Agent Skills: Skill: backend/domain-logic-module

Skill: backend/domain-logic-module

Purpose

Create or refactor domain logic so that business rules live in:

• a testable module boundary
• with explicit invariants
• and deterministic error behavior

This skill prevents domain rules from leaking into controllers, jobs, or adapters.

Inputs

• Domain concept to implement or change (1–3 sentences)
• Invariants that must always hold (must/must-not list)
• Entry points that will call this module:
  • endpoint handler / job / integration adapter
• Repo profile (preferred): <repo>/REPO_PROFILE.json

Outputs

• Domain module(s) with:
  • clear inputs/outputs
  • explicit invariants
  • deterministic error returns
• Unit tests that cover:
  • happy path
  • invariant violations
  • at least one edge case

Non-goals

• Wiring endpoints (use backend/endpoint-scaffold)
• Schema/migrations (use backend/persistence-layer-change)
• External API IO concerns (use backend/integration-adapter)
• UI logic

Workflow

1. Locate the correct domain boundary (existing module/folder pattern wins).
2. Define the public API of the module:
   • function(s) or class with explicit types
3. Encode invariants explicitly:
   • validate preconditions
   • validate postconditions where meaningful
4. Define error behavior:
   • typed error codes (align with shared/error-taxonomy if present)
   • avoid throwing raw exceptions across boundaries unless repo standard
5. Write unit tests focused on rules, not wiring.
6. Ensure callers orchestrate and do not re-implement rules.
7. Run required validations from profile.

Checks

• Invariants are explicit and enforced
• Errors are deterministic and mapped cleanly
• Callers become thinner after extraction
• Unit tests cover:
  • happy path
  • at least 2 invariant failures
  • edge case
• Typecheck/lint pass if configured

Failure modes

• Invariants unclear or contested → recommend $decision-capture before encoding.
• Rule sprawl across multiple modules → propose consolidation (but do not refactor widely).
• Tests hard to write due to coupling → extract dependencies and inject interfaces.

Telemetry

Log:

• skill: backend/domain-logic-module
• domain_area: <short label>
• invariants_added_or_changed: count
• tests_added: count
• files_touched
• outcome: success | partial | blocked