> **domain-modeling** — chapter 5 of 35 in [mattpocock/skills](https://skillsdocs.com/mattpocock/skills).
>
> Book (all chapters, one file): https://skillsdocs.com/mattpocock/skills.md
> Machine manifest: https://skillsdocs.com/mattpocock/skills/.well-known/agent-skills/index.json
> Install the book: `npx skills add mattpocock/skills`
> Upstream: https://github.com/mattpocock/skills/blob/main/skills/engineering/domain-modeling/SKILL.md @ `main`
> Raw bytes, no header: https://raw.githubusercontent.com/mattpocock/skills/main/skills/engineering/domain-modeling/SKILL.md
> Base for relative paths: https://raw.githubusercontent.com/mattpocock/skills/main/skills/engineering/domain-modeling/
> Licence: MIT — https://spdx.org/licenses/MIT.html
>
> Bundled files (3), referenced from this skill's directory:
>   - `ADR-FORMAT.md` — https://raw.githubusercontent.com/mattpocock/skills/main/skills/engineering/domain-modeling/ADR-FORMAT.md
>   - `agents/openai.yaml` — https://raw.githubusercontent.com/mattpocock/skills/main/skills/engineering/domain-modeling/agents/openai.yaml
>   - `CONTEXT-FORMAT.md` — https://raw.githubusercontent.com/mattpocock/skills/main/skills/engineering/domain-modeling/CONTEXT-FORMAT.md
>
> Content © its authors, served unmodified. Takedown: https://github.com/DreambaseAI/skillsdocs/issues/new?labels=takedown&title=Takedown+request

<!-- Verbatim upstream SKILL.md follows, YAML frontmatter included. -->

---
name: domain-modeling
description: Build and sharpen a project's domain model. Use when the user wants to pin down domain terminology or a ubiquitous language, record an architectural decision, or when another skill needs to maintain the domain model.
---

# 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* `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.)

## File structure

Most repos have a single context:

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

Create files lazily — only when you have something to write. If no `CONTEXT.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 `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 [CONTEXT-FORMAT.md](./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

Only offer to create an ADR 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. Use the format in [ADR-FORMAT.md](./ADR-FORMAT.md).