diff --git a/ui/src/game/Game.svelte b/ui/src/game/Game.svelte
index 9c95ce3..5652454 100644
--- a/ui/src/game/Game.svelte
+++ b/ui/src/game/Game.svelte
@@ -499,11 +499,13 @@
   // The composed play's word geometry (client-side, independent of the move evaluator): the cells a
   // formed word runs through — greened on the board — and the main word that anchors the score
   // badge. Derived only for a legal preview, and never while dragging a staged tile back over the
-  // rack (recallOverRack), so the highlight and badge clear the instant a recall begins.
+  // rack (recallOverRack), so the highlight and badge clear the instant a recall begins. The play's
+  // orientation is the preview's own (`dir`, which every preview source fills in for a legal play)
+  // rather than a second, independent guess — see lib/formed.ts.
   const EMPTY_FORMED = new Set<string>();
   const formedGeom = $derived(
     preview?.legal && !recallOverRack
-      ? formedGeometry(board, placement.pending, !!view?.game.multipleWordsPerTurn)
+      ? formedGeometry(board, placement.pending, !!view?.game.multipleWordsPerTurn, preview.dir === 'V' ? 'V' : 'H')
       : null,
   );
   const formedCells = $derived(formedGeom?.cells ?? EMPTY_FORMED);
diff --git a/ui/src/lib/formed.engine.test.ts b/ui/src/lib/formed.engine.test.ts
new file mode 100644
index 0000000..abe8bfc
--- /dev/null
+++ b/ui/src/lib/formed.engine.test.ts
@@ -0,0 +1,223 @@
+// Differential test: the board highlight (lib/formed.ts) against the engine port
+// (lib/dict/{direction,validate}.ts) that decides what the play actually is.
+//
+// The two are separate implementations by necessity — the engine names the words, the highlight
+// names the cells — and they drifted once already: formedGeometry used to infer the play's
+// orientation itself, and for a lone staged tile under the single-word rule its "longer run wins"
+// guess disagreed with the engine, which settles that case through the validator. The result was a
+// legal vertical play greening the horizontal run it merely abuts. Nothing in either unit suite
+// caught it, because each was right about its own half.
+//
+// So this walks random positions, asks the engine for every legal play it can find, and asserts the
+// highlighted cells are exactly the cells of the words the engine says the play forms — and that
+// the highlighted main word runs along the engine's orientation. Two dictionaries are used: one
+// that accepts every run (the widest set of legal plays) and one that accepts a pseudo-random half
+// of them (so the two orientations of a lone tile genuinely differ in legality, which is what makes
+// the engine's validator tie-break matter).
+
+import { describe, expect, it } from 'vitest';
+import { formedGeometry, type Dir } from './formed';
+import { emptyBoard, type Board } from './board';
+import { playDirection } from './dict/direction';
+import {
+  validatePlay,
+  Horizontal,
+  type Board as EngineBoard,
+  type Dict,
+  type Move,
+  type Placement,
+  type Ruleset,
+} from './dict/validate';
+import { BOARD_SIZE } from './premiums';
+
+/** Size of the toy alphabet the fuzz draws letters from (indices 0..LETTERS-1). */
+const LETTERS = 6;
+
+/** rng is a small deterministic PRNG (mulberry32), so a failure is always reproducible by seed. */
+function rng(seed: number): () => number {
+  let a = seed >>> 0;
+  return () => {
+    a = (a + 0x6d2b79f5) | 0;
+    let t = Math.imul(a ^ (a >>> 15), 1 | a);
+    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+  };
+}
+
+/** Grid maps a "row,col" key to the letter index committed there. */
+type Grid = Map<string, number>;
+
+// engineBoardOf adapts a grid into the validator's read view.
+function engineBoardOf(grid: Grid): EngineBoard {
+  const inBounds = (r: number, c: number): boolean => r >= 0 && r < BOARD_SIZE && c >= 0 && c < BOARD_SIZE;
+  return {
+    inBounds,
+    filled: (r, c) => inBounds(r, c) && grid.has(`${r},${c}`),
+    cellAt: (r, c) => ({ letter: grid.get(`${r},${c}`)!, blank: false }),
+    isEmpty: () => grid.size === 0,
+  };
+}
+
+// clientBoardOf adapts the same grid into the client's letter-space board.
+function clientBoardOf(grid: Grid): Board {
+  const b = emptyBoard();
+  for (const [k, letter] of grid) {
+    const [r, c] = k.split(',').map(Number);
+    b[r][c] = { letter: String.fromCharCode(97 + letter), blank: false };
+  }
+  return b;
+}
+
+// ruleset builds a flat-premium ruleset (the premiums are irrelevant to geometry) with distinct
+// letter values, so two legal orientations of one tile can differ in score and exercise the
+// engine's higher-scoring tie-break.
+function ruleset(ignoreCrossWords: boolean): Ruleset {
+  return {
+    cols: BOARD_SIZE,
+    center: 7 * BOARD_SIZE + 7,
+    rackSize: 7,
+    bingo: 50,
+    values: Array.from({ length: LETTERS }, (_, i) => i + 1),
+    letterMult: () => 1,
+    wordMult: () => 1,
+    ignoreCrossWords,
+  };
+}
+
+/** acceptAll is a dictionary holding every run of two or more letters. */
+const acceptAll: Dict = { indexOf: (w) => (w.length >= 2 ? 1 : -1) };
+
+/** acceptHalf holds a deterministic pseudo-random half of the runs, so legality is axis-dependent. */
