# Implementation Plan: Edit Combatant

**Branch**: `004-edit-combatant` | **Date**: 2026-03-03 | **Spec**: [spec.md](./spec.md)
**Input**: Feature specification from `/specs/004-edit-combatant/spec.md`

## Summary

Add the ability to rename a combatant by id within an encounter. A pure domain function `editCombatant` validates the id and new name, returns the updated encounter with a `CombatantUpdated` event, or a `DomainError`. Wired through an application use case and exposed via the existing `useEncounter` hook to a minimal UI control.

## Technical Context

**Language/Version**: TypeScript 5.x (strict mode, verbatimModuleSyntax)
**Primary Dependencies**: React 19, Vite
**Storage**: In-memory React state (local-first, single-user MVP)
**Testing**: Vitest
**Target Platform**: Browser (localhost:5173)
**Project Type**: Web application (monorepo: domain → application → web)
**Performance Goals**: N/A — single-user local state, instant updates
**Constraints**: Pure domain logic, no I/O in domain layer
**Scale/Scope**: Single-user encounter tracker

## Constitution Check

*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*

| Principle | Status | Notes |
|-----------|--------|-------|
| I. Deterministic Domain Core | PASS | `editCombatant` is a pure function: same encounter + id + name → same result |
| II. Layered Architecture | PASS | Domain function → use case → hook/UI. No layer violations. |
| III. Agent Boundary | N/A | No agent layer involvement |
| IV. Clarification-First | PASS | Spec is complete, no ambiguities remain |
| V. Escalation Gates | PASS | All work is within spec scope |
| VI. MVP Baseline Language | PASS | Spec uses "MVP baseline does not include" for out-of-scope items |
| VII. No Gameplay Rules | PASS | Constitution contains no gameplay logic |

## Project Structure

### Documentation (this feature)

```text
specs/004-edit-combatant/
├── plan.md
├── research.md
├── data-model.md
├── quickstart.md
├── contracts/
└── tasks.md
```

### Source Code (repository root)

```text
packages/domain/src/
├── edit-combatant.ts          # New: pure editCombatant function
├── events.ts                  # Modified: add CombatantUpdated event
├── types.ts                   # Unchanged (Combatant, Encounter, DomainError)
├── index.ts                   # Modified: re-export editCombatant
└── __tests__/
    └── edit-combatant.test.ts # New: acceptance + invariant tests

packages/application/src/
├── edit-combatant-use-case.ts # New: use case wiring
└── index.ts                   # Modified: re-export use case

apps/web/src/
├── hooks/use-encounter.ts     # Modified: add editCombatant action
└── App.tsx                    # Modified: add rename UI control
```

**Structure Decision**: Follows the existing monorepo layout (`packages/domain` → `packages/application` → `apps/web`). Each new file mirrors the pattern established by `add-combatant` and `remove-combatant`.