docs/ARCHITECTURE.md

@@ -691,6 +691,55 @@ stream opens is `event_type = gateway.server_time`, reusing the opening
 `request_id` as `event_id` and carrying `server_time_ms` so clients can
 calibrate offset without a separate time request.
 
+#### Unsigned `gateway.heartbeat` keepalive
+
+Browser fetch-streaming layers (notably WebKit/Safari) close response
+bodies they consider idle after roughly 15-30 seconds without
+incoming bytes. A push stream in a quiet game (no `game.turn.ready`,
+no diplomatic mail) would otherwise be torn down and reopened
+repeatedly; events that fire during the reconnect window vanish
+because `push.Hub` queues are not persisted across subscription
+closes. To keep the body active, the gateway emits a
+`gateway.heartbeat` event after `GATEWAY_PUSH_HEARTBEAT_INTERVAL` of
+stream silence (default `15s`; set to `0s` to disable). Every real
+event resets the silence timer, so the heartbeat fires rarely on
+busy streams.
+
+Heartbeats are sent **unsigned**: every field except `event_type` is
+left at its protobuf default and no Ed25519 signature is computed.
+The client short-circuits on the `gateway.heartbeat` type before
+calling `verifyEvent` / `verifyPayloadHash` and never dispatches the
+event to handlers. The security implication is intentional —
+heartbeats carry no payload that the UI acts on, so an injected
+heartbeat trivially fails to cause any user-visible state change.
+TLS still protects the wire and the rest of the signed envelope is
+unchanged for real events.
+
+##### Wire cost projections
+
+| Clients | 15 s | 30 s | 60 s |
+| ------: | ---: | ---: | ---: |
+|     100 |  25 MB/day |  13 MB/day |   6 MB/day |
+|   1 000 | 250 MB/day | 125 MB/day |  62 MB/day |
+|  10 000 | 2.5 GB/day | 1.3 GB/day | 0.6 GB/day |
+| 100 000 |  25 GB/day | 12.5 GB/day |  6 GB/day |
+
+Per-heartbeat budget at ~45 bytes on the wire (proto + Connect
+framing + HTTP/2 DATA header + amortised TLS overhead), worst case
+when no real event ever displaces a tick. Active streams trade
+heartbeat traffic for real-event traffic 1:1, so the table is the
+upper bound at the chosen interval. Larger deployments that are
+willing to take a marginally higher Safari reconnect risk should
+raise `GATEWAY_PUSH_HEARTBEAT_INTERVAL` toward 30 s before paying
+the full table; setting `0s` reclaims all bytes at the cost of the
+visible Safari reconnect loop returning.
+
+Observability: every emission increments the
+`gateway.push.heartbeats_sent{outcome}` counter, where
+`outcome=sent` is the steady-state line item the operator budgets
+bandwidth against and a sudden `outcome=error` bump means the
+upstream connection is failing before the gateway can flush.
+
 ### Verification order at gateway
 
 Before any payload is forwarded to backend, gateway must:

docs/FUNCTIONAL.md

@@ -820,6 +820,18 @@ internal hub. The first frame the client receives is a
 gateway-signed bootstrap event carrying the current server time, so
 the client can calibrate its local clock without a separate request.
 
+While the stream is open, gateway tracks a silence timer; if no real
+event has been forwarded for `GATEWAY_PUSH_HEARTBEAT_INTERVAL`
+(default `15s`, `0s` disables), gateway emits an unsigned
+`gateway.heartbeat` event to keep browser fetch-streaming layers
+from closing the response body as idle. Real events reset the
+timer, so on busy streams the heartbeat fires rarely. The UI client
+short-circuits the heartbeat type before signature verification and
+never dispatches it to handlers — see
+[`docs/ARCHITECTURE.md` § 15](ARCHITECTURE.md#15-transport-security-model-gateway-boundary)
+for the wire-cost projection and the security rationale of leaving
+the heartbeat unsigned.
+
 ### 7.3 Backend → gateway control
 
 Backend hosts a single gRPC service `Push.SubscribePush`, consumed

docs/FUNCTIONAL_ru.md

@@ -845,6 +845,18 @@ control-канал backend → gateway, который производит эт
 с текущим серверным временем, чтобы клиент мог калибровать свои
 локальные часы без отдельного запроса.
 
+Пока стрим открыт, gateway отслеживает таймер тишины; если за
+`GATEWAY_PUSH_HEARTBEAT_INTERVAL` (по умолчанию `15s`, `0s`
+отключает) не пришло ни одного реального события, gateway
+отправляет неподписанное `gateway.heartbeat`-событие, чтобы
+браузерные fetch-streaming слои не закрыли response body как idle.
+Реальные события сбрасывают таймер, поэтому на нагруженных
+стримах heartbeat срабатывает редко. UI-клиент короткозамыкает
+heartbeat-тип до верификации подписи и никогда не дотягивает его
+до handlers — см.
+[`docs/ARCHITECTURE.md` § 15](ARCHITECTURE.md#15-transport-security-model-gateway-boundary)
+для расчёта траффика и обоснования отсутствия подписи у heartbeat.
+
 ### 7.3 Управление backend → gateway
 
