p00318821-eng/skills-plugins-hooks-agents/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 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.