isamu · Oct 21, 2026 · Oct 21, 2026
diff --git a/.github/workflows/pull_request.yaml b/.github/workflows/pull_request.yaml
@@ -55,6 +55,7 @@ jobs:
     - run: yarn run format:check
       if: runner.os == 'Linux'
     - run: yarn run build
+    - run: yarn run test
     # Sanity-check `npm pack` only once (Linux + Node 24) to avoid
     # 6-way artifact-name collisions; lint/build coverage is what
     # the cross-OS matrix is actually for.

diff --git a/package.json b/package.json
@@ -32,9 +32,10 @@
     "build": "vite build && vue-tsc -p tsconfig.build.json --emitDeclarationOnly",
     "typecheck": "vue-tsc --noEmit",
     "lint": "eslint src demo",
+    "test": "tsx --test tests/*.test.ts",
     "quality": "scoria",
-    "format": "prettier --write \"{src,demo}/**/*.{ts,tsx,vue,json,css}\" \"*.{ts,js,json}\"",
-    "format:check": "prettier --check \"{src,demo}/**/*.{ts,tsx,vue,json,css}\" \"*.{ts,js,json}\""
+    "format": "prettier --write \"{src,demo,tests}/**/*.{ts,tsx,vue,json,css}\" \"*.{ts,js,json}\"",
+    "format:check": "prettier --check \"{src,demo,tests}/**/*.{ts,tsx,vue,json,css}\" \"*.{ts,js,json}\""
   },
   "peerDependencies": {
     "gui-chat-protocol": "^2.3.0",
@@ -53,6 +54,7 @@
     "prettier": "^3.9.10",
     "scoria": "^0.4.1",
     "tailwindcss": "^4.3.3",
+    "tsx": "^4.23.15",
     "typescript": "~6.0.3",
     "vite": "^8.3.4",
     "vue": "^3.5.44",

diff --git a/plans/fix-34-move-position-bounds.md b/plans/fix-34-move-position-bounds.md
@@ -0,0 +1,11 @@
+# #34 — reject fractional move coordinates
+
+The bounds check (`row < 0 || row > 2 || …`) let a fractional value through, so `row: 1.5` threw a `TypeError` and `col: 0.5` was reported as an occupied square.
+
+## How
+- `src/core/position.ts`: `isOnBoard(row, col, size)` — both are whole numbers inside the board. Pure, so it is tested on its own.
+- `playTicTacToe` uses it in place of the range comparison, so these moves get the out-of-bounds response.
+- `tests/position.test.ts` covers the check in both directions and `playTicTacToe` with off-board and fractional moves. Run with `yarn test`, the same `tsx --test` setup the other plugins use; CI now runs it.
+
+## Left out
+- Validating the shape of a caller-supplied `board`.
diff --git a/src/core/logic.ts b/src/core/logic.ts
@@ -3,6 +3,9 @@
  */
 
 import type { Cell, Side, TicTacToeBoard, Command, TicTacToeState } from "./types";
+import { isOnBoard } from "./position";
+
+const BOARD_SIZE = 3;
 
 // All possible winning lines (rows, columns, diagonals)
 const WINNING_LINES: { row: number; col: number }[][] = [
@@ -54,7 +57,7 @@
 function createEmptyBoard(): TicTacToeBoard {
  const board: TicTacToeBoard = [];
  for (let i = 0; i < 3; i++) {
    board[i] = new Array(3).fill(".") as Cell[];
  }
  return board;
 }
@@ -112,7 +115,7 @@
  for (const line of WINNING_LINES) {
    const cells = line.map((pos) => board[pos.row][pos.col]);
    if (cells[0] !== "." && cells[0] === cells[1] && cells[1] === cells[2]) {
      return { winner: cells[0] as Side, winningLine: line };
    }
  }
  return { winner: null, winningLine: null };
@@ -157,7 +160,7 @@
   const { row, col, board, currentSide, playerNames } = cmd;
 
   // Validate the move
-  if (row < 0 || row > 2 || col < 0 || col > 2) {
+  if (!isOnBoard(row, col, BOARD_SIZE)) {
     const legalMoves = getLegalMoves(board);
     const counts = countPieces(board);
 
