feat(chat): limit in-game chat to one message per turn

Enforce one chat message per turn on both ends. The backend rejects a
second message in the same turn (ErrChatAlreadySentThisTurn -> 409
chat_already_sent), keyed on the move-driven turn start (turn_started_at).
The UI derives "already wrote this turn" from the message list against
GameView.lastActivityUnix (no counter, survives reopening, resets on turn
change), hides the field behind a short caption once the limit is reached,
and now reloads the game state on turn/game-state events so the toggle and
the limit follow the live game. Enter is gated on !busy to avoid a
double-send in the in-flight window.

Backend: new game.TurnStartedAt; social GameReader gains it; PostMessage
enforces the limit reusing lastMessageAt. UI: new lib/chatlimit.ts pure
logic + unit tests; Chat/ChatScreen wiring; chat.sentThisTurn and
error.chat_already_sent i18n (en/ru); extended chat e2e. Docs: FUNCTIONAL
(+ru), ARCHITECTURE, UI_DESIGN.

backend/internal/game/service.go

@@ -408,6 +408,12 @@ func (svc *Service) LastMoveAt(ctx context.Context, gameID, accountID uuid.UUID)
 	return svc.store.LastMoveAt(ctx, gameID, accountID)
 }
 
+// TurnStartedAt returns the start time of a game's current turn. The social service uses
+// it as the per-turn boundary for the one-chat-message-per-turn limit.
+func (svc *Service) TurnStartedAt(ctx context.Context, gameID uuid.UUID) (time.Time, error) {
+	return svc.store.TurnStartedAt(ctx, gameID)
+}
+
 // transition validates the actor and turn, applies op under the per-game lock and
 // commits the result.
 func (svc *Service) transition(ctx context.Context, gameID, accountID uuid.UUID, op engineOp) (MoveResult, error) {

backend/internal/game/store.go

@@ -968,6 +968,19 @@ func (s *Store) LastMoveAt(ctx context.Context, gameID, accountID uuid.UUID) (ti
 	return at.Time, true, nil
 }
 
+// TurnStartedAt returns the start time of a game's current turn (games.turn_started_at).
+// The social service uses it as the per-turn boundary for the one-chat-message-per-turn
+// limit; it advances only on a move, never on chat or a nudge.
+func (s *Store) TurnStartedAt(ctx context.Context, gameID uuid.UUID) (time.Time, error) {
+	var at time.Time
+	err := s.db.QueryRowContext(ctx,
+		`SELECT turn_started_at FROM backend.games WHERE game_id = $1`, gameID).Scan(&at)
+	if err != nil {
+		return time.Time{}, fmt.Errorf("game: turn started at %s: %w", gameID, err)
+	}
+	return at, nil
+}
+
 // RobotSchedule returns a game's bag seed and current turn-start time. The admin console
 // combines them with the robot strategy to show a robot seat's play-to-win intent and its
 // next-move ETA. Both are server-only state, never part of the public game view.

backend/internal/inttest/social_test.go

@@ -333,6 +333,29 @@ func TestChatOnlyOnYourTurn(t *testing.T) {
 	}
 }
 
+// TestChatOnePerTurn checks the one-message-per-turn limit: a second message on the same
+// turn is refused, and a fresh turn (turn_started_at advancing past the sent message)
+// reopens chat for the player.
+func TestChatOnePerTurn(t *testing.T) {
+	ctx := context.Background()
+	svc := newSocialService()
+	gameID, seats := newGameWithSeats(t, 2) // seat 0 is to move at the opening
+	if _, err := svc.PostMessage(ctx, gameID, seats[0], "first", ""); err != nil {
+		t.Fatalf("first message: %v", err)
+	}
+	if _, err := svc.PostMessage(ctx, gameID, seats[0], "second", ""); !errors.Is(err, social.ErrChatAlreadySentThisTurn) {
+		t.Fatalf("second message = %v, want ErrChatAlreadySentThisTurn", err)
+	}
+	// Advancing the turn start past the sent message (as a real move does) reopens chat.
+	if _, err := testDB.ExecContext(ctx,
+		`UPDATE backend.games SET turn_started_at = now() WHERE game_id = $1`, gameID); err != nil {
+		t.Fatalf("advance turn: %v", err)
+	}
+	if _, err := svc.PostMessage(ctx, gameID, seats[0], "next turn", ""); err != nil {
+		t.Fatalf("message on the new turn: %v", err)
+	}
+}
+
 func TestNudgeRulesAndRateLimit(t *testing.T) {
 	ctx := context.Background()
 	svc := newSocialService()

