Merge pull request 'Stage 13: alphabet on the wire (UI alphabet-agnostic, TODO-4)' (#14) from feature/stage-13-alphabet-on-the-wire into master

PLAN.md

@@ -46,7 +46,7 @@ independent (see ARCHITECTURE §9.1).
 | 10 | Admin & dictionary ops (complaint review, version reload) | **done** |
 | 11 | Account linking & merge | **done** |
 | 12 | Observability & performance (telemetry, metrics, guest GC) | **done** |
-| 13 | Alphabet on the wire (UI alphabet-agnostic) | todo |
+| 13 | Alphabet on the wire (UI alphabet-agnostic) | **done** |
 | 14 | CI & deploy (multi-service, dictionary artifacts) | todo |
 
 Scaffolding is incremental: `go.work` lists only existing modules; each stage
@@ -893,6 +893,51 @@ dashboard stack; solver-publish vs clone-in-build; load expectations.
     `./backend/... ./gateway/... ./pkg/... ./platform/telegram/...`, integration stays
     `./backend/...`, and the default `none` exporter keeps CI collector-free.
 
+- **Stage 13** (interview + implementation, discharges TODO-4):
+  - **Scope = live play only** (interview): indices ride the wire for `StateView.rack`
+    (out) and `SubmitPlay`/`Evaluate`/`Exchange`/`CheckWord` (in). The **board path is
+    untouched** — `MoveRecord` (history, move results, hint), formed `words`,
+    `ComplaintRequest.word` (durable, admin-reviewed) and `WordCheckResult.word` (echo) stay
+    decoded concrete characters, so the durable journal / GCG and the §9.1 invariant are
+    unchanged. **Hard cutover**, no dual letter/index fields (single client; the fbs Go + TS
+    regenerate in one PR; no external wire consumer). Exchange moved fully to indices, a
+    blank = the shared sentinel index **255** (`engine.BlankIndex`).
+  - **Edge-mapping layering** (engineering): the engine gained a cached per-variant codec —
+    `AlphabetTable` (the `(index, letter, value)` table from the solver ruleset),
+    `LetterForIndex`, `EncodeRack`, `DecodeTiles`, `DecodeWord` — and the backend **server
+    edge** owns the index↔letter mapping. `game.Service`'s domain methods, `engine.Game` and
+    the **robot** keep a single **letter-based** play path (untouched); a new thin
+    `game.Service.GameVariant` (a single-column `SELECT variant`, cheaper than `GetGame`)
+    lets the inbound handlers resolve the variant without doubling the play-path read. The
+    **gateway carries no alphabet table** — it passes indices through verbatim; `check_word`
+    rides as repeated `?idx=` query params.
+  - **`include_alphabet` flag** (interview): `StateRequest.include_alphabet` gates the table
+    so it is not resent on every poll; the client sets it only on a **per-variant cache
+    miss** (first open of a variant), and the table then arrives with the index rack so the
+    rack is always decodable. The client caches the table in memory by variant
+    (`ui/src/lib/alphabet.ts`).
+  - **Letter case** (discovered): the solver emits **lower-case** letters and the rest of
+    the UI works in **upper case**. The wire and the journal stay lower case; the **UI
+    normalises display to upper case** (the codec upper-cases decoded board tiles and words,
+    and the alphabet cache upper-cases on ingest), so `placement.ts` / `board.ts` /
+    `checkword.ts` are unchanged and the latent real-backend lower-case display is fixed.
+  - **Parity rework** (interview): the real value/alphabet parity moved to a **Go engine
+    test** (`engine.AlphabetTable`: EN/RU/Эрудит sizes, EN a=1/q=10, **Эрудит ё=index 6,
+    value 0**); `ui/src/lib/premiums.ts` is now **geometry only** (its value tables,
+    `tileValue` and `alphabet` were removed, its parity test trimmed to the premium grid);
+    the codec test round-trips the index tiles + the alphabet table; the **mock keeps a
+    fixture table** (relocated from `premiums.ts`) seeded into the client cache, so the
+    mock-driven UI is alphabet-agnostic too.
+  - **Wire/codegen/CI**: new fbs `AlphabetEntry` + `PlayTile`; `StateView.rack`→`[ubyte]` +
+    `alphabet`; `StateRequest.include_alphabet`; `SubmitPlay`/`Eval` tiles→`[PlayTile]`;
+    `Exchange` tiles→`[ubyte]`; `CheckWord.word`→`[ubyte]` (committed Go + TS regenerated).
+    UI ~90 KB gzip JS (budget 100 KB). **No CI workflow change** — the Go workflows already
+    span `./backend/... ./gateway/... ./pkg/...` and the UI workflow runs check/unit/build +
+    a chromium/webkit e2e. `docs/FUNCTIONAL.md` is **untouched** (no user-visible behaviour
+    change — the UI looks and plays the same; like Stage 2). The index-drift caveat is
+    handled by construction (the running backend produces the table, so client↔server cannot
+    drift); the DAWG/solver build-time agreement remains **Stage 14 / TODO-2**.
+
 ## Deferred TODOs (cross-stage)
 
 - **TODO-1 — publish & version the solver.** Once `scrabble-solver` is stable,
@@ -931,17 +976,16 @@ dashboard stack; solver-publish vs clone-in-build; load expectations.
   `last_seen_at`, so a lingering session never expires and **account age** is the
   abandonment trigger, not "last session gone". The reaped guest's `sessions`/`identities`/
   `account_stats` fall away via their own `ON DELETE CASCADE`.
-- **TODO-4 — put the per-game alphabet on the wire (owner's idea, Stage 7).** Today the
-  client hardcodes each variant's letters/values (ported into `ui/src/lib/premiums.ts`
-  from `scrabble-solver/rules/rules.go`) and the edge exchanges plays/hints by concrete
-  letters. Consider extending `game.state` to carry the variant's `(letter, index,
-  value)` table so the UI stops duplicating it, and optionally moving tile exchange to
-  letter **indices** end-to-end. Caveat (as for the dictionaries, TODO-2): the wire table
-  must stay pinned to the same `rules.Alphabet` the engine uses, or indices drift.
-  **Planned for Stage 13**, expanded (owner) to a fully **alphabet-agnostic UI**: the
-  client caches the per-variant table (display only) behind an `include_alphabet` request
-  flag and exchanges indices both ways, word-check included; the durable journal stays
-  concrete characters (§9.1). See Stage 13.
+- ~~**TODO-4 — put the per-game alphabet on the wire (owner's idea, Stage 7).**~~ **Done in
+  Stage 13.** The client is now alphabet-agnostic: it caches each variant's `(index, letter,
+  value)` table — sent by the backend behind `StateRequest.include_alphabet` on a per-variant
+  cache miss — and live play exchanges **letter indices** both ways (rack, submit-play,
+  evaluate, exchange, word-check; a blank rides as the sentinel index 255). The table is
+  produced from the solver ruleset (`engine.AlphabetTable`), so it is pinned by the solver
+  version and cannot drift from the running backend, and `ui/src/lib/premiums.ts` is now
+  geometry only. The durable journal / history / GCG stay decoded concrete characters (§9.1,
+  unchanged). The DAWG/solver build-time agreement (the original caveat, shared with TODO-2)
+  remains Stage 14.
 - **TODO-5 — QR friend codes (owner's idea, Stage 8).** *Partially done in Stage 9:*
   the deep-link scheme now exists (`f<code>`, shared Go ↔ TS), the bot redeems it on
   launch, and the UI shows a **share-to-Telegram** link for an issued code when

backend/internal/engine/alphabet.go

@@ -0,0 +1,150 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+)
+
+// AlphabetEntry is one letter of a variant's alphabet: its alphabet-index byte, the
+// concrete character and its tile point value. It is the dictionary-independent display
+// table the edge sends to the client (Stage 13), produced from the variant's solver
+// ruleset (its alphabet and value table) and so pinned by the solver version, not by any
+// dictionary.
+type AlphabetEntry struct {
+	// Index is the alphabet-index byte the wire uses for this letter (0..Size-1).
+	Index byte
+	// Letter is the concrete character, in the case the solver ruleset emits (lower).
+	Letter string
+	// Value is the tile's point score.
+	Value int
+}
+
+// BlankIndex is the wire sentinel for a blank tile inside an alphabet-index sequence (a
+// rack or an exchange list). It is out of range of every offered variant's alphabet (the
+// largest has 33 letters), so it never collides with a real letter index. A placed blank
+// instead travels as an ordinary tile carrying its designated letter's index alongside a
+// separate blank flag. The constant is untyped so it serves both byte (FlatBuffers ubyte)
+// and int (the gateway/backend JSON edge) call sites.
+const BlankIndex = 0xFF
+
+// variantCodec is the cached per-variant alphabet data backing the wire helpers: the
+// ordered display table and a case-insensitive letter→index lookup. Both are derived once
+// from the solver ruleset (see variantCodecs).
+type variantCodec struct {
+	table         []AlphabetEntry
+	letterToIndex map[string]byte
+}
+
+// variantCodecs holds one codec per offered variant, built once at package load from each
+// ruleset's alphabet and value table. The rulesets are needed only here (not per request),
+// so the hot path never rebuilds them.
+var variantCodecs = buildVariantCodecs()
+
+func buildVariantCodecs() map[Variant]*variantCodec {
+	m := make(map[Variant]*variantCodec, len(Variants()))
+	for _, v := range Variants() {
+		rs, ok := v.ruleset()
+		if !ok {
+			continue
+		}
+		size := rs.Alphabet.Size()
+		table := make([]AlphabetEntry, size)
+		lut := make(map[string]byte, size)
+		for i := range size {
+			ch, err := rs.Alphabet.Character(byte(i))
+			if err != nil {
+				// An offered variant's alphabet never yields a bad index; skip defensively.
+				continue
+			}
+			table[i] = AlphabetEntry{Index: byte(i), Letter: ch, Value: rs.Values[i]}
+			lut[strings.ToLower(ch)] = byte(i)
+		}
+		m[v] = &variantCodec{table: table, letterToIndex: lut}
+	}
+	return m
+}
+
+// AlphabetTable returns a copy of variant's full alphabet as an ordered (index, letter,
+// value) table, or ErrUnknownVariant. Entry i has Index i, so the slice doubles as an
+// index→(letter, value) lookup. It needs no dictionary — the data comes from the solver
+// ruleset alone — so it is safe to build for any offered variant and is the same table the
+// client caches for display while live play exchanges bare indices.
+func AlphabetTable(v Variant) ([]AlphabetEntry, error) {
+	c, ok := variantCodecs[v]
+	if !ok {
+		return nil, fmt.Errorf("%w: %d", ErrUnknownVariant, v)
+	}
+	out := make([]AlphabetEntry, len(c.table))
+	copy(out, c.table)
+	return out, nil
+}
+
+// LetterForIndex maps one alphabet index to its concrete letter for variant. It is the
+// wire-decode primitive for a placed tile (a blank carries its designated letter's index).
+// An out-of-range index is an illegal play.
+func LetterForIndex(v Variant, idx int) (string, error) {
+	c, ok := variantCodecs[v]
+	if !ok {
+		return "", fmt.Errorf("%w: %d", ErrUnknownVariant, v)
+	}
+	if idx < 0 || idx >= len(c.table) {
+		return "", fmt.Errorf("%w: alphabet index %d for %s", ErrIllegalPlay, idx, v)
+	}
+	return c.table[idx].Letter, nil
+}
+
+// EncodeRack maps a decoded rack (the Game.Hand form: concrete letters with "?" for an
+// undesignated blank) to wire alphabet indices, using BlankIndex for each blank. It backs
+// the per-player state view, whose rack the client renders via the cached table.
+func EncodeRack(v Variant, letters []string) ([]int, error) {
+	c, ok := variantCodecs[v]
+	if !ok {
+		return nil, fmt.Errorf("%w: %d", ErrUnknownVariant, v)
+	}
+	out := make([]int, len(letters))
+	for i, l := range letters {
+		if l == blankLetter {
+			out[i] = BlankIndex
+			continue
+		}
+		idx, ok := c.letterToIndex[strings.ToLower(l)]
+		if !ok {
+			return nil, fmt.Errorf("%w: rack letter %q for %s", ErrTilesNotOnRack, l, v)
+		}
+		out[i] = int(idx)
+	}
+	return out, nil
+}
+
+// DecodeTiles maps a wire rack/exchange index list back to the decoded letter form ("?"
+// for a blank, BlankIndex), for handing to the existing letter-based exchange path.
+func DecodeTiles(v Variant, idx []int) ([]string, error) {
+	out := make([]string, len(idx))
+	for i, x := range idx {
+		if x == BlankIndex {
+			out[i] = blankLetter
+			continue
+		}
+		l, err := LetterForIndex(v, x)
+		if err != nil {
+			return nil, fmt.Errorf("%w (exchange)", err)
+		}
+		out[i] = l
+	}
+	return out, nil
+}
+
+// DecodeWord maps a sequence of alphabet indices to a concrete word (word-check carries no
+// blanks). The client constrains input to the variant's alphabet, so every index is a real
+// letter.
+func DecodeWord(v Variant, idx []int) (string, error) {
+	var sb strings.Builder
+	for _, x := range idx {
+		l, err := LetterForIndex(v, x)
+		if err != nil {
+			return "", fmt.Errorf("%w (word check)", err)
+		}
+		sb.WriteString(l)
+	}
+	return sb.String(), nil
+}

