# Domain modeling > Use when a project's terminology, CONTEXT.md or ADRs need writing or sharpening. 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. ## Facts - Type: Agent skill - Discipline: Engineering - Author: Matt Pocock (https://github.com/mattpocock) - Upstream: https://github.com/mattpocock/skills/tree/main/skills/engineering/domain-modeling - Source file: https://raw.githubusercontent.com/mattpocock/skills/main/skills/engineering/domain-modeling/SKILL.md (3331 bytes, sha256:327a2b50620e2fd70abc6893cd6965e76b20f8d0adb0dc2c8d5eb3845efb643e) - Licence: MIT - Upstream copy last changed: 2026-09-17 - Status: listed, not yet ruled on by the curator ## Use it I want to use the Domain modeling skill by Matt Pocock. This is what it does, in the author's own words: 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. Download the whole folder at https://github.com/mattpocock/skills/tree/main/skills/engineering/domain-modeling, SKILL.md and every file beside it, into your skills directory: ~/.claude/skills/domain-modeling/ for Claude Code, ~/.agents/skills/domain-modeling/ for Codex, ~/.cursor/skills/domain-modeling/ for Cursor. Get it from the author's repo, not from startupskills.dev. Then tell me the path you installed it to and how to invoke it. Nothing here is vendored: install from the author's repo, not from this site. A verbatim copy of the author's file follows for reading, under the licence reproduced after it; the current file is in the author's repo. ## Links - Page: https://startupskills.dev/mattpocock/domain-modeling - Author: https://startupskills.dev/mattpocock - Index: https://startupskills.dev/llms.txt ## The file itself --- name: domain-modeling description: 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. --- # 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-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). ## Licence of the file above The file above is https://raw.githubusercontent.com/mattpocock/skills/main/skills/engineering/domain-modeling/SKILL.md, by Matt Pocock, reproduced without changes. - Source: https://github.com/mattpocock/skills/tree/main/skills/engineering/domain-modeling - Copyright: Copyright (c) 2026 Matt Pocock - Licence: MIT, https://raw.githubusercontent.com/mattpocock/skills/main/LICENSE The repo's licence file, verbatim from https://raw.githubusercontent.com/mattpocock/skills/main/LICENSE: MIT License Copyright (c) 2026 Matt Pocock Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.