edkuksa/skills
1
0
Fork 0
mirror of https://github.com/mattpocock/skills.git synced 2026-09-12 10:28:06 +07:00
Code Issues Packages Projects Releases Wiki Activity

docs: rewrite the remaining 18 pages to the four-section standard

Browse Source 
One agent per page, each rewriting from its SKILL.md up and running its
own evidence hunt across the wiki, the issue tracker and the unreleased
changesets. Question counts track the evidence: wayfinder earns ten,
resolving-merge-conflicts earns three.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Matt Pocock
2026-08-05 12:41:23 +01:00
co-authored by Claude Opus 5
parent 14780a1f8d
commit c33ce1c187
18 changed files with 1263 additions and 217 deletions
Download Patch File Download Diff File Expand all files Collapse all files
docs/engineering/codebase-design.md
+70 -13
View File
@@ -1,31 +1,88 @@
## What it does
`codebase-design` gives you a shared, precise vocabulary for designing **deep modules** — a lot of behaviour hidden behind a small interface, placed at a clean seam, testable through that interface.
`codebase-design` fixes the words you use to design a module: **module**, **interface**, **depth**, **seam**, **adapter**, **leverage**, **locality**. It defines each one precisely, bans the loose substitutes ("component", "service", "API", "boundary"), and states the handful of principles that follow from them.
It is a **language, not a procedure**. It doesn't restructure your code or hand you a refactor plan — it fixes the words (module, interface, depth, seam, adapter, leverage, locality) so that every design conversation and every other skill that touches design speaks the same way. Consistent language is the whole point; "component," "service," "API," and "boundary" are deliberately banned because they blur the distinctions that matter.
It is a reference, not a process. There is no loop to run, no artifact it produces, no checkpoint where it asks you a question. Every other skill that touches design borrows its vocabulary; on its own it gives you the language and stops. That is the thing to know before you invoke it, because a skill with no process and no stopping rule will improvise one if you point a session at it and say "go" — see the questions below.
## When to reach for it
Type `/codebase-design`, or the agent reaches for it automatically when a task fits.
Type `/codebase-design`, or the agent reaches for it automatically when a design task fits.
Reach for it when you're designing or improving a module's interface, hunting for deepening opportunities, deciding where a seam goes, or making code more testable and AI-navigable. Other skills pull it in whenever they need the deep-module vocabulary. If you want to sharpen the project's *domain* terms rather than its module design, use [domain-modeling](https://aihero.dev/skills-domain-modeling) instead; to run a whole architecture pass over an existing codebase, use [improve-codebase-architecture](https://aihero.dev/skills-improve-codebase-architecture).
Reach for it when you already know which code you're redesigning and you need to think about its shape: where the seam goes, how small the interface can get, whether an extraction is earning its keep. It is also what you reach for to settle an argument about what a word means.
## Deep, not shallow
Several skills sit close to it. Which one you want depends on what the actual problem is:
A module is **deep** when a large amount of behaviour sits behind a small interface, and **shallow** when the interface is nearly as complex as the implementation. Depth is measured as **leverage** — how much a caller (or a test) can exercise per unit of interface they have to learn. Crucially, depth is a property of the *interface*, not the implementation: a deep module can be internally composed of small, swappable parts that just never surface to callers.
| The problem | The skill |
|---|---|
| The shape of one module — its interface, its seam, its depth | `codebase-design` |
| The *words of the domain* — "account" means three things, two people mean different things by "cancellation" | [domain-modeling](https://aihero.dev/skills-domain-modeling) |
| You don't yet know *which* module to redesign | [improve-codebase-architecture](https://aihero.dev/skills-improve-codebase-architecture) — the survey that finds candidates |
| You want the design argued with, not just named | [grilling](https://aihero.dev/skills-grilling) |
| There's a concrete behaviour to build and you want tests that survive a refactor | [tdd](https://aihero.dev/skills-tdd) |
Two checks do most of the work. The **deletion test**: imagine deleting the module — if complexity vanishes, it was a pass-through; if it reappears across N callers, it was earning its keep. And **one adapter means a hypothetical seam; two adapters means a real one** — don't cut a seam until something actually varies across it.
## The vocabulary
## The interface is the test surface
The glossary is the skill. Every term is defined against the others, and each one comes with the word it replaces.
