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 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

Single-context repo (most repos):

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

Multi-context repo (presence of CONTEXT-MAP.md at the root). The map points to where each context 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. Never scaffold either ahead of need, and never flag their absence: other skills read these docs and are told to proceed silently when they're missing.

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 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.

Adapted from Matt Pocock's "domain-modeling" skill (github.com/mattpocock/skills) — MIT © Matt Pocock. Description tightened to one trigger per branch, sibling format files moved to references/, and the file-structure trees aligned with the layout setup-skills documents for consuming skills — including the "don't scaffold, don't flag absence" rule that governs them there. See NOTICE.md.