diff --git a/deploy/caddy/Caddyfile b/deploy/caddy/Caddyfile
index 7a2b9f7..578bd81 100644
--- a/deploy/caddy/Caddyfile
+++ b/deploy/caddy/Caddyfile
@@ -86,7 +86,7 @@
 	# The game SPA and the Connect edge are served by the gateway. Strip any
 	# client-supplied X-Scrabble-Honeypot here so the gateway only ever honours the
 	# tag the honeypot block sets below (a client cannot self-tag a real request).
-	@gateway path /app /app/* /telegram /telegram/* /vk /vk/* /dict/* /dl/* /metrics/* /scrabble.edge.v1.Gateway/*
+	@gateway path /app /app/* /telegram /telegram/* /vk /vk/* /dict/* /dl/* /metrics/* /telemetry/* /scrabble.edge.v1.Gateway/*
 	handle @gateway {
 		reverse_proxy gateway:8081 {
 			header_up -X-Scrabble-Honeypot
diff --git a/deploy/grafana/dashboards/users.json b/deploy/grafana/dashboards/users.json
index e7eb072..6504813 100644
--- a/deploy/grafana/dashboards/users.json
+++ b/deploy/grafana/dashboards/users.json
@@ -56,6 +56,22 @@
       "gridPos": { "h": 8, "w": 24, "x": 0, "y": 24 },
       "datasource": { "type": "prometheus", "uid": "prometheus" },
       "targets": [{ "refId": "A", "expr": "sum(rate(local_eval_preview_total[1h])) by (path)", "legendFormat": "{{path}}" }]
+    },
+    {
+      "type": "timeseries",
+      "title": "Unsupported-engine screens (cumulative by reason)",
+      "description": "Clients turned away by the index.html boot guard because the engine cannot run the app (an old Android System WebView), by reason: no_bigint / no_proxy (a missing unpolyfillable primitive) or boot_error (an uncaught startup failure). Deduped per device/version, so this counts distinct blocked installs, not launches. The effective floor is Chrome 67 (BigInt).",
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 32 },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "sum(unsupported_engine_total) by (reason)", "legendFormat": "{{reason}}" }]
+    },
+    {
+      "type": "timeseries",
+      "title": "Unsupported engines by Chromium (rate)",
+      "description": "The same blocked clients by reported Chromium major version — which old in-app WebViews are still in the wild. \"other\" is an unparseable or out-of-range version.",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 32 },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "sum(rate(unsupported_engine_total[1h])) by (chromium)", "legendFormat": "Chromium {{chromium}}" }]
     }
   ]
 }
diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index 030165e..f053c97 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -1070,6 +1070,17 @@ browser), so an open in-app session reflects it at once.
   it fills IndexedDB — and is surfaced on the **Scrabble — Users** dashboard. Lossy by
   design (a dropped beacon just retries its batch on the next flush); it is telemetry, never
   a game input.
+- **Unsupported-engine screens (client-reported):** the ES5 boot guard in `index.html` (§13)
+  shows a full-screen "your device's OS or browser can't run the app" screen — instead of a white
+  screen — when the engine lacks an unpolyfillable essential (`BigInt`, the 64-bit FlatBuffers
+  decode; or `Proxy`, Svelte 5 runes) or an uncaught error aborts boot; the effective floor is
+  Chrome 67. It then fires one fire-and-forget beacon (`POST /telemetry/unsupported` —
+  unauthenticated, since the client never booted, but per-IP public-limited and body-capped),
+  deduped in `localStorage` by app version + reason + Chromium so a user reopening the app is one
+  report. The gateway folds it into `unsupported_engine_total` (`reason` =
+  no_bigint/no_proxy/boot_error/other; `chromium` = the major version reduced to a bounded range so
+  a spoofed beacon cannot inflate cardinality) and logs the full user agent (not a label). Surfaced
+  on the **Scrabble — Users** dashboard beside app opens.
 - **Rate-limit observability:** every limiter rejection increments the gateway
   counter `gateway_rate_limited_total` (`class` = user/public/email/admin — aggregate
   only, honouring the no-per-user-label discipline above) and logs one **Debug** line;
diff --git a/gateway/internal/connectsrv/metrics.go b/gateway/internal/connectsrv/metrics.go
index 9c619e7..c531e83 100644
--- a/gateway/internal/connectsrv/metrics.go
+++ b/gateway/internal/connectsrv/metrics.go
@@ -32,6 +32,8 @@ type serverMetrics struct {
 	localColdStart metric.Int64Counter
 	localDictLoad  metric.Int64Counter
 	localPreview   metric.Int64Counter
+	// Clients turned away by the unsupported-engine boot screen (see unsupportedEngineHandler).