 Backend хостит единственный gRPC-сервис `Push.SubscribePush`,

gateway/README.md

@@ -294,7 +294,17 @@ the stream to the verified `user_id` and `device_session_id`, sends one
 signed `gateway.server_time` bootstrap event whose FlatBuffers payload carries
 `server_time_ms`, registers the active stream in the in-memory `PushHub`, and
 then forwards signed client-facing events consumed from the configured client
-event Redis stream. User-targeted events fan out to every active stream for
+event Redis stream. After the bootstrap, the stream is wrapped with a
+silence-based heartbeat: when no real event has been forwarded for
+`GATEWAY_PUSH_HEARTBEAT_INTERVAL` (default `15s`, set to `0s` to disable),
+the gateway emits an unsigned `gateway.heartbeat` event so browser
+fetch-streaming layers (Safari is the most aggressive) keep the response
+body open and pending push events are not lost into the client-side
+reconnect window. Each emission is counted by the
+`gateway.push.heartbeats_sent{outcome}` metric; see
+[`docs/ARCHITECTURE.md` § 15](../docs/ARCHITECTURE.md#15-transport-security-model-gateway-boundary)
+for the bandwidth projection and the reasoning behind the unsigned
+envelope. User-targeted events fan out to every active stream for
 that user. Session-targeted events fan out only to streams whose
 `user_id` and `device_session_id` both match the event target. Each active
 stream uses a bounded in-memory queue; when that queue overflows, only the

gateway/internal/config/config.go

@@ -125,6 +125,14 @@ const (
 	// gRPC requests.
 	authenticatedGRPCFreshnessWindowEnvVar = "GATEWAY_AUTHENTICATED_GRPC_FRESHNESS_WINDOW"
 
+	// pushHeartbeatIntervalEnvVar names the environment variable that
+	// configures the silence-based heartbeat cadence for authenticated
+	// push streams. The heartbeat keeps idle SubscribeEvents responses
+	// alive across browser fetch-streaming idle timeouts (Safari is
+	// notably aggressive) so push events do not disappear into the
+	// reconnect window. A value of `0s` disables heartbeats entirely.
+	pushHeartbeatIntervalEnvVar = "GATEWAY_PUSH_HEARTBEAT_INTERVAL"
+
 	// authenticatedGRPCIPRateLimitRequestsEnvVar names the environment
 	// variable that configures the authenticated gRPC per-IP request budget per
 	// window.
@@ -321,6 +329,13 @@ const (
 	defaultAuthenticatedGRPCDownstreamTimeout = 5 * time.Second
 	defaultAuthenticatedGRPCFreshnessWindow = 5 * time.Minute
 
+	// defaultPushHeartbeatInterval is the silence window the push stream
+	// keeps open before emitting `gateway.heartbeat`. 15s is comfortably
+	// below the empirical Safari fetch-streaming idle threshold
+	// (~15-30s) and well above any realistic per-event rate, so the
+	// timer is almost always reset by a real event in active games.
+	defaultPushHeartbeatInterval = 15 * time.Second
+
 	defaultAuthenticatedGRPCIPRateLimitRequests = 120
 	defaultAuthenticatedGRPCIPRateLimitBurst = 40
 
@@ -549,6 +564,16 @@ type AuthenticatedGRPCConfig struct {
 	// used for client request timestamps.
 	FreshnessWindow time.Duration
 
+	// PushHeartbeatInterval is the silence window after which an open
+	// authenticated SubscribeEvents stream sends an unsigned
+	// `gateway.heartbeat` event. Every real Send resets the window, so
+	// in busy streams the heartbeat fires rarely. A zero or negative
+	// value disables the heartbeat — the stream then relies on
+	// transport-level keepalives only, which Safari's fetch-streaming
+	// layer ignores. See `docs/ARCHITECTURE.md` for the security
+	// rationale of leaving the heartbeat unsigned.
+	PushHeartbeatInterval time.Duration
+
 	// AntiAbuse configures the authenticated gRPC rate limits enforced after
 	// the request passes the transport authenticity checks.
 	AntiAbuse AuthenticatedGRPCAntiAbuseConfig
@@ -719,6 +744,7 @@ func DefaultAuthenticatedGRPCConfig() AuthenticatedGRPCConfig {
 		ConnectionTimeout: defaultAuthenticatedGRPCConnectionTimeout,
 		DownstreamTimeout: defaultAuthenticatedGRPCDownstreamTimeout,
 		FreshnessWindow: defaultAuthenticatedGRPCFreshnessWindow,
+		PushHeartbeatInterval: defaultPushHeartbeatInterval,
 		AntiAbuse: AuthenticatedGRPCAntiAbuseConfig{
 			IP: AuthenticatedRateLimitConfig{
 				Requests: defaultAuthenticatedGRPCIPRateLimitRequests,
@@ -928,6 +954,12 @@ func LoadFromEnv() (Config, error) {
 	}
 	cfg.AuthenticatedGRPC.DownstreamTimeout = authenticatedGRPCDownstreamTimeout
 
+	pushHeartbeatInterval, err := loadDurationEnvWithDefault(pushHeartbeatIntervalEnvVar, cfg.AuthenticatedGRPC.PushHeartbeatInterval)
+	if err != nil {
+		return Config{}, err
+	}
+	cfg.AuthenticatedGRPC.PushHeartbeatInterval = pushHeartbeatInterval
+
 	authenticatedGRPCFreshnessWindow, err := loadDurationEnvWithDefault(authenticatedGRPCFreshnessWindowEnvVar, cfg.AuthenticatedGRPC.FreshnessWindow)
 	if err != nil {
 		return Config{}, err
@@ -1156,6 +1188,9 @@ func LoadFromEnv() (Config, error) {
 	if cfg.AuthenticatedGRPC.FreshnessWindow <= 0 {
 		return Config{}, fmt.Errorf("load gateway config: %s must be positive", authenticatedGRPCFreshnessWindowEnvVar)
 	}
+	if cfg.AuthenticatedGRPC.PushHeartbeatInterval < 0 {
+		return Config{}, fmt.Errorf("load gateway config: %s must not be negative", pushHeartbeatIntervalEnvVar)
+	}
 	if err := validateRateLimitConfig(
 		cfg.AuthenticatedGRPC.AntiAbuse.IP,
 		authenticatedGRPCIPRateLimitRequestsEnvVar,