backend/internal/engine/alphabet_test.go

@@ -0,0 +1,110 @@
+package engine
+
+import (
+	"errors"
+	"slices"
+	"testing"
+)
+
+// TestAlphabetTableEnglish pins the English table against the solver ruleset: 26 letters,
+// contiguous indices, the concrete lower-case characters the solver emits and the standard
+// tile values. This is the real parity check the UI no longer carries (Stage 13).
+func TestAlphabetTableEnglish(t *testing.T) {
+	tab, err := AlphabetTable(VariantEnglish)
+	if err != nil {
+		t.Fatalf("AlphabetTable(english): %v", err)
+	}
+	if len(tab) != 26 {
+		t.Fatalf("size = %d, want 26", len(tab))
+	}
+	for i, e := range tab {
+		if int(e.Index) != i {
+			t.Errorf("entry %d has Index %d, want %d (index must equal position)", i, e.Index, i)
+		}
+	}
+	// a=index0/value1, q=index16/value10, z=index25/value10.
+	if tab[0].Letter != "a" || tab[0].Value != 1 {
+		t.Errorf("entry 0 = %q/%d, want a/1", tab[0].Letter, tab[0].Value)
+	}
+	if tab[16].Letter != "q" || tab[16].Value != 10 {
+		t.Errorf("entry 16 = %q/%d, want q/10", tab[16].Letter, tab[16].Value)
+	}
+	if tab[25].Letter != "z" || tab[25].Value != 10 {
+		t.Errorf("entry 25 = %q/%d, want z/10", tab[25].Letter, tab[25].Value)
+	}
+}
+
+// TestAlphabetTableRussianVariants pins both Russian variants: they share the 33-letter
+// alphabet but differ in tile values — most visibly ё (index 6), worth 3 in Russian
+// Scrabble and 0 in Эрудит.
+func TestAlphabetTableRussianVariants(t *testing.T) {
+	ru, err := AlphabetTable(VariantRussianScrabble)
+	if err != nil {
+		t.Fatalf("AlphabetTable(russian_scrabble): %v", err)
+	}
+	er, err := AlphabetTable(VariantErudit)
+	if err != nil {
+		t.Fatalf("AlphabetTable(erudit): %v", err)
+	}
+	if len(ru) != 33 || len(er) != 33 {
+		t.Fatalf("sizes = %d/%d, want 33/33", len(ru), len(er))
+	}
+	if ru[0].Letter != "а" || ru[0].Value != 1 {
+		t.Errorf("russian entry 0 = %q/%d, want а/1", ru[0].Letter, ru[0].Value)
+	}
+	if ru[6].Letter != "ё" || ru[6].Value != 3 {
+		t.Errorf("russian ё (entry 6) = %q/%d, want ё/3", ru[6].Letter, ru[6].Value)
+	}
+	if er[6].Letter != "ё" || er[6].Value != 0 {
+		t.Errorf("erudit ё (entry 6) = %q/%d, want ё/0", er[6].Letter, er[6].Value)
+	}
+	if ru[32].Letter != "я" || er[32].Letter != "я" {
+		t.Errorf("last letter = %q/%q, want я/я", ru[32].Letter, er[32].Letter)
+	}
+}
+
+// TestAlphabetTableUnknownVariant rejects a variant outside the catalogue.
+func TestAlphabetTableUnknownVariant(t *testing.T) {
+	if _, err := AlphabetTable(Variant(99)); !errors.Is(err, ErrUnknownVariant) {
+		t.Fatalf("got %v, want ErrUnknownVariant", err)
+	}
+}
+
+// TestRackCodecRoundTrip pins the rack/exchange index codec the edge uses: EncodeRack maps
+// concrete letters (with "?" for a blank) to indices (BlankIndex for the blank) and
+// DecodeTiles inverts it. EncodeRack is case-insensitive so it accepts the lower-case
+// Hand form and an upper-case letter alike.
+func TestRackCodecRoundTrip(t *testing.T) {
+	letters := []string{"c", "a", "t", "?"}
+	idx, err := EncodeRack(VariantEnglish, letters)
+	if err != nil {
+		t.Fatalf("EncodeRack: %v", err)
+	}
+	if want := []int{2, 0, 19, BlankIndex}; !slices.Equal(idx, want) {
+		t.Fatalf("EncodeRack = %v, want %v", idx, want)
+	}
+	back, err := DecodeTiles(VariantEnglish, idx)
+	if err != nil {
+		t.Fatalf("DecodeTiles: %v", err)
+	}
+	if !slices.Equal(back, letters) {
+		t.Fatalf("DecodeTiles = %v, want %v", back, letters)
+	}
+	if up, err := EncodeRack(VariantEnglish, []string{"C"}); err != nil || !slices.Equal(up, []int{2}) {
+		t.Errorf("EncodeRack upper-case = %v,%v; want [2],nil", up, err)
+	}
+}
+
+// TestDecodeWordAndBounds covers the word-check decode and the out-of-range guard.
+func TestDecodeWordAndBounds(t *testing.T) {
+	w, err := DecodeWord(VariantEnglish, []int{2, 0, 19})
+	if err != nil || w != "cat" {
+		t.Fatalf("DecodeWord = %q,%v; want cat,nil", w, err)
+	}
+	if _, err := LetterForIndex(VariantEnglish, 26); !errors.Is(err, ErrIllegalPlay) {
+		t.Errorf("out-of-range index: got %v, want ErrIllegalPlay", err)
+	}
+	if _, err := DecodeWord(VariantEnglish, []int{BlankIndex}); !errors.Is(err, ErrIllegalPlay) {
+		t.Errorf("blank in word: got %v, want ErrIllegalPlay", err)
+	}
+}

backend/internal/game/service.go

@@ -178,6 +178,13 @@ func (svc *Service) Resign(ctx context.Context, gameID, accountID uuid.UUID) (Mo
 	})
 }
 
+// GameVariant returns just a game's variant. The edge layer uses it to map wire alphabet
+// indices to concrete letters before delegating to the letter-based play, exchange and
+// word-check methods (Stage 13), keeping a single domain path shared with the robot.
+func (svc *Service) GameVariant(ctx context.Context, gameID uuid.UUID) (engine.Variant, error) {
+	return svc.store.GetGameVariant(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

@@ -135,6 +135,24 @@ func (s *Store) GetGame(ctx context.Context, id uuid.UUID) (Game, error) {
 	return projectGame(grow, srows)
 }
 
+// GetGameVariant reads just a game's variant — a cheap single-column lookup the edge uses
+// to map wire alphabet indices to concrete letters (Stage 13) without loading the whole
+// game and its seats.
+func (s *Store) GetGameVariant(ctx context.Context, id uuid.UUID) (engine.Variant, error) {
+	stmt := postgres.SELECT(table.Games.Variant).
+		FROM(table.Games).
+		WHERE(table.Games.GameID.EQ(postgres.UUID(id))).
+		LIMIT(1)
+	var row model.Games
+	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
+		if errors.Is(err, qrm.ErrNoRows) {
+			return 0, ErrNotFound
+		}
+		return 0, fmt.Errorf("game: get variant %s: %w", id, err)
+	}
+	return engine.ParseVariant(row.Variant)
+}
+
 // SharedGameExists reports whether accounts a and b are both seated in at least
 // one game (active or finished). It backs the social package's "befriend an
 // opponent" gate via a self-join on game_players.

backend/internal/inttest/game_test.go

@@ -428,6 +428,26 @@ func TestHintPolicy(t *testing.T) {
 	}
 }
 
+// TestGameVariant covers the edge's lightweight variant lookup (Stage 13): it returns the
+// created game's variant and ErrNotFound for an unknown id.
+func TestGameVariant(t *testing.T) {
+	ctx := context.Background()
+	svc := newGameService()
+	seats := []uuid.UUID{provisionAccount(t), provisionAccount(t)}
+	g, err := svc.Create(ctx, game.CreateParams{
+		Variant: engine.VariantEnglish, Seats: seats, TurnTimeout: time.Hour, Seed: 1,
+	})
+	if err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	if v, err := svc.GameVariant(ctx, g.ID); err != nil || v != engine.VariantEnglish {
+		t.Fatalf("GameVariant = %v, %v; want english, nil", v, err)
+	}
+	if _, err := svc.GameVariant(ctx, uuid.New()); !errors.Is(err, game.ErrNotFound) {
+		t.Errorf("GameVariant(unknown) = %v, want ErrNotFound", err)
+	}
+}
+
 // TestCheckWordAndComplaint covers the word-check tool and complaint capture.
 func TestCheckWordAndComplaint(t *testing.T) {
 	ctx := context.Background()

backend/internal/server/dto.go

@@ -101,13 +101,24 @@ type moveResultDTO struct {
 	Game gameDTO       `json:"game"`
 }
 
-// stateDTO is a player's view of a game.
+// alphabetEntryDTO is one letter of a variant's alphabet (its index, concrete letter and
+// tile value), embedded in the state view for display only when the client requests it
+// (Stage 13).
+type alphabetEntryDTO struct {
+	Index  int    `json:"index"`
+	Letter string `json:"letter"`
+	Value  int    `json:"value"`
+}
+
+// stateDTO is a player's view of a game. Rack carries wire alphabet indices (Stage 13; a
+// blank is engine.BlankIndex). Alphabet is present only when the request asked for it.
 type stateDTO struct {
-	Game           gameDTO  `json:"game"`
-	Seat           int      `json:"seat"`
-	Rack           []string `json:"rack"`
-	BagLen         int      `json:"bag_len"`
-	HintsRemaining int      `json:"hints_remaining"`
+	Game           gameDTO            `json:"game"`
+	Seat           int                `json:"seat"`
+	Rack           []int              `json:"rack"`
+	BagLen         int                `json:"bag_len"`
+	HintsRemaining int                `json:"hints_remaining"`
+	Alphabet       []alphabetEntryDTO `json:"alphabet,omitempty"`
 }
 
 // matchDTO reports whether the caller has been paired into a game.
@@ -217,15 +228,32 @@ func moveResultDTOFrom(r game.MoveResult) moveResultDTO {
 	return moveResultDTO{Move: moveRecordDTOFrom(r.Move), Game: gameDTOFromGame(r.Game)}
 }
 
-// stateDTOFrom projects a player's state view into its DTO.
-func stateDTOFrom(v game.StateView) stateDTO {
-	return stateDTO{
+// stateDTOFrom projects a player's state view into its DTO, encoding the rack as wire
+// alphabet indices (Stage 13). When includeAlphabet is set it also embeds the variant's
+// display table, which the client caches per variant and renders the rack with.
+func stateDTOFrom(v game.StateView, includeAlphabet bool) (stateDTO, error) {
+	rack, err := engine.EncodeRack(v.Game.Variant, v.Rack)
+	if err != nil {
+		return stateDTO{}, err
+	}
+	dto := stateDTO{
 		Game:           gameDTOFromGame(v.Game),
 		Seat:           v.Seat,
-		Rack:           v.Rack,
+		Rack:           rack,
 		BagLen:         v.BagLen,
 		HintsRemaining: v.HintsRemaining,
 	}
+	if includeAlphabet {
+		tab, err := engine.AlphabetTable(v.Game.Variant)
+		if err != nil {
+			return stateDTO{}, err
+		}
+		dto.Alphabet = make([]alphabetEntryDTO, len(tab))
+		for i, e := range tab {
+			dto.Alphabet[i] = alphabetEntryDTO{Index: int(e.Index), Letter: e.Letter, Value: e.Value}
+		}
+	}
+	return dto, nil
 }
 
 // matchDTOFrom projects an enqueue/poll result into its DTO.

backend/internal/server/handlers_game.go

@@ -3,6 +3,7 @@ package server
 import (
 	"context"
 	"net/http"
+	"strconv"
 
 	"github.com/gin-gonic/gin"
 	"github.com/google/uuid"
@@ -51,9 +52,10 @@ type chatListDTO struct {
 	Messages []chatDTO `json:"messages"`
 }
 
-// exchangeRequest swaps the given rack tiles back into the bag.
+// exchangeRequest swaps the given rack tiles back into the bag. Tiles are wire alphabet
+// indices (Stage 13); a blank is engine.BlankIndex.
 type exchangeRequest struct {
-	Tiles []string `json:"tiles"`
+	Tiles []int `json:"tiles"`
 }
 
 // complaintRequest disputes a word-check result.
@@ -125,7 +127,17 @@ func (s *Server) handleExchange(c *gin.Context) {
 		abortBadRequest(c, "invalid request body")
 		return
 	}
-	res, err := s.games.Exchange(c.Request.Context(), gameID, uid, req.Tiles)
+	variant, err := s.games.GameVariant(c.Request.Context(), gameID)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
+	tiles, err := engine.DecodeTiles(variant, req.Tiles)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
+	res, err := s.games.Exchange(c.Request.Context(), gameID, uid, tiles)
 	if err != nil {
 		s.abortErr(c, err)
 		return
@@ -180,9 +192,15 @@ func (s *Server) handleEvaluate(c *gin.Context) {
 		abortBadRequest(c, "dir must be H or V")
 		return
 	}
-	tiles := make([]engine.TileRecord, 0, len(req.Tiles))
-	for _, t := range req.Tiles {
-		tiles = append(tiles, engine.TileRecord{Row: t.Row, Col: t.Col, Letter: t.Letter, Blank: t.Blank})
+	variant, err := s.games.GameVariant(c.Request.Context(), gameID)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
+	tiles, err := tilesFromRequest(variant, req)
+	if err != nil {
+		s.abortErr(c, err)
+		return
 	}
 	ev, err := s.games.EvaluatePlay(c.Request.Context(), gameID, uid, dir, tiles)
 	if err != nil {
@@ -192,13 +210,29 @@ func (s *Server) handleEvaluate(c *gin.Context) {
 	c.JSON(http.StatusOK, evalResultDTO{Legal: ev.Valid, Score: ev.Score, Words: ev.Words})
 }
 
-// handleCheckWord looks a word up in the game's pinned dictionary.
+// handleCheckWord looks a word up in the game's pinned dictionary. The word arrives as
+// repeated ?idx= alphabet indices (Stage 13); the backend decodes them to the concrete
+// word for the lookup and echoes that concrete word back for the client's result cache.
 func (s *Server) handleCheckWord(c *gin.Context) {
 	_, gameID, ok := s.userGame(c)
 	if !ok {
 		return
 	}
-	word := c.Query("word")
+	idx, err := queryIndexes(c, "idx")
+	if err != nil {
+		abortBadRequest(c, "invalid word")
+		return
+	}
+	variant, err := s.games.GameVariant(c.Request.Context(), gameID)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
+	word, err := engine.DecodeWord(variant, idx)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
 	legal, err := s.games.CheckWord(c.Request.Context(), gameID, word)
 	if err != nil {
 		s.abortErr(c, err)
@@ -207,6 +241,21 @@ func (s *Server) handleCheckWord(c *gin.Context) {
 	c.JSON(http.StatusOK, wordCheckDTO{Word: word, Legal: legal})
 }
 
+// queryIndexes parses repeated integer query parameters (e.g. ?idx=2&idx=0) into a slice.
+// It carries a word-check query as alphabet indices on a GET (Stage 13).
+func queryIndexes(c *gin.Context, key string) ([]int, error) {
+	raw := c.QueryArray(key)
+	out := make([]int, 0, len(raw))
+	for _, s := range raw {
+		n, err := strconv.Atoi(s)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, n)
+	}
+	return out, nil
+}
+
 // handleComplaint files a word-check complaint into the admin review queue.
 func (s *Server) handleComplaint(c *gin.Context) {
 	uid, gameID, ok := s.userGame(c)

backend/internal/server/handlers_user.go

@@ -26,17 +26,33 @@ func (s *Server) handleProfile(c *gin.Context) {
 	c.JSON(http.StatusOK, profileResponseFor(acc))
 }
 
-// submitPlayRequest places tiles in a direction on the player's turn.
+// submitPlayRequest places tiles in a direction on the player's turn. Each tile's Letter
+// is a wire alphabet index (Stage 13); for a blank it is the designated letter's index.
 type submitPlayRequest struct {
 	Dir   string `json:"dir"`
 	Tiles []struct {
-		Row    int    `json:"row"`
-		Col    int    `json:"col"`
-		Letter string `json:"letter"`
-		Blank  bool   `json:"blank"`
+		Row    int  `json:"row"`
+		Col    int  `json:"col"`
+		Letter int  `json:"letter"`
+		Blank  bool `json:"blank"`
 	} `json:"tiles"`
 }
 
+// tilesFromRequest maps a play/evaluate request's index-addressed tiles to engine tile
+// records for the game's variant (Stage 13: a placed blank carries its designated letter's
+// index with Blank set). An out-of-range index surfaces as engine.ErrIllegalPlay (HTTP 400).
+func tilesFromRequest(variant engine.Variant, req submitPlayRequest) ([]engine.TileRecord, error) {
+	tiles := make([]engine.TileRecord, 0, len(req.Tiles))
+	for _, t := range req.Tiles {
+		letter, err := engine.LetterForIndex(variant, t.Letter)
+		if err != nil {
+			return nil, err
+		}
+		tiles = append(tiles, engine.TileRecord{Row: t.Row, Col: t.Col, Letter: letter, Blank: t.Blank})
+	}
+	return tiles, nil
+}
+
 // handleSubmitPlay validates, scores and commits a placement.
 func (s *Server) handleSubmitPlay(c *gin.Context) {
 	uid, ok := userID(c)
@@ -59,9 +75,15 @@ func (s *Server) handleSubmitPlay(c *gin.Context) {
 		abortBadRequest(c, "dir must be H or V")
 		return
 	}
-	tiles := make([]engine.TileRecord, 0, len(req.Tiles))
-	for _, t := range req.Tiles {
-		tiles = append(tiles, engine.TileRecord{Row: t.Row, Col: t.Col, Letter: t.Letter, Blank: t.Blank})
+	variant, err := s.games.GameVariant(c.Request.Context(), gameID)
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
+	tiles, err := tilesFromRequest(variant, req)
+	if err != nil {
+		s.abortErr(c, err)
+		return
 	}
 	res, err := s.games.SubmitPlay(c.Request.Context(), gameID, uid, dir, tiles)
 	if err != nil {
@@ -88,7 +110,11 @@ func (s *Server) handleGameState(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
-	dto := stateDTOFrom(view)
+	dto, err := stateDTOFrom(view, c.Query("include_alphabet") == "true")
+	if err != nil {
+		s.abortErr(c, err)
+		return
+	}
 	s.fillSeatNames(c.Request.Context(), &dto.Game, map[string]string{})
 	c.JSON(http.StatusOK, dto)
 }

docs/ARCHITECTURE.md

@@ -87,6 +87,18 @@ dropped). Horizontal scaling is explicit future work.
   operation's domain outcome rides back in `ExecuteResponse.result_code` (HTTP
   200); only edge failures (rate limit, missing session, unknown type, internal)
   surface as Connect error codes.
+- **Alphabet on the wire (Stage 13)**: live play exchanges **alphabet indices**, not
+  concrete letters. The rack (`StateView.rack`), the `SubmitPlay`/`Evaluate` tiles, the
+  `Exchange` tiles and the `CheckWord` word are `ubyte` indices into the variant's alphabet
+  (a blank is the sentinel index **255**). The client is **alphabet-agnostic**: on a
+  per-variant cache miss it sets `StateRequest.include_alphabet`, and the backend embeds the
+  variant's `(index, letter, value)` table (`engine.AlphabetTable`, derived from the solver
+  ruleset — no dictionary) for display; the client caches it by variant and renders the rack
+  and the blank chooser from it. The backend maps index↔letter at its REST edge, so the
+  gateway forwards indices **verbatim** (it holds no alphabet table) and the engine's
+  letter-based domain API — shared with the robot — is unchanged. The table is pinned by the
+  solver version, so it cannot drift from the running backend. The **move journal, history
+  and GCG are unaffected** (they stay decoded concrete characters, §9.1).
 - **gateway ↔ backend (sync)**: plain HTTP REST/JSON. The gateway injects
   `X-User-ID` for authenticated requests; `backend` never re-derives identity
   from the body.
@@ -407,6 +419,11 @@ does not cover. **GCG export is offered only on a finished game** (`game.ErrGame
 otherwise, Stage 8), so an in-progress journal is never leaked mid-play; the client
 shares the `.gcg` file via the Web Share API where available, else downloads it.
 