+	unsupportedEngine metric.Int64Counter
 }
 
 // newServerMetrics builds the instruments on meter (nil selects a no-op meter),
@@ -63,6 +65,8 @@ func newServerMetrics(meter metric.Meter) *serverMetrics {
 		localColdStart: counterOf(meter, "local_eval_cold_start_total", "App cold starts reported by clients — the denominator for local-move-preview adoption."),
 		localDictLoad:  counterOf(meter, "local_eval_dict_load_total", "Client dictionary loads for the local move preview, by result (fetched, cache_hit or miss)."),
 		localPreview:   counterOf(meter, "local_eval_preview_total", "Client move previews, by path (local on-device, or network fallback)."),
+		unsupportedEngine: counterOf(meter, "unsupported_engine_total",
+			"Clients that hit the unsupported-engine boot screen (the app cannot run), by reason (no_bigint, no_proxy, boot_error, other) and Chromium major — a deduped beacon from the index.html boot guard; the full user agent is logged, not labelled."),
 	}
 
 	gauge, err := meter.Int64ObservableGauge("active_users",
@@ -108,6 +112,16 @@ func (m *serverMetrics) recordBan(ctx context.Context, reason string) {
 	m.banned.Add(ctx, 1, metric.WithAttributes(attribute.String("reason", reason)))
 }
 
+// recordUnsupportedEngine counts one client turned away by the unsupported-engine boot screen,
+// labelled by reason and Chromium major. The caller passes both already reduced to bounded label
+// sets (see normalizeUnsupported) so a spoofed beacon cannot explode the metric cardinality.
+func (m *serverMetrics) recordUnsupportedEngine(ctx context.Context, reason, chromium string) {
+	m.unsupportedEngine.Add(ctx, 1, metric.WithAttributes(
+		attribute.String("reason", reason),
+		attribute.String("chromium", chromium),
+	))
+}
+
 // localEvalReport is the client-reported local move-preview telemetry batch — deltas since
 // the client's previous report. It backs the adoption dashboard: app cold starts vs cached
 // dictionaries vs on-device previews.
diff --git a/gateway/internal/connectsrv/metrics_test.go b/gateway/internal/connectsrv/metrics_test.go
index 9e9f314..c2cf8c8 100644
--- a/gateway/internal/connectsrv/metrics_test.go
+++ b/gateway/internal/connectsrv/metrics_test.go
@@ -128,3 +128,70 @@ func TestBannedMetric(t *testing.T) {
 		t.Errorf("banned counts = %v, want tripwire=2 honeytoken=1", counts)
 	}
 }
+
+// TestUnsupportedEngineMetric records unsupported-engine beacons through a manual reader and asserts
+// unsupported_engine_total splits by reason and Chromium major.
+func TestUnsupportedEngineMetric(t *testing.T) {
+	ctx := context.Background()
+	reader := sdkmetric.NewManualReader()
+	meter := sdkmetric.NewMeterProvider(sdkmetric.WithReader(reader)).Meter("test")
+	m := newServerMetrics(meter)
+
+	m.recordUnsupportedEngine(ctx, "no_bigint", "66")
+	m.recordUnsupportedEngine(ctx, "no_bigint", "66")
+	m.recordUnsupportedEngine(ctx, "boot_error", "74")
+
+	var rm metricdata.ResourceMetrics
+	if err := reader.Collect(ctx, &rm); err != nil {
+		t.Fatalf("collect: %v", err)
+	}
+	type key struct{ reason, chromium string }
+	counts := map[key]int64{}
+	for _, sm := range rm.ScopeMetrics {
+		for _, md := range sm.Metrics {
+			if md.Name != "unsupported_engine_total" {
+				continue
+			}
+			sum, ok := md.Data.(metricdata.Sum[int64])
+			if !ok {
+				t.Fatalf("unsupported_engine_total is not an int64 sum")
+			}
+			for _, dp := range sum.DataPoints {
+				reason, _ := dp.Attributes.Value(attribute.Key("reason"))
+				chromium, _ := dp.Attributes.Value(attribute.Key("chromium"))
+				counts[key{reason.AsString(), chromium.AsString()}] += dp.Value
+			}
+		}
+	}
+	if got := counts[key{"no_bigint", "66"}]; got != 2 {
+		t.Errorf("unsupported no_bigint/66 = %d, want 2", got)
+	}
+	if got := counts[key{"boot_error", "74"}]; got != 1 {
+		t.Errorf("unsupported boot_error/74 = %d, want 1", got)
+	}
+}
+
+// TestNormalizeUnsupported checks that a beacon's reason and Chromium are reduced to bounded label
+// values, so a spoofed beacon cannot explode the metric cardinality.
+func TestNormalizeUnsupported(t *testing.T) {
+	cases := []struct {
+		reason, chromium         string
+		wantReason, wantChromium string
+	}{
+		{"no_bigint", "66", "no_bigint", "66"},
+		{"no_proxy", " 74 ", "no_proxy", "74"},
+		{"boot_error", "105", "boot_error", "105"},
+		{"garbage", "66", "other", "66"},
+		{"", "", "other", "other"},
+		{"no_bigint", "not-a-number", "no_bigint", "other"},
+		{"no_bigint", "9999", "no_bigint", "other"},
+		{"no_bigint", "0", "no_bigint", "other"},
+		{"no_bigint", "-5", "no_bigint", "other"},
+	}
+	for _, c := range cases {
+		gotR, gotC := normalizeUnsupported(c.reason, c.chromium)
+		if gotR != c.wantReason || gotC != c.wantChromium {
+			t.Errorf("normalizeUnsupported(%q,%q) = %q,%q; want %q,%q", c.reason, c.chromium, gotR, gotC, c.wantReason, c.wantChromium)
+		}
+	}
+}
diff --git a/gateway/internal/connectsrv/server.go b/gateway/internal/connectsrv/server.go
index cfe93d3..1863390 100644
--- a/gateway/internal/connectsrv/server.go
+++ b/gateway/internal/connectsrv/server.go
@@ -15,6 +15,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"strconv"
 	"strings"
 	"time"
 
@@ -197,6 +198,9 @@ func (s *Server) HTTPHandler() http.Handler {
 	mux.Handle("/dl/", s.exportDownloadHandler())
 	// The client posts its local-move-preview adoption telemetry here (session-gated).
