Backend infers play direction; UI previews words and gates submit on legality

A single tile that only extended a word perpendicular to the client-declared
direction was rejected: the UI always sent dir=H for one-tile plays (the
dirOverride/Controls toggle was orphaned in the Stage 7 game rework), so placing
"А" above "БАК" to form "АБАК" failed the solver's main-word-length check even
though the word is in the dictionary.

Make the backend infer a play's orientation from the placed tiles and the board
(internal/engine.resolveDirection): two or more tiles by the line they share, a
lone tile by the axis it abuts (longer word wins, horizontal on a tie). Direction
becomes an output, not an input: drop dir from the SubmitPlay/Eval wire requests
and add it to EvalResult. Journal replay keeps trusting the stored "H"/"V"
(SubmitPlayDir) so a rebuilt game matches the one committed.

UI: stop computing/sending direction; the preview now shows the words a move
forms with its total score (game.previewWords); the make-move control is disabled
until the play is confirmed legal; the "your turn" label hides while tiles are
pending. Delete the orphaned Controls.svelte.

Regenerate the FlatBuffers bindings (Go + TS) and update the gateway transcode
and the loadtest edge client to the new contract. Bake the decision into
ARCHITECTURE.md (§5/§9.1), FUNCTIONAL.md (+ _ru) and the backend README.

ui/src/game/Controls.svelte

@@ -1,101 +0,0 @@
-<script lang="ts">
-  import type { EvalResult } from '../lib/model';
-  import { t } from '../lib/i18n/index.svelte';
-
-  let {
-    preview,
-    hints,
-    busy,
-    ambiguous,
-    dir,
-    ondraw,
-    onskip,
-    onshuffle,
-    onhint,
-    ondir,
-  }: {
-    preview: EvalResult | null;
-    hints: number;
-    busy: boolean;
-    ambiguous: boolean;
-    dir: 'H' | 'V';
-    ondraw: () => void;
-    onskip: () => void;
-    onshuffle: () => void;
-    onhint: () => void;
-    ondir: () => void;
-  } = $props();
-</script>
-
-<div class="controls">
-  <div class="preview">
-    {#if preview}
-      {#if preview.legal}
-        <span class="ok">{t('game.preview', { n: preview.score })}</span>
-      {:else}
-        <span class="bad">{t('game.previewIllegal')}</span>
-      {/if}
-    {/if}
-    {#if ambiguous}
-      <button class="dir" onclick={ondir} title="direction">{dir === 'H' ? '↔' : '↕'}</button>
-    {/if}
-  </div>
-  <div class="row">
-    <button onclick={ondraw} disabled={busy}>{t('game.draw')}</button>
-    <button onclick={onskip} disabled={busy}>{t('game.skip')}</button>
-    <button onclick={onshuffle} disabled={busy}>{t('game.shuffle')}</button>
-    <button class="hint" onclick={onhint} disabled={busy || hints <= 0}>
-      {t('game.hint')}{hints > 0 ? ` (${hints})` : ''}
-    </button>
-  </div>
-</div>
-
-<style>
-  .controls {
-    display: flex;
-    flex-direction: column;
-    gap: 8px;
-  }
-  .preview {
-    min-height: 22px;
-    display: flex;
-    align-items: center;
-    gap: 10px;
-    font-weight: 600;
-  }
-  .ok {
-    color: var(--ok);
-  }
-  .bad {
-    color: var(--danger);
-  }
-  .dir {
-    margin-left: auto;
-    width: 34px;
-    height: 28px;
-    border: 1px solid var(--border);
-    background: var(--surface);
-    color: var(--text);
-    border-radius: var(--radius-sm);
-    font-size: 1rem;
-  }
-  .row {
-    display: flex;
-    gap: 6px;
-  }
-  .row button {
-    flex: 1;
-    padding: 11px 6px;
-    border: 1px solid var(--border);
-    background: var(--surface);
-    color: var(--text);
-    border-radius: var(--radius-sm);
-    font-weight: 600;
-  }
-  .row button:disabled {
-    opacity: 0.45;
-  }
-  .hint {
-    color: var(--accent);
-  }
-</style>

