domain-model

# Domain modelling 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 is a one-line habit any skill can do. This skill is for when you are 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).