backend/internal/server/dto_test.go

@@ -25,6 +25,7 @@ func TestStatusForError(t *testing.T) {
 		"nudge own turn": {social.ErrNudgeOnOwnTurn, http.StatusConflict, "nudge_own_turn"},
 		"nudge too soon": {social.ErrNudgeTooSoon, http.StatusConflict, "nudge_too_soon"},
 		"chat off turn":  {social.ErrChatNotYourTurn, http.StatusConflict, "chat_not_your_turn"},
+		"chat one/turn":  {social.ErrChatAlreadySentThisTurn, http.StatusConflict, "chat_already_sent"},
 		"illegal play":   {engine.ErrIllegalPlay, http.StatusUnprocessableEntity, "illegal_play"},
 		"email taken":    {account.ErrEmailTaken, http.StatusConflict, "email_taken"},
 		"code mismatch":  {account.ErrCodeMismatch, http.StatusUnauthorized, "code_invalid"},

backend/internal/server/handlers.go

@@ -213,6 +213,8 @@ func statusForError(err error) (int, string) {
 		return http.StatusConflict, "nudge_too_soon"
 	case errors.Is(err, social.ErrChatNotYourTurn):
 		return http.StatusConflict, "chat_not_your_turn"
+	case errors.Is(err, social.ErrChatAlreadySentThisTurn):
+		return http.StatusConflict, "chat_already_sent"
 	case errors.Is(err, social.ErrSelfRelation):
 		return http.StatusBadRequest, "self_relation"
 	case errors.Is(err, social.ErrRequestExists):

backend/internal/social/chat.go

@@ -65,6 +65,18 @@ func (svc *Service) PostMessage(ctx context.Context, gameID, senderID uuid.UUID,
 	if idx != toMove {
 		return Message{}, ErrChatNotYourTurn
 	}
+	// At most one chat message per turn. The turn's start (move-driven, so stable for the
+	// whole turn) bounds the window: a prior message posted at or after it closes chat until
+	// the next turn.
+	turnStart, err := svc.games.TurnStartedAt(ctx, gameID)
+	if err != nil {
+		return Message{}, err
+	}
+	if last, ok, err := svc.store.lastMessageAt(ctx, gameID, senderID); err != nil {
+		return Message{}, err
+	} else if ok && !last.Before(turnStart) {
+		return Message{}, ErrChatAlreadySentThisTurn
+	}
 	sender, err := svc.accounts.GetByID(ctx, senderID)
 	if err != nil {
 		return Message{}, err

backend/internal/social/social.go

@@ -31,6 +31,10 @@ type GameReader interface {
 	// LastMoveAt is the time of an account's most recent move in a game (and whether it
 	// has moved); the nudge cooldown resets once the player has taken a turn.
 	LastMoveAt(ctx context.Context, gameID, accountID uuid.UUID) (time.Time, bool, error)
+	// TurnStartedAt is the start time of the game's current turn; it bounds the
+	// one-chat-message-per-turn limit (a message counts toward the current turn when it
+	// was posted at or after this time).
+	TurnStartedAt(ctx context.Context, gameID uuid.UUID) (time.Time, error)
 	// GameLanguage is the game's language tag ("en"/"ru"), so a nudge's out-of-app push routes
 	// to the game's bot rather than the recipient's last-login bot.
 	GameLanguage(ctx context.Context, gameID uuid.UUID) (string, error)
@@ -77,6 +81,9 @@ var (
 	// sender's turn — chat is allowed only on your own turn (the opponent's-turn control
 	// is the nudge).
 	ErrChatNotYourTurn = errors.New("social: cannot chat while it is not your turn")
+	// ErrChatAlreadySentThisTurn is returned when the sender has already posted a chat
+	// message during the current turn — at most one message is allowed per turn.
+	ErrChatAlreadySentThisTurn = errors.New("social: already sent a chat message this turn")
 )
 
