer this skill again on the next candidate.