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+# Handoff: Duplicate Chess v1 — built, tested, merged to main
+
+## Session Metadata
+- Created: 2026-05-19 06:01:41
+- Project: /home/claude/bin/duplicate_chess
+- Branch: main (all work merged and pushed to `git.sethpc.xyz/Seth/duplicate_chess`)
+- Session duration: one long session — brainstorm → spec → plan → full 14-task implementation → merge.
+
+### Recent Commits (for context)
+  - 5047ad7 docs: update CLAUDE.md — v1 implemented and merged
+  - fae9f8d docs: correct Task 4 test move data in the plan
+  - 5db0410 fix: real project README and save-file version validation
+  - ead4839 feat(ui): assemble the duplicate chess sandbox app
+  - bedb5a0 feat(ui): promotion dialog
+  - Full implementation range: `9611c0a` (scaffold + spec) → `5047ad7`. 18 commits.
+
+## Handoff Chain
+
+- **Continues from**: None (first handoff — this is a brand-new project).
+- **Supersedes**: None.
+
+## Current State Summary
+
+`duplicate_chess` is a **brand-new project created this session** — a local browser
+sandbox for "duplicate chess", a four-player chess variant invented by Andrew
+Freiberg (Seth's father; also the inventor behind the sibling `blind_chess`
+project). The session ran the full superpowers pipeline: brainstorming → design
+spec → implementation plan → subagent-driven execution of all 14 plan tasks → final
+review → merge. **v1 is code-complete, all 27 engine tests pass, the build and
+typecheck are clean, and it is merged to `main` and pushed.** The one thing not
+done: a human interactive browser test (clicking through a real game). The app
+mounts and renders correctly (verified via a headless smoke render).
+
+## Codebase Understanding
+
+### Architecture Overview
+
+Single Vite + Svelte 5 + TypeScript app, **no server** (duplicate chess is
+perfect-information, so everything runs client-side — this is the key difference
+from `blind_chess`, which needs a server as its trusted view boundary).
+
+- **Engine** (`src/engine/`, pure TypeScript, DOM-free, vitest-tested): four
+  `chess.js` games (NW/NE/SW/SE). A player's legal moves = the **intersection** of
+  the moves legal on their two boards, keyed by `(from,to,promotion)`. Ghost
+  immobility, the synchronized-checkmate definition, and en-passant/castling
+  divergence all fall out of the intersection — no special-case code.
+- **UI** (`src/lib/`): a reactive store wraps the engine; the compass renders the
+  four boards as a 45°-rotated pinwheel; the triple-highlight (green = playable on
+  both boards, grey = legal on one only) is the teaching feature.
+
+### Critical Files
+
+| File | Purpose | Relevance |
+|------|---------|-----------|
+| `docs/superpowers/specs/2026-05-19-duplicate-chess-design.md` | The full design spec | Read first — variant rules, engine model, provisional rules |
+| `docs/superpowers/plans/2026-05-19-duplicate-chess-sandbox.md` | The 14-task implementation plan | What was built, task by task |
+| `src/engine/legality.ts` | `legalSyncedMoves` + `selectionHighlight` | The intersection — the heart of the variant |
+| `src/engine/game.ts` | `DuplicateGame` — 4 chess.js, history, undo, draw clocks | The single source of truth for game state |
+| `src/engine/ghosts.ts` | Ghost derivation by cross-board comparison | |
+| `src/engine/endgame.ts` | checkmate/stalemate/threefold/fifty-move; **PROVISIONAL rules** | Andrew can revise the provisional rulings — grep `PROVISIONAL` |
+| `src/lib/stores/game.svelte.ts` | Reactive store wrapping the engine | `#game` is plain (non-reactive); `view` is the `$state` snapshot |
+| `src/lib/Compass.svelte` | The four-board pinwheel + click-to-move wiring | |
+
+### Key Patterns Discovered
+
+- **The engine is DOM-free and the single source of truth.** The UI never computes
+  legality; it calls the engine and renders the result.
+- **Store reactivity:** `chess.js` objects must NOT be wrapped in a Svelte `$state`
+  proxy. The store keeps `DuplicateGame` in a plain private `#game` field and
+  exposes a plain-data `view` snapshot in `$state`, rebuilt after every change.
+- **The pinwheel rotations** (NW 225°, NE 135°, SW 315°, SE 45°) put each player's
+  army on the board edge facing their seat. Confirmed against Andrew's sketch.
+- **Tests reach real positions** via `playSymmetric` (test-helpers.ts): when all
+  four players move symmetrically the four boards stay identical, so each board is
+  an ordinary chess game — that is how the checkmate/stalemate/threefold tests
+  reach real terminal positions.
+
+## Work Completed
+
+### Tasks Finished
+
+- [x] All 14 tasks of the implementation plan, executed via subagent-driven
+  development (fresh implementer subagent per task + a combined spec/quality review
+  per task + a final whole-implementation review by an opus reviewer).
+- [x] Engine: `types, boards, game, legality, ghosts, endgame, notation` + an
+  integration test. 27 vitest tests, all passing.
+- [x] UI: the reactive store + `Board`, `Compass`, `Panel`, `PromotionDialog`,
+  `App` components.
+- [x] Two post-review fixes: a real project README, and save-file `version`
+  validation in `deserialize`.
+- [x] Merged `build-sandbox` → `main`, pushed, feature branch deleted.
+
+### Files Modified
+
+The whole project was created this session. See `git log` on `main`. New trees:
+`src/engine/` (7 modules + 6 test files), `src/lib/` (store + 4 components),
+`src/App.svelte`, `src/app.css`, plus the Vite scaffold and project docs.