gateway/internal/config/config_test.go

@@ -164,6 +164,30 @@ func TestLoadFromEnvAppliesPublicAndAuthGRPCDefaults(t *testing.T) {
 	assert.Equal(t, defaultAuthenticatedGRPCConnectionTimeout, cfg.AuthenticatedGRPC.ConnectionTimeout)
 	assert.Equal(t, defaultAuthenticatedGRPCDownstreamTimeout, cfg.AuthenticatedGRPC.DownstreamTimeout)
 	assert.Equal(t, defaultAuthenticatedGRPCFreshnessWindow, cfg.AuthenticatedGRPC.FreshnessWindow)
+	assert.Equal(t, defaultPushHeartbeatInterval, cfg.AuthenticatedGRPC.PushHeartbeatInterval)
+}
+
+func TestLoadFromEnvParsesPushHeartbeatInterval(t *testing.T) {
+	configEnvMu.Lock()
+	defer configEnvMu.Unlock()
+
+	resetEnv(t)
+	setBaseRequiredEnv(t)
+	t.Setenv(pushHeartbeatIntervalEnvVar, "0s")
+
+	cfg, err := LoadFromEnv()
+	require.NoError(t, err)
+	assert.Equal(t, time.Duration(0), cfg.AuthenticatedGRPC.PushHeartbeatInterval, "0s explicitly disables the heartbeat")
+
+	t.Setenv(pushHeartbeatIntervalEnvVar, "25s")
+	cfg, err = LoadFromEnv()
+	require.NoError(t, err)
+	assert.Equal(t, 25*time.Second, cfg.AuthenticatedGRPC.PushHeartbeatInterval)
+
+	t.Setenv(pushHeartbeatIntervalEnvVar, "-1s")
+	_, err = LoadFromEnv()
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), pushHeartbeatIntervalEnvVar)
 }
 
 func TestLoadFromEnvParsesCORSAllowedOrigins(t *testing.T) {
@@ -211,6 +235,7 @@ func resetEnv(t *testing.T) {
 		authenticatedGRPCConnectionTimeoutEnvVar,
 		authenticatedGRPCDownstreamTimeoutEnvVar,
 		authenticatedGRPCFreshnessWindowEnvVar,
+		pushHeartbeatIntervalEnvVar,
 		gatewayRedisMasterAddrEnvVar,
 		gatewayRedisPasswordEnvVar,
 		replayRedisKeyPrefixEnvVar,

gateway/internal/grpcapi/push_heartbeat.go

@@ -0,0 +1,163 @@
+package grpcapi
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	"galaxy/gateway/internal/telemetry"
+	gatewayv1 "galaxy/gateway/proto/galaxy/gateway/v1"
+
+	"go.opentelemetry.io/otel/attribute"
+	"google.golang.org/grpc"
+)
+
+// heartbeatingStream wraps a server-streaming response so the inner
+// stream stays alive across browser fetch-streaming idle timeouts.
+// Every call to Send (a real event from a tail service) resets a
+// silence timer; when the timer fires, Run emits an unsigned
+// `gateway.heartbeat` event on its own. Send and the heartbeat
+// goroutine serialise on the same mutex because grpc.ServerStream.Send
+// is documented as not goroutine-safe.
+//
+// Wire-cost budgeting: each heartbeat is one GatewayEvent with only
+// EventType populated (~17 bytes + protobuf tag), framed by Connect
+// (~5 bytes) and HTTP/2 plus TLS overhead (~50 bytes). At the
+// 15-second default a fully-idle stream costs ~840 KB/day per client;
+// see `docs/ARCHITECTURE.md` for the per-scale projection.
+type heartbeatingStream struct {
+	grpc.ServerStreamingServer[gatewayv1.GatewayEvent]
+
+	interval time.Duration
+	metrics  *telemetry.Runtime
+
+	sendMu sync.Mutex
+	timer  *time.Timer
+
+	stopOnce sync.Once
+	done     chan struct{}
+}
+
+// newHeartbeatingStream wraps inner with a silence-based heartbeat
+// emitter. A non-positive interval returns nil so the caller can skip
+// the wrapping entirely; non-nil returns must have `Stop()` called once
+// the stream lifecycle ends.
+func newHeartbeatingStream(
+	inner grpc.ServerStreamingServer[gatewayv1.GatewayEvent],
+	interval time.Duration,
+	metrics *telemetry.Runtime,
+) *heartbeatingStream {
+	if interval <= 0 {
+		return nil
+	}
+
+	return &heartbeatingStream{
+		ServerStreamingServer: inner,
+		interval:              interval,
+		metrics:               metrics,
+		timer:                 time.NewTimer(interval),
+		done:                  make(chan struct{}),
+	}
+}
+
+// Send forwards event to the inner stream and resets the silence timer
+// so the heartbeat goroutine waits a fresh interval before firing
+// again. A Send that succeeds means the transport just delivered real
+// bytes; the silence window restarts from "now".
+func (s *heartbeatingStream) Send(event *gatewayv1.GatewayEvent) error {
+	s.sendMu.Lock()
+	defer s.sendMu.Unlock()
+	if err := s.ServerStreamingServer.Send(event); err != nil {
+		return err
+	}
+	s.resetTimerLocked()
+
+	return nil
+}
+
+// Run blocks until ctx is canceled or Stop is called, emitting one
+// `gateway.heartbeat` event whenever the silence timer fires. Intended
+// to run in its own goroutine alongside the tail service that owns the
+// stream. A Send failure from the heartbeat path is recorded in
+// telemetry and returned to the caller; production wiring discards it
+// because the tail service will see the same transport failure on its
+// next Send and propagate the real error to the gateway frame
+// observability layer.
+func (s *heartbeatingStream) Run(ctx context.Context) error {
+	for {
+		select {
+		case <-ctx.Done():
+			return nil
+		case <-s.done:
+			return nil
+		case <-s.timer.C:
+			err := s.sendHeartbeat()
+			if err != nil {
+				return err
+			}
+		}
+	}
+}
+
+// Stop halts the heartbeat goroutine and drains the silence timer.
+// Safe to call multiple times; subsequent calls are no-ops.
+func (s *heartbeatingStream) Stop() {
+	s.stopOnce.Do(func() {
+		close(s.done)
+		if !s.timer.Stop() {
+			select {
+			case <-s.timer.C:
+			default:
+			}
+		}
+	})
+}
+
+// sendHeartbeat emits one heartbeat event, records the outcome in
+// telemetry, and re-arms the silence timer. The outcome attribute
+// makes a sudden bump of `error` easy to spot in dashboards — it
+// usually means the upstream connection is failing before the gateway
+// can flush, while a steady `sent` rate is the normal idle baseline
+// the deployment operator budgets bandwidth against.
+func (s *heartbeatingStream) sendHeartbeat() error {
+	s.sendMu.Lock()
+	defer s.sendMu.Unlock()
+
+	err := s.ServerStreamingServer.Send(buildHeartbeatEvent())
+	outcome := attribute.String("outcome", "sent")
+	if err != nil {
+		outcome = attribute.String("outcome", "error")
+	}
+	s.metrics.RecordPushHeartbeat(context.Background(), outcome)
+	if err != nil {
+		return err
+	}
+	s.resetTimerLocked()
+
+	return nil
+}
+
+// resetTimerLocked re-arms the silence timer. Caller must hold sendMu.
+// The drain follows the canonical pattern from the time.Timer
+// docstring: Stop may report `false` either because the timer already
+// fired or because nothing was queued, so the non-blocking drain
+// handles both states without deadlocking when the channel was already
+// emptied by Run.
+func (s *heartbeatingStream) resetTimerLocked() {
+	if !s.timer.Stop() {
+		select {
+		case <-s.timer.C:
+		default:
+		}
+	}
+	s.timer.Reset(s.interval)
+}
+
+// buildHeartbeatEvent returns the minimal `gateway.heartbeat`
+// GatewayEvent emitted into the push stream. Every field except
+// EventType is left at its proto3 default so the wire frame stays as
+// small as Connect framing allows. See `gatewayHeartbeatEventType` for
+// the security rationale of leaving the event unsigned.
+func buildHeartbeatEvent() *gatewayv1.GatewayEvent {
+	return &gatewayv1.GatewayEvent{EventType: gatewayHeartbeatEventType}
+}