ui/src/game/Game.svelte

@@ -12,7 +12,7 @@
   import { connection } from '../lib/connection.svelte';
   import { GatewayError } from '../lib/client';
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
-  import type { Direction, EvalResult, MoveRecord, MoveResult, StateView, Tile } from '../lib/model';
+  import type { EvalResult, MoveRecord, MoveResult, StateView, Tile } from '../lib/model';
   import { lastMoveCells, replay } from '../lib/board';
   import { historyGrid } from '../lib/history';
   import { centre, premiumGrid } from '../lib/premiums';
@@ -42,7 +42,6 @@
   let moves = $state<MoveRecord[]>([]);
   let placement = $state<Placement>(newPlacement([]));
   let preview = $state<EvalResult | null>(null);
-  let dirOverride = $state<Direction | undefined>(undefined);
   let busy = $state(false);
   let zoomed = $state(false);
   let selected = $state<number | null>(null);
@@ -130,7 +129,6 @@
       moves = hist.moves;
       setCachedGame(id, st, hist.moves);
       selected = null;
-      dirOverride = undefined;
       await applyDraft(st);
       recompute();
       refreshRecent();
@@ -491,11 +489,11 @@
     if (previewTimer) clearTimeout(previewTimer);
     // Off-turn the composition is position-only: no score preview or evaluate.
     if (!isMyTurn) return;
-    const sub = toSubmit(placement, dirOverride);
+    const sub = toSubmit(placement);
     if (!sub) return;
     previewTimer = setTimeout(async () => {
       try {
-        preview = await gateway.evaluate(id, sub.dir, sub.tiles, variant);
+        preview = await gateway.evaluate(id, sub.tiles, variant);
       } catch {
         /* best-effort */
       }
@@ -511,17 +509,16 @@
     rackIds = r.rack.map((_, i) => i);
     placement = newPlacement(r.rack);
     selected = null;
-    dirOverride = undefined;
     recompute();
     refreshRecent();
   }
 
   async function commit() {
-    const sub = toSubmit(placement, dirOverride);
+    const sub = toSubmit(placement);
     if (!sub) return;
     busy = true;
     try {
-      applyMoveResult(await gateway.submitPlay(id, sub.dir, sub.tiles, variant));
+      applyMoveResult(await gateway.submitPlay(id, sub.tiles, variant));
       telegramHaptic('success');
       zoomed = false;
     } catch (e) {
@@ -534,7 +531,6 @@
     placement = reset(placement);
     preview = null;
     selected = null;
-    dirOverride = undefined;
     scheduleDraftSave();
   }
 
@@ -857,11 +853,11 @@
       <span>{view.bagLen === 0 ? t('game.bagEmpty') : t('game.bag', { n: view.bagLen })}</span>
       {#if gameOver}
         <strong class="over">{t('game.over')} — {resultText()}</strong>
-      {:else}
+      {:else if placement.pending.length === 0}
         <span class="turn-ind">{isMyTurn ? t('game.yourTurn') : view.game.seats[view.game.toMove]?.displayName ?? ''}</span>
       {/if}
       <span class="scores">
-        {#if preview}{preview.legal ? t('game.scores', { n: preview.score }) : t('game.previewIllegal')}{/if}
+        {#if preview}{preview.legal ? t('game.previewWords', { words: preview.words.join(', '), n: preview.score }) : t('game.previewIllegal')}{/if}
       </span>
     </div>
 
@@ -880,7 +876,7 @@
         />
       </div>
       {#if !gameOver && placement.pending.length > 0}
-        <button class="make" onclick={commit} disabled={busy || !isMyTurn || !connection.online || (preview !== null && !preview.legal)} aria-label={t('game.makeMove')}>✅</button>
+        <button class="make" onclick={commit} disabled={busy || !isMyTurn || !connection.online || !preview?.legal} aria-label={t('game.makeMove')}>✅</button>
       {/if}
     </div>
   {:else}

ui/src/gen/fbs/scrabblefb/eval-request.ts

@@ -30,37 +30,26 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-dir():string|null
-dir(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
-dir(optionalEncoding?:any):string|Uint8Array|null {
-  const offset = this.bb!.__offset(this.bb_pos, 6);
-  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
-}
-
 tiles(index: number, obj?:PlayTile):PlayTile|null {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? (obj || new PlayTile()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
 }
 
 tilesLength():number {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
 static startEvalRequest(builder:flatbuffers.Builder) {
-  builder.startObject(3);
+  builder.startObject(2);
 }
 
 static addGameId(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset) {
   builder.addFieldOffset(0, gameIdOffset, 0);
 }
 