 	mux.Handle("/metrics/local-eval", s.localEvalMetricsHandler())
+	// The index.html boot guard beacons here when it turns a client away on the unsupported-engine
+	// screen (the app cannot run). Unauthenticated — the client never booted — but rate-limited.
+	mux.Handle("/telemetry/unsupported", s.unsupportedEngineHandler())
 	// The embedded UI: the game SPA under /app/ (web), /telegram/ (the Telegram Mini
 	// App) and /vk/ (the VK Mini App) — the single-origin model (docs/ARCHITECTURE.md
 	// §13). All sit below the h2c wrap so the Connect edge (a more specific prefix) keeps
@@ -582,6 +586,76 @@ func clampReport(r *localEvalReport) {
 	clamp(&r.PreviewNetwork)
 }
 
+// unsupportedEngineBeacon is the small fire-and-forget report the index.html boot guard sends when
+// it turns a client away on the unsupported-engine screen (no BigInt/Proxy, or an uncaught boot
+// error). It is deduped client-side (one per device / app version / reason).
+type unsupportedEngineBeacon struct {
+	Reason   string `json:"reason"`
+	Chromium string `json:"chromium"`
+	Version  string `json:"version"`
+	UA       string `json:"ua"`
+}
+
+// unsupportedEngineHandler folds one unsupported-engine beacon into the edge counter. It is
+// unauthenticated (the client never booted, so it carries no session) but per-IP rate-limited with
+// the public limiter and body-capped. reason and the Chromium major are reduced to bounded label
+// sets (normalizeUnsupported) so a spoofed beacon cannot inflate the metric cardinality; the full
+// user agent is logged, not labelled. Only POST; the reply is always 204.
+func (s *Server) unsupportedEngineHandler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+			return
+		}
+		ip := peerIP(r.RemoteAddr, r.Header)
+		if !s.limiter.Allow("public:"+ip, s.publicPolicy) {
+			s.noteRateLimited(r.Context(), classPublic, ip, "unsupported")
+			http.Error(w, "rate limited", http.StatusTooManyRequests)
+			return
+		}
+		var b unsupportedEngineBeacon
+		if err := json.NewDecoder(io.LimitReader(r.Body, 2048)).Decode(&b); err != nil {
+			http.Error(w, "bad request", http.StatusBadRequest)
+			return
+		}
+		reason, chromium := normalizeUnsupported(b.Reason, b.Chromium)
+		s.metrics.recordUnsupportedEngine(r.Context(), reason, chromium)
+		s.log.Info("unsupported engine",
+			zap.String("reason", reason),
+			zap.String("chromium", chromium),
+			zap.String("app_version", truncate(b.Version, 40)),
+			zap.String("user_agent", truncate(b.UA, 400)),
+		)
+		w.WriteHeader(http.StatusNoContent)
+	})
+}
+
+// normalizeUnsupported reduces a beacon's reason and Chromium fields to bounded label values, so a
+// spoofed beacon cannot explode the metric cardinality: reason is allow-listed, and Chromium is
+// parsed as a major version kept only within a plausible range, otherwise "other".
+func normalizeUnsupported(reason, chromium string) (string, string) {
+	switch reason {
+	case "no_bigint", "no_proxy", "boot_error":
+		// a recognised reason — keep as-is
+	default:
+		reason = "other"
+	}
+	major := "other"
+	if n, err := strconv.Atoi(strings.TrimSpace(chromium)); err == nil && n >= 1 && n <= 199 {
+		major = strconv.Itoa(n)
+	}
+	return reason, major
+}
+
+// truncate bounds a logged, client-supplied string to n bytes (a spoofed beacon field is not
+// trusted to be small).
+func truncate(s string, n int) string {
+	if len(s) > n {
+		return s[:n]
+	}
+	return s
+}
+
 // resolve extracts and resolves the Authorization bearer token to an account id
 // and its guest flag, returning a Connect Unauthenticated error when it is missing
 // or unknown.
diff --git a/gateway/internal/connectsrv/telemetry_test.go b/gateway/internal/connectsrv/telemetry_test.go
new file mode 100644
index 0000000..5d08aee
--- /dev/null
+++ b/gateway/internal/connectsrv/telemetry_test.go
@@ -0,0 +1,81 @@
+package connectsrv_test
+
+import (
+	"bytes"
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"go.opentelemetry.io/otel/attribute"
+	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+
+	"scrabble/gateway/internal/config"
+	"scrabble/gateway/internal/connectsrv"
+	"scrabble/gateway/internal/ratelimit"
+)
+
+// TestUnsupportedEngineHandler drives the /telemetry/unsupported beacon route end to end: a POST
+// increments unsupported_engine_total with the normalised labels and replies 204; a GET is 405.