gateway/internal/grpcapi/push_heartbeat_test.go

@@ -0,0 +1,185 @@
+package grpcapi
+
+import (
+	"context"
+	"errors"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	gatewayv1 "galaxy/gateway/proto/galaxy/gateway/v1"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/metadata"
+)
+
+func TestNewHeartbeatingStreamZeroIntervalReturnsNil(t *testing.T) {
+	t.Parallel()
+
+	stream := newHeartbeatingStream(newCapturingStream(t), 0, nil)
+	assert.Nil(t, stream, "zero interval must not allocate a wrapper")
+
+	negative := newHeartbeatingStream(newCapturingStream(t), -time.Second, nil)
+	assert.Nil(t, negative, "negative interval must not allocate a wrapper")
+}
+
+func TestHeartbeatingStreamSendsHeartbeatAfterSilence(t *testing.T) {
+	t.Parallel()
+
+	inner := newCapturingStream(t)
+	hb := newHeartbeatingStream(inner, 30*time.Millisecond, nil)
+	require.NotNil(t, hb)
+	defer hb.Stop()
+
+	go func() { _ = hb.Run(t.Context()) }()
+
+	event := inner.recv(t, 200*time.Millisecond)
+	assert.Equal(t, gatewayHeartbeatEventType, event.GetEventType())
+	// Heartbeat envelope: only the event type travels. Every other
+	// field stays at proto3 default so the wire frame stays minimal.
+	assert.Empty(t, event.GetEventId())
+	assert.Zero(t, event.GetTimestampMs())
+	assert.Empty(t, event.GetPayloadBytes())
+	assert.Empty(t, event.GetPayloadHash())
+	assert.Empty(t, event.GetSignature())
+	assert.Empty(t, event.GetRequestId())
+	assert.Empty(t, event.GetTraceId())
+}
+
+func TestHeartbeatingStreamRealSendResetsSilenceTimer(t *testing.T) {
+	t.Parallel()
+
+	inner := newCapturingStream(t)
+	hb := newHeartbeatingStream(inner, 50*time.Millisecond, nil)
+	require.NotNil(t, hb)
+	defer hb.Stop()
+
+	go func() { _ = hb.Run(t.Context()) }()
+
+	// Reset the timer every 20ms for 120ms — the silence window never
+	// elapses, so the heartbeat goroutine must stay quiet and the
+	// channel must only carry the manual real-event Sends.
+	go func() {
+		ticker := time.NewTicker(20 * time.Millisecond)
+		defer ticker.Stop()
+		for range 6 {
+			<-ticker.C
+			if err := hb.Send(&gatewayv1.GatewayEvent{EventType: "real.event"}); err != nil {
+				t.Errorf("real Send failed: %v", err)
+				return
+			}
+		}
+	}()
+
+	for range 6 {
+		event := inner.recv(t, 100*time.Millisecond)
+		assert.Equal(t, "real.event", event.GetEventType(), "only real events should appear while Send keeps resetting the silence window")
+	}
+}
+
+func TestHeartbeatingStreamStopHaltsRun(t *testing.T) {
+	t.Parallel()
+
+	inner := newCapturingStream(t)
+	hb := newHeartbeatingStream(inner, 20*time.Millisecond, nil)
+	require.NotNil(t, hb)
+
+	runDone := make(chan error, 1)
+	go func() { runDone <- hb.Run(context.Background()) }()
+
+	hb.Stop()
+	select {
+	case err := <-runDone:
+		require.NoError(t, err)
+	case <-time.After(200 * time.Millisecond):
+		t.Fatal("Run did not exit after Stop")
+	}
+
+	// Stop is idempotent; the second call must not panic on the
+	// already-closed done channel.
+	assert.NotPanics(t, hb.Stop)
+}
+
+func TestHeartbeatingStreamContextCancelHaltsRun(t *testing.T) {
+	t.Parallel()
+
+	inner := newCapturingStream(t)
+	hb := newHeartbeatingStream(inner, 20*time.Millisecond, nil)
+	require.NotNil(t, hb)
+	defer hb.Stop()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	runDone := make(chan error, 1)
+	go func() { runDone <- hb.Run(ctx) }()
+
+	cancel()
+	select {
+	case err := <-runDone:
+		require.NoError(t, err)
+	case <-time.After(200 * time.Millisecond):
+		t.Fatal("Run did not exit after context cancel")
+	}
+}
+
+func TestHeartbeatingStreamSendErrorPropagates(t *testing.T) {