-static addDir(builder:flatbuffers.Builder, dirOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(1, dirOffset, 0);
-}
-
 static addTiles(builder:flatbuffers.Builder, tilesOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(2, tilesOffset, 0);
+  builder.addFieldOffset(1, tilesOffset, 0);
 }
 
 static createTilesVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
@@ -80,10 +69,9 @@ static endEvalRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   return offset;
 }
 
-static createEvalRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, dirOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createEvalRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
   EvalRequest.startEvalRequest(builder);
   EvalRequest.addGameId(builder, gameIdOffset);
-  EvalRequest.addDir(builder, dirOffset);
   EvalRequest.addTiles(builder, tilesOffset);
   return EvalRequest.endEvalRequest(builder);
 }

ui/src/gen/fbs/scrabblefb/eval-result.ts

@@ -42,8 +42,15 @@ wordsLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+dir():string|null
+dir(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+dir(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 10);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
 static startEvalResult(builder:flatbuffers.Builder) {
-  builder.startObject(3);
+  builder.startObject(4);
 }
 
 static addLegal(builder:flatbuffers.Builder, legal:boolean) {
@@ -70,16 +77,21 @@ static startWordsVector(builder:flatbuffers.Builder, numElems:number) {
   builder.startVector(4, numElems, 4);
 }
 
+static addDir(builder:flatbuffers.Builder, dirOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(3, dirOffset, 0);
+}
+
 static endEvalResult(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createEvalResult(builder:flatbuffers.Builder, legal:boolean, score:number, wordsOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createEvalResult(builder:flatbuffers.Builder, legal:boolean, score:number, wordsOffset:flatbuffers.Offset, dirOffset:flatbuffers.Offset):flatbuffers.Offset {
   EvalResult.startEvalResult(builder);
   EvalResult.addLegal(builder, legal);
   EvalResult.addScore(builder, score);
   EvalResult.addWords(builder, wordsOffset);
+  EvalResult.addDir(builder, dirOffset);
   return EvalResult.endEvalResult(builder);
 }
 }

ui/src/gen/fbs/scrabblefb/submit-play-request.ts

@@ -30,37 +30,26 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-dir():string|null
-dir(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
-dir(optionalEncoding?:any):string|Uint8Array|null {
-  const offset = this.bb!.__offset(this.bb_pos, 6);
-  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
-}
-
 tiles(index: number, obj?:PlayTile):PlayTile|null {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? (obj || new PlayTile()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
 }
 
 tilesLength():number {
-  const offset = this.bb!.__offset(this.bb_pos, 8);
+  const offset = this.bb!.__offset(this.bb_pos, 6);
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
 static startSubmitPlayRequest(builder:flatbuffers.Builder) {
-  builder.startObject(3);
+  builder.startObject(2);
 }
 
 static addGameId(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset) {
   builder.addFieldOffset(0, gameIdOffset, 0);
 }
 
-static addDir(builder:flatbuffers.Builder, dirOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(1, dirOffset, 0);
-}
-
 static addTiles(builder:flatbuffers.Builder, tilesOffset:flatbuffers.Offset) {
-  builder.addFieldOffset(2, tilesOffset, 0);
+  builder.addFieldOffset(1, tilesOffset, 0);
 }
 
 static createTilesVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
@@ -80,10 +69,9 @@ static endSubmitPlayRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   return offset;
 }
 
-static createSubmitPlayRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, dirOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createSubmitPlayRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, tilesOffset:flatbuffers.Offset):flatbuffers.Offset {
   SubmitPlayRequest.startSubmitPlayRequest(builder);
   SubmitPlayRequest.addGameId(builder, gameIdOffset);
-  SubmitPlayRequest.addDir(builder, dirOffset);
   SubmitPlayRequest.addTiles(builder, tilesOffset);
   return SubmitPlayRequest.endSubmitPlayRequest(builder);
 }

ui/src/lib/client.ts

@@ -74,12 +74,12 @@ export interface GatewayClient {
   // table), and gameState's includeAlphabet asks the server to embed that table.
   gameState(gameId: string, includeAlphabet: boolean): Promise<StateView>;
   gameHistory(gameId: string): Promise<History>;
-  submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<MoveResult>;
+  submitPlay(gameId: string, tiles: PlacedTile[], variant: Variant): Promise<MoveResult>;
   pass(gameId: string): Promise<MoveResult>;
   exchange(gameId: string, tiles: string[], variant: Variant): Promise<MoveResult>;
   resign(gameId: string): Promise<MoveResult>;
   hint(gameId: string): Promise<HintResult>;
-  evaluate(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<EvalResult>;
+  evaluate(gameId: string, tiles: PlacedTile[], variant: Variant): Promise<EvalResult>;
   checkWord(gameId: string, word: string, variant: Variant): Promise<WordCheckResult>;
   complaint(gameId: string, word: string, note: string): Promise<void>;
   /** Hide a finished game from the caller's own lobby list; per-account, irreversible. */