 // Service is the social domain. It is the only writer of the friendships, blocks

docs/ARCHITECTURE.md

@@ -444,7 +444,10 @@ English game the Latin pool.
 - **Chat**: per-game, persisted (kept with the game's archive), **≤ 60 runes**,
   and **validated on input** — links, email addresses and phone numbers (including
   lightly obfuscated forms) are rejected, since the chat is for quick reactions,
-  not contact exchange. Each message stores the sender's IP (forwarded by the
+  not contact exchange. Chat is allowed **only on the sender's own turn** and
+  **at most once per turn** (the turn boundary is the move-driven turn start; the
+  opponent's-turn control is the nudge); the backend enforces both and the client
+  mirrors them by hiding the field. Each message stores the sender's IP (forwarded by the
   gateway) for moderation. A sender who has disabled chat cannot post,
   and messages from a blocked sender are hidden from the viewer. The operator console
   has a **Messages** section that lists posted messages (nudges excluded)

docs/FUNCTIONAL.md

@@ -155,7 +155,9 @@ and/or friend requests — and block individual players (a per-user block hides
 person's chat and stops requests and game invitations both ways; it also ends any
 existing friendship). Per-game chat is for quick reactions: messages are short
 (up to 60 characters) and may not contain links, email addresses or phone numbers,
-even disguised. Nudge the player whose turn is awaited at most once per hour (the
+even disguised. You may send **one message per turn, on your own turn**; once it is sent
+the field gives way to a short caption until your next turn. Nudge the player whose turn
+is awaited at most once per hour (the
 nudge is part of the game chat); the out-of-app push is delivered via the platform.
 Chat and the word-check tool share one **comms screen** with **💬 chat** / **🔎 dictionary**
 tabs, reached from the 💬 in the move-history header (with a back to the game); a new chat

docs/FUNCTIONAL_ru.md

@@ -159,7 +159,9 @@ nudge) приходят от бота **этой партии** — по язы
 и блокировка конкретного игрока (пер-юзер блок скрывает его чат и запрещает заявки
 и приглашения в игру в обе стороны, а также расторгает уже имеющуюся дружбу). Чат
 партии — для быстрых реакций: сообщения короткие (до 60 символов) и не должны
-содержать ссылок, email и телефонов, даже завуалированных. Nudge ожидаемого
+содержать ссылок, email и телефонов, даже завуалированных. В свой ход можно отправить
+**одно сообщение за ход**; после отправки поле сменяется короткой подписью до следующего
+хода. Nudge ожидаемого
 соперника — не чаще раза в час (nudge — часть игрового чата); внеприложенческий
 push доставляется через платформу.
 Чат и инструмент проверки слова — один **экран связи** со вкладками **💬 чат** / **🔎 словарь**,

docs/UI_DESIGN.md

@@ -248,7 +248,10 @@ IV 🏅; active games show Your move 🟢 / Opponent's move ⏳; invitations use
 - **Finished game**: the board keeps no last-word highlight and no zoom; the history header
   offers *Export GCG* (not *Drop game*) and the comms hub hides the 🔎 *Dictionary* tab; and
   the footer (rack + tab bar) is **drawn but inert** (greyed, non-interactive) rather than
-  hidden, so the layout does not jump. Chat send / nudge are the ⬆️ / 🛎️ icons.
+  hidden, so the layout does not jump. Chat send / nudge are the ⬆️ / 🛎️ icons. The input
+  row is turn-driven: on your turn the message field shows until your one allowed message is
+  sent, then a short caption replaces it until the next turn; on the opponent's turn the 🛎️
+  nudge takes the field's place.
 