+The Stage 13 alphabet-on-the-wire change does **not** touch this invariant: the live edge
+exchanges alphabet indices, but the persisted journal (and everything derived from it —
+replay, history, GCG) keeps the decoded concrete letters described above, so an archived
+game still replays with the variant's `rules.Alphabet` alone, independent of any dictionary.
+
 ## 10. Notifications
 
 Two channels: the **in-app live stream** (delivered from Stage 6) and

gateway/internal/backendclient/api.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net/http"
 	"net/url"
+	"strconv"
 )
 
 // The structs below mirror the backend's JSON DTOs (backend/internal/server
@@ -47,7 +48,8 @@ type LinkResultResp struct {
 	Profile          *ProfileResp `json:"profile"`
 }
 
-// TileJSON is one placed tile, used in both play requests and move responses.
+// TileJSON is one tile in a decoded move response (history, move result, hint); its Letter
+// is a concrete character (Stage 13 keeps the move journal in letters).
 type TileJSON struct {
 	Row    int    `json:"row"`
 	Col    int    `json:"col"`
@@ -55,6 +57,15 @@ type TileJSON struct {
 	Blank  bool   `json:"blank"`
 }
 
+// PlayTileJSON is one inbound tile to place, addressed by alphabet index (Stage 13). For a
+// blank, Letter is the designated letter's index and Blank is true.
+type PlayTileJSON struct {
+	Row    int  `json:"row"`
+	Col    int  `json:"col"`
+	Letter int  `json:"letter"`
+	Blank  bool `json:"blank"`
+}
+
 // MoveRecordResp is a decoded move.
 type MoveRecordResp struct {
 	Player  int        `json:"player"`
@@ -99,13 +110,23 @@ type MoveResultResp struct {
 	Game GameResp       `json:"game"`
 }
 
-// StateResp is a player's view of a game.
+// AlphabetEntryJSON is one letter of a variant's alphabet (its index, concrete letter and
+// tile value), present in StateResp only when the client requested it (Stage 13).
+type AlphabetEntryJSON struct {
+	Index  int    `json:"index"`
+	Letter string `json:"letter"`
+	Value  int    `json:"value"`
+}
+
+// StateResp is a player's view of a game. Rack carries wire alphabet indices (Stage 13);
+// Alphabet is present only when the request asked for it.
 type StateResp struct {
-	Game           GameResp `json:"game"`
-	Seat           int      `json:"seat"`
-	Rack           []string `json:"rack"`
-	BagLen         int      `json:"bag_len"`
-	HintsRemaining int      `json:"hints_remaining"`
+	Game           GameResp            `json:"game"`
+	Seat           int                 `json:"seat"`
+	Rack           []int               `json:"rack"`
+	BagLen         int                 `json:"bag_len"`
+	HintsRemaining int                 `json:"hints_remaining"`
+	Alphabet       []AlphabetEntryJSON `json:"alphabet,omitempty"`
 }
 
 // MatchResp reports an auto-match outcome.
@@ -194,18 +215,25 @@ func (c *Client) Profile(ctx context.Context, userID string) (ProfileResp, error
 	return out, err
 }
 
-// SubmitPlay commits a placement on the player's turn.
-func (c *Client) SubmitPlay(ctx context.Context, userID, gameID, dir string, tiles []TileJSON) (MoveResultResp, error) {
+// SubmitPlay commits a placement on the player's turn. The tiles are addressed by alphabet
+// index (Stage 13).
+func (c *Client) SubmitPlay(ctx context.Context, userID, gameID, dir string, tiles []PlayTileJSON) (MoveResultResp, error) {
 	var out MoveResultResp
 	body := map[string]any{"dir": dir, "tiles": tiles}
 	err := c.do(ctx, http.MethodPost, "/api/v1/user/games/"+url.PathEscape(gameID)+"/play", userID, "", body, &out)
 	return out, err
 }
 
-// GameState returns the player's view of a game.
-func (c *Client) GameState(ctx context.Context, userID, gameID string) (StateResp, error) {
+// GameState returns the player's view of a game. When includeAlphabet is set the backend
+// embeds the variant's alphabet table (Stage 13); the client asks for it on a per-variant
+// cache miss only.
+func (c *Client) GameState(ctx context.Context, userID, gameID string, includeAlphabet bool) (StateResp, error) {
 	var out StateResp
-	err := c.do(ctx, http.MethodGet, "/api/v1/user/games/"+url.PathEscape(gameID)+"/state", userID, "", nil, &out)
+	path := "/api/v1/user/games/" + url.PathEscape(gameID) + "/state"
+	if includeAlphabet {
+		path += "?include_alphabet=true"
+	}
+	err := c.do(ctx, http.MethodGet, path, userID, "", nil, &out)
 	return out, err
 }
 
@@ -278,8 +306,9 @@ func (c *Client) Pass(ctx context.Context, userID, gameID string) (MoveResultRes
 	return out, err
 }
 
-// Exchange swaps the chosen rack tiles back into the bag.
-func (c *Client) Exchange(ctx context.Context, userID, gameID string, tiles []string) (MoveResultResp, error) {
+// Exchange swaps the chosen rack tiles back into the bag. Tiles are wire alphabet indices
+// (Stage 13; a blank is engine.BlankIndex).
+func (c *Client) Exchange(ctx context.Context, userID, gameID string, tiles []int) (MoveResultResp, error) {
 	var out MoveResultResp
 	err := c.do(ctx, http.MethodPost, c.gamePath(gameID, "/exchange"), userID, "",
 		map[string]any{"tiles": tiles}, &out)
@@ -300,18 +329,24 @@ func (c *Client) Hint(ctx context.Context, userID, gameID string) (HintResultRes
 	return out, err
 }
 
-// Evaluate previews a tentative play's legality and score.
-func (c *Client) Evaluate(ctx context.Context, userID, gameID, dir string, tiles []TileJSON) (EvalResultResp, error) {
+// Evaluate previews a tentative play's legality and score. The tiles are addressed by
+// alphabet index (Stage 13).
+func (c *Client) Evaluate(ctx context.Context, userID, gameID, dir string, tiles []PlayTileJSON) (EvalResultResp, error) {
 	var out EvalResultResp
 	err := c.do(ctx, http.MethodPost, c.gamePath(gameID, "/evaluate"), userID, "",
 		map[string]any{"dir": dir, "tiles": tiles}, &out)
 	return out, err
 }
 
-// CheckWord looks a word up in the game's pinned dictionary.
-func (c *Client) CheckWord(ctx context.Context, userID, gameID, word string) (WordCheckResp, error) {
+// CheckWord looks a word up in the game's pinned dictionary. The word is carried as
+// repeated ?idx= alphabet indices (Stage 13); the backend echoes the decoded concrete word.
+func (c *Client) CheckWord(ctx context.Context, userID, gameID string, word []int) (WordCheckResp, error) {
 	var out WordCheckResp
-	err := c.do(ctx, http.MethodGet, c.gamePath(gameID, "/check_word")+"?word="+url.QueryEscape(word), userID, "", nil, &out)
+	q := url.Values{}
+	for _, x := range word {
+		q.Add("idx", strconv.Itoa(x))
+	}
+	err := c.do(ctx, http.MethodGet, c.gamePath(gameID, "/check_word")+"?"+q.Encode(), userID, "", nil, &out)
 	return out, err
 }
 

gateway/internal/transcode/encode.go

@@ -107,21 +107,53 @@ func encodeMoveResult(r backendclient.MoveResultResp) []byte {
 	return b.FinishedBytes()
 }
 
-// encodeState builds a StateView payload.
+// encodeState builds a StateView payload. The rack is a vector of alphabet indices and the
+// alphabet display table is included only when the backend returned it (Stage 13: the
+// client requests it on a per-variant cache miss).
 func encodeState(s backendclient.StateResp) []byte {
 	b := flatbuffers.NewBuilder(512)
 	game := buildGameView(b, s.Game)
-	rack := buildStringVector(b, s.Rack, fb.StateViewStartRackVector)
+	rackBytes := make([]byte, len(s.Rack))
+	for i, v := range s.Rack {
+		rackBytes[i] = byte(v)
+	}
+	rack := b.CreateByteVector(rackBytes)
+	hasAlphabet := len(s.Alphabet) > 0
+	var alphabet flatbuffers.UOffsetT
+	if hasAlphabet {
+		alphabet = buildAlphabet(b, s.Alphabet)
+	}
 	fb.StateViewStart(b)
 	fb.StateViewAddGame(b, game)
 	fb.StateViewAddSeat(b, int32(s.Seat))
 	fb.StateViewAddRack(b, rack)
 	fb.StateViewAddBagLen(b, int32(s.BagLen))
 	fb.StateViewAddHintsRemaining(b, int32(s.HintsRemaining))
+	if hasAlphabet {
+		fb.StateViewAddAlphabet(b, alphabet)
+	}
 	b.Finish(fb.StateViewEnd(b))
 	return b.FinishedBytes()
 }
 
+// buildAlphabet builds the AlphabetEntry vector for a StateView and returns its offset.
+func buildAlphabet(b *flatbuffers.Builder, entries []backendclient.AlphabetEntryJSON) flatbuffers.UOffsetT {
+	offs := make([]flatbuffers.UOffsetT, len(entries))
+	for i, e := range entries {
+		letter := b.CreateString(e.Letter)
+		fb.AlphabetEntryStart(b)
+		fb.AlphabetEntryAddIndex(b, byte(e.Index))
+		fb.AlphabetEntryAddLetter(b, letter)
+		fb.AlphabetEntryAddValue(b, int32(e.Value))
+		offs[i] = fb.AlphabetEntryEnd(b)
+	}
+	fb.StateViewStartAlphabetVector(b, len(offs))
+	for i := len(offs) - 1; i >= 0; i-- {
+		b.PrependUOffsetT(offs[i])
+	}
+	return b.EndVector(len(offs))
+}
+
 // encodeMatch builds a MatchResult payload.
 func encodeMatch(m backendclient.MatchResp) []byte {
 	b := flatbuffers.NewBuilder(512)

gateway/internal/transcode/transcode.go

@@ -198,7 +198,7 @@ func submitPlayHandler(backend *backendclient.Client) Handler {
 func gameStateHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsStateRequest(req.Payload, 0)
-		st, err := backend.GameState(ctx, req.UserID, string(in.GameId()))
+		st, err := backend.GameState(ctx, req.UserID, string(in.GameId()), in.IncludeAlphabet())
 		if err != nil {
 			return nil, err
 		}
@@ -238,17 +238,17 @@ func chatPostHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-// decodeTiles reads the placed tiles from a SubmitPlayRequest.
-func decodeTiles(in *fb.SubmitPlayRequest) []backendclient.TileJSON {
+// decodeTiles reads the index-addressed tiles to place from a SubmitPlayRequest (Stage 13).
+func decodeTiles(in *fb.SubmitPlayRequest) []backendclient.PlayTileJSON {
 	n := in.TilesLength()
-	tiles := make([]backendclient.TileJSON, 0, n)
-	var t fb.TileRecord
+	tiles := make([]backendclient.PlayTileJSON, 0, n)
+	var t fb.PlayTile
 	for i := 0; i < n; i++ {
 		if in.Tiles(&t, i) {
-			tiles = append(tiles, backendclient.TileJSON{
+			tiles = append(tiles, backendclient.PlayTileJSON{
 				Row:    int(t.Row()),
 				Col:    int(t.Col()),
-				Letter: string(t.Letter()),
+				Letter: int(t.Letter()),
 				Blank:  t.Blank(),
 			})
 		}
@@ -256,17 +256,17 @@ func decodeTiles(in *fb.SubmitPlayRequest) []backendclient.TileJSON {
 	return tiles
 }
 
-// decodeEvalTiles reads the tentative tiles from an EvalRequest.
-func decodeEvalTiles(in *fb.EvalRequest) []backendclient.TileJSON {
+// decodeEvalTiles reads the index-addressed tentative tiles from an EvalRequest (Stage 13).
+func decodeEvalTiles(in *fb.EvalRequest) []backendclient.PlayTileJSON {
 	n := in.TilesLength()
-	tiles := make([]backendclient.TileJSON, 0, n)
-	var t fb.TileRecord
+	tiles := make([]backendclient.PlayTileJSON, 0, n)
+	var t fb.PlayTile
 	for i := 0; i < n; i++ {
 		if in.Tiles(&t, i) {
-			tiles = append(tiles, backendclient.TileJSON{
+			tiles = append(tiles, backendclient.PlayTileJSON{
 				Row:    int(t.Row()),
 				Col:    int(t.Col()),
-				Letter: string(t.Letter()),
+				Letter: int(t.Letter()),
 				Blank:  t.Blank(),
 			})
 		}
@@ -274,12 +274,12 @@ func decodeEvalTiles(in *fb.EvalRequest) []backendclient.TileJSON {
 	return tiles
 }
 