ui/src/lib/codec.test.ts

@@ -43,7 +43,6 @@ describe('codec', () => {
     // A placed blank carries its designated letter's index with the blank flag set.
     const buf = encodeSubmitPlay(
       'g1',
-      'H',
       [
         { row: 7, col: 7, letter: 'A', blank: false },
         { row: 7, col: 8, letter: 'B', blank: true },
@@ -52,7 +51,6 @@ describe('codec', () => {
     );
     const r = fb.SubmitPlayRequest.getRootAsSubmitPlayRequest(new ByteBuffer(buf));
     expect(r.gameId()).toBe('g1');
-    expect(r.dir()).toBe('H');
     expect(r.tilesLength()).toBe(2);
     expect(r.tiles(0)?.letter()).toBe(0);
     expect(r.tiles(1)?.letter()).toBe(1);

ui/src/lib/codec.ts

@@ -87,7 +87,6 @@ export function encodeDraftSave(gameId: string, json: string): Uint8Array {
 
 export function encodeSubmitPlay(
   gameId: string,
-  dir: 'H' | 'V',
   tiles: PlacedTile[],
   variant: Variant,
 ): Uint8Array {
@@ -95,28 +94,19 @@ export function encodeSubmitPlay(
   const tileOffs = tiles.map((t) => buildPlayTile(b, t, variant));
   const vec = fb.SubmitPlayRequest.createTilesVector(b, tileOffs);
   const gid = b.createString(gameId);
-  const d = b.createString(dir);
   fb.SubmitPlayRequest.startSubmitPlayRequest(b);
   fb.SubmitPlayRequest.addGameId(b, gid);
-  fb.SubmitPlayRequest.addDir(b, d);
   fb.SubmitPlayRequest.addTiles(b, vec);
   return finish(b, fb.SubmitPlayRequest.endSubmitPlayRequest(b));
 }
 
-export function encodeEval(
-  gameId: string,
-  dir: 'H' | 'V',
-  tiles: PlacedTile[],
-  variant: Variant,
-): Uint8Array {
+export function encodeEval(gameId: string, tiles: PlacedTile[], variant: Variant): Uint8Array {
   const b = new Builder(256);
   const tileOffs = tiles.map((t) => buildPlayTile(b, t, variant));
   const vec = fb.EvalRequest.createTilesVector(b, tileOffs);
   const gid = b.createString(gameId);
-  const d = b.createString(dir);
   fb.EvalRequest.startEvalRequest(b);
   fb.EvalRequest.addGameId(b, gid);
-  fb.EvalRequest.addDir(b, d);
   fb.EvalRequest.addTiles(b, vec);
   return finish(b, fb.EvalRequest.endEvalRequest(b));
 }
@@ -377,7 +367,7 @@ export function decodeEvalResult(buf: Uint8Array): EvalResult {
   const r = fb.EvalResult.getRootAsEvalResult(new ByteBuffer(buf));
   const words: string[] = [];
   for (let i = 0; i < r.wordsLength(); i++) words.push(s(r.words(i)));
-  return { legal: r.legal(), score: r.score(), words };
+  return { legal: r.legal(), score: r.score(), words, dir: s(r.dir()) };
 }
 
 export function decodeWordCheck(buf: Uint8Array): WordCheckResult {

ui/src/lib/i18n/en.ts

@@ -67,7 +67,7 @@ export const en = {
   'game.checkWord': 'Check word',
   'game.dictionary': 'Dictionary',
   'game.dropGame': 'Drop game',
-  'game.preview': 'Scores {n}',
+  'game.previewWords': '{words}: {n}',
   'game.previewIllegal': 'Not a legal move',
   'game.chooseBlank': 'Choose a letter for the blank',
   'game.exchangeTitle': 'Select tiles to exchange',
@@ -86,7 +86,6 @@ export const en = {
   'game.check': 'Check',
   'game.checkWait': 'Please wait a moment.',
   'game.noHintOptions': 'No options with your letters.',
-  'game.scores': 'Scores: {n}',
   'game.thinking': 'thinking…',
 
