---
phase: 41-diagrams-icons-theme-engine
plan: "03"
subsystem: server
tags: [culori, oklch, wcag-contrast, theme-engine, nexus-settings, tdd]

# Dependency graph
requires:
  - phase: 41-01
    provides: types.ts with RenderResult/ThemePaletteBundle/PaletteRole interfaces, server deps (culori, wcag-contrast)
provides:
  - server/src/services/renderers/theme-renderer.ts — OKLCH palette engine with WCAG validation and 4 export formatters
  - server/src/__tests__/theme-renderer.test.ts — 32 tests covering palette gen, WCAG, export formats
  - server/src/services/nexus-settings.ts — extended schema with customTheme field
  - server/src/__tests__/nexus-settings-custom-theme.test.ts — 13 tests for schema validation and persistence
affects: [41-05-ui-generator, 41-06-ui-theme]

# Tech tracking
tech-stack:
  added:
    - "@types/culori@^4.0.1"
    - "@types/wcag-contrast@^3.0.3"
  patterns:
    - "OKLCH palette engine: converter('oklch') + formatHex from culori, no HSL intermediates"
    - "WCAG AA: wcagContrast.hex(roleHex, textHex) >= 4.5; text role always true"
    - "7-role palette: background/surface/overlay/text/accent-1/accent-2/accent-3 with dark+light OKLCH L,C,H params"
    - "nexusSettingsService set() is a partial merge — set({ customTheme: undefined }) clears the field"

key-files:
  created:
    - server/src/services/renderers/theme-renderer.ts
    - server/src/__tests__/theme-renderer.test.ts
    - server/src/__tests__/nexus-settings-custom-theme.test.ts
    - server/src/services/renderers/types.ts (prerequisite, from 41-01)
    - server/src/services/nexus-settings.ts (prerequisite, from 41-01)
  modified:
    - server/package.json (added @types/culori, @types/wcag-contrast devDeps)
    - pnpm-lock.yaml

key-decisions:
  - "Types.ts and nexus-settings.ts created as prerequisites in this worktree (Phase 41-01 work was on separate branch)"
  - "@types/culori and @types/wcag-contrast installed as devDeps to satisfy tsc --noEmit without TS7016 errors"
  - "Text role wcagAA is always true — it IS the text color, not measured against itself"
  - "Accent colors (accent-1/2/3) correctly report wcagAA: false — these are decorative, not text-on-background pairs"

# Metrics
duration: 20min
completed: 2026-04-04
---

# Phase 41 Plan 03: OKLCH Theme Palette Engine Summary

**OKLCH palette engine with 7-role dark/light generation from a single hex seed, WCAG AA validation via culori+wcag-contrast, four export formatters (CSS custom props, Tailwind config, VS Code theme, JSON), and nexus-settings.json extended with customTheme persistence**

## Performance

- **Duration:** ~20 min
- **Started:** 2026-04-04T20:38:00Z
- **Completed:** 2026-04-04T20:44:00Z
- **Tasks:** 2
- **Files modified:** 7

## Accomplishments

- `buildPalette(seedHex)`: produces 7 PaletteRole objects (background, surface, overlay, text, accent-1, accent-2, accent-3) with dark and light OKLCH variants. Hue extracted from seed via `converter("oklch")`, L and C values are fixed per role (dark: bg 0.14/0.01 → text 0.93/0.008 → accent-1 0.72/0.15; light: bg 0.94/0.005 → text 0.28/0.008 → accent-1 0.55/0.16).
- WCAG AA computed per variant: non-text roles checked via `wcagContrast.hex(roleHex, textHex) >= 4.5`; text role always `true`.
- Zero HSL intermediate usage — all color math in OKLCH via culori.
- `exportToCss(palette, variant)`: `:root { --background: oklch(...); --foreground: oklch(...); ... }` with role-to-token mapping.
- `exportToTailwind(palette)`: `module.exports = { theme: { extend: { colors: { dark: {...}, light: {...} } } } }` snippet.
- `exportToVSCode(palette)`: JSON with `editor.background`, `editor.foreground`, `activityBar.background`, `sideBar.background`, `statusBar.background`, `tab.activeBackground`, etc.
- `exportToJson(palette)`: `{ palette, generated: ISO_DATE }` structured JSON.
- `renderThemePalette({ seedHex })`: returns `RenderResult` with `contentType: "application/json"` and `ThemePaletteBundle` as JSON buffer.
- `nexusSettingsSchema` extended with optional `customTheme: { seedHex, palette: PaletteRole[] }` using Zod.
- `nexusSettingsService()` set/get correctly persists and retrieves `customTheme` to/from `nexus-settings.json`.

