Domain Modeling

---
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.)

Two files, two different contents: sharpened glossary terms go to
`CONTEXT.md`; decisions that constrain architecture or behaviour go to the
repository's resolved ADR home. With active OpenSpec, that is the change's
`design.md`; otherwise follow `references/ADR-FORMAT.md`. Don't
conflate them — a term is not a decision, and a decision is not a definition.

## File structure

Most repos have a single context (the `docs/adr/` trees below illustrate a repo
with no existing decision record; resolve the ADR's home first, per
[references/ADR-FORMAT.md](references/ADR-FORMAT.md)):

```
/
├── CONTEXT.md
├── docs/
│   └── adr/
│       ├── adr-42-event-sourced-orders.md
│       └── adr-57-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. An ADR goes
wherever the repo already records decisions ([references/ADR-FORMAT.md](references/ADR-FORMAT.md)),
and a new `docs/adr/` is created only when that resolves to `docs/adr/`.

## 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?"

When the model touches module shape — interfaces, seams, adapters — use the
`codebase-design` skill's vocabulary rather than inventing your own terms for
architecture; keep `CONTEXT.md` itself limited to domain language (see
[references/CONTEXT-FORMAT.md](references/CONTEXT-FORMAT.md)).

### 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

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
[references/ADR-FORMAT.md](references/ADR-FORMAT.md).