The client's eye learns the VISIONSTONE

The engine always honored the stone — its bearer casts through exactly
one wall or door — but the targeting shadow never did: sightedCellsFor
kept every square behind a wall dimmed, so the aid told the bearer the
stone wasn't working. The sighted set now retries each unseen square
with a single edge removed, mirroring casterLos; the automaton shares
the function, so bots see through their stones too. Two tests pin the
engine ruling and the client parity.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015RCWSTnb1KYTPyL4GmhGnF

packages/engine/src/view.ts

@@ -200,6 +200,26 @@ export function sightedCellsFor(view: GameView): Set<string> {
     const [x, y] = key.split(",").map(Number) as [number, number];
     if (sightBetween(board, me.position, { x, y }, blockers)) out.add(key);
   }
+  // VISIONSTONE lets its bearer see through exactly one wall or door —
+  // any one — so a square is also sighted if removing a single edge
+  // reveals it, mirroring the engine's casterLos.
+  if (me.displayed.some((c) => c.cardId === "visionstone")) {
+    const unseen = Object.keys(board.cells).filter((k) => !out.has(k));
+    for (const edgeK of Object.keys(board.edges)) {
+      if (unseen.length === 0) break;
+      if ((board.edges[edgeK] ?? "open") === "open") continue;
+      const edges = { ...board.edges };
+      delete edges[edgeK];
+      const opened = { ...board, edges };
+      for (let i = unseen.length - 1; i >= 0; i--) {
+        const [x, y] = unseen[i]!.split(",").map(Number) as [number, number];
+        if (sightBetween(opened, me.position, { x, y }, blockers)) {
+          out.add(unseen[i]!);
+          unseen.splice(i, 1);
+        }
+      }
+    }
+  }
   return out;
 }
 

packages/engine/test/casting.test.ts

@@ -2,7 +2,7 @@ import { describe, expect, it } from "vitest";
 import { applyCommand, activePlayer, boardView, createGame, type GameState } from "../src/game";
 import { cellKey, edgeKey, hasLineOfSight, neighbor, type Cell, type Side, SIDES, stepTarget } from "../src/board";
 import type { CardInstance } from "../src/cards";
-import { viewFor } from "../src/view";
+import { sightedCellsFor, viewFor } from "../src/view";
 import { newGame, must, giveCard, toRound2, faceOff } from "./helpers";
 
 /** Test surgery: put a specific card into a player's hand (swapping one out). */
@@ -500,6 +500,42 @@ describe("zero-damage utility attacks", () => {
   });
 });
 
+describe("the VISIONSTONE pierces one wall", () => {
+  function stoneRig() {
+    let { state } = newGame();
+    state = toRound2(state);
+    const a = activePlayer(state);
+    const b = state.players.find((p) => p.id !== a.id)!;
+    a.position = { x: 2, y: 4 };
+    b.position = { x: 2, y: 5 };
+    state.edgeOverrides[edgeKey({ x: 2, y: 4 }, "S")] = "wall";
+    return { state, a, b };
+  }
+
+  it("its bearer casts an attack straight through the wall", () => {
+    const { state, a, b } = stoneRig();
+    const fb = giveCard(state, a.id, "fireball", "FB", 1);
+    const refused = applyCommand(state, a.id, {
+      type: "cast", instanceId: fb.instanceId, target: { kind: "player", playerId: b.id },
+    });
+    expect(refused.ok).toBe(false);
+    const stone = giveCard(state, a.id, "visionstone", "VS");
+    a.displayed.push(stone.instanceId);
+    const cast = applyCommand(state, a.id, {
+      type: "cast", instanceId: fb.instanceId, target: { kind: "player", playerId: b.id },
+    });
+    expect(cast.ok).toBe(true);
+  });
+
+  it("the client's sighted squares agree with the stone", () => {
+    const { state, a, b } = stoneRig();
+    expect(sightedCellsFor(viewFor(state, a.id)).has(cellKey(b.position))).toBe(false);
+    const stone = giveCard(state, a.id, "visionstone", "VS");
+    a.displayed.push(stone.instanceId);
+    expect(sightedCellsFor(viewFor(state, a.id)).has(cellKey(b.position))).toBe(true);
+  });
+});
+
 describe("teleport wraps the maze's rim", () => {
   it("four spaces counted through a warp mouth reach the far side", () => {
     let { state } = newGame();