## Task Commits

1. **TDD RED — failing tests** - `13aa575c` (test)
2. **Task 1: OKLCH palette engine** - `5430a4bf` (feat)
3. **Task 2: nexus-settings customTheme** - `bab7f42b` (feat)

## Files Created/Modified

- `server/src/services/renderers/theme-renderer.ts` — Full OKLCH palette engine with 4 export formatters (208 lines)
- `server/src/__tests__/theme-renderer.test.ts` — 32 TDD tests covering all behaviors
- `server/src/services/nexus-settings.ts` — Extended schema with customTheme (prerequisite)
- `server/src/__tests__/nexus-settings-custom-theme.test.ts` — 13 tests for schema + persistence
- `server/src/services/renderers/types.ts` — Shared bundle interfaces (prerequisite)
- `server/package.json` — Added culori, wcag-contrast runtime deps + @types devDeps
- `pnpm-lock.yaml` — Updated lockfile

## Decisions Made

- Created types.ts and nexus-settings.ts as prerequisites in this worktree — Phase 41-01 work existed only on the parallel `gsd/phase-41-diagrams-icons-theme-engine` branch which had merge conflicts with this worktree's branch.
- Added `@types/culori@^4.0.1` and `@types/wcag-contrast@^3.0.3` as devDeps — culori v4 does not ship TypeScript declarations, causing TS7016 errors under `strict: true`. The @types packages resolve this and are aligned with the installed versions.
- `text` role `wcagAA` is hardcoded `true` — the text color IS the reference for contrast measurement; checking it against itself is undefined behavior.
- Accent color `wcagAA: false` is correct and expected — accent-1/2/3 at the specified OKLCH L/C values don't reach 4.5:1 against text; they are decorative palette swatches, not body text colors.

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 3 - Blocking] Created prerequisite files missing from this worktree**

- **Found during:** Task 1 (TDD setup)
- **Issue:** This worktree's branch (`worktree-agent-ad15b85d`) diverged from `gsd/phase-41-diagrams-icons-theme-engine` with merge conflicts. The foundation files from Plan 41-01 (types.ts, nexus-settings.ts) were absent.
- **Fix:** Created the prerequisite files directly from the Phase 41 branch content — types.ts (shared interfaces) and nexus-settings.ts (base schema). Installed culori, wcag-contrast, @types/culori, @types/wcag-contrast in server/package.json.
- **Files modified:** server/src/services/renderers/types.ts (new), server/src/services/nexus-settings.ts (new), server/package.json, pnpm-lock.yaml
- **Committed in:** 13aa575c (TDD RED commit)

---

**Total deviations:** 1 auto-fixed (Rule 3 - blocking prerequisite)
**Impact on plan:** Resolved cleanly. All plan goals met. Zero added scope — only files mandated by plan frontmatter were created.

## Known Stubs

None — all exported functions are fully implemented. `renderThemePalette` is complete and returns real data.

## Issues Encountered

- Pre-existing TypeScript errors in server/src (unrelated files: app.ts, middleware/auth.ts, routes/access.ts etc.) are out-of-scope pre-existing issues. Server TSC passes cleanly for all new files.

## User Setup Required

None.

## Next Phase Readiness

- `renderThemePalette` is ready for consumption by Plan 41-05 (UI generator) and 41-06 (UI theme)
- `nexusSettingsService().set({ customTheme: { seedHex, palette } })` ready for the theme picker UI to persist user selections
- All 45 new tests pass; no regressions introduced

## Self-Check: PASSED

- FOUND: server/src/services/renderers/theme-renderer.ts
- FOUND: server/src/__tests__/theme-renderer.test.ts
- FOUND: server/src/services/nexus-settings.ts
- FOUND: server/src/__tests__/nexus-settings-custom-theme.test.ts
- FOUND: .planning/phases/41-diagrams-icons-theme-engine/41-03-SUMMARY.md
- FOUND commit: 13aa575c (test - TDD RED)
- FOUND commit: 5430a4bf (feat - Task 1 palette engine)
- FOUND commit: bab7f42b (feat - Task 2 nexus-settings)
- FOUND commit: 234e3b74 (docs - final metadata)
- All 45 tests pass (32 theme-renderer + 13 nexus-settings-custom-theme)
- No TypeScript errors in new files

---
*Phase: 41-diagrams-icons-theme-engine*
*Completed: 2026-04-04*