+const acceptHalf: Dict = {
+  indexOf: (w) => {
+    if (w.length < 2) return -1;
+    let h = 2166136261;
+    for (let i = 0; i < w.length; i++) h = Math.imul(h ^ (w[i] as number), 16777619);
+    return (h >>> 0) % 2 === 0 ? 1 : -1;
+  },
+};
+
+// engineCells collects every cell covered by the words the engine says the play forms.
+function engineCells(m: Move): Set<string> {
+  const out = new Set<string>();
+  for (const w of [m.main, ...m.cross]) {
+    for (let i = 0; i < w.letters.length; i++) {
+      const r = w.dir === Horizontal ? w.row : w.row + i;
+      const c = w.dir === Horizontal ? w.col + i : w.col;
+      out.add(`${r},${c}`);
+    }
+  }
+  return out;
+}
+
+// position lays a handful of random runs around the centre, giving hooks and abutments to play off.
+function position(rnd: () => number): Grid {
+  const grid: Grid = new Map();
+  const runs = 1 + Math.floor(rnd() * 5);
+  for (let i = 0; i < runs; i++) {
+    const horiz = rnd() < 0.5;
+    const len = 2 + Math.floor(rnd() * 4);
+    const row = 4 + Math.floor(rnd() * 7);
+    const col = 4 + Math.floor(rnd() * 7);
+    for (let j = 0; j < len; j++) {
+      const r = horiz ? row : row + j;
+      const c = horiz ? col + j : col;
+      if (r >= BOARD_SIZE || c >= BOARD_SIZE) break;
+      grid.set(`${r},${c}`, Math.floor(rnd() * LETTERS));
+    }
+  }
+  return grid;
+}
+
+// candidates enumerates every lone-tile play on the position plus a batch of random multi-tile line
+// plays (walking along the axis over occupied squares, so bridging plays appear too).
+function candidates(grid: Grid, rnd: () => number): Placement[][] {
+  const out: Placement[][] = [];
+  for (let r = 0; r < BOARD_SIZE; r++) {
+    for (let c = 0; c < BOARD_SIZE; c++) {
+      if (grid.has(`${r},${c}`)) continue;
+      out.push([{ row: r, col: c, letter: Math.floor(rnd() * LETTERS), blank: false }]);
+    }
+  }
+  for (let k = 0; k < 300; k++) {
+    const horiz = rnd() < 0.5;
+    const n = 2 + Math.floor(rnd() * 3);
+    const row = 2 + Math.floor(rnd() * 11);
+    const col = 2 + Math.floor(rnd() * 11);
+    const tiles: Placement[] = [];
+    let step = 0;
+    for (let i = 0; i < n; i++) {
+      for (let guard = 0; guard < 6; guard++) {
+        const r = horiz ? row : row + step;
+        const c = horiz ? col + step : col;
+        step++;
+        if (r >= BOARD_SIZE || c >= BOARD_SIZE) break;
+        if (!grid.has(`${r},${c}`)) {
+          tiles.push({ row: r, col: c, letter: Math.floor(rnd() * LETTERS), blank: false });
+          break;
+        }
+      }
+    }
+    if (tiles.length >= 2) out.push(tiles);
+  }
+  return out;
+}
+
+// compare walks `seeds` random positions under one rule and dictionary, returning how many legal
+// plays were checked and a description of every disagreement found.
+function compare(
+  multipleWords: boolean,
+  dict: Dict,
+  seeds: number,
+): { checked: number; failures: string[] } {
+  const rs = ruleset(!multipleWords);
+  let checked = 0;
+  const failures: string[] = [];
+
+  for (let seed = 1; seed <= seeds; seed++) {
+    const rnd = rng(seed);
+    const grid = position(rnd);
+    if (grid.size === 0) continue;
+    const eb = engineBoardOf(grid);
+    const cb = clientBoardOf(grid);
+
+    for (const tiles of candidates(grid, rnd)) {
+      const dir = playDirection(eb, rs, dict, tiles);
+      const res = validatePlay(eb, rs, dict, dir, tiles);
+      if (!res.legal || !res.move) continue;
+      checked++;
+
+      const wantDir: Dir = res.move.dir === Horizontal ? 'H' : 'V';
+      const want = engineCells(res.move);
+      const geom = formedGeometry(cb, tiles, multipleWords, wantDir);
+      const got = geom?.cells ?? new Set<string>();
+      const sameCells = want.size === got.size && [...want].every((k) => got.has(k));
+      if (sameCells && geom?.main.dir === wantDir) continue;
+
+      if (failures.length < 5) {
+        const at = tiles.map((t) => `${t.row},${t.col}`).join(' ');
+        failures.push(
+          `seed ${seed}: staged [${at}] — engine ${wantDir} ${[...want].sort().join(' ')}` +
+            ` / highlight ${geom?.main.dir ?? 'none'} ${[...got].sort().join(' ')}`,
+        );
+      }
+    }
+  }
+  return { checked, failures };
+}
+
+describe('formedGeometry matches the engine port', () => {
+  for (const multipleWords of [false, true]) {
+    for (const [dictName, dict] of [
+      ['every run a word', acceptAll],
+      ['half the runs words', acceptHalf],
+    ] as const) {
+      const rule = multipleWords ? 'multiple words per turn' : 'one word per turn';
+      it(`highlights exactly the engine's words (${rule}, ${dictName})`, () => {
+        const { checked, failures } = compare(multipleWords, dict, 40);
+        expect(checked).toBeGreaterThan(500); // the sweep must actually find legal plays
+        expect(failures).toEqual([]);
+      });
+    }
+  }
+});
diff --git a/ui/src/lib/formed.test.ts b/ui/src/lib/formed.test.ts
index bde1577..57ae97b 100644
--- a/ui/src/lib/formed.test.ts
+++ b/ui/src/lib/formed.test.ts
@@ -20,14 +20,24 @@ const cells = (g: ReturnType<typeof formedGeometry>) => [...(g?.cells ?? new Set
 