-// decodeStringVector reads the exchange tiles from an ExchangeRequest.
-func decodeStringVector(in *fb.ExchangeRequest) []string {
-	n := in.TilesLength()
-	out := make([]string, 0, n)
-	for i := 0; i < n; i++ {
-		out = append(out, string(in.Tiles(i)))
+// bytesToInts widens a FlatBuffers ubyte vector (an alphabet-index list) to []int for the
+// backend JSON edge (Stage 13: rack-exchange tiles and the word-check query).
+func bytesToInts(bs []byte) []int {
+	out := make([]int, len(bs))
+	for i, b := range bs {
+		out[i] = int(b)
 	}
 	return out
 }
@@ -319,7 +319,7 @@ func resignHandler(backend *backendclient.Client) Handler {
 func exchangeHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsExchangeRequest(req.Payload, 0)
-		res, err := backend.Exchange(ctx, req.UserID, string(in.GameId()), decodeStringVector(in))
+		res, err := backend.Exchange(ctx, req.UserID, string(in.GameId()), bytesToInts(in.TilesBytes()))
 		if err != nil {
 			return nil, err
 		}
@@ -352,7 +352,7 @@ func evaluateHandler(backend *backendclient.Client) Handler {
 func checkWordHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsCheckWordRequest(req.Payload, 0)
-		res, err := backend.CheckWord(ctx, req.UserID, string(in.GameId()), string(in.Word()))
+		res, err := backend.CheckWord(ctx, req.UserID, string(in.GameId()), bytesToInts(in.WordBytes()))
 		if err != nil {
 			return nil, err
 		}

gateway/internal/transcode/transcode_alphabet_test.go

@@ -0,0 +1,199 @@
+package transcode_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"testing"
+
+	flatbuffers "github.com/google/flatbuffers/go"
+
+	"scrabble/gateway/internal/transcode"
+	fb "scrabble/pkg/fbs/scrabblefb"
+)
+
+// TestGameStateIncludesAlphabet checks the include_alphabet flag reaches the backend and
+// the returned alphabet table plus the index rack (a blank is 255) are encoded into the
+// StateView (Stage 13).
+func TestGameStateIncludesAlphabet(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		if got := r.URL.Query().Get("include_alphabet"); got != "true" {
+			t.Errorf("include_alphabet query = %q, want true", got)
+		}
+		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":0,"seats":[]},"seat":0,"rack":[0,255],"bag_len":50,"hints_remaining":0,"alphabet":[{"index":0,"letter":"a","value":1},{"index":1,"letter":"b","value":3}]}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, _ := reg.Lookup(transcode.MsgGameState)
+
+	b := flatbuffers.NewBuilder(32)
+	gid := b.CreateString("g-1")
+	fb.StateRequestStart(b)
+	fb.StateRequestAddGameId(b, gid)
+	fb.StateRequestAddIncludeAlphabet(b, true)
+	b.Finish(fb.StateRequestEnd(b))
+
+	payload, err := op.Handler(context.Background(), transcode.Request{Payload: b.FinishedBytes(), UserID: "u-1"})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	st := fb.GetRootAsStateView(payload, 0)
+	if st.RackLength() != 2 || st.Rack(0) != 0 || st.Rack(1) != 255 {
+		t.Fatalf("rack indices wrong: len=%d [0]=%d [1]=%d", st.RackLength(), st.Rack(0), st.Rack(1))
+	}
+	if st.AlphabetLength() != 2 {
+		t.Fatalf("alphabet length = %d, want 2", st.AlphabetLength())
+	}
+	var e fb.AlphabetEntry
+	st.Alphabet(&e, 0)
+	if e.Index() != 0 || string(e.Letter()) != "a" || e.Value() != 1 {
+		t.Errorf("alphabet[0] = %d/%q/%d, want 0/a/1", e.Index(), e.Letter(), e.Value())
+	}
+}
+
+// TestGameStateOmitsAlphabetByDefault checks the table is neither requested nor encoded on
+// the steady-state poll (no include_alphabet flag).
+func TestGameStateOmitsAlphabetByDefault(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Query().Get("include_alphabet") == "true" {
+			t.Error("include_alphabet should be unset")
+		}
+		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":0,"seats":[]},"seat":0,"rack":[2,0,19],"bag_len":50,"hints_remaining":0}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, _ := reg.Lookup(transcode.MsgGameState)
+	b := flatbuffers.NewBuilder(32)
+	gid := b.CreateString("g-1")
+	fb.StateRequestStart(b)
+	fb.StateRequestAddGameId(b, gid)
+	b.Finish(fb.StateRequestEnd(b))
+	payload, err := op.Handler(context.Background(), transcode.Request{Payload: b.FinishedBytes(), UserID: "u-1"})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	st := fb.GetRootAsStateView(payload, 0)
+	if st.AlphabetLength() != 0 {
+		t.Errorf("alphabet length = %d, want 0", st.AlphabetLength())
+	}
+	if st.RackLength() != 3 {
+		t.Errorf("rack length = %d, want 3", st.RackLength())
+	}
+}
+
+// TestSubmitPlayForwardsIndexTiles checks PlayTile indices reach the backend as integer
+// letter fields in the JSON body, blank flag preserved (Stage 13).
+func TestSubmitPlayForwardsIndexTiles(t *testing.T) {
+	var body struct {
+		Dir   string `json:"dir"`
+		Tiles []struct {
+			Row    int  `json:"row"`
+			Col    int  `json:"col"`
+			Letter int  `json:"letter"`
+			Blank  bool `json:"blank"`
+		} `json:"tiles"`
+	}
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		raw, _ := io.ReadAll(r.Body)
+		if err := json.Unmarshal(raw, &body); err != nil {
+			t.Fatalf("decode body: %v", err)
+		}
+		_, _ = w.Write([]byte(`{"move":{"player":0,"action":"play","words":["CAT"],"score":9},"game":{"id":"g-5","status":"active","seats":[]}}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, _ := reg.Lookup(transcode.MsgGameSubmitPlay)
+
+	b := flatbuffers.NewBuilder(64)
+	gid := b.CreateString("g-5")
+	dir := b.CreateString("H")
+	fb.PlayTileStart(b)
+	fb.PlayTileAddRow(b, 7)
+	fb.PlayTileAddCol(b, 7)
+	fb.PlayTileAddLetter(b, 2)
+	fb.PlayTileAddBlank(b, true)
+	tile := fb.PlayTileEnd(b)
+	fb.SubmitPlayRequestStartTilesVector(b, 1)
+	b.PrependUOffsetT(tile)
+	tiles := b.EndVector(1)
+	fb.SubmitPlayRequestStart(b)
+	fb.SubmitPlayRequestAddGameId(b, gid)
+	fb.SubmitPlayRequestAddDir(b, dir)
+	fb.SubmitPlayRequestAddTiles(b, tiles)
+	b.Finish(fb.SubmitPlayRequestEnd(b))
+
+	if _, err := op.Handler(context.Background(), transcode.Request{Payload: b.FinishedBytes(), UserID: "u-1"}); err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	if len(body.Tiles) != 1 || body.Tiles[0].Letter != 2 || !body.Tiles[0].Blank || body.Tiles[0].Row != 7 {
+		t.Fatalf("forwarded tiles wrong: %+v", body.Tiles)
+	}
+}
+
+// TestCheckWordForwardsIndices checks the word-check query rides as repeated ?idx= params
+// and the decoded concrete word echoes back (Stage 13).
+func TestCheckWordForwardsIndices(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		if got := r.URL.Query()["idx"]; len(got) != 3 || got[0] != "2" || got[2] != "19" {
+			t.Errorf("idx params = %v, want [2 0 19]", got)
+		}
+		_, _ = w.Write([]byte(`{"word":"cat","legal":true}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, _ := reg.Lookup(transcode.MsgGameCheckWord)
+
+	b := flatbuffers.NewBuilder(32)
+	gid := b.CreateString("g-1")
+	word := b.CreateByteVector([]byte{2, 0, 19})
+	fb.CheckWordRequestStart(b)
+	fb.CheckWordRequestAddGameId(b, gid)
+	fb.CheckWordRequestAddWord(b, word)
+	b.Finish(fb.CheckWordRequestEnd(b))
+
+	payload, err := op.Handler(context.Background(), transcode.Request{Payload: b.FinishedBytes(), UserID: "u-1"})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	res := fb.GetRootAsWordCheckResult(payload, 0)
+	if string(res.Word()) != "cat" || !res.Legal() {
+		t.Errorf("word check = %q/%v, want cat/true", res.Word(), res.Legal())
+	}
+}
+
+// TestExchangeForwardsIndices checks rack-exchange indices (blank 255) reach the backend
+// body (Stage 13).
+func TestExchangeForwardsIndices(t *testing.T) {
+	var body struct {
+		Tiles []int `json:"tiles"`
+	}
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		raw, _ := io.ReadAll(r.Body)
+		_ = json.Unmarshal(raw, &body)
+		_, _ = w.Write([]byte(`{"move":{"player":0,"action":"exchange","count":2},"game":{"id":"g-1","status":"active","seats":[]}}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, _ := reg.Lookup(transcode.MsgGameExchange)
+
+	b := flatbuffers.NewBuilder(32)
+	gid := b.CreateString("g-1")
+	tiles := b.CreateByteVector([]byte{0, 255})
+	fb.ExchangeRequestStart(b)
+	fb.ExchangeRequestAddGameId(b, gid)
+	fb.ExchangeRequestAddTiles(b, tiles)
+	b.Finish(fb.ExchangeRequestEnd(b))
+
+	if _, err := op.Handler(context.Background(), transcode.Request{Payload: b.FinishedBytes(), UserID: "u-1"}); err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	if len(body.Tiles) != 2 || body.Tiles[0] != 0 || body.Tiles[1] != 255 {
+		t.Errorf("forwarded exchange tiles = %v, want [0 255]", body.Tiles)
+	}
+}

gateway/internal/transcode/transcode_test.go

@@ -59,7 +59,7 @@ func TestGameStateRoundTripForwardsUserID(t *testing.T) {
 		if r.URL.Path != "/api/v1/user/games/g-1/state" {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
-		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":1,"seats":[{"seat":0,"account_id":"u-7","score":5}]},"seat":0,"rack":["A","B"],"bag_len":80,"hints_remaining":1}`))
+		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"english","status":"active","players":2,"to_move":1,"seats":[{"seat":0,"account_id":"u-7","score":5}]},"seat":0,"rack":[0,1],"bag_len":80,"hints_remaining":1}`))
 	})
 	defer cleanup()
 

pkg/fbs/scrabble.fbs

@@ -14,8 +14,9 @@ namespace scrabblefb;
 
 // --- shared building blocks ---
 
-// TileRecord is one placed (or to-place) tile: its board coordinate, the concrete
-// letter ("?" when read from a hand for a blank) and whether it came from a blank.
+// TileRecord is one tile in a decoded move record (history, move result, hint): its
+// board coordinate, the concrete letter ("?" when read from a hand for a blank) and
+// whether it came from a blank. Inbound tiles to place use PlayTile (alphabet indices).
 table TileRecord {
   row:int;
   col:int;
@@ -23,6 +24,25 @@ table TileRecord {
   blank:bool;
 }
 
+// PlayTile is one inbound tile to place, addressed by its alphabet index rather than a
+// concrete letter (Stage 13). For a blank, letter carries the designated letter's index
+// and blank is true. The board coordinate is its target square.
+table PlayTile {
+  row:int;
+  col:int;
+  letter:ubyte;
+  blank:bool;
+}
+
+// AlphabetEntry is one letter of a variant's alphabet, sent for display only (Stage 13):
+// index is the engine alphabet-index byte the wire uses for this letter, letter is the
+// concrete character and value is its tile score. The client caches the table per variant.
+table AlphabetEntry {
+  index:ubyte;
+  letter:string;
+  value:int;
+}
+
 // SeatView is one seat's public standing in a game. display_name is resolved by the
 // backend from the account store (added trailing — backward-compatible).
 table SeatView {
@@ -123,11 +143,12 @@ table Profile {
 
 // --- game (authenticated) ---
 
-// SubmitPlayRequest places tiles in a direction on the player's turn.
+// SubmitPlayRequest places tiles in a direction on the player's turn. tiles are addressed
+// by alphabet index (Stage 13).
 table SubmitPlayRequest {
   game_id:string;
   dir:string;
-  tiles:[TileRecord];
+  tiles:[PlayTile];
 }
 
 // MoveResult is the outcome of a committed move: the move and the post-move game.
@@ -136,19 +157,25 @@ table MoveResult {
   game:GameView;
 }
 
-// StateRequest asks for the requesting player's view of a game.
+// StateRequest asks for the requesting player's view of a game. include_alphabet asks the
+// backend to embed the variant's AlphabetEntry table in the reply (Stage 13); the client
+// sets it only on a per-variant cache miss so the table is not resent on every poll.
 table StateRequest {
   game_id:string;
+  include_alphabet:bool = false;
 }
 
-// StateView is a player's view of a game: the shared summary plus their private
-// rack, the bag size and their remaining hint budget.
+// StateView is a player's view of a game: the shared summary plus their private rack, the
+// bag size and their remaining hint budget. rack carries alphabet indices (Stage 13); a
+// blank tile is the sentinel index 255. alphabet is present only when the request set
+// include_alphabet (a display table the client caches per variant).
 table StateView {
   game:GameView;
   seat:int;
-  rack:[string];
+  rack:[ubyte];
   bag_len:int;
   hints_remaining:int;
+  alphabet:[AlphabetEntry];
 }
 
 // GameActionRequest carries just a game id (pass / resign / hint / history).
@@ -156,17 +183,19 @@ table GameActionRequest {
   game_id:string;
 }
 
-// ExchangeRequest swaps the listed rack tiles back into the bag.
+// ExchangeRequest swaps the listed rack tiles back into the bag. tiles are alphabet
+// indices (Stage 13); a blank is the sentinel index 255.
 table ExchangeRequest {
   game_id:string;
-  tiles:[string];
+  tiles:[ubyte];
 }
 
-// EvalRequest previews a tentative play without committing it.
+// EvalRequest previews a tentative play without committing it. tiles are addressed by
+// alphabet index (Stage 13).
 table EvalRequest {
   game_id:string;
   dir:string;
-  tiles:[TileRecord];
+  tiles:[PlayTile];
 }
 
 // EvalResult is an unlimited move preview: legality, score and the words formed.
@@ -176,10 +205,11 @@ table EvalResult {
   words:[string];
 }
 
-// CheckWordRequest looks a word up in the game's pinned dictionary.
+// CheckWordRequest looks a word up in the game's pinned dictionary. word is a sequence of
+// alphabet indices (Stage 13); the client constrains input to the variant's alphabet.
 table CheckWordRequest {
   game_id:string;
-  word:string;
+  word:[ubyte];
 }
 
 // WordCheckResult is the dictionary lookup outcome.

pkg/fbs/scrabblefb/AlphabetEntry.go

@@ -0,0 +1,90 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type AlphabetEntry struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsAlphabetEntry(buf []byte, offset flatbuffers.UOffsetT) *AlphabetEntry {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &AlphabetEntry{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishAlphabetEntryBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsAlphabetEntry(buf []byte, offset flatbuffers.UOffsetT) *AlphabetEntry {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &AlphabetEntry{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedAlphabetEntryBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *AlphabetEntry) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *AlphabetEntry) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *AlphabetEntry) Index() byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.GetByte(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *AlphabetEntry) MutateIndex(n byte) bool {
+	return rcv._tab.MutateByteSlot(4, n)
+}
+
+func (rcv *AlphabetEntry) Letter() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *AlphabetEntry) Value() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *AlphabetEntry) MutateValue(n int32) bool {
+	return rcv._tab.MutateInt32Slot(8, n)
+}
+
+func AlphabetEntryStart(builder *flatbuffers.Builder) {
+	builder.StartObject(3)
+}
+func AlphabetEntryAddIndex(builder *flatbuffers.Builder, index byte) {
+	builder.PrependByteSlot(0, index, 0)
+}
+func AlphabetEntryAddLetter(builder *flatbuffers.Builder, letter flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(letter), 0)
+}
+func AlphabetEntryAddValue(builder *flatbuffers.Builder, value int32) {
+	builder.PrependInt32Slot(2, value, 0)
+}
+func AlphabetEntryEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}

pkg/fbs/scrabblefb/CheckWordRequest.go

@@ -49,7 +49,24 @@ func (rcv *CheckWordRequest) GameId() []byte {
 	return nil
 }
 