+func TestUnsupportedEngineHandler(t *testing.T) {
+	reader := sdkmetric.NewManualReader()
+	meter := sdkmetric.NewMeterProvider(sdkmetric.WithReader(reader)).Meter("test")
+	edge := connectsrv.NewServer(connectsrv.Deps{
+		Limiter:   ratelimit.New(),
+		RateLimit: config.DefaultRateLimit(),
+		Meter:     meter,
+	})
+	srv := httptest.NewServer(edge.HTTPHandler())
+	defer srv.Close()
+	url := srv.URL + "/telemetry/unsupported"
+
+	// A GET is rejected.
+	getResp, err := http.Get(url)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	getResp.Body.Close()
+	if getResp.StatusCode != http.StatusMethodNotAllowed {
+		t.Errorf("GET status = %d, want 405", getResp.StatusCode)
+	}
+
+	// A POST is accepted (204) and folded into the counter with normalised labels.
+	body := `{"reason":"no_bigint","chromium":"66","version":"v1.2.3","ua":"Mozilla/5.0 ... Chrome/66"}`
+	resp, err := http.Post(url, "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatalf("post: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusNoContent {
+		t.Errorf("POST status = %d, want 204", resp.StatusCode)
+	}
+
+	var rm metricdata.ResourceMetrics
+	if err := reader.Collect(context.Background(), &rm); err != nil {
+		t.Fatalf("collect: %v", err)
+	}
+	var total int64
+	for _, sm := range rm.ScopeMetrics {
+		for _, md := range sm.Metrics {
+			if md.Name != "unsupported_engine_total" {
+				continue
+			}
+			sum, ok := md.Data.(metricdata.Sum[int64])
+			if !ok {
+				t.Fatalf("unsupported_engine_total is not an int64 sum")
+			}
+			for _, dp := range sum.DataPoints {
+				reason, _ := dp.Attributes.Value(attribute.Key("reason"))
+				chromium, _ := dp.Attributes.Value(attribute.Key("chromium"))
+				if reason.AsString() != "no_bigint" || chromium.AsString() != "66" {
+					t.Errorf("labels = %s/%s, want no_bigint/66", reason.AsString(), chromium.AsString())
+				}
+				total += dp.Value
+			}
+		}
+	}
+	if total != 1 {
+		t.Errorf("unsupported_engine_total = %d, want 1", total)
+	}
+}
diff --git a/ui/index.html b/ui/index.html
index e6167a2..5095a37 100644
--- a/ui/index.html
+++ b/ui/index.html
@@ -2,6 +2,240 @@
 <html lang="ru">
   <head>
     <meta charset="UTF-8" />
+    <!-- Boot capability guard. Runs before the deferred ES module, in plain ES5 so it survives even
+         on an engine too old to parse the bundle. Three jobs, as early as possible:
+           1. Hard gate — if an UNPOLYFILLABLE essential is missing (BigInt for the 64-bit
+              FlatBuffers wire decode, Proxy for Svelte 5 runes) the app cannot run at all: show the
+              unsupported-engine screen (an old Android System WebView, e.g. Chromium 66, is the case
+              this guards) instead of a white screen.
+           2. Soft gate — if only polyfillable es2020+ globals are missing (globalThis,
+              structuredClone, Array.at, …) pull core-js (emitted as polyfills.js) BEFORE the module
+              runs. document.write is deliberate: the only way to inject a parser-blocking <script>
+              guaranteed to run ahead of a deferred module; its argument is a static literal, so no
+              injection surface.
+           3. Reactive net — record any uncaught error/rejection during boot; if the app has not
+              signalled window.__booted within a grace period AND an error fired, show the same
+              screen with the captured cause (covers a bundle that fails to parse, or an unforeseen
+              incompatibility). __booted is set in App.svelte once bootstrap resolves.
+         The screen has a "Diagnostic information" view (engine + feature table + reason + version)
+         with a Copy button. On a capable engine nothing here renders, so neither the bundle-size
+         budget nor the mock e2e is affected. -->
+    <script>
+      (function () {
+        'use strict';
+        var nav = navigator;
+        var ua = nav.userAgent || '';
+        var VERSION = '__BOOT_VERSION__'; // replaced at build (vite.config injectBootVersion)
+        var RU = (nav.language || '').toLowerCase().indexOf('ru') === 0;
+        var MINIAPP = /\/(?:telegram|vk)\//.test(location.pathname);
+        var WEBURL = location.protocol + '//' + location.host + '/app/';
+
+        // [label, test, Chrome version it landed in, hard?] — hard = required and unpolyfillable.