 describe('formedGeometry', () => {
   it('returns null with nothing staged', () => {
-    expect(formedGeometry(board(), [], true)).toBeNull();
+    expect(formedGeometry(board(), [], true, 'H')).toBeNull();
+  });
+
+  it('returns null when the staged tiles are off the named line', () => {
+    // Two tiles on different rows cannot form a horizontal play; only an illegal shape does this,
+    // and the caller never previews one.
+    const g = formedGeometry(board(), [
+      { row: 7, col: 7 },
+      { row: 8, col: 8 },
+    ], true, 'H');
+    expect(g).toBeNull();
   });
 
   it('finds a fresh horizontal word with no cross words', () => {
     const g = formedGeometry(board(), [
       { row: 7, col: 7 },
       { row: 7, col: 8 },
-    ], true);
+    ], true, 'H');
     expect(g?.main).toEqual({ row: 7, col: 7, dir: 'H', len: 2 });
     expect(cells(g)).toEqual(['7,7', '7,8']);
   });
@@ -36,14 +46,14 @@ describe('formedGeometry', () => {
     const g = formedGeometry(board(), [
       { row: 7, col: 7 },
       { row: 8, col: 7 },
-    ], true);
+    ], true, 'V');
     expect(g?.main).toEqual({ row: 7, col: 7, dir: 'V', len: 2 });
     expect(cells(g)).toEqual(['7,7', '8,7']);
   });
 