-func (rcv *CheckWordRequest) Word() []byte {
+func (rcv *CheckWordRequest) Word(j int) byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.GetByte(a + flatbuffers.UOffsetT(j*1))
+	}
+	return 0
+}
+
+func (rcv *CheckWordRequest) WordLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
+func (rcv *CheckWordRequest) WordBytes() []byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
 	if o != 0 {
 		return rcv._tab.ByteVector(o + rcv._tab.Pos)
@@ -57,6 +74,15 @@ func (rcv *CheckWordRequest) Word() []byte {
 	return nil
 }
 
+func (rcv *CheckWordRequest) MutateWord(j int, n byte) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.MutateByte(a+flatbuffers.UOffsetT(j*1), n)
+	}
+	return false
+}
+
 func CheckWordRequestStart(builder *flatbuffers.Builder) {
 	builder.StartObject(2)
 }
@@ -66,6 +92,9 @@ func CheckWordRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.
 func CheckWordRequestAddWord(builder *flatbuffers.Builder, word flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(word), 0)
 }
+func CheckWordRequestStartWordVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(1, numElems, 1)
+}
 func CheckWordRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }

pkg/fbs/scrabblefb/EvalRequest.go

@@ -57,7 +57,7 @@ func (rcv *EvalRequest) Dir() []byte {
 	return nil
 }
 
-func (rcv *EvalRequest) Tiles(obj *TileRecord, j int) bool {
+func (rcv *EvalRequest) Tiles(obj *PlayTile, j int) bool {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
 	if o != 0 {
 		x := rcv._tab.Vector(o)

pkg/fbs/scrabblefb/ExchangeRequest.go

@@ -49,13 +49,13 @@ func (rcv *ExchangeRequest) GameId() []byte {
 	return nil
 }
 
-func (rcv *ExchangeRequest) Tiles(j int) []byte {
+func (rcv *ExchangeRequest) Tiles(j int) byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
 	if o != 0 {
 		a := rcv._tab.Vector(o)
-		return rcv._tab.ByteVector(a + flatbuffers.UOffsetT(j*4))
+		return rcv._tab.GetByte(a + flatbuffers.UOffsetT(j*1))
 	}
-	return nil
+	return 0
 }
 
 func (rcv *ExchangeRequest) TilesLength() int {
@@ -66,6 +66,23 @@ func (rcv *ExchangeRequest) TilesLength() int {
 	return 0
 }
 
+func (rcv *ExchangeRequest) TilesBytes() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *ExchangeRequest) MutateTiles(j int, n byte) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.MutateByte(a+flatbuffers.UOffsetT(j*1), n)
+	}
+	return false
+}
+
 func ExchangeRequestStart(builder *flatbuffers.Builder) {
 	builder.StartObject(2)
 }
@@ -76,7 +93,7 @@ func ExchangeRequestAddTiles(builder *flatbuffers.Builder, tiles flatbuffers.UOf
 	builder.PrependUOffsetTSlot(1, flatbuffers.UOffsetT(tiles), 0)
 }
 func ExchangeRequestStartTilesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
-	return builder.StartVector(4, numElems, 4)
+	return builder.StartVector(1, numElems, 1)
 }
 func ExchangeRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()

pkg/fbs/scrabblefb/PlayTile.go

@@ -0,0 +1,109 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type PlayTile struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsPlayTile(buf []byte, offset flatbuffers.UOffsetT) *PlayTile {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &PlayTile{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishPlayTileBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsPlayTile(buf []byte, offset flatbuffers.UOffsetT) *PlayTile {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &PlayTile{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedPlayTileBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *PlayTile) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *PlayTile) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *PlayTile) Row() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *PlayTile) MutateRow(n int32) bool {
+	return rcv._tab.MutateInt32Slot(4, n)
+}
+
+func (rcv *PlayTile) Col() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *PlayTile) MutateCol(n int32) bool {
+	return rcv._tab.MutateInt32Slot(6, n)
+}
+
+func (rcv *PlayTile) Letter() byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.GetByte(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *PlayTile) MutateLetter(n byte) bool {
+	return rcv._tab.MutateByteSlot(8, n)
+}
+
+func (rcv *PlayTile) Blank() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(10))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *PlayTile) MutateBlank(n bool) bool {
+	return rcv._tab.MutateBoolSlot(10, n)
+}
+
+func PlayTileStart(builder *flatbuffers.Builder) {
+	builder.StartObject(4)
+}
+func PlayTileAddRow(builder *flatbuffers.Builder, row int32) {
+	builder.PrependInt32Slot(0, row, 0)
+}
+func PlayTileAddCol(builder *flatbuffers.Builder, col int32) {
+	builder.PrependInt32Slot(1, col, 0)
+}
+func PlayTileAddLetter(builder *flatbuffers.Builder, letter byte) {
+	builder.PrependByteSlot(2, letter, 0)
+}
+func PlayTileAddBlank(builder *flatbuffers.Builder, blank bool) {
+	builder.PrependBoolSlot(3, blank, false)
+}
+func PlayTileEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}

pkg/fbs/scrabblefb/StateRequest.go

@@ -49,12 +49,27 @@ func (rcv *StateRequest) GameId() []byte {
 	return nil
 }
 
+func (rcv *StateRequest) IncludeAlphabet() bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
+	}
+	return false
+}
+
+func (rcv *StateRequest) MutateIncludeAlphabet(n bool) bool {
+	return rcv._tab.MutateBoolSlot(6, n)
+}
+
 func StateRequestStart(builder *flatbuffers.Builder) {
-	builder.StartObject(1)
+	builder.StartObject(2)
 }
 func StateRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(gameId), 0)
 }
+func StateRequestAddIncludeAlphabet(builder *flatbuffers.Builder, includeAlphabet bool) {
+	builder.PrependBoolSlot(1, includeAlphabet, false)
+}
 func StateRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }

pkg/fbs/scrabblefb/StateView.go

@@ -66,13 +66,13 @@ func (rcv *StateView) MutateSeat(n int32) bool {
 	return rcv._tab.MutateInt32Slot(6, n)
 }
 
-func (rcv *StateView) Rack(j int) []byte {
+func (rcv *StateView) Rack(j int) byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
 	if o != 0 {
 		a := rcv._tab.Vector(o)
-		return rcv._tab.ByteVector(a + flatbuffers.UOffsetT(j*4))
+		return rcv._tab.GetByte(a + flatbuffers.UOffsetT(j*1))
 	}
-	return nil
+	return 0
 }
 
 func (rcv *StateView) RackLength() int {
@@ -83,6 +83,23 @@ func (rcv *StateView) RackLength() int {
 	return 0
 }
 
+func (rcv *StateView) RackBytes() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *StateView) MutateRack(j int, n byte) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.MutateByte(a+flatbuffers.UOffsetT(j*1), n)
+	}
+	return false
+}
+
 func (rcv *StateView) BagLen() int32 {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(10))
 	if o != 0 {
@@ -107,8 +124,28 @@ func (rcv *StateView) MutateHintsRemaining(n int32) bool {
 	return rcv._tab.MutateInt32Slot(12, n)
 }
 
+func (rcv *StateView) Alphabet(obj *AlphabetEntry, j int) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		x := rcv._tab.Vector(o)
+		x += flatbuffers.UOffsetT(j) * 4
+		x = rcv._tab.Indirect(x)
+		obj.Init(rcv._tab.Bytes, x)
+		return true
+	}
+	return false
+}
+
+func (rcv *StateView) AlphabetLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
 func StateViewStart(builder *flatbuffers.Builder) {
-	builder.StartObject(5)
+	builder.StartObject(6)
 }
 func StateViewAddGame(builder *flatbuffers.Builder, game flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(game), 0)
@@ -120,7 +157,7 @@ func StateViewAddRack(builder *flatbuffers.Builder, rack flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(2, flatbuffers.UOffsetT(rack), 0)
 }
 func StateViewStartRackVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
-	return builder.StartVector(4, numElems, 4)
+	return builder.StartVector(1, numElems, 1)
 }
 func StateViewAddBagLen(builder *flatbuffers.Builder, bagLen int32) {
 	builder.PrependInt32Slot(3, bagLen, 0)
@@ -128,6 +165,12 @@ func StateViewAddBagLen(builder *flatbuffers.Builder, bagLen int32) {
 func StateViewAddHintsRemaining(builder *flatbuffers.Builder, hintsRemaining int32) {
 	builder.PrependInt32Slot(4, hintsRemaining, 0)
 }
+func StateViewAddAlphabet(builder *flatbuffers.Builder, alphabet flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(5, flatbuffers.UOffsetT(alphabet), 0)
+}
+func StateViewStartAlphabetVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
 func StateViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }

pkg/fbs/scrabblefb/SubmitPlayRequest.go

@@ -57,7 +57,7 @@ func (rcv *SubmitPlayRequest) Dir() []byte {
 	return nil
 }
 
-func (rcv *SubmitPlayRequest) Tiles(obj *TileRecord, j int) bool {
+func (rcv *SubmitPlayRequest) Tiles(obj *PlayTile, j int) bool {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
 	if o != 0 {
 		x := rcv._tab.Vector(o)

ui/README.md

@@ -42,9 +42,14 @@ out of production). Both speak the plain `lib/model.ts` types via `lib/codec.ts`
 
 **No board on the wire:** `StateView` is a summary + rack only, so the client
 reconstructs the 15×15 board by replaying the decoded move journal (`game.history`).
-Premium squares and tile values (`lib/premiums.ts`) are a client-side map **ported from
-`scrabble-solver/rules/rules.go`** (pinned by a Vitest parity test). Board, tiles and
-effects are pure CSS + Unicode — no image/font/SVG assets.
+**The play loop is alphabet-agnostic (Stage 13):** the rack and the play / exchange /
+word-check requests carry **alphabet indices**, and the client caches each variant's
+`(index, letter, value)` table — sent once behind `StateRequest.include_alphabet` — in
+`lib/alphabet.ts`, rendering the rack and blank chooser from it. **Premium squares**
+(`lib/premiums.ts`) stay a client-side geometry map **ported from
+`scrabble-solver/rules/rules.go`** (pinned by a Vitest parity test); **tile values and the
+alphabet now come from the server table** (their parity lives in the Go `engine.AlphabetTable`
+test). Board, tiles and effects are pure CSS + Unicode — no image/font/SVG assets.
 