+        function has(fn) {
+          try {
+            return !!fn();
+          } catch (e) {
+            return false;
+          }
+        }
+        var probes = [
+          ['BigInt', function () { return typeof BigInt !== 'undefined'; }, 67, true],
+          ['Proxy', function () { return typeof Proxy !== 'undefined'; }, 49, true],
+          ['globalThis', function () { return typeof globalThis !== 'undefined'; }, 71, false],
+          ['structuredClone', function () { return typeof structuredClone === 'function'; }, 98, false],
+          ['Array.prototype.at', function () { return typeof [].at === 'function'; }, 92, false],
+          ['Array.prototype.findLast', function () { return typeof [].findLast === 'function'; }, 97, false],
+          ['Object.hasOwn', function () { return typeof Object.hasOwn === 'function'; }, 93, false],
+          ['Object.fromEntries', function () { return typeof Object.fromEntries === 'function'; }, 73, false],
+          ['Promise.allSettled', function () { return !!Promise.allSettled; }, 76, false],
+          ['Promise.any', function () { return !!Promise.any; }, 85, false],
+          ['WeakRef', function () { return typeof WeakRef !== 'undefined'; }, 84, false],
+          ['queueMicrotask', function () { return typeof queueMicrotask === 'function'; }, 71, false]
+        ];
+        var results = [];
+        var hardMissing = [];
+        var softMissing = false;
+        for (var i = 0; i < probes.length; i++) {
+          var ok = has(probes[i][1]);
+          results.push({ name: probes[i][0], ok: ok, chrome: probes[i][2], hard: probes[i][3] });
+          if (!ok) {
+            if (probes[i][3]) hardMissing.push(probes[i][0]);
+            else softMissing = true;
+          }
+        }
+
+        // The diagnostic report (built on demand, behind the button).
+        function diag(reason) {
+          var cm = /Chrom(?:e|ium)\/(\d+)/.exec(ua);
+          var wv = /;\s*wv\)/.test(ua) || /\bwv\b/.test(ua);
+          var out = [];
+          out.push('reason    : ' + reason);
+          out.push('app       : ' + VERSION);
+          out.push('chromium  : ' + (cm ? cm[1] : 'n/a') + (wv ? ' (Android WebView)' : ''));
+          out.push('userAgent : ' + ua);
+          out.push('url       : ' + location.href);
+          out.push('viewport  : ' + window.innerWidth + 'x' + window.innerHeight + ' @' + (window.devicePixelRatio || 1));
+          out.push('lang      : ' + (nav.language || '?') + '   online: ' + nav.onLine);
+          out.push('');
+          out.push('features:');
+          for (var k = 0; k < results.length; k++) {
+            var r = results[k];
+            out.push('  ' + (r.ok ? 'OK' : 'NO') + '  ' + r.name + '  (' + (r.hard ? 'required' : 'polyfilled') + ', Chrome ' + r.chrome + ')');
+          }
+          return out.join('\n');
+        }
+
+        function el(tag, style, text) {
+          var e = document.createElement(tag);
+          if (style) e.setAttribute('style', style);
+          if (text != null) e.appendChild(document.createTextNode(text));
+          return e;
+        }
+        function btn(bg, fg) {
+          return 'display:inline-block;margin:0 10px 10px 0;padding:11px 18px;border:0;border-radius:8px;' +
+            'font:600 15px/1 inherit;cursor:pointer;color:' + fg + ';background:' + bg + ';';
+        }
+
+        var shown = false;
+        // Fire-and-forget beacon so the gateway can count who hits this screen (Grafana). Deduped
+        // in localStorage by app version + reason + Chromium, so a user reopening the app ten times
+        // is one report. sendBeacon (Chrome 39+, present on the engines that reach here) with a
+        // fetch fallback; both are best-effort and never block or throw.
+        function beacon(code) {
+          try {
+            var cm = /Chrom(?:e|ium)\/(\d+)/.exec(ua);
+            var chromium = cm ? cm[1] : '';
+            var sig = VERSION + '|' + code + '|' + chromium;
+            try {
+              if (localStorage.getItem('scrabble_unsupp') === sig) return;
+            } catch (e) {}
+            var payload = JSON.stringify({ reason: code, chromium: chromium, version: VERSION, ua: ua });
+            var sent = false;
+            try {
+              if (nav.sendBeacon) sent = nav.sendBeacon('/telemetry/unsupported', new Blob([payload], { type: 'application/json' }));
+            } catch (e) {}
+            if (!sent && typeof fetch === 'function') {
+              try {
+                fetch('/telemetry/unsupported', { method: 'POST', body: payload, headers: { 'Content-Type': 'application/json' }, keepalive: true }).catch(function () {});
+                sent = true;
+              } catch (e) {}
+            }
+            if (sent) {
+              try {
+                localStorage.setItem('scrabble_unsupp', sig);
+              } catch (e) {}
+            }
+          } catch (e) {}
+        }
+        function show(reason, code) {
+          if (shown) return;
+          shown = true;
+          beacon(code);
+          var root = el('div', 'position:fixed;left:0;top:0;right:0;bottom:0;z-index:2147483647;overflow:auto;' +
+            '-webkit-overflow-scrolling:touch;background:#0f1115;color:#e8eaed;box-sizing:border-box;padding:24px;' +
+            'font:16px/1.5 -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Arial,sans-serif;');
+          var wrap = el('div', 'max-width:520px;margin:0 auto;');
+          root.appendChild(wrap);
+
+          var msg = el('div', null);
+          msg.appendChild(el('div', 'font-size:22px;font-weight:700;margin-bottom:14px;', 'Эрудит'));
+          msg.appendChild(el('p', 'margin:0 0 14px;', RU
+            ? 'На вашей версии операционной системы или браузера приложение не сможет работать.'
