# Domain Modeling

> Use when pinning down a project's domain terminology — building or sharpening a CONTEXT.md ubiquitous-language glossary, resolving vague or conflicting terms, or when another skill needs the domain model maintained. Triggers on "ubiquitous language", "domain glossary", "what should we call this", "add this to CONTEXT.md", or a term that conflicts with the existing glossary. Not for recording decisions (that's [[adr]] — this skill hands off to it).

- Skill: `mickzijdel/domain-modeling` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add mickzijdel/domain-modeling`
- Raw SKILL.md: https://api.skillmd.com/api/skills/mickzijdel/domain-modeling/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- License: MIT; copyright Matt Pocock; see ../matt-pocock-skills-LICENSE.tx
- Author: mickzijdel (https://skillmd.com/u/mickzijdel)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/mickzijdel/domain-modeling

---


# 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 down the moment a term crystallises. (Merely *reading* `CONTEXT.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.)

The payoff compounds: a shared language makes variables, functions, and files consistently named, the codebase easier to navigate, and every future session terser — one precise term where twenty vague words used to go.

## File structure

Most repos have a single context:

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

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

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

Create files lazily — only when you have something to write. If no `CONTEXT.md` exists, create one when the first term is resolved.

## During the session

### Challenge against the glossary

When the user uses a term that conflicts with the existing language in `CONTEXT.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

When the user states how something works, check whether the code agrees. If you find a contradiction, surface it: "Your code cancels entire Orders, but you just said partial cancellation is possible — which is right?"

### Update CONTEXT.md inline

When a term is resolved, update `CONTEXT.md` right there. Don't batch these up — capture them as they happen. Use the format in [references/context-format.md](references/context-format.md).

`CONTEXT.md` should be totally devoid of implementation details. Do not treat `CONTEXT.md` as a spec, a scratch pad, or a repository for implementation decisions. It is a glossary and nothing else.

### Offer ADRs sparingly

Decisions go in ADRs, not the glossary. Only offer to create one when all three are true:

1. **Hard to reverse** — the cost of changing your mind later is meaningful
2. **Surprising without context** — a future reader will wonder "why did they do it this way?"
3. **The result of a real trade-off** — there were genuine alternatives and you picked one for specific reasons

If any of the three is missing, skip the ADR. When all three hold, record it with [[adr]] (Nygard format, `docs/decisions/`).