domain-designer

name	domain-designer
description	Use after the external spec of the domain is know and before writing domain code. Internal design process for domain-builders. Guides file structure, design guard creation, and internal decomposition decisions before writing implementation code.
disable-model-invocation	true

Domain Designer (Internal Design Skill)

This skill guides the domain-builder's internal design process. It is invoked after receiving the external interface spec from the domain-interface-designer, and before writing any implementation code.

When to Use

The domain-builder reads this skill when it needs to:

• Decide how to decompose the interface spec into files and modules
• Write design guards for each file
• Make internal implementation decisions (rare ADRs)

Input Context

By the time you invoke this skill, you should already have:

1. The interface spec — command/query types, events, structural invariants (from domain-interface-designer)
2. The domain subdirectory — where to write code
3. The build request — what capability or fix to implement

Procedure

1. Decide file structure

Given the interface spec, decide which files to create. Each file should have a single responsibility. Common patterns:

• Command handler — one file per command (validate, authorize, write, emit event)
• Query handler — one file per query (read from table/read model, return shaped response)
• Types — shared type definitions for the domain's API surface (input/output types, event payloads)
• Validators — input validation logic, separated from handler orchestration
• Repository/data access — database interaction, isolated behind an interface for testability

Do not add layers, factories, wrappers, or patterns that the current scope doesn't require. Build exactly what the interface spec asks for.

2. Write design guards

Before writing any implementation code in a file, write the @design-guard comment block at the top. Every TypeScript file must have one. The guard documents invariants, not implementation details. Required keys:

/**
 * @design-guard
 * role: <what this file is responsible for>
 * layer: <ui | service | domain | infrastructure>
 * non_goals:
 *   - <what this file must NOT do>
 * boundaries:
 *   depends_on_layers: [<layers this file may depend on>]
 *   exposes: [<public API surface>]
 * invariants:
 *   - <structural truths that must always hold>
 * authority:
 *   decides: [<what this file is the single authority on>]
 *   delegates: [<what it hands off and to whom>]
 * extension_policy:
 *   - <how this file should grow over time>
 * failure_contract:
 *   - <how errors are handled/surfaced>
 * testing_contract:
 *   - <how to test this file>
 */

Design guard rules:

• Invariants must be structural and stable — they should survive implementation refactors.
• No business rules that change over time (those belong in workflows or configuration).
• No implementation details (specific class names of dependencies, algorithm choices, etc.).
• The guard is a contract. Code beneath it must conform. If you find a conflict, update the guard deliberately, not silently.

3. Record implementation ADRs (rare)

If you face a genuinely important internal implementation decision (e.g., choosing between two data structures, or a non-obvious error handling strategy), write an ADR to design/domains/<domain-name>/implementation/ADR-<name>.md.

Use the standard ADR template:

### Summary statement
### Context
### Problem to solve
### Options
#### Option 1
#### Option 2
### Reasoning

Most builds will not need implementation ADRs. Do not manufacture them. The design guards and code should speak for themselves.