+            : "This app can't run on your device's operating system or browser version."));
+          if (MINIAPP) {
+            msg.appendChild(el('p', 'margin:0 0 6px;', RU
+              ? 'Откройте веб-версию в обычном браузере (Chrome, Firefox):'
+              : 'Open the web version in a regular browser (Chrome, Firefox):'));
+            var a = el('a', 'color:#8ab4ff;word-break:break-all;', WEBURL);
+            a.setAttribute('href', WEBURL);
+            a.setAttribute('target', '_blank');
+            a.setAttribute('rel', 'noopener');
+            var lp = el('p', 'margin:0 0 18px;');
+            lp.appendChild(a);
+            msg.appendChild(lp);
+          }
+          var infoBtn = el('button', btn('#2a2f3a', '#e8eaed'), RU ? 'Диагностическая информация' : 'Diagnostic information');
+          msg.appendChild(infoBtn);
+          wrap.appendChild(msg);
+
+          var dv = el('div', 'display:none;');
+          var pre = el('pre', 'white-space:pre-wrap;word-break:break-word;background:#0b0d11;border-radius:8px;padding:12px;' +
+            'font:12.5px/1.45 ui-monospace,Menlo,Consolas,monospace;overflow:auto;', diag(reason));
+          dv.appendChild(pre);
+          var copyBtn = el('button', btn('#8ab4ff', '#0b0d11'), RU ? 'Копировать' : 'Copy');
+          var backBtn = el('button', btn('#2a2f3a', '#e8eaed'), RU ? 'Назад' : 'Back');
+          var row = el('div', 'margin-top:12px;');
+          row.appendChild(copyBtn);
+          row.appendChild(backBtn);
+          dv.appendChild(row);
+          wrap.appendChild(dv);
+
+          infoBtn.onclick = function () {
+            msg.style.display = 'none';
+            dv.style.display = 'block';
+          };
+          backBtn.onclick = function () {
+            dv.style.display = 'none';
+            msg.style.display = 'block';
+          };
+          copyBtn.onclick = function () {
+            var txt = diag(reason);
+            try {
+              if (nav.clipboard && nav.clipboard.writeText) {
+                nav.clipboard.writeText(txt);
+                copyBtn.firstChild.nodeValue = RU ? 'Скопировано' : 'Copied';
+                return;
+              }
+            } catch (e) {}
+            try {
+              var ta = document.createElement('textarea');
+              ta.value = txt;
+              ta.setAttribute('style', 'position:fixed;left:-9999px;top:0;');
+              document.body.appendChild(ta);
+              ta.select();
+              document.execCommand('copy');
+              document.body.removeChild(ta);
+              copyBtn.firstChild.nodeValue = RU ? 'Скопировано' : 'Copied';
+            } catch (e2) {
+              copyBtn.firstChild.nodeValue = RU ? 'Выделите и скопируйте текст' : 'Select and copy the text';
+            }
+          };
+
+          function mount() {
+            document.body.insertBefore(root, document.body.firstChild);
+          }
+          if (document.body) mount();
+          else document.addEventListener('DOMContentLoaded', mount);
+        }
+
+        // 1 + 2: the gate.
+        if (hardMissing.length) {
+          show((RU ? 'нет: ' : 'missing: ') + hardMissing.join(', '), hardMissing.indexOf('BigInt') >= 0 ? 'no_bigint' : 'no_proxy');
+        } else if (softMissing) {
+          document.write('<script src="polyfills.js"><\/script>');
+        }
+
+        // 3: the reactive net for an unforeseen boot failure.