@@ -171,7 +174,7 @@
       winner: null,
       winningLine: null,
       lastAction: { type: "move", row, col },
-      error: `Invalid move: (${row}, ${col}) is out of bounds. Row and column must be 0-2.`,
+      error: `Invalid move: (${row}, ${col}) is out of bounds. Row and column must be whole numbers from 0 to 2.`,
     };
   }
 

diff --git a/src/core/position.ts b/src/core/position.ts
@@ -0,0 +1,4 @@
+/** Whether (row, col) names a square of a size×size board: whole numbers, inside it. */
+export function isOnBoard(row: number, col: number, size: number): boolean {
+  return [row, col].every((index) => Number.isInteger(index) && index >= 0 && index < size);
+}
diff --git a/tests/position.test.ts b/tests/position.test.ts
@@ -0,0 +1,60 @@
+import { test, describe } from "node:test";
+import assert from "node:assert";
+import { isOnBoard } from "../src/core/position.js";
+import { playTicTacToe } from "../src/core/logic.js";
+
+const SIZE = 3;
+const playerNames = { X: "user", O: "computer" };
+const opening = () => playTicTacToe({ action: "new_game", playerNames });
+const moveTo = (row: number, col: number) => playTicTacToe({ action: "move", row, col, board: opening().board, currentSide: "X", playerNames });
+
+describe("isOnBoard", () => {
+  test("accepts every square", () => {
+    [0, 1, 2].forEach((row) => [0, 1, 2].forEach((col) => assert.equal(isOnBoard(row, col, SIZE), true)));
+  });
+
+  test("rejects positions just outside each edge", () => {
+    [
+      [-1, 0],
+      [0, -1],
+      [SIZE, 0],
+      [0, SIZE],
+    ].forEach(([row, col]) => assert.equal(isOnBoard(row, col, SIZE), false));
+  });
+
+  test("rejects values that are not whole numbers", () => {
+    [0.5, Number.NaN, Number.POSITIVE_INFINITY, Number.NEGATIVE_INFINITY, -0.5].forEach((value) => {
+      assert.equal(isOnBoard(value, 0, SIZE), false);
+      assert.equal(isOnBoard(0, value, SIZE), false);
+    });
+  });
+
+  test("rejects a non-number from an untyped caller", () => {
+    const fromJson: { row: number } = JSON.parse('{"row":"__proto__"}');
+    assert.equal(isOnBoard(fromJson.row, 0, SIZE), false);
+  });
+});
+
+describe("playTicTacToe move coordinates", () => {
+  test("a square on the board is played", () => {
+    const state = moveTo(1, 1);
+    assert.equal(state.error, undefined);
+    assert.equal(state.board[1][1], "X");
+  });
+
+  test("a square off the board or between squares is out of bounds, not a throw", () => {
+    [
+      [-1, 0],
+      [0, -1],
+      [SIZE, 0],
+      [0, SIZE],
+      [1.5, 0],
+      [0, 0.5],
+      [Number.NaN, 0],
+    ].forEach(([row, col]) => {
+      const state = moveTo(row, col);
+      assert.match(state.error ?? "", /out of bounds/);
+      assert.deepEqual(state.board, opening().board);
+    });
+  });
+});
diff --git a/yarn.lock b/yarn.lock
@@ -107,6 +107,136 @@
   dependencies:
     tslib "^2.4.0"
 