+	t.Parallel()
+
+	wantErr := errors.New("send failed")
+	inner := newCapturingStream(t)
+	inner.sendErr.Store(&errorBox{err: wantErr})
+
+	hb := newHeartbeatingStream(inner, time.Minute, nil)
+	require.NotNil(t, hb)
+	defer hb.Stop()
+
+	err := hb.Send(&gatewayv1.GatewayEvent{EventType: "real.event"})
+	require.ErrorIs(t, err, wantErr)
+}
+
+// capturingStream is a minimal grpc.ServerStreamingServer that pushes
+// every Send into a channel so tests can assert on the wire frame.
+type capturingStream struct {
+	grpc.ServerStreamingServer[gatewayv1.GatewayEvent]
+
+	events  chan *gatewayv1.GatewayEvent
+	sendErr atomic.Pointer[errorBox]
+}
+
+type errorBox struct{ err error }
+
+func newCapturingStream(t *testing.T) *capturingStream {
+	t.Helper()
+
+	return &capturingStream{events: make(chan *gatewayv1.GatewayEvent, 16)}
+}
+
+func (s *capturingStream) Send(event *gatewayv1.GatewayEvent) error {
+	if box := s.sendErr.Load(); box != nil {
+		return box.err
+	}
+	s.events <- event
+
+	return nil
+}
+
+func (s *capturingStream) Context() context.Context { return context.Background() }
+
+func (s *capturingStream) SetHeader(metadata.MD) error  { return nil }
+func (s *capturingStream) SendHeader(metadata.MD) error { return nil }
+func (s *capturingStream) SetTrailer(metadata.MD)       {}
+func (s *capturingStream) SendMsg(any) error            { return errors.New("capturingStream.SendMsg: unused") }
+func (s *capturingStream) RecvMsg(any) error            { return errors.New("capturingStream.RecvMsg: unused") }
+
+func (s *capturingStream) recv(t *testing.T, timeout time.Duration) *gatewayv1.GatewayEvent {
+	t.Helper()
+
+	select {
+	case event := <-s.events:
+		return event
+	case <-time.After(timeout):
+		t.Fatalf("no event captured within %s", timeout)
+		return nil
+	}
+}

gateway/internal/grpcapi/push_stream.go

@@ -4,9 +4,11 @@ import (
 	"bytes"
 	"context"
 	"crypto/sha256"
+	"time"
 
 	"galaxy/gateway/authn"
 	"galaxy/gateway/internal/clock"
+	"galaxy/gateway/internal/telemetry"
 	gatewayv1 "galaxy/gateway/proto/galaxy/gateway/v1"
 	gatewayfbs "galaxy/schema/fbs/gateway"
 
@@ -16,7 +18,31 @@ import (
 	"google.golang.org/grpc/status"
 )
 
-const serverTimeEventType = "gateway.server_time"
+const (
+	serverTimeEventType = "gateway.server_time"
+
+	// gatewayHeartbeatEventType labels the silence-filling event the
+	// authenticated push stream emits when no real event has been Send'd
+	// within `AuthenticatedGRPCConfig.PushHeartbeatInterval`. Browser
+	// fetch-streaming layers (notably Safari) close response bodies they
+	// consider idle; the heartbeat keeps the body active so push events
+	// land on the live stream instead of disappearing into the
+	// client-side reconnect window.
+	//
+	// Heartbeat events are sent UNSIGNED — `EventID`, `RequestID`,
+	// `TraceID`, `PayloadBytes`, `PayloadHash`, `Signature`, and
+	// `TimestampMs` are all left at their proto3 defaults so the wire
+	// frame stays under ~50 bytes. The UI's EventStream short-circuits
+	// on this event type before signature verification (see
+	// `ui/frontend/src/api/events.svelte.ts`) and never dispatches it to
+	// handlers. The security implication is intentional and documented
+	// in `docs/ARCHITECTURE.md` (§ authenticated edge): an attacker who
+	// could inject heartbeats gains nothing — they carry no payload and
+	// trigger no UI behaviour, the only practical effect is keeping a
+	// stream marginally more alive than transport-level keepalives
+	// would. Real events keep the signed envelope unchanged.
+	gatewayHeartbeatEventType = "gateway.heartbeat"
+)
 