+        var bootErr = null;
+        function note(m) {
+          if (!bootErr) bootErr = m;
+        }
+        window.onerror = function (m, s, l, c, e) {
+          note(String(m) + (e && e.stack ? '\n' + e.stack : s ? ' @ ' + s + ':' + l : ''));
+          return false;
+        };
+        window.addEventListener('error', function (e) {
+          if (e && e.message) note(e.message + (e.filename ? ' @ ' + e.filename + ':' + e.lineno : ''));
+        }, true);
+        window.addEventListener('unhandledrejection', function (e) {
+          var r = e && e.reason;
+          note(r && (r.stack || r.message) ? r.stack || r.message : String(r));
+        });
+        setTimeout(function () {
+          if (!window.__booted && bootErr && !shown) show((RU ? 'ошибка запуска: ' : 'boot error: ') + bootErr, 'boot_error');
+        }, 8000);
+      })();
+    </script>
     <!-- The SPA shell is an empty client-rendered document behind the public landing — keep it
          out of search indexes (robots.txt deliberately does NOT disallow /app/, so crawlers can
          reach this tag). -->
diff --git a/ui/package.json b/ui/package.json
index 11fa26e..9138db1 100644
--- a/ui/package.json
+++ b/ui/package.json
@@ -27,6 +27,7 @@
     "@playwright/test": "^1.49.0",
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "@types/node": "^22.10.0",
+    "core-js-bundle": "^3.49.0",
     "svelte": "^5.15.0",
     "svelte-check": "^4.1.0",
     "typescript": "^5.7.0",
diff --git a/ui/pnpm-lock.yaml b/ui/pnpm-lock.yaml
index 5f813f2..8b4434c 100644
--- a/ui/pnpm-lock.yaml
+++ b/ui/pnpm-lock.yaml
@@ -36,6 +36,9 @@ importers:
       '@types/node':
         specifier: ^22.10.0
         version: 22.19.19
+      core-js-bundle:
+        specifier: ^3.49.0
+        version: 3.49.0
       svelte:
         specifier: ^5.15.0
         version: 5.56.0
@@ -508,6 +511,9 @@ packages:
     resolution: {integrity: sha512-eYm0QWBtUrBWZWG0d386OGAw16Z995PiOVo2B7bjWSbHedGl5e0ZWaq65kOGgUSNesEIDkB9ISbTg/JK9dhCZA==}
     engines: {node: '>=6'}
 
+  core-js-bundle@3.49.0:
+    resolution: {integrity: sha512-WXc7oOsePN3aKFOJVG5zQdi+h/Jm2W0WIPYvRc4IG3vkNcbC2w6LlSzTmnhOl6N1xmOJEzCSNieX3mwF+3zBGw==}
+
   debug@4.4.3:
     resolution: {integrity: sha512-RGwwWnwQvkVfavKVt22FGLw+xYSdzARwm0ru6DhTVA3umU5hZc28V3kO4stgYryrTlLpuvgI9GiijltAjNbcqA==}
     engines: {node: '>=6.0'}
@@ -1132,6 +1138,8 @@ snapshots:
 
   clsx@2.1.1: {}
 
+  core-js-bundle@3.49.0: {}
+
   debug@4.4.3:
     dependencies:
       ms: 2.1.3
diff --git a/ui/pnpm-workspace.yaml b/ui/pnpm-workspace.yaml
index 34a05e7..14571e3 100644
--- a/ui/pnpm-workspace.yaml
+++ b/ui/pnpm-workspace.yaml
@@ -1,4 +1,8 @@
 # pnpm 11 records build-script approval here. esbuild's postinstall materialises
 # its CLI shim; the platform binary itself ships as an optional dependency.
+# core-js-bundle ships a prebuilt minified.js (we only read that file at build time,
+# via vite.config emitPolyfills) and its install script is just a funding banner, so
+# it is denied — nothing to build.
 allowBuilds:
+  core-js-bundle: false
   esbuild: true
diff --git a/ui/src/App.svelte b/ui/src/App.svelte
index ad3f36d..39bd17e 100644
--- a/ui/src/App.svelte
+++ b/ui/src/App.svelte
@@ -26,7 +26,12 @@
   import TelegramLaunchError from './screens/TelegramLaunchError.svelte';
 
   onMount(() => {
-    void bootstrap();
+    // Tell the index.html boot guard that startup completed, so its reactive net does not raise the
+    // unsupported-engine screen on a healthy boot — it only fires when an uncaught error occurred
+    // AND this flag never sets (a bundle that failed to parse, or an unforeseen incompatibility).
+    void bootstrap().then(() => {
+      (window as unknown as { __booted?: boolean }).__booted = true;
+    });
   });
 
   // The lobby is the cold-start landing (an empty hash and Telegram launch params both parse
diff --git a/ui/src/game/Board.svelte b/ui/src/game/Board.svelte
index 7288220..3e94fa3 100644
--- a/ui/src/game/Board.svelte
+++ b/ui/src/game/Board.svelte
@@ -479,6 +479,11 @@
     position: absolute;
     top: 5%;
     left: 8%;
+    /* Chrome < 105 has no container-query units: a dropped `cqw` would inherit the .cell
+       `font-size: 0` and the glyph would vanish. Fall back to a viewport-relative size that tracks
+       the board (≈ the viewport-fitted query container) and the zoom (--z), so old Android System
+       WebViews still draw the tile glyphs. Chrome 105+ takes the exact `cqw` line below. */
+    font-size: calc(4.2vmin * var(--z, 1));
     font-size: 4.2cqw;
     font-weight: 700;
     line-height: 1;
@@ -487,12 +492,14 @@
     position: absolute;
     right: 5%;
     bottom: 3%;
+    font-size: calc(2.4vmin * var(--z, 1)); /* cqw fallback for Chrome < 105 — see .letter */
     font-size: 2.4cqw;
     font-weight: 600;
   }