   'move.pass': 'pass',

ui/src/lib/i18n/ru.ts

@@ -68,7 +68,7 @@ export const ru: Record<MessageKey, string> = {
   'game.checkWord': 'Проверить слово',
   'game.dictionary': 'Словарь',
   'game.dropGame': 'Покинуть игру',
-  'game.preview': 'Очков: {n}',
+  'game.previewWords': '{words}: {n}',
   'game.previewIllegal': 'Недопустимый ход',
   'game.chooseBlank': 'Выберите букву для бланка',
   'game.exchangeTitle': 'Выберите фишки для обмена',
@@ -87,7 +87,6 @@ export const ru: Record<MessageKey, string> = {
   'game.check': 'Проверить',
   'game.checkWait': 'Секунду, пожалуйста.',
   'game.noHintOptions': 'Нет вариантов с вашим набором.',
-  'game.scores': 'Очков: {n}',
   'game.thinking': 'думает…',
 
   'move.pass': 'пас',

ui/src/lib/mock/client.ts

@@ -205,7 +205,7 @@ export class MockGateway implements GatewayClient {
     return { gameId, moves: structuredClone(g.moves) };
   }
 
-  async submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], _variant: Variant): Promise<MoveResult> {
+  async submitPlay(gameId: string, tiles: PlacedTile[], _variant: Variant): Promise<MoveResult> {
     const g = this.game(gameId);
     const seat = this.mySeat(g);
     if (g.view.toMove !== seat) throw new GatewayError('not_your_turn');
@@ -213,6 +213,7 @@ export class MockGateway implements GatewayClient {
     let score = tiles.reduce((s, t) => s + valueForLetter(variant, t.blank ? '?' : t.letter), 0);
     if (tiles.length === 7) score += 50;
     const total = g.view.seats[seat].score + score;
+    const dir = new Set(tiles.map((t) => t.row)).size === 1 ? 'H' : 'V';
     const move = {
       player: seat,
       action: 'play' as const,
@@ -311,12 +312,13 @@ export class MockGateway implements GatewayClient {
     };
   }
 
-  async evaluate(gameId: string, _dir: 'H' | 'V', tiles: PlacedTile[], _variant: Variant): Promise<EvalResult> {
+  async evaluate(gameId: string, tiles: PlacedTile[], _variant: Variant): Promise<EvalResult> {
     const g = this.game(gameId);
-    if (tiles.length === 0) return { legal: false, score: 0, words: [] };
+    if (tiles.length === 0) return { legal: false, score: 0, words: [], dir: '' };
     let score = tiles.reduce((s, t) => s + valueForLetter(g.view.variant, t.blank ? '?' : t.letter), 0);
     if (tiles.length === 7) score += 50;
-    return { legal: true, score, words: [tiles.map((t) => t.letter).join('')] };
+    const dir = new Set(tiles.map((t) => t.row)).size === 1 ? 'H' : 'V';
+    return { legal: true, score, words: [tiles.map((t) => t.letter).join('')], dir };
   }
 
   async checkWord(_gameId: string, word: string, _variant: Variant): Promise<WordCheckResult> {

ui/src/lib/model.ts

@@ -84,6 +84,8 @@ export interface EvalResult {
   legal: boolean;
   score: number;
   words: string[];
+  /** Orientation the backend inferred for the play ("H"/"V"), empty when illegal. */
+  dir: string;
 }
 
 export interface WordCheckResult {

ui/src/lib/placement.test.ts

@@ -2,7 +2,6 @@ import { describe, expect, it } from 'vitest';
 import {
   BLANK,
   cellOccupied,
-  direction,
   isBlankSlot,
   newPlacement,
   place,
@@ -47,23 +46,11 @@ describe('placement state machine', () => {
     expect(reset(place(p, 0, 7, 7)).pending).toHaveLength(0);
   });
 