 // authenticatedStreamBinding captures the verified identity bound to one
 // authenticated SubscribeEvents stream after the full ingress pipeline
@@ -47,12 +73,21 @@ func authenticatedStreamBindingFromContext(ctx context.Context) (authenticatedSt
 // authenticatedPushStreamService owns SubscribeEvents bootstrap behavior:
 // bind the authenticated stream, send the initial signed server-time event,
 // and then hand the stream lifecycle to the configured tail delegate.
+//
+// A positive `heartbeatInterval` wraps the bound stream with
+// `heartbeatingStream` before delegating, so any tail implementation
+// (fan-out, hold-open, future variants) gets the silence-based
+// `gateway.heartbeat` for free. The wrapper observes every real Send
+// the tail performs and only emits a heartbeat when the silence window
+// elapses; tails remain heartbeat-unaware.
 type authenticatedPushStreamService struct {
 	gatewayv1.UnimplementedEdgeGatewayServer
 
-	tailDelegate   gatewayv1.EdgeGatewayServer
+	tailDelegate      gatewayv1.EdgeGatewayServer
-	responseSigner authn.ResponseSigner
+	responseSigner    authn.ResponseSigner
-	clock          clock.Clock
+	clock             clock.Clock
+	heartbeatInterval time.Duration
+	metrics           *telemetry.Runtime
 }
 
 // SubscribeEvents binds the verified stream identity, sends the initial signed
@@ -112,18 +147,40 @@ func (s authenticatedPushStreamService) SubscribeEvents(req *gatewayv1.Subscribe
 		return err
 	}
 
-	return s.tailDelegate.SubscribeEvents(req, boundStream)
+	var streamForTail grpc.ServerStreamingServer[gatewayv1.GatewayEvent] = boundStream
+	if hbStream := newHeartbeatingStream(boundStream, s.heartbeatInterval, s.metrics); hbStream != nil {
+		defer hbStream.Stop()
+		go func() {
+			// Heartbeat Send failures imply the transport is already
+			// dead — the tail's next Send will hit the same error and
+			// surface through the gateway observability layer, so we
+			// discard the returned error here and rely on that path
+			// for the canonical failure record.
+			_ = hbStream.Run(stream.Context())
+		}()
+		streamForTail = hbStream
+	}
+
+	return s.tailDelegate.SubscribeEvents(req, streamForTail)
 }
 
-func newAuthenticatedPushStreamService(tailDelegate gatewayv1.EdgeGatewayServer, responseSigner authn.ResponseSigner, clk clock.Clock) gatewayv1.EdgeGatewayServer {
+func newAuthenticatedPushStreamService(
+	tailDelegate gatewayv1.EdgeGatewayServer,
+	responseSigner authn.ResponseSigner,
+	clk clock.Clock,
+	heartbeatInterval time.Duration,
+	metrics *telemetry.Runtime,
+) gatewayv1.EdgeGatewayServer {
 	if tailDelegate == nil {
 		tailDelegate = holdOpenSubscribeEventsService{}
 	}
 
 	return authenticatedPushStreamService{
-		tailDelegate:   tailDelegate,
+		tailDelegate:      tailDelegate,
-		responseSigner: responseSigner,
+		responseSigner:    responseSigner,
-		clock:          clk,
+		clock:             clk,
+		heartbeatInterval: heartbeatInterval,
+		metrics:           metrics,
 	}
 }
 

gateway/internal/grpcapi/server.go

@@ -111,7 +111,13 @@ func NewServer(cfg config.AuthenticatedGRPCConfig, deps ServerDependencies) *Ser
 	deps = normalizeServerDependencies(deps)
 
 	finalService := newCommandRoutingService(
-		newAuthenticatedPushStreamService(deps.Service, deps.ResponseSigner, deps.Clock),
+		newAuthenticatedPushStreamService(
+			deps.Service,
+			deps.ResponseSigner,
+			deps.Clock,
+			cfg.PushHeartbeatInterval,
+			deps.Telemetry,
+		),
 		deps.Router,
 		deps.ResponseSigner,
 		deps.Clock,

gateway/internal/grpcapi/server_test.go

@@ -174,6 +174,102 @@ func TestSubscribeEventsValidEnvelopeSendsBootstrapEventAndWaitsForCancellation(
 	assert.Equal(t, connect.CodeCanceled, connect.CodeOf(recvErr))
 }
 