Callers and tests cross the same seam, so a well-placed interface gives tests something durable to aim at while the code underneath moves freely. That's why the vocabulary insists on **seam** (Feathers' term — a place you can change behaviour without editing there) over the overloaded "boundary," and why "interface" here means *every fact a caller must know*: signatures, yes, but also invariants, ordering, error modes, and performance — not just the type-level surface.
| Term | What it means | Don't say |
|---|---|---|
| **Module** | Anything with an interface and an implementation. Deliberately scale-agnostic — a function, a class, a package, a slice spanning tiers. | unit, component, service |
| **Interface** | Everything a caller must know to use it correctly: the type signature, plus invariants, ordering constraints, error modes, required config, performance characteristics. | API, signature |
| **Depth** | Leverage at the interface — how much behaviour a caller or a test can exercise per unit of interface they have to learn. **Deep**: a lot of behaviour behind a small interface. **Shallow**: the interface is nearly as complex as the implementation. | — |
| **Seam** | Michael Feathers' term: a place you can alter behaviour without editing in that place. It is the *location* of an interface, and where to put it is its own decision, separate from what goes behind it. | boundary |
| **Adapter** | A concrete thing satisfying an interface at a seam. Names a role, not a substance — an in-memory fake and a Postgres repo are both adapters. | — |
| **Leverage** | What callers get from depth: more capability per unit of interface learned. | — |
| **Locality** | What maintainers get from depth: change, bugs and verification concentrate in one place. Fix once, fixed everywhere. | — |
## Pulled out on purpose
Depth is deliberately *not* defined as the ratio of implementation lines to interface lines, which is Ousterhout's own definition. That metric rewards padding the implementation. Depth-as-leverage is used instead.
`codebase-design` is the **single source of truth** for the deep-module vocabulary, split out as its own model-invoked skill so anything can reach it. Other skills point at it rather than restating the words: [tdd](https://aihero.dev/skills-tdd) borrows it to place a seam before writing the test, [improve-codebase-architecture](https://aihero.dev/skills-improve-codebase-architecture) leans on it while restructuring existing code, and [to-spec](https://aihero.dev/skills-to-spec) speaks it when it sketches seams and deepening opportunities before writing a spec.
## The four principles
The point of keeping it standalone is that you can also reach for it on its own — as a **reference** for how to think about module design — without triggering the larger process any of those skills mandate. Fix the words once, in one place, and every design conversation inherits them.
- **Depth is a property of the interface, not the implementation.** A deep module can be built internally from small swappable parts. They just don't surface to callers. A module can have internal seams its own tests use, and one external seam at its interface.
- **The deletion test.** Imagine deleting the module. If complexity vanishes, it was a pass-through. If it reappears across N callers, it was earning its keep.
- **The interface is the test surface.** Callers and tests cross the same seam. If you want to test *past* the interface, the module is the wrong shape.
- **One adapter means a hypothetical seam. Two adapters means a real one.** Don't cut a seam until something actually varies across it. A single-adapter seam is just indirection.
Two supporting files go further, and the skill reads them on demand rather than up front. [DEEPENING.md](https://github.com/mattpocock/skills/blob/main/skills/engineering/codebase-design/DEEPENING.md) classifies a candidate's dependencies — in-process, local-substitutable, remote-but-owned, true-external — because the category decides how the deepened module gets tested across its seam. [DESIGN-IT-TWICE.md](https://github.com/mattpocock/skills/blob/main/skills/engineering/codebase-design/DESIGN-IT-TWICE.md) spins up parallel sub-agents to produce three or more radically different interfaces for the same module, then compares them on depth, locality and seam placement.
## Common questions
**How do I actually build a deep module in TypeScript?**
This is the most-asked question about the skill and the skill does not answer it. It defines what a deep module *is*; it says nothing about how to stop a stray import from reaching past the interface. [Issue #458](https://github.com/mattpocock/skills/issues/458) put it plainly: "let's say we're happy with the interface, it hides the details, etc. But how do we enforce it? I think without linting or clear guardrails, humans and LLMs alike will start making it messy over time." Matt's answer, in that thread, was three options: wrap it in a class or IIFE and accept that the class gets enormous; make it a package in a monorepo and accept the monorepo tooling; or use a linter like [dependency-cruiser](https://github.com/sverweij/dependency-cruiser) to forbid imports that bypass the interface. He has separately called Effect the best mechanism and dependency-cruiser the second-best. There is a `setup-ts-deep-modules` skill in the repo's `in-progress/` bucket that lays down a `src/packages/<name>/index.ts` convention, but it is a beta-channel skill with no docs page, and it has no lint rule shipped with it.