 ## Codegen
 
@@ -65,7 +70,8 @@ runtime; the Telegram SDK itself is wired in the Telegram stage.
 ```
 src/
   lib/         model, client facade, transport (+ mock), codec, board replay,
-               placement state machine, premiums, stats, share, i18n, theme, session, router, app store
+               placement state machine, premiums (geometry), alphabet cache, stats, share,
+               i18n, theme, session, router, app store
   components/  Header, Menu (+ badge), Modal, Toast, TabBar, Screen
   screens/     Login, Lobby, NewGame, Profile, Settings, About, Friends, Stats
   game/        Game, Board, Rack, Controls, MakeMove, Chat

ui/src/game/Board.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import type { BoardCell } from '../lib/board';
   import type { Premium } from '../lib/premiums';
-  import { tileValue } from '../lib/premiums';
+  import { valueForLetter } from '../lib/alphabet';
   import type { Variant } from '../lib/model';
   import { bonusLabel, type BoardLabelMode } from '../lib/boardlabels';
   import type { Locale } from '../lib/i18n/catalog';
@@ -98,7 +98,7 @@
           >
             {#if letter}
               <span class="letter">{letter}</span>
-              {#if !blank}<span class="val">{tileValue(variant, letter)}</span>{/if}
+              {#if !blank}<span class="val">{valueForLetter(variant, letter)}</span>{/if}
             {:else if r === centre.row && c === centre.col}
               <span class="star">★</span>
             {:else if bl?.kind === 'single'}

ui/src/game/Game.svelte

@@ -14,7 +14,8 @@
   import { t } from '../lib/i18n/index.svelte';
   import type { ChatMessage, Direction, EvalResult, MoveRecord, StateView } from '../lib/model';
   import { replay } from '../lib/board';
-  import { alphabet, centre, premiumGrid } from '../lib/premiums';
+  import { centre, premiumGrid } from '../lib/premiums';
+  import { alphabetLetters, hasAlphabet } from '../lib/alphabet';
   import { canCheckWord, sanitizeCheckWord } from '../lib/checkword';
   import { shareOrDownloadGcg } from '../lib/share';
   import {
@@ -84,7 +85,13 @@
 
   async function load() {
     try {
-      const [st, hist] = await Promise.all([gateway.gameState(id), gateway.gameHistory(id)]);
+      // Ask for the alphabet table only on a per-variant cache miss (the first open of a
+      // game whose variant the client has not cached yet); steady-state polls omit it.
+      const includeAlphabet = !view || !hasAlphabet(view.game.variant);
+      const [st, hist] = await Promise.all([
+        gateway.gameState(id, includeAlphabet),
+        gateway.gameHistory(id),
+      ]);
       view = st;
       moves = hist.moves;
       placement = newPlacement(st.rack);
@@ -206,7 +213,7 @@
     if (!sub) return;
     previewTimer = setTimeout(async () => {
       try {
-        preview = await gateway.evaluate(id, sub.dir, sub.tiles);
+        preview = await gateway.evaluate(id, sub.dir, sub.tiles, variant);
       } catch {
         /* best-effort */
       }
@@ -218,7 +225,7 @@
     if (!sub) return;
     busy = true;
     try {
-      await gateway.submitPlay(id, sub.dir, sub.tiles);
+      await gateway.submitPlay(id, sub.dir, sub.tiles, variant);
       zoomed = false;
       await load();
     } catch (e) {
@@ -298,7 +305,7 @@
     exchangeOpen = false;
     busy = true;
     try {
-      await gateway.exchange(id, tiles);
+      await gateway.exchange(id, tiles, variant);
       await load();
     } catch (e) {
       handleError(e);
@@ -313,7 +320,7 @@
     checkOpen = true;
   }
   function onCheckInput(e: Event) {
-    checkWord = sanitizeCheckWord((e.target as HTMLInputElement).value, alphabet(variant));
+    checkWord = sanitizeCheckWord((e.target as HTMLInputElement).value, alphabetLetters(variant));
   }
   // Check is disabled while cooling down, for an already-checked word, or an out-of-range
   // length. The input filter already restricts to the variant's alphabet.
@@ -326,9 +333,11 @@
     cooling = true;
     setTimeout(() => (cooling = false), 5000);
     try {
-      const r = await gateway.checkWord(id, w);
-      checkedWords.set(r.word.toUpperCase(), r.legal);
-      checkResult = r;
+      const r = await gateway.checkWord(id, w, variant);
+      // Key the cache and the displayed result on the upper-case word the player typed; the
+      // server echoes the decoded concrete word in the solver's lower case.
+      checkedWords.set(w, r.legal);
+      checkResult = { word: w, legal: r.legal };
     } catch (e) {
       handleError(e);
     }
@@ -535,7 +544,7 @@
 {#if blankPrompt}
   <Modal title={t('game.chooseBlank')} onclose={() => (blankPrompt = null)}>
     <div class="alpha">
-      {#each alphabet(variant) as ch (ch)}
+      {#each alphabetLetters(variant) as ch (ch)}
         <button onclick={() => chooseBlank(ch)}>{ch}</button>
       {/each}
     </div>

ui/src/game/Rack.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import type { RackSlot } from '../lib/placement';
   import { BLANK } from '../lib/placement';
-  import { tileValue } from '../lib/premiums';
+  import { valueForLetter } from '../lib/alphabet';
   import type { Variant } from '../lib/model';
 
   let {
@@ -30,7 +30,7 @@
       onpointerdown={(e) => ondown(e, slot.index)}
     >
       <span class="letter">{slot.letter === BLANK ? '' : slot.letter}</span>
-      {#if slot.letter !== BLANK}<span class="val">{tileValue(variant, slot.letter)}</span>{/if}
+      {#if slot.letter !== BLANK}<span class="val">{valueForLetter(variant, slot.letter)}</span>{/if}
     </button>
   {/each}
 </div>

ui/src/gen/fbs/scrabblefb.ts

@@ -2,6 +2,7 @@
 
 export { AccountRef } from './scrabblefb/account-ref.js';
 export { Ack } from './scrabblefb/ack.js';
+export { AlphabetEntry } from './scrabblefb/alphabet-entry.js';
 export { BlockList } from './scrabblefb/block-list.js';
 export { ChatList } from './scrabblefb/chat-list.js';
 export { ChatMessage } from './scrabblefb/chat-message.js';
@@ -41,6 +42,7 @@ export { MoveResult } from './scrabblefb/move-result.js';
 export { NotificationEvent } from './scrabblefb/notification-event.js';
 export { NudgeEvent } from './scrabblefb/nudge-event.js';
 export { OpponentMovedEvent } from './scrabblefb/opponent-moved-event.js';
+export { PlayTile } from './scrabblefb/play-tile.js';
 export { Profile } from './scrabblefb/profile.js';
 export { RedeemCodeRequest } from './scrabblefb/redeem-code-request.js';
 export { RedeemResult } from './scrabblefb/redeem-result.js';

ui/src/gen/fbs/scrabblefb/alphabet-entry.ts

@@ -0,0 +1,68 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import * as flatbuffers from 'flatbuffers';
+
+export class AlphabetEntry {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):AlphabetEntry {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsAlphabetEntry(bb:flatbuffers.ByteBuffer, obj?:AlphabetEntry):AlphabetEntry {
+  return (obj || new AlphabetEntry()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsAlphabetEntry(bb:flatbuffers.ByteBuffer, obj?:AlphabetEntry):AlphabetEntry {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new AlphabetEntry()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+index():number {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? this.bb!.readUint8(this.bb_pos + offset) : 0;
+}
+
+letter():string|null
+letter(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+letter(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;
+}
+
+value():number {
+  const offset = this.bb!.__offset(this.bb_pos, 8);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+static startAlphabetEntry(builder:flatbuffers.Builder) {
+  builder.startObject(3);
+}
+
+static addIndex(builder:flatbuffers.Builder, index:number) {
+  builder.addFieldInt8(0, index, 0);
+}
+
+static addLetter(builder:flatbuffers.Builder, letterOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(1, letterOffset, 0);
+}
+
+static addValue(builder:flatbuffers.Builder, value:number) {
+  builder.addFieldInt32(2, value, 0);
+}
+
+static endAlphabetEntry(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createAlphabetEntry(builder:flatbuffers.Builder, index:number, letterOffset:flatbuffers.Offset, value:number):flatbuffers.Offset {
+  AlphabetEntry.startAlphabetEntry(builder);
+  AlphabetEntry.addIndex(builder, index);
+  AlphabetEntry.addLetter(builder, letterOffset);
+  AlphabetEntry.addValue(builder, value);
+  return AlphabetEntry.endAlphabetEntry(builder);
+}
+}

ui/src/gen/fbs/scrabblefb/check-word-request.ts

@@ -27,11 +27,19 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-word():string|null
-word(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
-word(optionalEncoding?:any):string|Uint8Array|null {
+word(index: number):number|null {
   const offset = this.bb!.__offset(this.bb_pos, 6);
-  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+  return offset ? this.bb!.readUint8(this.bb!.__vector(this.bb_pos + offset) + index) : 0;
+}
+
+wordLength():number {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
+}
+
+wordArray():Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? new Uint8Array(this.bb!.bytes().buffer, this.bb!.bytes().byteOffset + this.bb!.__vector(this.bb_pos + offset), this.bb!.__vector_len(this.bb_pos + offset)) : null;
 }
 
 static startCheckWordRequest(builder:flatbuffers.Builder) {
@@ -46,6 +54,18 @@ static addWord(builder:flatbuffers.Builder, wordOffset:flatbuffers.Offset) {
   builder.addFieldOffset(1, wordOffset, 0);
 }
 
+static createWordVector(builder:flatbuffers.Builder, data:number[]|Uint8Array):flatbuffers.Offset {
+  builder.startVector(1, data.length, 1);
+  for (let i = data.length - 1; i >= 0; i--) {
+    builder.addInt8(data[i]!);
+  }
+  return builder.endVector();
+}
+
+static startWordVector(builder:flatbuffers.Builder, numElems:number) {
+  builder.startVector(1, numElems, 1);
+}
+
 static endCheckWordRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;

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

@@ -2,7 +2,7 @@
 
 import * as flatbuffers from 'flatbuffers';
 
-import { TileRecord } from '../scrabblefb/tile-record.js';
+import { PlayTile } from '../scrabblefb/play-tile.js';
 
 
 export class EvalRequest {
@@ -37,9 +37,9 @@ dir(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-tiles(index: number, obj?:TileRecord):TileRecord|null {
+tiles(index: number, obj?:PlayTile):PlayTile|null {
   const offset = this.bb!.__offset(this.bb_pos, 8);
-  return offset ? (obj || new TileRecord()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
+  return offset ? (obj || new PlayTile()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
 }
 
 tilesLength():number {

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

@@ -27,11 +27,9 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-tiles(index: number):string
-tiles(index: number,optionalEncoding:flatbuffers.Encoding):string|Uint8Array
-tiles(index: number,optionalEncoding?:any):string|Uint8Array|null {
+tiles(index: number):number|null {
   const offset = this.bb!.__offset(this.bb_pos, 6);
-  return offset ? this.bb!.__string(this.bb!.__vector(this.bb_pos + offset) + index * 4, optionalEncoding) : null;
+  return offset ? this.bb!.readUint8(this.bb!.__vector(this.bb_pos + offset) + index) : 0;
 }
 
 tilesLength():number {
@@ -39,6 +37,11 @@ tilesLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+tilesArray():Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? new Uint8Array(this.bb!.bytes().buffer, this.bb!.bytes().byteOffset + this.bb!.__vector(this.bb_pos + offset), this.bb!.__vector_len(this.bb_pos + offset)) : null;
+}
+
 static startExchangeRequest(builder:flatbuffers.Builder) {
   builder.startObject(2);
 }
@@ -51,16 +54,16 @@ static addTiles(builder:flatbuffers.Builder, tilesOffset:flatbuffers.Offset) {
   builder.addFieldOffset(1, tilesOffset, 0);
 }
 
-static createTilesVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
-  builder.startVector(4, data.length, 4);
+static createTilesVector(builder:flatbuffers.Builder, data:number[]|Uint8Array):flatbuffers.Offset {
+  builder.startVector(1, data.length, 1);
   for (let i = data.length - 1; i >= 0; i--) {
-    builder.addOffset(data[i]!);
+    builder.addInt8(data[i]!);
   }
   return builder.endVector();
 }
 
 static startTilesVector(builder:flatbuffers.Builder, numElems:number) {
-  builder.startVector(4, numElems, 4);
+  builder.startVector(1, numElems, 1);
 }
 
 static endExchangeRequest(builder:flatbuffers.Builder):flatbuffers.Offset {

ui/src/gen/fbs/scrabblefb/play-tile.ts

@@ -0,0 +1,76 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import * as flatbuffers from 'flatbuffers';
+
+export class PlayTile {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):PlayTile {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsPlayTile(bb:flatbuffers.ByteBuffer, obj?:PlayTile):PlayTile {
+  return (obj || new PlayTile()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsPlayTile(bb:flatbuffers.ByteBuffer, obj?:PlayTile):PlayTile {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new PlayTile()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+row():number {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+col():number {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+letter():number {
+  const offset = this.bb!.__offset(this.bb_pos, 8);
+  return offset ? this.bb!.readUint8(this.bb_pos + offset) : 0;
+}
+
+blank():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 10);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
+static startPlayTile(builder:flatbuffers.Builder) {
+  builder.startObject(4);
+}
+
+static addRow(builder:flatbuffers.Builder, row:number) {
+  builder.addFieldInt32(0, row, 0);
+}
+
+static addCol(builder:flatbuffers.Builder, col:number) {
+  builder.addFieldInt32(1, col, 0);
+}
+
+static addLetter(builder:flatbuffers.Builder, letter:number) {
+  builder.addFieldInt8(2, letter, 0);
+}
+
+static addBlank(builder:flatbuffers.Builder, blank:boolean) {
+  builder.addFieldInt8(3, +blank, +false);
+}
+
+static endPlayTile(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createPlayTile(builder:flatbuffers.Builder, row:number, col:number, letter:number, blank:boolean):flatbuffers.Offset {
+  PlayTile.startPlayTile(builder);
+  PlayTile.addRow(builder, row);
+  PlayTile.addCol(builder, col);
+  PlayTile.addLetter(builder, letter);
+  PlayTile.addBlank(builder, blank);
+  return PlayTile.endPlayTile(builder);
+}
+}

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

@@ -27,22 +27,32 @@ gameId(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
+includeAlphabet():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
 static startStateRequest(builder:flatbuffers.Builder) {
-  builder.startObject(1);
+  builder.startObject(2);
 }
 
 static addGameId(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset) {
   builder.addFieldOffset(0, gameIdOffset, 0);
 }
 
+static addIncludeAlphabet(builder:flatbuffers.Builder, includeAlphabet:boolean) {
+  builder.addFieldInt8(1, +includeAlphabet, +false);
+}
+
 static endStateRequest(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createStateRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createStateRequest(builder:flatbuffers.Builder, gameIdOffset:flatbuffers.Offset, includeAlphabet:boolean):flatbuffers.Offset {
   StateRequest.startStateRequest(builder);
   StateRequest.addGameId(builder, gameIdOffset);
+  StateRequest.addIncludeAlphabet(builder, includeAlphabet);
   return StateRequest.endStateRequest(builder);
 }
 }

ui/src/gen/fbs/scrabblefb/state-view.ts

@@ -2,6 +2,7 @@
 
 import * as flatbuffers from 'flatbuffers';
 
+import { AlphabetEntry } from '../scrabblefb/alphabet-entry.js';
 import { GameView } from '../scrabblefb/game-view.js';
 
 
@@ -33,11 +34,9 @@ seat():number {
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
 }
 
-rack(index: number):string
-rack(index: number,optionalEncoding:flatbuffers.Encoding):string|Uint8Array
-rack(index: number,optionalEncoding?:any):string|Uint8Array|null {
+rack(index: number):number|null {
   const offset = this.bb!.__offset(this.bb_pos, 8);
-  return offset ? this.bb!.__string(this.bb!.__vector(this.bb_pos + offset) + index * 4, optionalEncoding) : null;
+  return offset ? this.bb!.readUint8(this.bb!.__vector(this.bb_pos + offset) + index) : 0;
 }
 
 rackLength():number {
@@ -45,6 +44,11 @@ rackLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+rackArray():Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 8);