+func TestSubscribeEventsEmitsHeartbeatAfterSilenceWindow(t *testing.T) {
+	t.Parallel()
+
+	grpcCfg := config.DefaultAuthenticatedGRPCConfig()
+	grpcCfg.Addr = "127.0.0.1:0"
+	grpcCfg.FreshnessWindow = testFreshnessWindow
+	// 30 ms keeps the test inside the standard 60-second go test
+	// timeout while still giving the heartbeat goroutine enough
+	// headroom to fire after the bootstrap server-time event lands.
+	grpcCfg.PushHeartbeatInterval = 30 * time.Millisecond
+
+	server, runGateway := newTestGatewayWithGRPCConfig(t, grpcCfg, ServerDependencies{
+		SessionCache: staticSessionCache{
+			lookupFunc: func(context.Context, string) (session.Record, error) {
+				return newActiveSessionRecord(), nil
+			},
+		},
+		ReplayStore: staticReplayStore{},
+	})
+	defer runGateway.stop(t)
+
+	addr := waitForListenAddr(t, server)
+	client := newEdgeClient(t, addr)
+
+	stream, err := client.SubscribeEvents(t.Context(), connect.NewRequest(newValidSubscribeEventsRequest()))
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = stream.Close() })
+
+	bootstrap := recvBootstrapEvent(t, stream)
+	assertServerTimeBootstrapEvent(t, bootstrap, newTestResponseSignerPublicKey(), "request-123", "trace-123", testCurrentTime.UnixMilli())
+
+	// The next frame must be the unsigned heartbeat. Every field
+	// except EventType is left at its proto3 default — the UI side
+	// short-circuits on EventType BEFORE signature verification, so
+	// any non-empty signature would be wasted bytes on the wire.
+	require.True(t, stream.Receive(), "stream did not deliver a heartbeat: %v", stream.Err())
+	heartbeat := stream.Msg()
+	assert.Equal(t, gatewayHeartbeatEventType, heartbeat.GetEventType())
+	assert.Empty(t, heartbeat.GetEventId())
+	assert.Zero(t, heartbeat.GetTimestampMs())
+	assert.Empty(t, heartbeat.GetPayloadBytes())
+	assert.Empty(t, heartbeat.GetPayloadHash())
+	assert.Empty(t, heartbeat.GetSignature())
+	assert.Empty(t, heartbeat.GetRequestId())
+	assert.Empty(t, heartbeat.GetTraceId())
+}
+
+func TestSubscribeEventsZeroHeartbeatIntervalDisablesEmission(t *testing.T) {
+	t.Parallel()
+
+	grpcCfg := config.DefaultAuthenticatedGRPCConfig()
+	grpcCfg.Addr = "127.0.0.1:0"
+	grpcCfg.FreshnessWindow = testFreshnessWindow
+	grpcCfg.PushHeartbeatInterval = 0
+
+	server, runGateway := newTestGatewayWithGRPCConfig(t, grpcCfg, ServerDependencies{
+		SessionCache: staticSessionCache{
+			lookupFunc: func(context.Context, string) (session.Record, error) {
+				return newActiveSessionRecord(), nil
+			},
+		},
+		ReplayStore: staticReplayStore{},
+	})
+	defer runGateway.stop(t)
+
+	addr := waitForListenAddr(t, server)
+	client := newEdgeClient(t, addr)
+
+	stream, err := client.SubscribeEvents(t.Context(), connect.NewRequest(newValidSubscribeEventsRequest()))
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = stream.Close() })
+
+	bootstrap := recvBootstrapEvent(t, stream)
+	assertServerTimeBootstrapEvent(t, bootstrap, newTestResponseSignerPublicKey(), "request-123", "trace-123", testCurrentTime.UnixMilli())
+
+	// No heartbeat is expected — the stream must stay silent. A
+	// background Receive races a deadline check so the test fails
+	// fast if the gateway ever sends a second frame on this stream.
+	recvResult := make(chan error, 1)
+	go func() {
+		if stream.Receive() {
+			recvResult <- errors.New("stream produced unexpected event")
+			return
+		}
+		recvResult <- stream.Err()
+	}()
+	require.Never(t, func() bool {
+		select {
+		case <-recvResult:
+			return true
+		default:
+			return false
+		}
+	}, 200*time.Millisecond, 20*time.Millisecond, "heartbeat fired despite zero interval")
+}
+
 func TestSubscribeEventsMissingReplayStoreFailsClosed(t *testing.T) {
 	t.Parallel()
 

gateway/internal/telemetry/runtime.go

@@ -46,6 +46,7 @@ type Runtime struct {
 	// Push instruments.
 	pushActiveStreams metric.Int64UpDownCounter
 	pushStreamClosers metric.Int64Counter
+	pushHeartbeats    metric.Int64Counter
 
 	// Internal event consumer instruments.
 	internalEventDrops metric.Int64Counter
@@ -120,6 +121,10 @@ func New(ctx context.Context, logger *zap.Logger) (*Runtime, error) {
 	if err != nil {
 		return nil, err
 	}
+	pushHeartbeats, err := meter.Int64Counter("gateway.push.heartbeats_sent")
+	if err != nil {
+		return nil, err
+	}
 	internalEventDrops, err := meter.Int64Counter("gateway.internal_event_drops")
 	if err != nil {
 		return nil, err
@@ -136,6 +141,7 @@ func New(ctx context.Context, logger *zap.Logger) (*Runtime, error) {
 		grpcDuration:       grpcDuration,
 		pushActiveStreams:  pushActiveStreams,
 		pushStreamClosers:  pushStreamClosers,
+		pushHeartbeats:     pushHeartbeats,
 		internalEventDrops: internalEventDrops,
 	}, nil
 }
@@ -228,6 +234,19 @@ func (r *Runtime) RecordPushStreamClosure(ctx context.Context, attrs ...attribut
 	r.pushStreamClosers.Add(ctx, 1, metric.WithAttributes(attrs...))
 }
 
