model-domain

Model Domain

Distinct from define-language and deepen-architecture: Use this skill to stress-test a plan through a grilling interview that resolves domain model decisions and captures invariants. Use define-language to produce a canonical glossary of terms. Use deepen-architecture to find module-level refactoring opportunities in code.

Interview me relentlessly about every aspect of this plan until we reach a shared understanding. Walk down each branch of the design tree, resolving dependencies between decisions one-by-one. For each question, provide your recommended answer.

HARD GATE — Capture invariants (what MUST always be true) and state machines (what transitions are legal) for core entities. If these are fuzzy, design will fail.

Ask the questions one at a time, waiting for feedback on each question before continuing.

If a question can be answered by exploring the codebase, explore the codebase instead.

Domain awareness

During codebase exploration, also look for existing documentation:

File structure

Most repos have a single context:

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

If a specs/tech-architecture/tech-stack.md exists, the repo has multiple contexts. The map points to where each one lives:

/
├── specs/
│   ├── CONTEXT-MAP.md
│   └── adr/                          ← system-wide decisions
└── src/
    ├── ordering/
    │   └── specs/
    │       ├── CONTEXT.md
    │       └── adr/                  ← context-specific decisions
    └── billing/
        └── specs/
            ├── CONTEXT.md
            └── adr/

Create files lazily — only when you have something to write. If no specs/tech-architecture/tech-stack.md exists, create it when the first term is resolved. If no specs/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 specs/tech-architecture/tech-stack.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 specs/tech-architecture/tech-stack.md inline

When a term is resolved, update specs/tech-architecture/tech-stack.md right there. Don't batch these up — capture them as they happen. Use the format in CONTEXT-FORMAT.md.

Don't couple specs/tech-architecture/tech-stack.md to implementation details. Only include terms that are meaningful to domain experts.

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.

Concurrency safety audit

When the plan touches shared state, async, or multi-threaded code:

• [ ] List every shared mutable location (globals, singletons, module-level caches).
• [ ] For each: who reads, who writes, synchronization mechanism (lock, actor, immutable copy).
• [ ] Flag race risks (check-then-act, non-atomic read-modify-write) with severity.
• [ ] Record findings in specs/tech-architecture/tech-stack.md under ## Concurrency or in an ADR if architectural.

<!-- story: e07s03 -->

---

ADR Format

ADRs live in docs/adr/ and use sequential numbering: 0001-slug.md, 0002-slug.md, etc.

Create the docs/adr/ directory lazily — only when the first ADR is needed.

Template

# {Short title of the decision}

{1-3 sentences: what's the context, what did we decide, and why.}

That's it. An ADR can be a single paragraph. The value is in recording that a decision was made and why — not in filling out sections.

Optional sections

Only include these when they add genuine value. Most ADRs won't need them.

• Status frontmatter (proposed | accepted | deprecated | superseded by ADR-NNNN) — useful when decisions are revisited
• Considered Options — only when the rejected alternatives are worth remembering
• Consequences — only when non-obvious downstream effects need to be called out

Numbering

Scan docs/adr/ for the highest existing number and increment by one.

When to offer an ADR

All three of these must be true:

1. Hard to reverse — the cost of changing your mind later is meaningful
2. Surprising without context — a future reader will look at the code and wonder "why on earth 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 a decision is easy to reverse, skip it — you'll just reverse it. If it's not surprising, nobody will wonder why. If there was no real alternative, there's nothing to record beyond "we did the obvious thing."

What qualifies

• Architectural shape. "We're using a monorepo." "The write model is event-sourced, the read model is projected into Postgres."
• Integration patterns between contexts. "Ordering and Billing communicate via domain events, not synchronous HTTP."
• Technology choices that carry lock-in. Database, message bus, auth provider, deployment target. Not every library — just the ones that would take a quarter to swap out.
• Boundary and scope decisions. "Customer data is owned by the Customer context; other contexts reference it by ID only." The explicit no-s are as valuable as the yes-s.
• Deliberate deviations from the obvious path. "We're using manual SQL instead of an ORM because X." Anything where a reasonable reader would assume the opposite. These stop the next engineer from "fixing" something that was deliberate.
• Constraints not visible in the code. "We can't use AWS because of compliance requirements." "Response times must be under 200ms because of the partner API contract."
• Rejected alternatives when the rejection is non-obvious. If you considered GraphQL and picked REST for subtle reasons, record it — otherwise someone will suggest GraphQL again in six months.

---

CONTEXT.md Format

Structure

# {Context Name}

{One or two sentence description of what this context is and why it exists.}

## Language

**Order**:
A customer's request to purchase one or more items.
_Avoid_: Purchase, transaction

**Invoice**:
A request for payment sent to a customer after delivery.
_Avoid_: Bill, payment request

**Customer**:
A person or organization that places orders.
_Avoid_: Client, buyer, account

## Relationships

- An **Order** produces one or more **Invoices**
- An **Invoice** belongs to exactly one **Customer**

## Example dialogue

> **Dev:** "When a **Customer** places an **Order**, do we create the **Invoice** immediately?"
> **Domain expert:** "No — an **Invoice** is only generated once a **Fulfillment** is confirmed."

## Flagged ambiguities

- "account" was used to mean both **Customer** and **User** — resolved: these are distinct concepts.

Rules

• Be opinionated. When multiple words exist for the same concept, pick the best one and list the others as aliases to avoid.
• Flag conflicts explicitly. If a term is used ambiguously, call it out in "Flagged ambiguities" with a clear resolution.
• Keep definitions tight. One sentence max. Define what it IS, not what it does.
• Show relationships. Use bold term names and express cardinality where obvious.
• Only include terms specific to this project's context. General programming concepts (timeouts, error types, utility patterns) don't belong even if the project uses them extensively. Before adding a term, ask: is this a concept unique to this context, or a general programming concept? Only the former belongs.
• Group terms under subheadings when natural clusters emerge. If all terms belong to a single cohesive area, a flat list is fine.
• Write an example dialogue. A conversation between a dev and a domain expert that demonstrates how the terms interact naturally and clarifies boundaries between related concepts.

Single vs multi-context repos

Single context (most repos): One CONTEXT.md at the repo root.

Multiple contexts: A CONTEXT-MAP.md at the repo root lists the contexts, where they live, and how they relate to each other:

# Context Map

## Contexts

- Ordering — `src/ordering/CONTEXT.md` — receives and tracks customer orders
- Billing — `src/billing/CONTEXT.md` — generates invoices and processes payments
- Fulfillment — `src/fulfillment/CONTEXT.md` — manages warehouse picking and shipping

## Relationships

- **Ordering → Fulfillment**: Ordering emits `OrderPlaced` events; Fulfillment consumes them to start picking
- **Fulfillment → Billing**: Fulfillment emits `ShipmentDispatched` events; Billing consumes them to generate invoices
- **Ordering ↔ Billing**: Shared types for `CustomerId` and `Money`

The skill infers which structure applies:

• If CONTEXT-MAP.md exists, read it to find contexts
• If only a root CONTEXT.md exists, single context
• If neither exists, create a root CONTEXT.md lazily when the first term is resolved

When multiple contexts exist, infer which one the current topic relates to. If unclear, ask.