   /* A placed Erudit blank ("звёздочка") shows its star where the (absent) point value sits,
      its ink centred on the same line as a neighbouring tile's value digit. */
   .blankmark {
+    font-size: calc(2.8vmin * var(--z, 1)); /* cqw fallback for Chrome < 105 — see .letter */
     font-size: 2.8cqw;
     bottom: 0;
   }
@@ -501,6 +508,7 @@
     inset: 0;
     display: grid;
     place-items: center;
+    font-size: calc(3.6vmin * var(--z, 1)); /* cqw fallback for Chrome < 105 — see .letter */
     font-size: 3.6cqw;
     opacity: 0.7;
   }
@@ -509,6 +517,7 @@
     inset: 0;
     display: grid;
     place-items: center;
+    font-size: calc(2.7vmin * var(--z, 1)); /* cqw fallback for Chrome < 105 — see .letter */
     font-size: 2.7cqw;
     font-weight: 600;
     opacity: 0.9;
@@ -526,10 +535,12 @@
     padding: 0 1px;
   }
   .bt {
+    font-size: calc(1.7vmin * var(--z, 1)); /* cqw fallback for Chrome < 105 — see .letter */
     font-size: 1.7cqw;
     font-weight: 600;
   }
   .bb {
+    font-size: calc(1.9vmin * var(--z, 1)); /* cqw fallback for Chrome < 105 — see .letter */
     font-size: 1.9cqw;
     font-weight: 700;
     white-space: nowrap;
diff --git a/ui/vite.config.ts b/ui/vite.config.ts
index 53258e6..3395ad0 100644
--- a/ui/vite.config.ts
+++ b/ui/vite.config.ts
@@ -1,7 +1,42 @@
+import { readFileSync } from 'node:fs';
 import { resolve } from 'node:path';
-import { defineConfig } from 'vite';
+import { defineConfig, type Plugin } from 'vite';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
 
+/**
+ * injectBootVersion stamps the app version into index.html's boot-capability guard, replacing its
+ * __BOOT_VERSION__ placeholder with the same VITE_APP_VERSION build-arg that feeds __APP_VERSION__.
+ * The guard runs before the bundle, so it cannot read the bundle's version; this lets its on-demand
+ * diagnostic report name the client version anyway. Falls back to "dev" for a local build.
+ */
+function injectBootVersion(): Plugin {
+  const version = process.env.VITE_APP_VERSION || 'dev';
+  return {
+    name: 'inject-boot-version',
+    transformIndexHtml(html) {
+      return html.replace(/__BOOT_VERSION__/g, version);
+    },
+  };
+}
+
+/**
+ * emitPolyfills writes the prebuilt core-js bundle to `dist/polyfills.js`. The index.html gate
+ * script loads it via document.write only on an old engine that lacks the es2020+ runtime APIs the
+ * app uses (globalThis, structuredClone, Array.at, …) — e.g. the Chromium 66 Android System WebView
+ * behind the Telegram/VK in-app browser — before the deferred module runs. A modern engine never
+ * requests it, so it adds nothing to that payload and stays out of the module graph the bundle-size
+ * gate measures.
+ */
+function emitPolyfills(): Plugin {
+  return {
+    name: 'emit-polyfills',
+    generateBundle() {
+      const src = readFileSync(resolve(import.meta.dirname, 'node_modules/core-js-bundle/minified.js'), 'utf8');
+      this.emitFile({ type: 'asset', fileName: 'polyfills.js', source: src });
+    },
+  };
+}
+
 // The edge Connect service is scrabble.edge.v1.Gateway; the gateway serves it over
 // h2c on :8081 by default. In dev we proxy the RPC path so the browser (which can
 // not speak h2c directly) talks to the dev server on the same origin. In `mock`
@@ -19,7 +54,9 @@ export default defineConfig(({ mode }) => ({
     // so a missing build-arg never breaks the build.
     __APP_VERSION__: JSON.stringify(process.env.VITE_APP_VERSION || 'dev'),
   },
-  plugins: [svelte()],
+  // emitPolyfills ships dist/polyfills.js (loaded only by old engines; see its docstring + the
+  // index.html boot guard). injectBootVersion stamps the app version into that guard's diagnostic.
+  plugins: [svelte(), emitPolyfills(), injectBootVersion()],
   server: {
     port: 5173,
     proxy:
@@ -33,7 +70,14 @@ export default defineConfig(({ mode }) => ({
           },
   },
   build: {
-    target: 'es2022',
+    // Down-level to es2019 so an old Android System WebView (seen: Telegram/VK in-app WebView on
+    // Chromium 66, Android 9) can parse the bundle — esbuild lowers es2020+ syntax (?., ??, private
+    // fields, static blocks, …) that Chrome 66 rejects with "Unexpected token ?", which left the SPA
+    // on a white screen. esbuild lowers syntax only, not runtime APIs — the es2020+ globals the
+    // bundle/deps call at runtime (globalThis, structuredClone, Array.at, …) are covered separately
+    // by the conditional core-js loader (emitPolyfills + the index.html gate), loaded only on an
+    // engine that actually lacks them.
+    target: 'es2019',
     // Emit sourcemaps everywhere except the production build. A shipped `.map`
     // carries full `sourcesContent` — the entire TypeScript/Svelte source — and the
     // gateway/landing images serve `dist/` verbatim, so production maps would expose