 ## Caveat
 

ui/e2e/social.spec.ts

@@ -230,7 +230,7 @@ test('link account: the Telegram web sign-in control is offered in a browser', a
   await expect(page.getByRole('button', { name: 'Link Telegram' })).toBeVisible();
 });
 
-test('chat: the message field shows on your turn, the nudge replaces it otherwise', async ({ page }) => {
+test('chat: one message per turn — the field shows, then a caption replaces it after sending', async ({ page }) => {
   await loginLobby(page);
   await page.getByRole('button', { name: /Ann/ }).click(); // g1: your turn
   await page.locator('.scoreboard').click(); // open the history
@@ -241,4 +241,11 @@ test('chat: the message field shows on your turn, the nudge replaces it otherwis
   // through the aria-label.
   await expect(page.getByRole('button', { name: 'Send' })).toBeVisible();
   await expect(page.getByRole('button', { name: 'Nudge' })).toHaveCount(0);
+  // The limit is one message per turn: after sending, the field is hidden behind a caption.
+  // It is still your turn, so the nudge does not appear either.
+  await page.getByPlaceholder(/Quick message/).fill('hi there');
+  await page.getByRole('button', { name: 'Send' }).click();
+  await expect(page.getByText('You can write again next turn.')).toBeVisible();
+  await expect(page.getByRole('button', { name: 'Send' })).toHaveCount(0);
+  await expect(page.getByRole('button', { name: 'Nudge' })).toHaveCount(0);
 });

ui/src/game/Chat.svelte

@@ -8,6 +8,7 @@
     myId,
     busy,
     myTurn = false,
+    canSend = false,
     waiting = false,
     nudgeOnCooldown = false,
     onsend,
@@ -16,11 +17,16 @@
     messages: ChatMessage[];
     myId: string;
     busy: boolean;
-    // Chat and nudge are mutually exclusive by turn: on the player's own turn the
-    // message field + send are shown (and nudging makes no sense — there is no one to
-    // hurry); on the opponent's turn only the nudge button shows. While the hourly nudge
-    // cooldown is active the nudge is disabled with an "awaiting reply" caption.
+    // The input area has three turn-driven states: on your own turn the message field +
+    // send show while you may still post (canSend); once the one allowed message is sent a
+    // short caption replaces them until the next turn; on the opponent's turn only the
+    // nudge button shows (nudging your own turn makes no sense — there is no one to hurry).
+    // While the hourly nudge cooldown is active the nudge is disabled with an "awaiting
+    // reply" caption.
     myTurn?: boolean;
+    // canSend is true while the player may still post this turn (own turn and the per-turn
+    // limit not yet reached); it gates the message field.
+    canSend?: boolean;
     // waiting is true while an auto-match game still has no opponent: both send and nudge
     // are disabled (there is no one to message or hurry yet).
     waiting?: boolean;
@@ -53,14 +59,18 @@
     {/each}
   </div>
   <div class="input">
-    {#if myTurn}
+    {#if canSend}
       <input
         maxlength="60"
         placeholder={t('chat.placeholder')}
         bind:value={text}
-        onkeydown={(e) => e.key === 'Enter' && send()}
+        onkeydown={(e) => e.key === 'Enter' && !busy && send()}
       />
       <button class="iconbtn" onclick={send} disabled={busy || waiting || !connection.online} aria-label={t('chat.send')}>⬆️</button>
+    {:else if myTurn}
+      <!-- Own turn, the one allowed message already sent: a caption holds the row until the
+           next turn (no nudge — there is no one to hurry on your own turn). -->
+      <span class="cooldown">{t('chat.sentThisTurn')}</span>
     {:else}
       <!-- A flex:1 caption keeps the nudge pinned right whether or not the cooldown text shows. -->
       <span class="cooldown">{nudgeOnCooldown ? t('chat.awaitingReply') : ''}</span>

ui/src/game/ChatScreen.svelte

@@ -3,6 +3,7 @@
   import Chat from './Chat.svelte';
   import { gateway } from '../lib/gateway';
   import { app, handleError, clearChatUnread } from '../lib/app.svelte';
+  import { canSendChat, sentThisTurn } from '../lib/chatlimit';
   import type { ChatMessage, StateView } from '../lib/model';
 