+"@esbuild/aix-ppc64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/aix-ppc64/-/aix-ppc64-0.28.2.tgz#bf6e10303bcf2e7c686975fa52f937ec2728d8bc"
+  integrity sha512-XExcO+dvLKvVtNTibSTBej1NCAbaGhWn9Ww1ZPx80qsahhPFe/8jgWP0IchNe0F3HwkU7n8ejhH8bjonqht8mQ==
+
+"@esbuild/android-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/android-arm64/-/android-arm64-0.28.2.tgz#0c6246bc8d2c4d172aac2db3fb1190d72bd65504"
+  integrity sha512-5YfKeeI8qWfBZIX+u2xZC3Zlb3Os/gLS2sbEKM+I4ZOcsWmHS2WLysCcQZDAFRslDUU5Oiq44gf6PYN1vGwG5A==
+
+"@esbuild/android-arm@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/android-arm/-/android-arm-0.28.2.tgz#2d84ece6a4e2684d92be26ee13d42757d831c381"
+  integrity sha512-kXXoiPVVGQcnIYGOeaovwOURpniDBpSq4A03qkQ+BMQqtGG6HYap3xne9C1O1yo4TR3qxlCX5IqqmX6fFo2Lqg==
+
+"@esbuild/android-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/android-x64/-/android-x64-0.28.2.tgz#fc38d4d6358d8dc1cf53f09f7589fe436eb64801"
+  integrity sha512-O387ite7SzUyCcy3JQX4P4bLtEA7bLLkx+esve5JHnyYfNTxcVpXZo9jhdB0lTKN44gztELTdU7nS8Nr16Fs1Q==
+
+"@esbuild/darwin-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/darwin-arm64/-/darwin-arm64-0.28.2.tgz#f83afeeac1d7dac01c7a2fd012b3e451a0591fcc"
+  integrity sha512-n4KqkOQrraxHJcgjM1RvwbigfQKIKJVpM7xp+KsxiyUSrRdIXnt73VhrPAx0fV44hgfmIVKjxMN9J1t5jySVkw==
+
+"@esbuild/darwin-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/darwin-x64/-/darwin-x64-0.28.2.tgz#510147c055a795588dbbe14fd6b1b8ad0a2f30de"
+  integrity sha512-uq6suIWYP37qzGddBKPw5QEQPi6HiLGsO7UmkpfyaYNQ3D+rN6w6WfwH+nuqcGXWvawGwxOEroO4YGnFh95azw==
+
+"@esbuild/freebsd-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/freebsd-arm64/-/freebsd-arm64-0.28.2.tgz#093b9200ecf0b115ba4e5e248a7485c9c5f8bd5e"
+  integrity sha512-n+I0BTSRIoy+d6RPKnEVwql5UwBJolytvY4mAOIEJorKlqgPII8ix6slVVrfZ5Tnj7glIZvloylbB/EJPMWEXw==
+
+"@esbuild/freebsd-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/freebsd-x64/-/freebsd-x64-0.28.2.tgz#0be22b6df925d213e841ea87123af5df80b0faf7"
+  integrity sha512-78XJTJkvPs0kz2w61301PJjXl4g7q3JqiYMZ/M/yVI73EHBrCRTgkhu9oqG7vPqq+a/yadEW8aD+agKlk5xrmg==
+
+"@esbuild/linux-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-arm64/-/linux-arm64-0.28.2.tgz#1bdbc651cda9ba9995c53ed9c71ceaa65094762d"
+  integrity sha512-pW4AC0P3it8c7do9MVM4p51FzHzdM/TZrerurgRcHJ2WTa1VQ1CIq18xncfpBJw4ojkiZZrKW2yIBWBP92j6Ug==
+
+"@esbuild/linux-arm@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-arm/-/linux-arm-0.28.2.tgz#beb12ad72b84f72d28488cc1b8ee9f7eb141d753"
+  integrity sha512-XlDnu2q5yoqems+xay6wSAcg9DDD7K9RLKZEBOMZm3ckNpJBvOX20tSfby8KfrrhINDyv9V2YVZKY/SpoGJI8w==
+
+"@esbuild/linux-ia32@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-ia32/-/linux-ia32-0.28.2.tgz#b81f9d55529b45c206a46a138214b1aa6879696b"
+  integrity sha512-CYbnj78HsIeA+DhgUKgFCfvNsTHFhMMrinUrMZpDXJXKN8T3XViTZ/+wtHeVxEWY8ewSzTFN+nRmSwO2tZaLUQ==
+
+"@esbuild/linux-loong64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-loong64/-/linux-loong64-0.28.2.tgz#598667241a04c99b76ed6ef940ac50038c419f98"
+  integrity sha512-buwkd8nsph4R+ajRvw0qM5Hja/TXQow3ptzWO2EbG/cqcIkHloRrdlBtQlshyYGTNFvfkfJ5tpPLVkY4DtsPfQ==
+
+"@esbuild/linux-mips64el@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-mips64el/-/linux-mips64el-0.28.2.tgz#1c51eb9cea903f53d97b5af3b1841db70f5596ca"
+  integrity sha512-ZVykbDyk7519VwiNb9Lcj9m8XM6v5V9uKPvrEMkkEedVewf+0itkhahp4HDpgERXhwLRpWFypsGbG/J8s0QjJA==
+
+"@esbuild/linux-ppc64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-ppc64/-/linux-ppc64-0.28.2.tgz#63dd61f17ceb31a81227f413feac8a71bc2c51f2"
