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 .ai/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.)

Fork note: this skill is vendored from github.com/mattpocock/skills (skills/engineering/domain-modeling) with one deliberate modification — upstream targets root-level CONTEXT.md/docs/adr/; this fork targets .ai/CONTEXT.md/.ai/adr/ instead, to stay inside this Standard's single .ai/ umbrella (see project-memory-template's .ai/LINEAGE.md). Tracked in manifests/origins.json the same way caveman's local modification is tracked. Everything else below is unchanged from upstream.

File structure

Most repos have a single context:

/
├── .ai/
│   ├── CONTEXT.md
│   └── adr/
│       ├── 0001-event-sourced-orders.md
│       └── 0002-postgres-for-write-model.md
└── src/

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

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

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

Check for an existing owner before adding a term

Before adding a term to .ai/CONTEXT.md, check whether a runtime/consumer-facing glossary already owns it in this repo (mechanical signal: a references/hisd-power-bi-context.md-style file, or similar). If one exists, cross-reference it instead of re-deriving the definition — .ai/CONTEXT.md is a planning-time, engineering-facing glossary; a file like hisd-power-bi-context.md is runtime, Copilot/FDA-facing. They can name-collide on the same term without being the same system, so link rather than duplicate.

Update .ai/CONTEXT.md inline

When a term is resolved, update .ai/CONTEXT.md right there. Don't batch these up — capture them as they happen. Use the format in CONTEXT-FORMAT.md.

.ai/CONTEXT.md should be totally devoid of implementation details. Do not treat it 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.