Domain Modeling

Domain Modeling

Build and sharpen a project's domain model. Use when discussing codebase terminology, writing or editing a CONTEXT.md, or recording or editing an ADR.

This skill is imported from mattpocock/skills (MIT) and is model-invoked upstream.

When to use this skill

• Build and sharpen a project's domain model.
• Use when discussing codebase terminology, writing or editing a CONTEXT.md, or recording or editing an ADR.

Instructions

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

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.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.

Examples

• Apply this skill to one narrow scope first, confirm the output matches the shape described above, then widen to the full task.
• When a step needs a fact from the repository or the environment, look it up instead of asking the user for it.

Best practices

• Keep the upstream procedure intact; record deviations explicitly instead of silently improvising.
• Stop and hand control back to the user at every decision point this skill marks as theirs.
• Prefer small reversible changes, and state assumptions rather than burying them.

References

• Upstream skill: mattpocock/skills skills/engineering/domain-modeling/SKILL.md (commit 3cca18b, MIT)
• Companion doc: references/ADR-FORMAT.md
• Companion doc: references/CONTEXT-FORMAT.md
• Project standards: .agent-skills/skill-standardization/SKILL.md
• Validator script: .agent-skills/skill-standardization/scripts/validate_skill.sh