+  integrity sha512-CAXl+Dtd9UUuJd8pKKdwh6MLm3MUMiqMPmhZ3tTSXPqfyQ3vDl6R5hZdZ/kYojK4ofXtdfSv1tFq8XzWx3heNQ==
+
+"@esbuild/linux-riscv64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-riscv64/-/linux-riscv64-0.28.2.tgz#3763b08fde5cf25ab1facb8e7752edfe45fbfc27"
+  integrity sha512-GeXCej4IQtU1B+QlDV8W/RRvbzI3O/Stss+/bCXv4lZls5WGRtu2a+3JkA3i4qIUlMXpcHebWpF8AkJhATowuA==
+
+"@esbuild/linux-s390x@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-s390x/-/linux-s390x-0.28.2.tgz#1a137ff293a82906eb3176385bd7e8e0e5cfb7cb"
+  integrity sha512-3H1weTYZPxt/WOhByszQZybS9w5lKzUn1FDMsgEChbHWQwHYQQRfBxgCcZvPhjHfKyJjIievvMmEUawJrdY9Dg==
+
+"@esbuild/linux-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/linux-x64/-/linux-x64-0.28.2.tgz#268b36211c146ca54f8fe12c578a8d6ef8979485"
+  integrity sha512-4xTZr1FUmSoQW4XIWmit3tzQrUTZM+N3P0XV8xROKYF50XfI7xeO90+1bZvNwxIufQ9hDQVRJH5YhgPVF8A/HQ==
+
+"@esbuild/netbsd-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/netbsd-arm64/-/netbsd-arm64-0.28.2.tgz#22571ad951d62bb6accc82d8d1fad5c8c1ac0ba1"
+  integrity sha512-sSATRjPeDBg3pdgHoQfoYBob11Kk1FGa9lui5RIHZCoCkJa9QKlvl3/vKz2usCmYYjs7ymJR/2Nnsqe+Hjt5nw==
+
+"@esbuild/netbsd-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/netbsd-x64/-/netbsd-x64-0.28.2.tgz#42fcc57297eb0a0ca3f5fc475291f4c1a3f7c0de"
+  integrity sha512-lqnzCV+mM0gIADaKihiCg6ifgfU2L3h5E33rNQBN1Y4MaVGnzryzmvvf7UHxprpQdE8hpqLolJ9Rl+SkIRDpyw==
+
+"@esbuild/openbsd-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/openbsd-arm64/-/openbsd-arm64-0.28.2.tgz#9eb32af104ac3dacf4edca01f596664aab0c73ef"
+  integrity sha512-AL2qJILH7lNjrDmCQDvdxMfAUIv8KMNZOvrwAQ8i8//ntL9FflhOyMJ8OZSMBb8/AWXe3/5v5S20y3zCoZWKoQ==
+
+"@esbuild/openbsd-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/openbsd-x64/-/openbsd-x64-0.28.2.tgz#febed2402d6088225e91f20fb4ce2522ad0a4efd"
+  integrity sha512-QtiuPytchRyC4rwUKhexJdQKvDuZ6hWloi3igqPQNUJCS1/v9EiO3UTOXR6A3FoMo4fnAKbWJdqaIwhOzh8qEw==
+
+"@esbuild/openharmony-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/openharmony-arm64/-/openharmony-arm64-0.28.2.tgz#85641c3d466428bfbccea5f21c26836663fef5ce"
+  integrity sha512-WkhYDmpTjLvGlScA1rwjRUmhl4k8oXR3cIbtqWmELgU/dFeHHlEllxDvdWcNJV9rbzCexB5vz8gtNewWLgCT7Q==
+
+"@esbuild/sunos-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/sunos-x64/-/sunos-x64-0.28.2.tgz#a736f9d8962481045fc4c3e54f5479f22c870fb4"
+  integrity sha512-GPMSkTOtMnv2U2F8gxe4Io6qmVs+YKyp832Etqqxr0hFngmXQ3rzwytelm3GIn7T4VviRUlf3sOgBOiTdvaf7g==
+
+"@esbuild/win32-arm64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/win32-arm64/-/win32-arm64-0.28.2.tgz#ee5ab40fad186201b652a33f8a5eb149e9e42532"
+  integrity sha512-PIhhEkE9uPBleRBrQEJpUn7MBnibZzbGzYWPmY3x+YoVg/95zbjB4CxPPOQ8l5tYYM4mMaCthF8/1DIfBQQyWQ==
+
+"@esbuild/win32-ia32@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/win32-ia32/-/win32-ia32-0.28.2.tgz#c40d28a6d99a127da6711f2afd74b11cb63b06a7"
+  integrity sha512-YmJbfTlvU7Sdn9BB+4PRES4oB6pxgS37MAONj+hBr/cpXS1aBPKXxNnDbu+QCWPj0o9dgyxeq79g6c5P8KeuYA==
+
+"@esbuild/win32-x64@0.28.2":
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/@esbuild/win32-x64/-/win32-x64-0.28.2.tgz#b21affb804cc167c133d95f45b3a1dc1323b9a87"
+  integrity sha512-5ebpxr3nWMzrL/rnUI755Jkuee0bHL/Gq0WTF9lvcpv73wAp5eu8MfBUgWK9bhWvZjj7yX8etf/8tI8Ney695g==
+
 "@eslint-community/eslint-utils@^4.8.0", "@eslint-community/eslint-utils@^4.9.1":
   version "4.10.1"
   resolved "https://registry.yarnpkg.com/@eslint-community/eslint-utils/-/eslint-utils-4.10.1.tgz#8911bd72b2c3640a543609e0400b8c4d2e7e7cb6"
@@ -1515,6 +1645,38 @@ es-object-atoms@^1.0.0, es-object-atoms@^1.1.1:
   dependencies:
     es-errors "^1.3.0"
 