   it('extends a committed tile into the main word (committed cell in the set)', () => {
     // A committed A at (7,7); a lone tile to its right forms the two-letter main word.
-    const g = formedGeometry(board([[7, 7, 'A']]), [{ row: 7, col: 8 }], true);
+    const g = formedGeometry(board([[7, 7, 'A']]), [{ row: 7, col: 8 }], true, 'H');
     expect(g?.main).toEqual({ row: 7, col: 7, dir: 'H', len: 2 });
     expect(cells(g)).toEqual(['7,7', '7,8']);
   });
@@ -54,7 +64,7 @@ describe('formedGeometry', () => {
     const g = formedGeometry(board([[6, 8, 'X'], [8, 8, 'Y']]), [
       { row: 7, col: 7 },
       { row: 7, col: 8 },
-    ], true);
+    ], true, 'H');
     expect(g?.main).toEqual({ row: 7, col: 7, dir: 'H', len: 2 });
     expect(cells(g)).toEqual(['6,8', '7,7', '7,8', '8,8']);
   });
@@ -66,14 +76,15 @@ describe('formedGeometry', () => {
     const g = formedGeometry(board([[6, 8, 'X'], [8, 8, 'Y']]), [
       { row: 7, col: 7 },
       { row: 7, col: 8 },
-    ], false);
+    ], false, 'H');
     expect(g?.main).toEqual({ row: 7, col: 7, dir: 'H', len: 2 });
     expect(cells(g)).toEqual(['7,7', '7,8']);
   });
 
-  it('takes the longer axis as the main word for a lone connecting tile', () => {
-    // (7,8) joins a length-3 horizontal (cols 6-8) and a length-4 vertical (rows 5-8): the vertical
-    // is longer, so it is the main word and the horizontal becomes a cross word.
+  it('takes the named axis as the main word for a lone connecting tile', () => {
+    // (7,8) joins a length-3 horizontal (cols 6-8) and a length-4 vertical (rows 5-8). The engine
+    // resolved the play as vertical, so the vertical is the main word and the horizontal becomes a
+    // cross word.
     const g = formedGeometry(
       board([
         [7, 6, 'A'],
@@ -84,10 +95,32 @@ describe('formedGeometry', () => {
       ]),
       [{ row: 7, col: 8 }],
       true,
+      'V',
     );
     expect(g?.main).toEqual({ row: 5, col: 8, dir: 'V', len: 4 });
     expect(cells(g)).toEqual(['5,8', '6,8', '7,6', '7,7', '7,8', '8,8']);
   });
+
+  it('follows the engine over the longer run for a lone tile under the single-word rule', () => {
+    // The reported bug: a committed horizontal DOM (row 7, cols 7-9) and a committed X at (8,10). A
+    // lone tile at (7,10) plays the vertical two-letter word down column 10 — it merely abuts DOM's
+    // last letter without crossing it. The horizontal run through the tile is the longer one (4
+    // cells) but is no word at all, so the engine resolves the play vertical; the highlight must
+    // follow, not light up the four-cell horizontal run.
+    const g = formedGeometry(
+      board([
+        [7, 7, 'D'],
+        [7, 8, 'O'],
+        [7, 9, 'M'],
+        [8, 10, 'X'],
+      ]),
+      [{ row: 7, col: 10 }],
+      false,
+      'V',
+    );
+    expect(g?.main).toEqual({ row: 7, col: 10, dir: 'V', len: 2 });
+    expect(cells(g)).toEqual(['7,10', '8,10']);
+  });
 });
 
