# Domain Modeling > Use when project terminology, product workflow, data contracts, or architectural decisions are causing ambiguity, or when a feature/RCA/refactor needs a shared domain vocabulary. In local Codex work, read existing repo docs first and write durable docs only when the repo convention or user request justifies it. - Skill: `masih-0x3/domain-modeling` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add masih-0x3/domain-modeling` - Raw SKILL.md: https://api.skillmd.com/api/skills/masih-0x3/domain-modeling/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Productivity - Author: Masih-0x3 (https://skillmd.com/u/masih-0x3) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/masih-0x3/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.) ## Local Codex use Use this skill when language is load-bearing. Good triggers are overloaded product terms, unclear actors, confusing lifecycle states, data model naming, event names, queue/job meanings, billing/auth concepts, or an RCA where the code and the user's words disagree. Do not turn every task into domain modeling. For small implementation or UI work, consume existing vocabulary and keep moving. Before writing anything durable, read the repo's current source of truth: `AGENTS.md`, README, existing `CONTEXT.md`, `CONTEXT-MAP.md`, `docs/`, ADRs, issue templates, schemas, routes, and nearby code. Existing project docs outrank new conventions. Write durable domain docs only when at least one is true: - The user asked for glossary/domain/ADR work. - The repo already uses `CONTEXT.md`, `CONTEXT-MAP.md`, or ADRs. - The resolved term or decision will clearly matter across future sessions. - Another local skill or repo instruction explicitly depends on those docs. Otherwise, capture the resolved terminology in the final answer, issue/PR body, handoff, or implementation notes instead of creating new repo files. ## 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 and only under the local write rules above. If no `CONTEXT.md` exists, do not create one just because a single term was clarified. If no `docs/adr/` exists, offer an ADR only when the decision is durable enough to justify the new convention. ## 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?" ### Capture resolved terms When a term is resolved and the repo already has a domain doc convention, 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. When the repo has no domain doc convention, keep the term in the working plan or final response unless the user approves creating the convention. ### 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).