+esbuild@~0.28.0:
+  version "0.28.2"
+  resolved "https://registry.yarnpkg.com/esbuild/-/esbuild-0.28.2.tgz#0f43bd1bad955b72d24e2261e3abe5957ccf0816"
+  integrity sha512-HKVLS8dvII+xoKW9kmqxbRKrnWEXfJJr/FZhhJmiqIB0e053QNYFqOBouTMO/k5sID4MvCiUCvv8b9M4h32wIA==
+  optionalDependencies:
+    "@esbuild/aix-ppc64" "0.28.2"
+    "@esbuild/android-arm" "0.28.2"
+    "@esbuild/android-arm64" "0.28.2"
+    "@esbuild/android-x64" "0.28.2"
+    "@esbuild/darwin-arm64" "0.28.2"
+    "@esbuild/darwin-x64" "0.28.2"
+    "@esbuild/freebsd-arm64" "0.28.2"
+    "@esbuild/freebsd-x64" "0.28.2"
+    "@esbuild/linux-arm" "0.28.2"
+    "@esbuild/linux-arm64" "0.28.2"
+    "@esbuild/linux-ia32" "0.28.2"
+    "@esbuild/linux-loong64" "0.28.2"
+    "@esbuild/linux-mips64el" "0.28.2"
+    "@esbuild/linux-ppc64" "0.28.2"
+    "@esbuild/linux-riscv64" "0.28.2"
+    "@esbuild/linux-s390x" "0.28.2"
+    "@esbuild/linux-x64" "0.28.2"
+    "@esbuild/netbsd-arm64" "0.28.2"
+    "@esbuild/netbsd-x64" "0.28.2"
+    "@esbuild/openbsd-arm64" "0.28.2"
+    "@esbuild/openbsd-x64" "0.28.2"
+    "@esbuild/openharmony-arm64" "0.28.2"
+    "@esbuild/sunos-x64" "0.28.2"
+    "@esbuild/win32-arm64" "0.28.2"
+    "@esbuild/win32-ia32" "0.28.2"
+    "@esbuild/win32-x64" "0.28.2"
+
 escape-string-regexp@^4.0.0:
   version "4.0.0"
   resolved "https://registry.yarnpkg.com/escape-string-regexp/-/escape-string-regexp-4.0.0.tgz#14ba83a5d373e3d311e5afca29cf5bfad965bf34"
@@ -3137,6 +3299,15 @@ tslib@^2.4.0, tslib@^2.8.1:
   resolved "https://registry.yarnpkg.com/tslib/-/tslib-2.8.1.tgz#612efe4ed235d567e8aba5f2a5fab70280ade83f"
   integrity sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w==
 
+tsx@^4.23.15:
+  version "4.23.15"
+  resolved "https://registry.yarnpkg.com/tsx/-/tsx-4.23.15.tgz#f53901695685ad567fed0f08532f7782e631c251"
+  integrity sha512-Yiex1Ovn8z2xPpOWckIiysV1SSyRMY9BkLF++q0yKiDxCqRhosKfMg3janKkiLBwZ5c/YryloKwGZcrEmtwxKw==
+  dependencies:
+    esbuild "~0.28.0"
+  optionalDependencies:
+    fsevents "~2.3.3"
+
 type-check@^0.4.0, type-check@~0.4.0:
   version "0.4.0"
   resolved "https://registry.yarnpkg.com/type-check/-/type-check-0.4.0.tgz#07b8203bfa7056c0657050e3ccd2c37730bab8f1"