 describe('badgePlacement', () => {
diff --git a/ui/src/lib/formed.ts b/ui/src/lib/formed.ts
index 924371b..91d64d7 100644
--- a/ui/src/lib/formed.ts
+++ b/ui/src/lib/formed.ts
@@ -1,10 +1,13 @@
 // Client-side geometry of the play being composed: which cells the formed word(s) cover, and
 // where to anchor the move-score badge. This is deliberately independent of the move evaluator
 // (lib/dict/eval.ts) — the network `evaluate` returns legality/score/word strings but no cell
-// coordinates, and the board highlight plus the score badge must work even while the local
-// dictionary is still warming up. Legality and the score come from the preview; the geometry is
-// derived here purely from the board and the staged tiles. Both functions are pure (testable in
-// lib/formed.test.ts); the board stays the source of truth for legality.
+// coordinates. Legality, the score and the play's ORIENTATION come from the preview (every
+// preview source carries it: the on-device evaluator, the network `evaluate` and a reused hint);
+// the geometry is derived here from the board, the staged tiles and that orientation. The
+// orientation is never re-derived here — inferring it a second time is what made a lone staged
+// tile under the single-word rule highlight the perpendicular run instead of the word actually
+// played (lib/formed.engine.test.ts pins the two against each other). Both functions are pure
+// (testable in lib/formed.test.ts); the board stays the source of truth for legality.
 
 import type { Board } from './board';
 import { BOARD_SIZE } from './premiums';
@@ -74,39 +77,36 @@ function span(
 }
 
 /**
- * formedGeometry derives the composed play's word geometry from the board and the staged tiles.
- * It returns the main word (the maximal run along the play axis through the staged tiles) and the
- * set of every cell the main word and each cross word (a perpendicular run of two or more cells
- * through a staged tile) cover. Under the **single-word rule** (multipleWords = false) cross words
- * are omitted, matching the engine, which ignores them entirely — a perpendicular run there is
- * neither validated nor scored (and need not even be a real word), so highlighting it would be
- * misleading. It returns null when nothing is staged, or when the staged tiles are not on a single
- * line — a shape only an illegal play produces, which the caller never asks about (it is invoked
- * only for a legal preview). The play axis is fixed by the staged tiles when two or more are
- * colinear; a lone tile takes whichever axis forms the longer word.
+ * formedGeometry derives the composed play's word geometry from the board, the staged tiles and
+ * dir — the orientation the engine resolved for this very placement, taken from the preview. It
+ * returns the main word (the maximal run along dir through the staged tiles) and the set of every
+ * cell the main word and each cross word (a perpendicular run of two or more cells through a
+ * staged tile) cover. Under the **single-word rule** (multipleWords = false) cross words are
+ * omitted, matching the engine, which ignores them entirely — a perpendicular run there is neither
+ * validated nor scored (and need not even be a real word), so highlighting it would be misleading.
+ *
+ * dir must be the engine's own answer rather than a local guess: for a lone staged tile under the
+ * single-word rule the two orientations can differ in legality, so the engine settles it through
+ * the validator (lib/dict/direction.ts playDirection) and the geometry must follow that verdict or
+ * it lights up a run the play does not form.
+ *
+ * It returns null when nothing is staged, or when the staged tiles do not all sit on one line
+ * along dir — a shape only an illegal play produces, which the caller never asks about (it is
+ * invoked only for a legal preview).
  */
 export function formedGeometry(
   board: Board,
   pending: readonly Cell[],
   multipleWords: boolean,
+  dir: Dir,
 ): FormedGeometry | null {
   if (pending.length === 0) return null;
-  const filled = filledPredicate(board, pending);
   const first = pending[0];
+  const colinear =
+    dir === 'H' ? pending.every((p) => p.row === first.row) : pending.every((p) => p.col === first.col);
+  if (!colinear) return null;
 
-  let dir: Dir;
-  if (pending.length === 1) {
-    const h = span(filled, 'H', first.row, first.col);
-    const v = span(filled, 'V', first.row, first.col);
-    dir = h.len >= v.len ? 'H' : 'V';
-  } else if (pending.every((p) => p.row === first.row)) {
-    dir = 'H';
-  } else if (pending.every((p) => p.col === first.col)) {
-    dir = 'V';
-  } else {
-    return null;
-  }
-
+  const filled = filledPredicate(board, pending);
   const cells = new Set<string>();
 
   // Main word: the maximal run along the play axis through the staged tiles (contiguous via any