   // The Chat tab body, hosted by CommsHub (which supplies the nav bar + tab bar). The
@@ -20,6 +21,11 @@
   const isMyTurn = $derived(
     !!view && (view.game.status === 'active' || view.game.status === 'open') && view.game.toMove === view.seat,
   );
+  // At most one chat message per turn: "already wrote this turn" is derived from the
+  // message list against the move-driven turn start (lastActivityUnix), so it resets when
+  // the turn advances. canSend hides the field once the limit is reached (and always on
+  // the opponent's turn).
+  const canSend = $derived(canSendChat(isMyTurn, view ? sentThisTurn(messages, myId, view.game.lastActivityUnix) : 0));
   // While the auto-match game still has no opponent, chat and nudge are both disabled.
   const waiting = $derived(!!view && view.game.status === 'open');
   const nudgeCooldownSecs = 3600;
@@ -46,22 +52,35 @@
       /* best-effort */
     }
   }
-  onMount(async () => {
+  // loadState (re)reads the game state that drives the chat/nudge toggle and the per-turn
+  // limit. The initial load surfaces an error; live reloads are best-effort.
+  async function loadState(initial = false) {
     try {
       view = await gateway.gameState(id, false);
     } catch (e) {
-      handleError(e);
+      if (initial) handleError(e);
     }
+  }
+  onMount(async () => {
+    await loadState(true);
     await refresh();
   });
 
-  // Live: refresh (and keep the unread cleared) on a chat / nudge for this game.
+  // Live: refresh the message list on a chat / nudge for this game, and reload the state on
+  // a turn / game-state change so the chat/nudge toggle and the per-turn limit (keyed on the
+  // turn start) follow the live game.
   $effect(() => {
     const e = app.lastEvent;
     if (!e) return;
     if ((e.kind === 'chat_message' && e.message.gameId === id) || (e.kind === 'nudge' && e.gameId === id)) {
       void refresh();
     }
+    if (
+      (e.kind === 'your_turn' || e.kind === 'opponent_moved' || e.kind === 'game_over' || e.kind === 'opponent_joined') &&
+      e.gameId === id
+    ) {
+      void loadState();
+    }
   });
   // Re-evaluate the nudge cooldown on a timer so the control re-enables on time.
   $effect(() => {
@@ -91,4 +110,4 @@
   }
 </script>
 
-<Chat {messages} {myId} {busy} myTurn={isMyTurn} {waiting} {nudgeOnCooldown} onsend={sendChat} onnudge={nudge} />
+<Chat {messages} {myId} {busy} myTurn={isMyTurn} {canSend} {waiting} {nudgeOnCooldown} onsend={sendChat} onnudge={nudge} />

ui/src/lib/chatlimit.test.ts

@@ -0,0 +1,41 @@
+import { describe, expect, it } from 'vitest';
+import { canSendChat, sentThisTurn, turnMessageLimit } from './chatlimit';
+import type { ChatMessage } from './model';
+
+function msg(senderId: string, kind: string, createdAtUnix: number): ChatMessage {
+  return { id: `${senderId}-${kind}-${createdAtUnix}`, gameId: 'g1', senderId, kind, body: kind === 'nudge' ? '' : 'hi', createdAtUnix };
+}
+
+const ME = 'me';
+const TURN_START = 1000;
+
+describe('turnMessageLimit', () => {
+  it('allows one message on your turn and none on the opponent\'s', () => {
+    expect(turnMessageLimit(true)).toBe(1);
+    expect(turnMessageLimit(false)).toBe(0);
+  });
+});
+
+describe('sentThisTurn', () => {
+  it('counts my messages posted at or after the turn start', () => {
+    const messages = [msg(ME, 'message', TURN_START), msg(ME, 'message', TURN_START + 5)];
+    expect(sentThisTurn(messages, ME, TURN_START)).toBe(2);
+  });
+  it('ignores my messages from a previous turn', () => {
+    expect(sentThisTurn([msg(ME, 'message', TURN_START - 1)], ME, TURN_START)).toBe(0);
+  });
+  it("ignores other players' messages and my own nudges", () => {
+    const messages = [msg('ann', 'message', TURN_START + 1), msg(ME, 'nudge', TURN_START + 2)];
+    expect(sentThisTurn(messages, ME, TURN_START)).toBe(0);
+  });
+});
+
+describe('canSendChat', () => {
+  it("is false on the opponent's turn", () => {
+    expect(canSendChat(false, 0)).toBe(false);
+  });
+  it('is true on your turn before sending and false after one message', () => {
+    expect(canSendChat(true, 0)).toBe(true);
+    expect(canSendChat(true, 1)).toBe(false);
+  });
+});