**I pointed a session at it and it burned 100k tokens redesigning things I never asked about.**
Known, and filed as [issue #449](https://github.com/mattpocock/skills/issues/449). The skill is model-invoked and describes itself as vocabulary, but nothing in it hard-stops an agent from treating it as a runnable process. Told to "resume in /codebase-design and drive the open decisions", an agent reached for the most action-shaped content it could find — the parallel sub-agents in `DESIGN-IT-TWICE.md` — re-explored code a previous session had already mapped, and ran a long way before asking anything. None of the guardrails a driver skill has (checkpoints, one question at a time, no auto-advance) are present here, because a reference has none. The workaround is to name a driver skill and let this one sit underneath it: `/grill-with-docs`, `/improve-codebase-architecture` or `/tdd` with `codebase-design` as the vocabulary. The issue is open.
**Where did `design-an-interface` go? And is there an `/interface-design` skill?**
`design-an-interface` was removed and absorbed into this skill. Nothing was lost: its "design it twice" technique — parallel sub-agents generating radically different designs, from Ousterhout — ships here as `DESIGN-IT-TWICE.md`. Separately, several people have asked for a dedicated `/interface-design` skill for the deep-module/thin-interface philosophy; Matt's reply each time has been that `/codebase-design` is in there. If you came looking for either name, this is the page.
**Isn't this a file-structure convention — folders, barrel files, feature slices?**
No, and Matt has been consistent about that under repeated pushback. [Issue #95](https://github.com/mattpocock/skills/issues/95) proposed a formalised fractal-tree file structure as the concrete implementation of deep modules; the reply was that the two are orthogonal — "deep modules are about the design of the interface and accessing through a strict interface, no matter what the file system looks like. It seems perfectly possible that you could have shallow modules with this approach." The same came up in #458: "I think you might be tying the concept of modules too closely to the file system. The file system can certainly be a useful hint to the shape of modules, but there's no need to use the file system in the construction of deep modules." The glossary defines **module** as scale-agnostic on purpose.
**Does `tdd` actually use this vocabulary?**
Not yet, in the sense you'd expect. `tdd` talks about seams heavily, but it defines "seam" itself — as "the public boundary you test at" — and contains no reference to `codebase-design`. It uses "boundary", the word this skill's glossary bans. The inline deep-module notes that used to live inside `tdd` were removed in favour of this shared skill, but the pointer replacing them was never added. Matt has acknowledged the gap publicly: "Yeah I have a /codebase-design skill, might be that TDD needs to point there too." In practice that means invoking `/tdd` will not reliably pull this vocabulary in — say so explicitly if you want it.
**Does the design-it-twice pattern work outside Claude Code?**
Not cleanly. `DESIGN-IT-TWICE.md` says "spawn 3+ sub-agents in parallel using the Agent tool", which is Claude Code's tool by Claude Code's name. The repo ships metadata for other harnesses, including Codex, and those may expose nothing under that name — so the parallel-design phase is less portable than the skill's metadata suggests. Tracked in [issue #564](https://github.com/mattpocock/skills/issues/564), open.
**Can I add my own concepts to the glossary — connascence, module secrets, progressive disclosure?**
People have proposed exactly those. [Issue #180](https://github.com/mattpocock/skills/issues/180) adds Parnas's module secrets and Page-Jones's connascence as a naming layer for *what* is leaking across a seam, with a working diff attached; [issue #303](https://github.com/mattpocock/skills/issues/303) proposes progressive disclosure inside the implementation, so a module that is deep at its public interface isn't one undifferentiated slab underneath. Both are open and unmerged. The glossary as shipped is deliberately small, and the reason it stays small is stated in the skill itself: consistent language is the whole point, and a term nobody uses consistently is worse than no term.
## It's working if
- The design conversation stops producing the words "component", "service" and "boundary", and starts producing "module", "interface" and "seam".
- Someone can point at a proposed extraction and say whether it passes the deletion test, without hedging.
- A proposed seam comes with a second adapter named, not just the first one.
- Discussion of an interface covers invariants, ordering and error modes — not only the type signature.
- Invoking it does not start a session. If the agent begins reading files and proposing refactors off the back of `/codebase-design` alone, it has mistaken the reference for a driver.
## Where it fits
`codebase-design` is a **reach-for-it-anytime standalone** — the shared vocabulary layer under the engineering skills. Its closest neighbour is [domain-modeling](https://aihero.dev/skills-domain-modeling), the parallel vocabulary skill for the problem domain rather than the module structure. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
`codebase-design` is a **reach-for-it-anytime standalone**, and the vocabulary layer underneath the engineering skills rather than a step in any chain. Its closest neighbour is [domain-modeling](https://aihero.dev/skills-domain-modeling), the parallel reference for the *problem domain*'s words rather than the module's shape — the two are usually wanted together, since naming a deep module well needs both. [improve-codebase-architecture](https://aihero.dev/skills-improve-codebase-architecture) is the other: it surveys a codebase for deepening candidates and writes every one of them in this glossary, so it finds the module and this skill is the bench you design it on. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.