+  return offset ? new Uint8Array(this.bb!.bytes().buffer, this.bb!.bytes().byteOffset + this.bb!.__vector(this.bb_pos + offset), this.bb!.__vector_len(this.bb_pos + offset)) : null;
+}
+
 bagLen():number {
   const offset = this.bb!.__offset(this.bb_pos, 10);
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
@@ -55,8 +59,18 @@ hintsRemaining():number {
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
 }
 
+alphabet(index: number, obj?:AlphabetEntry):AlphabetEntry|null {
+  const offset = this.bb!.__offset(this.bb_pos, 14);
+  return offset ? (obj || new AlphabetEntry()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
+}
+
+alphabetLength():number {
+  const offset = this.bb!.__offset(this.bb_pos, 14);
+  return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
+}
+
 static startStateView(builder:flatbuffers.Builder) {
-  builder.startObject(5);
+  builder.startObject(6);
 }
 
 static addGame(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset) {
@@ -71,16 +85,16 @@ static addRack(builder:flatbuffers.Builder, rackOffset:flatbuffers.Offset) {
   builder.addFieldOffset(2, rackOffset, 0);
 }
 
-static createRackVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
-  builder.startVector(4, data.length, 4);
+static createRackVector(builder:flatbuffers.Builder, data:number[]|Uint8Array):flatbuffers.Offset {
+  builder.startVector(1, data.length, 1);
   for (let i = data.length - 1; i >= 0; i--) {
-    builder.addOffset(data[i]!);
+    builder.addInt8(data[i]!);
   }
   return builder.endVector();
 }
 
 static startRackVector(builder:flatbuffers.Builder, numElems:number) {
-  builder.startVector(4, numElems, 4);
+  builder.startVector(1, numElems, 1);
 }
 
 static addBagLen(builder:flatbuffers.Builder, bagLen:number) {
@@ -91,18 +105,35 @@ static addHintsRemaining(builder:flatbuffers.Builder, hintsRemaining:number) {
   builder.addFieldInt32(4, hintsRemaining, 0);
 }
 
+static addAlphabet(builder:flatbuffers.Builder, alphabetOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(5, alphabetOffset, 0);
+}
+
+static createAlphabetVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
+  builder.startVector(4, data.length, 4);
+  for (let i = data.length - 1; i >= 0; i--) {
+    builder.addOffset(data[i]!);
+  }
+  return builder.endVector();
+}
+
+static startAlphabetVector(builder:flatbuffers.Builder, numElems:number) {
+  builder.startVector(4, numElems, 4);
+}
+
 static endStateView(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createStateView(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset, seat:number, rackOffset:flatbuffers.Offset, bagLen:number, hintsRemaining:number):flatbuffers.Offset {
+static createStateView(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset, seat:number, rackOffset:flatbuffers.Offset, bagLen:number, hintsRemaining:number, alphabetOffset:flatbuffers.Offset):flatbuffers.Offset {
   StateView.startStateView(builder);
   StateView.addGame(builder, gameOffset);
   StateView.addSeat(builder, seat);
   StateView.addRack(builder, rackOffset);
   StateView.addBagLen(builder, bagLen);
   StateView.addHintsRemaining(builder, hintsRemaining);
+  StateView.addAlphabet(builder, alphabetOffset);
   return StateView.endStateView(builder);
 }
 }

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

@@ -2,7 +2,7 @@
 
 import * as flatbuffers from 'flatbuffers';
 
-import { TileRecord } from '../scrabblefb/tile-record.js';
+import { PlayTile } from '../scrabblefb/play-tile.js';
 
 
 export class SubmitPlayRequest {
@@ -37,9 +37,9 @@ dir(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
-tiles(index: number, obj?:TileRecord):TileRecord|null {
+tiles(index: number, obj?:PlayTile):PlayTile|null {
   const offset = this.bb!.__offset(this.bb_pos, 8);
-  return offset ? (obj || new TileRecord()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
+  return offset ? (obj || new PlayTile()).__init(this.bb!.__indirect(this.bb!.__vector(this.bb_pos + offset) + index * 4), this.bb!) : null;
 }
 
 tilesLength():number {

ui/src/lib/alphabet.test.ts

@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest';
+import {
+  alphabetLetters,
+  BLANK_INDEX,
+  hasAlphabet,
+  indexForLetter,
+  letterForIndex,
+  setAlphabet,
+  valueForLetter,
+} from './alphabet';
+
+// The cache module is per-file-isolated by vitest, so only what these tests seed exists.
+describe('alphabet cache (Stage 13)', () => {
+  it('upper-cases letters for display and maps indices and values case-insensitively', () => {
+    setAlphabet('english', [
+      { index: 0, letter: 'a', value: 1 },
+      { index: 16, letter: 'q', value: 10 },
+    ]);
+    expect(hasAlphabet('english')).toBe(true);
+    expect(letterForIndex('english', 0)).toBe('A');
+    expect(letterForIndex('english', 16)).toBe('Q');
+    expect(indexForLetter('english', 'a')).toBe(0);
+    expect(indexForLetter('english', 'Q')).toBe(16);
+    expect(valueForLetter('english', 'a')).toBe(1);
+    expect(valueForLetter('english', 'Q')).toBe(10);
+  });
+
+  it('handles the blank sentinel and unknown letters/indices', () => {
+    setAlphabet('english', [{ index: 0, letter: 'a', value: 1 }]);
+    expect(letterForIndex('english', BLANK_INDEX)).toBe('?');
+    expect(indexForLetter('english', '?')).toBe(BLANK_INDEX);
+    expect(valueForLetter('english', '?')).toBe(0);
+    expect(letterForIndex('english', 99)).toBe(''); // out of range
+    expect(valueForLetter('english', 'Z')).toBe(0); // not in this alphabet
+    expect(() => indexForLetter('english', 'Z')).toThrow();
+  });
+
+  it('lists the alphabet for the blank chooser and is empty for an uncached variant', () => {
+    setAlphabet('english', [
+      { index: 0, letter: 'a', value: 1 },
+      { index: 1, letter: 'b', value: 3 },
+    ]);
+    expect(alphabetLetters('english')).toEqual(['A', 'B']);
+    expect(hasAlphabet('erudit')).toBe(false);
+    expect(alphabetLetters('erudit')).toEqual([]);
+    expect(valueForLetter('erudit', 'A')).toBe(0);
+  });
+});

ui/src/lib/alphabet.ts

@@ -0,0 +1,84 @@
+// Per-variant alphabet table cache (Stage 13). The client is alphabet-agnostic: it caches
+// each variant's (index, letter, value) table — sent by the server on a per-variant cache
+// miss, behind game.state's include_alphabet flag — and renders the rack and the blank
+// chooser with it while live play exchanges bare alphabet indices on the wire. Letters are
+// stored upper-cased for display (the rest of the UI works in upper case) and index lookups
+// are case-insensitive. A blank rides as the sentinel index 255 in a rack/exchange list; a
+// placed blank instead travels as its designated letter's index with a separate blank flag.
+
+import type { Variant } from './model';
+
+/** BLANK_INDEX is the wire sentinel for a blank tile in a rack/exchange index list. */
+export const BLANK_INDEX = 255;
+
+/** BLANK_LETTER is the glyph a blank rack tile decodes to (matches placement.BLANK). */
+const BLANK_LETTER = '?';
+
+/** AlphabetEntryWire is one raw alphabet row as received from the wire or a mock fixture. */
+export interface AlphabetEntryWire {
+  index: number;
+  letter: string;
+  value: number;
+}
+
+interface Table {
+  letters: string[]; // by index, upper-cased
+  values: number[]; // by index
+  indexByLetter: Map<string, number>; // upper-cased letter -> index
+}
+
+const cache = new Map<Variant, Table>();
+
+/** setAlphabet caches a variant's table, upper-casing letters for display. */
+export function setAlphabet(variant: Variant, entries: AlphabetEntryWire[]): void {
+  let size = 0;
+  for (const e of entries) size = Math.max(size, e.index + 1);
+  const letters = new Array<string>(size).fill('');
+  const values = new Array<number>(size).fill(0);
+  const indexByLetter = new Map<string, number>();
+  for (const e of entries) {
+    const up = e.letter.toUpperCase();
+    letters[e.index] = up;
+    values[e.index] = e.value;
+    indexByLetter.set(up, e.index);
+  }
+  cache.set(variant, { letters, values, indexByLetter });
+}
+
+/** hasAlphabet reports whether a variant's table is cached (so the client can skip asking
+ * the server to resend it). */
+export function hasAlphabet(variant: Variant): boolean {
+  return cache.has(variant);
+}
+
+/** alphabetLetters lists a variant's letters (upper-cased) for the blank chooser and the
+ * word-check input filter; empty when the table is not yet cached. */
+export function alphabetLetters(variant: Variant): string[] {
+  return cache.get(variant)?.letters.slice() ?? [];
+}
+
+/** letterForIndex maps a wire rack index to its display letter: the blank sentinel maps to
+ * "?", an unknown index to "". */
+export function letterForIndex(variant: Variant, index: number): string {
+  if (index === BLANK_INDEX) return BLANK_LETTER;
+  return cache.get(variant)?.letters[index] ?? '';
+}
+
+/** valueForLetter returns a tile's point value; a blank ("?") and an unknown letter score 0. */
+export function valueForLetter(variant: Variant, letter: string): number {
+  if (!letter || letter === BLANK_LETTER) return 0;
+  const t = cache.get(variant);
+  if (!t) return 0;
+  const i = t.indexByLetter.get(letter.toUpperCase());
+  return i === undefined ? 0 : t.values[i];
+}
+
+/** indexForLetter maps a display letter to its wire index; a blank ("?") maps to the blank
+ * sentinel. It throws when the letter is outside the cached alphabet — a placement bug, not
+ * user input (the UI constrains every entry point to the variant's alphabet). */
+export function indexForLetter(variant: Variant, letter: string): number {
+  if (letter === BLANK_LETTER) return BLANK_INDEX;
+  const i = cache.get(variant)?.indexByLetter.get(letter.toUpperCase());
+  if (i === undefined) throw new Error(`alphabet: no index for "${letter}" in ${variant}`);
+  return i;
+}

ui/src/lib/client.ts

@@ -67,15 +67,18 @@ export interface GatewayClient {
   lobbyPoll(): Promise<MatchResult>;
 
   // --- game ---
-  gameState(gameId: string): Promise<StateView>;
+  // Stage 13: the play loop exchanges alphabet indices, so submit/evaluate/exchange/
+  // check-word take the game's variant (to map letters<->indices via the cached alphabet
+  // 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[]): Promise<MoveResult>;
+  submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<MoveResult>;
   pass(gameId: string): Promise<MoveResult>;
-  exchange(gameId: string, tiles: 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[]): Promise<EvalResult>;
-  checkWord(gameId: string, word: string): Promise<WordCheckResult>;
+  evaluate(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<EvalResult>;
+  checkWord(gameId: string, word: string, variant: Variant): Promise<WordCheckResult>;
   complaint(gameId: string, word: string, note: string): Promise<void>;
 
   // --- chat ---

ui/src/lib/codec.test.ts

@@ -1,28 +1,44 @@
 import { Builder, ByteBuffer } from 'flatbuffers';
 import { describe, expect, it } from 'vitest';
 import * as fb from '../gen/fbs/scrabblefb';
+import { BLANK_INDEX, setAlphabet } from './alphabet';
 import {
   decodeFriendList,
   decodeGameList,
   decodeInvitation,
   decodeLinkResult,
   decodeSession,
+  decodeStateView,
   decodeStats,
+  encodeCheckWord,
+  encodeExchange,
+  encodeStateRequest,
   encodeSubmitPlay,
   encodeTarget,
 } from './codec';
 
 describe('codec', () => {
-  it('encodes a SubmitPlayRequest the gateway can read', () => {
-    const buf = encodeSubmitPlay('g1', 'H', [
-      { row: 7, col: 7, letter: 'A', blank: false },
-      { row: 7, col: 8, letter: 'B', blank: true },
+  it('encodes a SubmitPlayRequest with alphabet indices (Stage 13)', () => {
+    setAlphabet('english', [
+      { index: 0, letter: 'a', value: 1 },
+      { index: 1, letter: 'b', value: 3 },
     ]);
+    // 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 },
+      ],
+      'english',
+    );
     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('A');
+    expect(r.tiles(0)?.letter()).toBe(0);
+    expect(r.tiles(1)?.letter()).toBe(1);
     expect(r.tiles(1)?.blank()).toBe(true);
   });
 
@@ -214,3 +230,72 @@ describe('codec', () => {
     expect(inv.variant).toBe('english');
   });
 });
+
+// Stage 13: the live play loop exchanges alphabet indices, mapped through the per-variant
+// table cached in lib/alphabet. Each test seeds the cache it needs (setAlphabet replaces
+// the whole table), so they are independent of order.
+describe('codec — alphabet on the wire (Stage 13)', () => {
+  it('encodes an ExchangeRequest as alphabet indices, blank as the sentinel', () => {
+    setAlphabet('english', [
+      { index: 0, letter: 'a', value: 1 },
+      { index: 1, letter: 'b', value: 3 },
+    ]);
+    const r = fb.ExchangeRequest.getRootAsExchangeRequest(
+      new ByteBuffer(encodeExchange('g1', ['A', '?'], 'english')),
+    );
+    expect(r.tilesLength()).toBe(2);
+    expect(r.tiles(0)).toBe(0);
+    expect(r.tiles(1)).toBe(BLANK_INDEX);
+  });
+
+  it('encodes a CheckWordRequest as alphabet indices', () => {
+    setAlphabet('english', [
+      { index: 0, letter: 'a', value: 1 },
+      { index: 2, letter: 'c', value: 3 },
+      { index: 19, letter: 't', value: 1 },
+    ]);
+    const r = fb.CheckWordRequest.getRootAsCheckWordRequest(
+      new ByteBuffer(encodeCheckWord('g1', 'CAT', 'english')),
+    );
+    expect(r.wordLength()).toBe(3);
+    expect([r.word(0), r.word(1), r.word(2)]).toEqual([2, 0, 19]);
+  });
+
+  it('carries the include_alphabet flag on a StateRequest', () => {
+    const on = fb.StateRequest.getRootAsStateRequest(new ByteBuffer(encodeStateRequest('g1', true)));
+    expect(on.gameId()).toBe('g1');
+    expect(on.includeAlphabet()).toBe(true);
+    const off = fb.StateRequest.getRootAsStateRequest(new ByteBuffer(encodeStateRequest('g1', false)));
+    expect(off.includeAlphabet()).toBe(false);
+  });
+
+  it('caches the alphabet table from a StateView and decodes the index rack to letters', () => {
+    const b = new Builder(128);
+    const la = b.createString('a');
+    fb.AlphabetEntry.startAlphabetEntry(b);
+    fb.AlphabetEntry.addIndex(b, 0);
+    fb.AlphabetEntry.addLetter(b, la);
+    fb.AlphabetEntry.addValue(b, 1);
+    const ea = fb.AlphabetEntry.endAlphabetEntry(b);
+    const lb = b.createString('b');
+    fb.AlphabetEntry.startAlphabetEntry(b);
+    fb.AlphabetEntry.addIndex(b, 1);
+    fb.AlphabetEntry.addLetter(b, lb);
+    fb.AlphabetEntry.addValue(b, 3);
+    const eb = fb.AlphabetEntry.endAlphabetEntry(b);
+    const alpha = fb.StateView.createAlphabetVector(b, [ea, eb]);
+    const rack = fb.StateView.createRackVector(b, [0, BLANK_INDEX]);
+    fb.StateView.startStateView(b);
+    fb.StateView.addSeat(b, 0);
+    fb.StateView.addRack(b, rack);
+    fb.StateView.addBagLen(b, 10);
+    fb.StateView.addHintsRemaining(b, 0);
+    fb.StateView.addAlphabet(b, alpha);
+    b.finish(fb.StateView.endStateView(b));
+
+    // No GameView on the buffer, so decode falls back to the default variant 'english';
+    // the embedded table is cached under it and the rack [0, blank] decodes to letters.
+    const sv = decodeStateView(b.asUint8Array());
+    expect(sv.rack).toEqual(['A', '?']);
+  });
+});

ui/src/lib/codec.ts

@@ -5,6 +5,7 @@
 
 import { Builder, ByteBuffer, type Offset } from 'flatbuffers';
 import * as fb from '../gen/fbs/scrabblefb';
+import { indexForLetter, letterForIndex, setAlphabet, type AlphabetEntryWire } from './alphabet';
 import type { PlacedTile } from './client';
 import type {
   AccountRef,
@@ -37,14 +38,15 @@ import type {
 
 // --- request encoders ---
 
-function buildTile(b: Builder, t: PlacedTile): Offset {
-  const letter = b.createString(t.letter);
-  fb.TileRecord.startTileRecord(b);
-  fb.TileRecord.addRow(b, t.row);
-  fb.TileRecord.addCol(b, t.col);
-  fb.TileRecord.addLetter(b, letter);
-  fb.TileRecord.addBlank(b, t.blank);
-  return fb.TileRecord.endTileRecord(b);
+// buildPlayTile encodes one to-place tile by its alphabet index (Stage 13); a placed blank
+// carries its designated letter's index with blank set.
+function buildPlayTile(b: Builder, t: PlacedTile, variant: Variant): Offset {
+  fb.PlayTile.startPlayTile(b);
+  fb.PlayTile.addRow(b, t.row);
+  fb.PlayTile.addCol(b, t.col);
+  fb.PlayTile.addLetter(b, indexForLetter(variant, t.letter));
+  fb.PlayTile.addBlank(b, t.blank);
+  return fb.PlayTile.endPlayTile(b);
 }
 
 function finish(b: Builder, root: Offset): Uint8Array {
@@ -62,17 +64,23 @@ export function encodeGameAction(gameId: string): Uint8Array {
   return finish(b, fb.GameActionRequest.endGameActionRequest(b));