+// RecordPushHeartbeat records one outbound push-stream heartbeat event.
+// The `outcome` attribute should distinguish a successful Send from a
+// transport-level failure so the metric stays useful for bandwidth
+// budgeting (most heartbeats are `sent`; a sudden bump of `error` means
+// the upstream connection is failing before the gateway can flush).
+func (r *Runtime) RecordPushHeartbeat(ctx context.Context, attrs ...attribute.KeyValue) {
+	if r == nil {
+		return
+	}
+
+	r.pushHeartbeats.Add(ctx, 1, metric.WithAttributes(attrs...))
+}
+
 // RecordInternalEventDrop records one malformed or rejected internal event.
 func (r *Runtime) RecordInternalEventDrop(ctx context.Context, attrs ...attribute.KeyValue) {
 	if r == nil {

ui/frontend/src/api/events.svelte.ts

@@ -24,7 +24,6 @@ import { ConnectError } from "@connectrpc/connect";
 import type { Core } from "../platform/core/index";
 import type { DeviceKeypair } from "../platform/store/index";
 import {
-	GatewayEventSchema,
 	SubscribeEventsRequestSchema,
 	type GatewayEvent,
 } from "../proto/galaxy/gateway/v1/edge_gateway_pb";
@@ -35,6 +34,17 @@ import { createEdgeGatewayClient, type EdgeGatewayClient } from "./connect";
 const PROTOCOL_VERSION = "v1";
 const SUBSCRIBE_MESSAGE_TYPE = "gateway.subscribe";
 
+// HEARTBEAT_EVENT_TYPE matches `gatewayHeartbeatEventType` on the
+// gateway side. The server emits this unsigned event with only the
+// `eventType` field set when the push stream has been silent for the
+// configured interval — its sole purpose is to keep browser fetch
+// streaming layers (Safari is notably aggressive on idle timeouts)
+// from closing the response body and causing push events to disappear
+// into the reconnect window. The client SHORT-CIRCUITS on this event
+// type before signature verification or handler dispatch: the payload
+// is empty by design and there is no signature to validate.
+const HEARTBEAT_EVENT_TYPE = "gateway.heartbeat";
+
 // Connect error code numerical values used by the watcher. The full
 // enum lives in `@connectrpc/connect` but importing the runtime enum
 // would pull a large surface into this small module.
@@ -207,6 +217,20 @@ export class EventStream {
 					if (signal.aborted) {
 						return;
 					}
+					if (event.eventType === HEARTBEAT_EVENT_TYPE) {
+						// Heartbeats are unsigned by design (see the
+						// constant's docstring) and carry no payload — skip
+						// verification + dispatch. They still count as
+						// connection proof: receiving one means the
+						// stream is healthy, so reset backoff like any
+						// other first event.
+						if (!firstEventSeen) {
+							firstEventSeen = true;
+							this.connectionStatus = "connected";
+							attempt = 0;
+						}
+						continue;
+					}
 					this.verifyEvent(event, opts);
 					if (!firstEventSeen) {
 						firstEventSeen = true;

ui/frontend/tests/events.test.ts

@@ -321,6 +321,88 @@ describe("EventStream", () => {
 		eventStream.stop();
 	});
 
+	test("gateway.heartbeat is short-circuited before verification and dispatch", async () => {
+		const handler = vi.fn();
+		eventStream.on("game.turn.ready", handler);
+		// A heartbeat with an empty signature (matching the unsigned
+		// wire shape the gateway emits) would normally trip
+		// `verifyEvent` and tear the stream down with a SignatureError.
+		// The short-circuit skips both verification and dispatch.
+		const heartbeat = create(GatewayEventSchema, {
+			eventType: "gateway.heartbeat",
+			eventId: "",
+			timestampMs: 0n,
+			payloadBytes: new Uint8Array(0),
+			payloadHash: new Uint8Array(0),
+			signature: new Uint8Array(0),
+			requestId: "",
+			traceId: "",
+		});
+		const realEvent = buildEvent(
+			"game.turn.ready",
+			new TextEncoder().encode("{}"),
+		);
+
+		const verifyEventSpy = vi.fn(() => true);
+		const verifyPayloadHashSpy = vi.fn(() => true);
+		const core = mockCore({
+			verifyEvent: verifyEventSpy,
+			verifyPayloadHash: verifyPayloadHashSpy,
+		});
+
+		const client = makeRouter(async function* () {
+			yield heartbeat;
+			yield realEvent;
+		});
+		eventStream.start({
+			core,
+			keypair: mockKeypair(),
+			deviceSessionId: "device-1",
+			gatewayResponsePublicKey: new Uint8Array(32),
+			client,
+			sleep: async () => {},
+			random: () => 0,
+		});
+
+		await vi.waitFor(() => {
+			expect(handler).toHaveBeenCalled();
+		});
+		// Verification ran exactly once — for the real event, never
+		// for the heartbeat. The handler also only sees the real one.
+		expect(verifyEventSpy).toHaveBeenCalledTimes(1);
+		expect(verifyPayloadHashSpy).toHaveBeenCalledTimes(1);
+		expect(handler).toHaveBeenCalledTimes(1);
+		expect(handler.mock.calls[0]?.[0].eventType).toBe("game.turn.ready");
+		eventStream.stop();
+	});
+
+	test("a leading heartbeat still flips connectionStatus to connected", async () => {
+		const heartbeat = create(GatewayEventSchema, {
+			eventType: "gateway.heartbeat",
+		});
+		// The stream yields only a heartbeat then waits for the test
+		// to abort the consumer — the `connected` transition must not
+		// require a real event behind the heartbeat.
+		const pending = new Promise<never>(() => {});
+		const client = makeRouter(async function* () {
+			yield heartbeat;
+			await pending;
+		});
+		eventStream.start({
+			core: mockCore(),
+			keypair: mockKeypair(),
+			deviceSessionId: "device-1",
+			gatewayResponsePublicKey: new Uint8Array(32),
+			client,
+			sleep: async () => {},
+			random: () => 0,
+		});
+		await vi.waitFor(() => {
+			expect(eventStream.connectionStatus).toBe("connected");
+		});
+		eventStream.stop();
+	});
+
 	test("connectionStatus transitions through connecting → connected → idle", async () => {
 		expect(eventStream.connectionStatus).toBe("idle");
 		const event = buildEvent(