+
+### Decisions Made
+
+All recorded in `DECISIONS.md`. Key ones: local sandbox first (not networked);
+single Vite app, no server; engine = 4× chess.js + intersection; compass UI as a
+pinwheel of diamonds; coordinate notation; provisional endgame rules picked by
+Claude and marked `PROVISIONAL`. One decision surfaced during the build and is NOT
+yet in DECISIONS.md — see "Blockers/Open Questions".
+
+## Pending Work
+
+## Immediate Next Steps
+
+1. **Manual interactive browser test.** Run `pnpm install && pnpm dev`, open the
+   URL, and play a real game: click a piece on a glowing board → confirm the
+   green/grey triple-highlight → click a green square → move applies to both that
+   player's boards → turn advances. Verify ghosts appear after a one-sided capture,
+   promotion dialog fires, undo / Prev / Next / Live work, Save downloads JSON and
+   Load restores it. The engine is well-tested; the UI interaction is verified only
+   by `svelte-check` + a headless smoke render so far.
+2. **Decide the scrubbing semantics** (see Open Questions) and reconcile spec §4.3.
+3. (Optional) The remaining minor follow-ups below, if they matter.
+
+### Blockers/Open Questions
+
+- [ ] **Scrub semantics — spec vs shipped code disagree.** Spec §4.3 says "making a
+  new move while scrubbed truncates history." The shipped code instead makes
+  scrubbing **view-only** (you must click "● Live" before moving). The final review
+  flagged this; the view-only behaviour is arguably cleaner. Seth to confirm which
+  to keep; then update spec §4.3 (or the code) to match.
+- [ ] **The provisional endgame rules** (spec §6) are Claude's defaults, not
+  Andrew's rulings — double-board-mate = two winners, any stalemate ends the game
+  all-draw, threefold/50-move tracked on the whole system. Andrew should confirm.
+
+### Deferred Items
+
+- `deserialize` trusts the `player` field in a save file rather than recomputing it
+  from turn order — a corrupt/hand-edited save could desync. App-written saves are
+  always consistent, so this is robustness-only. Fix: have `DuplicateGame`'s
+  constructor ignore `entry.player` and use the turn-order default.
+- Move log has no round-number column (within spec, but a nicety).
+- Spec §4.3 names a `replayTo(n)` primitive; the code uses
+  `new DuplicateGame(history.slice(0,n))` instead — functionally equivalent,
+  cosmetic naming mismatch only.
+- Networking / AI / position editor — explicitly out of v1 scope (spec §7).
+
+## Context for Resuming Agent
+
+## Important Context
+
+- **The project is DONE for v1 and merged to `main`.** There is nothing half-built.
+  A resuming agent's job is the manual browser test (#1 above) and then deciding
+  whether to ship/deploy or extend.
+- **`blind_chess` is the sibling project** (`~/bin/blind_chess`) — same inventor,
+  same homelab conventions, similar shared-engine + view-filter shape. The original
+  inventor conversation that defines duplicate chess is
+  `~/bin/blind_chess/USERFILES/4-person-chess.txt`, and Andrew's compass sketch is
+  `~/bin/blind_chess/USERFILES/4personchess.png`.
+- **Provisional rules** are isolated in `src/engine/endgame.ts` and commented
+  `PROVISIONAL (spec §6)` — grep for it to find every spot a future ruling lands.
+- v1 is **local only** — no deploy. Hosting the static `pnpm build` output behind
+  Caddy is a trivial later option (it is just static files), not done.
+
+### Assumptions Made
+
+- The interactive browser test passing is assumed but unverified — the headless
+  smoke render confirmed the app mounts and renders all four boards with no console
+  errors, but no clicks were exercised.
+- 27 is the expected test count (all in `src/engine/`; the UI has no test harness
+  by design — `svelte-check` + manual, same as `blind_chess`).
+
+### Potential Gotchas
+
+- `pnpm test` uses `--passWithNoTests` (vitest 4.x exits 1 on no test files) — a
+  deliberate scaffold choice; harmless now that tests exist.
+- A `.secrets.baseline` file exists for the global `detect-secrets` pre-commit hook
+  (it flags `pnpm-lock.yaml` SHA-512 integrity hashes as false positives).
+- `svelte-check` reports **5 warnings** — all pre-existing Vite-template `tsconfig`
+  warnings (deprecated `moduleResolution`, missing `composite`). 0 errors. The
+  warnings are not defects; ignore them or fix the template tsconfig if desired.
+- The brainstorming visual-companion mockups for the compass live under
+  `~/bin/blind_chess/.superpowers/brainstorm/.../content/` (`layout-v6.html` is the
+  approved layout) — they are in the `blind_chess` repo, not this one.
+
+## Environment State
+
+### Tools/Services Used
+
+- pnpm workspace tooling (Node 22, pnpm 10). `gitea` CLI for push.
+- Subagent-driven development for the build (sonnet implementers/reviewers, an opus
+  final reviewer).
+
+### Active Processes
+
+- None. No dev server left running.
+
+### Environment Variables
+
+- None added or required.
+
+## Related Resources
+
+- Spec: `docs/superpowers/specs/2026-05-19-duplicate-chess-design.md`
+- Plan: `docs/superpowers/plans/2026-05-19-duplicate-chess-sandbox.md`
+- `DECISIONS.md`, `IDEA.md`
+- Repo: https://git.sethpc.xyz/Seth/duplicate_chess (`main` at `5047ad7`)
+- Inventor conversation + sketch: `~/bin/blind_chess/USERFILES/4-person-chess.txt`,
+  `~/bin/blind_chess/USERFILES/4personchess.png`
+
+---
+
+**Security Reminder**: No credentials or secrets are included in this handoff.