ui/src/lib/chatlimit.ts

@@ -0,0 +1,37 @@
+import type { ChatMessage } from './model';
+
+// The in-game chat is a per-turn reaction, not a running conversation: a player may post
+// at most one message on their own turn and none on the opponent's. The turn boundary is
+// the move-driven turn start (GameView.lastActivityUnix), so "already wrote this turn" is
+// derived from the loaded message list rather than a counter — it survives reopening the
+// chat and resets by itself when the turn advances. The backend enforces the same rule.
+
+/**
+ * turnMessageLimit is how many chat messages a player may post in the given turn state:
+ * one on the player's own turn, none on the opponent's.
+ */
+export function turnMessageLimit(isMyTurn: boolean): number {
+  return isMyTurn ? 1 : 0;
+}
+
+/**
+ * sentThisTurn counts the chat messages (nudges excluded) that the viewer myId has posted
+ * since the current turn began. turnStartUnix is the move-driven turn start in Unix
+ * seconds; a message counts toward the current turn when it was created at or after it.
+ */
+export function sentThisTurn(messages: ChatMessage[], myId: string, turnStartUnix: number): number {
+  let n = 0;
+  for (const m of messages) {
+    if (m.senderId === myId && m.kind === 'message' && m.createdAtUnix >= turnStartUnix) n += 1;
+  }
+  return n;
+}
+
+/**
+ * canSendChat reports whether the viewer may still post a chat message this turn: the count
+ * already sent is below the turn's limit. It is false on the opponent's turn (limit zero)
+ * and once the single own-turn message has been sent.
+ */
+export function canSendChat(isMyTurn: boolean, sent: number): boolean {
+  return sent < turnMessageLimit(isMyTurn);
+}

ui/src/lib/i18n/en.ts

@@ -113,6 +113,7 @@ export const en = {
   'chat.awaitingReply': "Waiting for the opponent's reply",
   'chat.empty': 'No messages yet.',
   'chat.nudged': '{name} nudged you',
+  'chat.sentThisTurn': 'You can write again next turn.',
 
   'profile.title': 'Profile',
   'profile.language': 'Language',
@@ -177,6 +178,7 @@ export const en = {
   'error.chat_rejected': 'Message rejected (too long or contains contact info).',
   'error.nudge_too_soon': "Please don't rush your opponent so often.",
   'error.chat_not_your_turn': 'You can chat only on your turn.',
+  'error.chat_already_sent': 'You can send only one message per turn.',
   'error.game_finished': 'This game is finished.',
   'error.not_a_player': 'You are not a player in this game.',
   'error.already_queued': 'You are already in the queue.',

ui/src/lib/i18n/ru.ts

@@ -114,6 +114,7 @@ export const ru: Record<MessageKey, string> = {
   'chat.awaitingReply': 'Ждём реакцию соперника',
   'chat.empty': 'Сообщений пока нет.',
   'chat.nudged': '{name} торопит вас',
+  'chat.sentThisTurn': 'Можно написать снова в следующем ходу.',
 
   'profile.title': 'Профиль',
   'profile.language': 'Язык',
@@ -178,6 +179,7 @@ export const ru: Record<MessageKey, string> = {
   'error.chat_rejected': 'Сообщение отклонено (слишком длинное или содержит контакты).',
   'error.nudge_too_soon': 'Не стоит торопить соперника так часто.',
   'error.chat_not_your_turn': 'Писать в чат можно только в свой ход.',
+  'error.chat_already_sent': 'За один ход можно отправить только одно сообщение.',
   'error.game_finished': 'Эта игра уже завершена.',
   'error.not_a_player': 'Вы не участник этой игры.',
   'error.already_queued': 'Вы уже в очереди.',