-  it('infers direction H for a row, V for a column, null for a single tile', () => {
-    let h = place(newPlacement(rack), 0, 7, 7);
-    h = place(h, 1, 7, 8);
-    expect(direction(h)).toBe('H');
-    let v = place(newPlacement(rack), 0, 7, 7);
-    v = place(v, 1, 8, 7);
-    expect(direction(v)).toBe('V');
-    expect(direction(place(newPlacement(rack), 0, 7, 7))).toBeNull();
-  });
-
-  it('builds a sorted submit payload and honours a direction override', () => {
+  it('builds a sorted submit payload and returns null when empty', () => {
     let p = place(newPlacement(rack), 1, 7, 9);
     p = place(p, 0, 7, 7);
     const sub = toSubmit(p);
-    expect(sub?.dir).toBe('H');
     expect(sub?.tiles.map((t) => t.col)).toEqual([7, 9]);
-    expect(toSubmit(place(newPlacement(rack), 0, 7, 7), 'V')?.dir).toBe('V');
     expect(toSubmit(newPlacement(rack))).toBeNull();
   });
 
@@ -78,15 +65,8 @@ describe('placement state machine', () => {
     expect(isBlankSlot(newPlacement(rack), 0)).toBe(false);
   });
 
-  it('treats a non-linear placement as no inferred direction', () => {
-    let p = place(newPlacement(rack), 0, 7, 7);
-    p = place(p, 1, 8, 8); // diagonal
-    expect(direction(p)).toBeNull();
-  });
-
-  it('defaults a single-tile submit to H without an override', () => {
+  it('submits a single tile as a one-tile payload', () => {
     const sub = toSubmit(place(newPlacement(rack), 0, 7, 7));
-    expect(sub?.dir).toBe('H');
     expect(sub?.tiles).toHaveLength(1);
   });
 });

ui/src/lib/placement.ts

@@ -4,7 +4,7 @@
 // payload. It is board-agnostic (the gateway/engine does full legality validation at
 // submit), which keeps it trivially unit-testable.
 
-import type { Direction, Tile } from './model';
+import type { Tile } from './model';
 import type { PlacedTile } from './client';
 
 export interface PendingTile {
@@ -119,30 +119,13 @@ export function reorderIndices(n: number, from: number, toSlot: number): number[
   return order;
 }
 
-/**
- * direction infers the play orientation from the pending tiles: H if they share a row,
- * V if they share a column, null if a single tile (ambiguous) or non-linear (invalid).
- */
-export function direction(p: Placement): Direction | null {
-  if (p.pending.length < 2) return null;
-  const rows = new Set(p.pending.map((t) => t.row));
-  const cols = new Set(p.pending.map((t) => t.col));
-  if (rows.size === 1 && cols.size === p.pending.length) return 'H';
-  if (cols.size === 1 && rows.size === p.pending.length) return 'V';
-  return null;
-}
-
-/** toSubmit builds the submit payload. dirOverride resolves a single-tile play, where
- * the orientation cannot be inferred; otherwise the inferred direction is used. */
-export function toSubmit(
-  p: Placement,
-  dirOverride?: Direction,
-): { dir: Direction; tiles: PlacedTile[] } | null {
+/** toSubmit builds the submit payload: the placed tiles in board order. The backend
+ * infers the play's orientation from the tiles and the board, so none is sent. */
+export function toSubmit(p: Placement): { tiles: PlacedTile[] } | null {
   if (p.pending.length === 0) return null;
-  const dir = dirOverride ?? direction(p) ?? 'H';
   const tiles: PlacedTile[] = p.pending
     .slice()
     .sort((a, b) => a.row - b.row || a.col - b.col)
     .map((t) => ({ row: t.row, col: t.col, letter: t.letter, blank: t.blank }));
-  return { dir, tiles };
+  return { tiles };
 }

ui/src/lib/transport.ts

@@ -97,8 +97,8 @@ export function createTransport(baseUrl: string): GatewayClient {
     async gameHistory(id) {
       return codec.decodeHistory(await exec('game.history', codec.encodeGameAction(id)));
     },
-    async submitPlay(id, dir, tiles, variant) {
-      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, dir, tiles, variant)));
+    async submitPlay(id, tiles, variant) {
+      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, tiles, variant)));
     },
     async pass(id) {
       return codec.decodeMoveResult(await exec('game.pass', codec.encodeGameAction(id)));
@@ -112,8 +112,8 @@ export function createTransport(baseUrl: string): GatewayClient {
     async hint(id) {
       return codec.decodeHintResult(await exec('game.hint', codec.encodeGameAction(id)));
     },
-    async evaluate(id, dir, tiles, variant) {
-      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, dir, tiles, variant)));
+    async evaluate(id, tiles, variant) {
+      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, tiles, variant)));
     },
     async checkWord(id, word, variant) {
       return codec.decodeWordCheck(await exec('game.check_word', codec.encodeCheckWord(id, word, variant)));