 }
 
-export function encodeStateRequest(gameId: string): Uint8Array {
+export function encodeStateRequest(gameId: string, includeAlphabet: boolean): Uint8Array {
   const b = new Builder(64);
   const gid = b.createString(gameId);
   fb.StateRequest.startStateRequest(b);
   fb.StateRequest.addGameId(b, gid);
+  fb.StateRequest.addIncludeAlphabet(b, includeAlphabet);
   return finish(b, fb.StateRequest.endStateRequest(b));
 }
 
-export function encodeSubmitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[]): Uint8Array {
+export function encodeSubmitPlay(
+  gameId: string,
+  dir: 'H' | 'V',
+  tiles: PlacedTile[],
+  variant: Variant,
+): Uint8Array {
   const b = new Builder(256);
-  const tileOffs = tiles.map((t) => buildTile(b, t));
+  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);
@@ -83,9 +91,14 @@ export function encodeSubmitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTi
   return finish(b, fb.SubmitPlayRequest.endSubmitPlayRequest(b));
 }
 
-export function encodeEval(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[]): Uint8Array {
+export function encodeEval(
+  gameId: string,
+  dir: 'H' | 'V',
+  tiles: PlacedTile[],
+  variant: Variant,
+): Uint8Array {
   const b = new Builder(256);
-  const tileOffs = tiles.map((t) => buildTile(b, t));
+  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);
@@ -96,10 +109,12 @@ export function encodeEval(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[]):
   return finish(b, fb.EvalRequest.endEvalRequest(b));
 }
 
-export function encodeExchange(gameId: string, tiles: string[]): Uint8Array {
+export function encodeExchange(gameId: string, tiles: string[], variant: Variant): Uint8Array {
   const b = new Builder(128);
-  const offs = tiles.map((s) => b.createString(s));
-  const vec = fb.ExchangeRequest.createTilesVector(b, offs);
+  const vec = fb.ExchangeRequest.createTilesVector(
+    b,
+    tiles.map((l) => indexForLetter(variant, l)),
+  );
   const gid = b.createString(gameId);
   fb.ExchangeRequest.startExchangeRequest(b);
   fb.ExchangeRequest.addGameId(b, gid);
@@ -107,13 +122,16 @@ export function encodeExchange(gameId: string, tiles: string[]): Uint8Array {
   return finish(b, fb.ExchangeRequest.endExchangeRequest(b));
 }
 
-export function encodeCheckWord(gameId: string, word: string): Uint8Array {
+export function encodeCheckWord(gameId: string, word: string, variant: Variant): Uint8Array {
   const b = new Builder(128);
+  const vec = fb.CheckWordRequest.createWordVector(
+    b,
+    Array.from(word).map((ch) => indexForLetter(variant, ch)),
+  );
   const gid = b.createString(gameId);
-  const w = b.createString(word);
   fb.CheckWordRequest.startCheckWordRequest(b);
   fb.CheckWordRequest.addGameId(b, gid);
-  fb.CheckWordRequest.addWord(b, w);
+  fb.CheckWordRequest.addWord(b, vec);
   return finish(b, fb.CheckWordRequest.endCheckWordRequest(b));
 }
 
@@ -188,7 +206,8 @@ function s(v: string | null): string {
 }
 
 function decodeTile(t: fb.TileRecord): Tile {
-  return { row: t.row(), col: t.col(), letter: s(t.letter()), blank: t.blank() };
+  // The wire keeps the move journal in the solver's lower case; the UI renders upper case.
+  return { row: t.row(), col: t.col(), letter: s(t.letter()).toUpperCase(), blank: t.blank() };
 }
 
 function decodeSeat(v: fb.SeatView): Seat {
@@ -229,7 +248,7 @@ function decodeMove(m: fb.MoveRecord): MoveRecord {
     if (t) tiles.push(decodeTile(t));
   }
   const words: string[] = [];
-  for (let i = 0; i < m.wordsLength(); i++) words.push(s(m.words(i)));
+  for (let i = 0; i < m.wordsLength(); i++) words.push(s(m.words(i)).toUpperCase());
   return {
     player: m.player(),
     action: s(m.action()),
@@ -280,8 +299,19 @@ export function decodeProfile(buf: Uint8Array): Profile {
 export function decodeStateView(buf: Uint8Array): StateView {
   const v = fb.StateView.getRootAsStateView(new ByteBuffer(buf));
   const g = v.game();
+  const variant = (g ? s(g.variant()) : 'english') as Variant;
+  // Cache the alphabet table when the server included it (a per-variant cache miss), then
+  // decode the index rack to display letters with it (Stage 13).
+  if (v.alphabetLength() > 0) {
+    const entries: AlphabetEntryWire[] = [];
+    for (let i = 0; i < v.alphabetLength(); i++) {
+      const e = v.alphabet(i);
+      if (e) entries.push({ index: e.index(), letter: s(e.letter()), value: e.value() });
+    }
+    setAlphabet(variant, entries);
+  }
   const rack: string[] = [];
-  for (let i = 0; i < v.rackLength(); i++) rack.push(s(v.rack(i)));
+  for (let i = 0; i < v.rackLength(); i++) rack.push(letterForIndex(variant, v.rack(i) ?? 0));
   return {
     game: g ? decodeGameView(g) : emptyGame(),
     seat: v.seat(),

ui/src/lib/mock/alphabet.ts

@@ -0,0 +1,41 @@
+// Mock alphabet fixtures (Stage 13). In production the per-variant (index, letter, value)
+// table comes from the server; the mock seeds the same client cache from a local copy so
+// the rack, the blank chooser and the mock's scoring work with no backend. The data is the
+// solver's value tables (scrabble-solver/rules/rules.go), in alphabet-index order, so a
+// token's position is its index — the same shape the wire delivers.
+
+import { setAlphabet, type AlphabetEntryWire } from '../alphabet';
+import type { Variant } from '../model';
+
+// "letter+value" tokens in alphabet-index order. English Latin a..z; Russian а..я incl. ё;
+// Эрудит а..я incl. ё=0.
+const SPECS: Record<Variant, string> = {
+  english:
+    'a1 b3 c3 d2 e1 f4 g2 h4 i1 j8 k5 l1 m3 n1 o1 p3 q10 r1 s1 t1 u1 v4 w4 x8 y4 z10',
+  russian:
+    'а1 б3 в1 г3 д2 е1 ё3 ж5 з5 и1 й4 к2 л2 м2 н1 о1 п2 р1 с1 т1 у2 ф10 х5 ц5 ч5 ш8 щ10 ъ10 ы4 ь3 э8 ю8 я3',
+  erudit:
+    'а1 б3 в2 г3 д2 е1 ё0 ж5 з5 и1 й2 к2 л2 м2 н1 о1 п2 р2 с2 т2 у3 ф10 х5 ц10 ч5 ш10 щ10 ъ10 ы5 ь5 э10 ю10 я3',
+};
+
+function parse(spec: string): AlphabetEntryWire[] {
+  return spec
+    .trim()
+    .split(/\s+/)
+    .map((tok, index) => {
+      const m = tok.match(/^(.+?)(\d+)$/);
+      return { index, letter: m ? m[1] : tok, value: m ? Number(m[2]) : 0 };
+    });
+}
+
+let seeded = false;
+
+/** seedMockAlphabets populates the alphabet cache for every variant once, mirroring the
+ * server-sent tables so the mock-driven UI is alphabet-agnostic too. */
+export function seedMockAlphabets(): void {
+  if (seeded) return;
+  for (const variant of Object.keys(SPECS) as Variant[]) {
+    setAlphabet(variant, parse(SPECS[variant]));
+  }
+  seeded = true;
+}

ui/src/lib/mock/client.ts

@@ -34,7 +34,8 @@ import type {
   Variant,
   WordCheckResult,
 } from '../model';
-import { tileValue } from '../premiums';
+import { valueForLetter } from '../alphabet';
+import { seedMockAlphabets } from './alphabet';
 import {
   ME,
   MOCK_FRIENDS,
@@ -93,6 +94,12 @@ export class MockGateway implements GatewayClient {
   private invitations: Invitation[] = mockInvitations();
   private readonly stats: Stats = { ...MOCK_STATS };
 
+  constructor() {
+    // Seed the per-variant alphabet cache the rack, blank chooser and scoring read, so the
+    // mock-driven UI is alphabet-agnostic without a backend (Stage 13).
+    seedMockAlphabets();
+  }
+
   setToken(_token: string | null): void {
     // The mock needs no auth; the real transport stores the bearer token.
   }
@@ -174,7 +181,7 @@ export class MockGateway implements GatewayClient {
   }
 
   // --- game ---
-  async gameState(gameId: string): Promise<StateView> {
+  async gameState(gameId: string, _includeAlphabet: boolean): Promise<StateView> {
     const g = this.game(gameId);
     return {
       game: structuredClone(g.view),
@@ -190,12 +197,12 @@ export class MockGateway implements GatewayClient {
     return { gameId, moves: structuredClone(g.moves) };
   }
 
-  async submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[]): Promise<MoveResult> {
+  async submitPlay(gameId: string, dir: 'H' | 'V', 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');
     const variant = g.view.variant;
-    let score = tiles.reduce((s, t) => s + tileValue(variant, t.blank ? '?' : t.letter), 0);
+    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 move = {
@@ -265,7 +272,7 @@ export class MockGateway implements GatewayClient {
   pass(gameId: string): Promise<MoveResult> {
     return this.simpleAction(gameId, 'pass');
   }
-  exchange(gameId: string, tiles: string[]): Promise<MoveResult> {
+  exchange(gameId: string, tiles: string[], _variant: Variant): Promise<MoveResult> {
     return this.simpleAction(gameId, 'exchange', tiles);
   }
   resign(gameId: string): Promise<MoveResult> {
@@ -287,22 +294,22 @@ export class MockGateway implements GatewayClient {
         tiles: [{ row: 7, col: 7, letter, blank: false }],
         words: [letter],
         count: 1,
-        score: tileValue(g.view.variant, letter),
+        score: valueForLetter(g.view.variant, letter),
         total: 0,
       },
       hintsRemaining: g.hintsRemaining,
     };
   }
 
-  async evaluate(gameId: string, _dir: 'H' | 'V', tiles: PlacedTile[]): Promise<EvalResult> {
+  async evaluate(gameId: string, _dir: 'H' | 'V', tiles: PlacedTile[], _variant: Variant): Promise<EvalResult> {
     const g = this.game(gameId);
     if (tiles.length === 0) return { legal: false, score: 0, words: [] };
-    let score = tiles.reduce((s, t) => s + tileValue(g.view.variant, t.blank ? '?' : t.letter), 0);
+    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('')] };
   }
 
-  async checkWord(_gameId: string, word: string): Promise<WordCheckResult> {
+  async checkWord(_gameId: string, word: string, _variant: Variant): Promise<WordCheckResult> {
     return { word, legal: word.trim().length >= 2 };
   }
   async complaint(): Promise<void> {}

ui/src/lib/premiums.test.ts

@@ -1,8 +1,10 @@
 import { describe, expect, it } from 'vitest';
-import { alphabet, BOARD_SIZE, centre, premiumGrid, tileValue } from './premiums';
+import { BOARD_SIZE, centre, premiumGrid } from './premiums';
 
-// Parity with scrabble-solver/rules/rules.go: english/russian share standardBoard
-// (centre is a double word); erudit shares the geometry but a non-doubling centre.
+// Premium-square geometry parity with scrabble-solver/rules/rules.go: english/russian
+// share standardBoard (centre is a double word); erudit shares the geometry but a
+// non-doubling centre. Tile-value and alphabet parity moved to the Go engine test
+// (backend/internal/engine AlphabetTable) in Stage 13 — the server now owns that table.
 describe('premium layout', () => {
   it('is a 15x15 grid with TW corners', () => {
     const g = premiumGrid('english');
@@ -35,19 +37,3 @@ describe('premium layout', () => {
     expect(count('DW')).toBe(17); // 16 double-word squares + the centre
   });
 });
-
-describe('tile values', () => {
-  it('scores letters per variant and zero for a blank', () => {
-    expect(tileValue('english', 'A')).toBe(1);
-    expect(tileValue('english', 'Q')).toBe(10);
-    expect(tileValue('english', '?')).toBe(0);
-    expect(tileValue('russian', 'Ф')).toBe(10);
-    expect(tileValue('erudit', 'Ё')).toBe(0);
-  });
-
-  it('exposes the full alphabet for the blank chooser', () => {
-    expect(alphabet('english')).toHaveLength(26);
-    expect(alphabet('russian')).toHaveLength(33);
-    expect(alphabet('erudit')).toHaveLength(33);
-  });
-});

ui/src/lib/premiums.ts

@@ -1,8 +1,9 @@
-// Board premium layout and tile values — ported verbatim from the engine source of
-// truth, scrabble-solver/rules/rules.go (standardBoard / eruditBoard, and the
-// per-variant value tables). These are NOT transmitted on the wire (StateView has
-// no board), so the client renders them locally. A Vitest parity test pins the
-// layout against the known geometry. Keep this in lockstep with the solver.
+// Board premium layout — the 15x15 premium-square geometry, ported from the engine source
+// of truth, scrabble-solver/rules/rules.go (standardBoard / eruditBoard). The board is not
+// transmitted on the wire (StateView has no board), so the client renders the premiums
+// locally; only the centre differs by variant. A Vitest parity test pins the geometry.
+// Tile values and the alphabet moved to the server-sent per-variant table in Stage 13 (see
+// lib/alphabet.ts), so this file is geometry only.
 
 import type { Variant } from './model';
 
@@ -84,43 +85,5 @@ export function centre(variant: Variant): { row: number; col: number } {
   return { row: 7, col: 7 };
 }
 
-// --- tile values (points shown on the tile face); blank scores 0 ---
-
-// English Latin a..z (rules.go English()).
-const enValues =
-  'a1 b3 c3 d2 e1 f4 g2 h4 i1 j8 k5 l1 m3 n1 o1 p3 q10 r1 s1 t1 u1 v4 w4 x8 y4 z10';
-// Russian а..я incl. ё (rules.go RussianScrabble()).
-const ruValues =
-  'а1 б3 в1 г3 д2 е1 ё3 ж5 з5 и1 й4 к2 л2 м2 н1 о1 п2 р1 с1 т1 у2 ф10 х5 ц5 ч5 ш8 щ10 ъ10 ы4 ь3 э8 ю8 я3';
-// Эрудит а..я incl. ё=0 (rules.go Erudit()).
-const eruditValues =
-  'а1 б3 в2 г3 д2 е1 ё0 ж5 з5 и1 й2 к2 л2 м2 н1 о1 п2 р2 с2 т2 у3 ф10 х5 ц10 ч5 ш10 щ10 ъ10 ы5 ь5 э10 ю10 я3';
-
-// Split each "letter+value" token into its letter (all but trailing digits) and its
-// integer value (the trailing digits).
-function valueTable(spec: string): Map<string, number> {
-  const m = new Map<string, number>();
-  for (const pair of spec.trim().split(/\s+/)) {
-    const match = pair.match(/^(.+?)(\d+)$/);
-    if (!match) continue;
-    m.set(match[1].toUpperCase(), Number(match[2]));
-  }
-  return m;
-}
-
-const VALUES: Record<Variant, Map<string, number>> = {
-  english: valueTable(enValues),
-  russian: valueTable(ruValues),
-  erudit: valueTable(eruditValues),
-};
-
-/** tileValue returns the point value of a concrete letter; a blank ("?") is 0. */
-export function tileValue(variant: Variant, letter: string): number {
-  if (!letter || letter === '?') return 0;
-  return VALUES[variant]?.get(letter.toUpperCase()) ?? 0;
-}
-
-/** alphabet lists a variant's letters in dictionary order (used by the blank chooser). */
-export function alphabet(variant: Variant): string[] {
-  return [...VALUES[variant].keys()];
-}
+// Tile values and the per-variant alphabet now arrive from the server (lib/alphabet.ts,
+// Stage 13); the board geometry above is all this module owns.

ui/src/lib/transport.ts

@@ -81,20 +81,20 @@ export function createTransport(baseUrl: string): GatewayClient {
       return codec.decodeMatchResult(await exec('lobby.poll', codec.empty()));
     },
 
-    async gameState(id) {
-      return codec.decodeStateView(await exec('game.state', codec.encodeStateRequest(id)));
+    async gameState(id, includeAlphabet) {
+      return codec.decodeStateView(await exec('game.state', codec.encodeStateRequest(id, includeAlphabet)));
     },
     async gameHistory(id) {
       return codec.decodeHistory(await exec('game.history', codec.encodeGameAction(id)));
     },
-    async submitPlay(id, dir, tiles) {
-      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, dir, tiles)));
+    async submitPlay(id, dir, tiles, variant) {
+      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, dir, tiles, variant)));
     },
     async pass(id) {
       return codec.decodeMoveResult(await exec('game.pass', codec.encodeGameAction(id)));
     },
-    async exchange(id, tiles) {
-      return codec.decodeMoveResult(await exec('game.exchange', codec.encodeExchange(id, tiles)));
+    async exchange(id, tiles, variant) {
+      return codec.decodeMoveResult(await exec('game.exchange', codec.encodeExchange(id, tiles, variant)));
     },
     async resign(id) {
       return codec.decodeMoveResult(await exec('game.resign', codec.encodeGameAction(id)));
@@ -102,11 +102,11 @@ export function createTransport(baseUrl: string): GatewayClient {
     async hint(id) {
       return codec.decodeHintResult(await exec('game.hint', codec.encodeGameAction(id)));
     },
-    async evaluate(id, dir, tiles) {
-      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, dir, tiles)));
+    async evaluate(id, dir, tiles, variant) {
+      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, dir, tiles, variant)));
     },
-    async checkWord(id, word) {
-      return codec.decodeWordCheck(await exec('game.check_word', codec.encodeCheckWord(id, word)));
+    async checkWord(id, word, variant) {
+      return codec.decodeWordCheck(await exec('game.check_word', codec.encodeCheckWord(id, word, variant)));
     },
     async complaint(id, word, note) {
       await exec('game.complaint', codec.encodeComplaint(id, word, note));