# Domain Modeling

Sharpen project terms in a glossary and record decisions as ADRs.

- Repository: https://github.com/mattpocock/skills (309182 stars)
- Category: Engineering
- License: MIT
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add mattpocock/skills --skill domain-modeling -g -a claude-code -y`
Codex: `npx skills add mattpocock/skills --skill domain-modeling -g -a codex -y`
Cursor: `npx skills add mattpocock/skills --skill domain-modeling -g -a cursor -y`
Gemini CLI: `npx skills add mattpocock/skills --skill domain-modeling -g -a gemini-cli -y`
Claude Code plugin marketplace: `/plugin marketplace add mattpocock/skills`

## Description

Build and sharpen a project's domain model. Use when discussing codebase terminology, writing or editing a GLOSSARY.md, or recording or editing an ADR.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Domain Modeling

Actively build and sharpen the project's domain model as you design. This is the *active* discipline: challenging terms, inventing edge-case scenarios, and writing the glossary and decisions down the moment they crystallise. (Merely *reading* `GLOSSARY.md` for vocabulary is not this skill: that's a one-line habit any skill can do. This skill is for when you're changing the model, not just consuming it.)

## File structure

Most repos have a single context:

```
/
├── GLOSSARY.md
├── docs/
│   └── adr/
│       ├── 0001-event-sourced-orders.md
│       └── 0002-postgres-for-write-model.md
└── src/
```

If a `GLOSSARY-MAP.md` exists at the root, the repo has multiple contexts. The map points to where each one lives:

```
/
├── GLOSSARY-MAP.md
├── docs/
│   └── adr/                          ← system-wide decisions
├── src/
│   ├── ordering/
│   │   ├── GLOSSARY.md
│   │   └── docs/adr/                 ← context-specific decisions
│   └── billing/
│       ├── GLOSSARY.md
│       └── docs/adr/
```

Create files lazily: only when you have something to write. If no `GLOSSARY.md` exists, create one when the first term is resolved. If no `docs/adr/` exists, create it when the first ADR is needed.

## During the session

### Challenge against the glossary

When the user uses a term that conflicts with the existing language in `GLOSSARY.md`, call it out immediately. "Your glossary defines 'cancellation' as X, but you seem to mean Y. Which is it?"

### Sharpen fuzzy language

When the user uses vague or overloaded terms, propose a precise canonical term. "You're saying 'account': do you mean the Customer or the User? Those are different things."

### Discuss concrete scenarios

When domain relationships are being discussed, stress-test them with specific scenarios. Invent scenarios that probe edge cases and force the user to be precise about the boundaries between concepts.

### Cross-reference with code

The rest: https://github.com/mattpocock/skills/blob/HEAD/skills/engineering/domain-modeling/SKILL.md