feat(proxy): restore SyncMappings bidirectional stream with ack back-pressure

Reinstates the SyncMappings RPC that landed on origin/main and the
client-side fallback to GetMappingUpdate.

- proto: SyncMappings RPC + SyncMappingsRequest{Init|Ack} +
  SyncMappingsResponse messages.
- management proxy.go: SyncMappings server handler, recvSyncInit,
  sendSnapshotSync (per-batch send-then-wait-for-ack), drainRecv,
  waitForAck; proxyConnection.syncStream + sendResponse routes the
  same sendChan onto the bidi stream when set.
- proxy/server.go: trySyncMappings + handleSyncMappingsStream that
  acks after each batch is processed; outer loop tries SyncMappings
  first and falls back to GetMappingUpdate on Unimplemented.
  Capabilities lifted into proxyCapabilities() so both code paths
  use the same flags.

proxy/server.go

@@ -32,9 +32,11 @@ import (
 	"go.opentelemetry.io/otel/sdk/metric"
 	"golang.org/x/exp/maps"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/credentials/insecure"
 	"google.golang.org/grpc/keepalive"
+	grpcstatus "google.golang.org/grpc/status"
 	"google.golang.org/protobuf/encoding/protojson"
 	"google.golang.org/protobuf/types/known/timestamppb"
 
@@ -1145,6 +1147,10 @@ func (s *Server) newManagementMappingWorker(ctx context.Context, client proto.Pr
 		Clock:               backoff.SystemClock,
 	}
 
+	// syncSupported tracks whether management supports SyncMappings.
+	// Starts true; set to false on the first Unimplemented error so
+	// subsequent retries skip straight to GetMappingUpdate.
+	syncSupported := true
 	initialSyncDone := false
 
 	operation := func() error {
@@ -1156,41 +1162,25 @@ func (s *Server) newManagementMappingWorker(ctx context.Context, client proto.Pr
 			s.healthChecker.SetManagementConnected(false)
 		}
 
-		supportsCrowdSec := s.crowdsecRegistry.Available()
-		privateCapability := s.Private
-		// Always true: this build enforces ProxyMapping.private via the auth middleware.
-		supportsPrivateService := true
-		mappingClient, err := client.GetMappingUpdate(ctx, &proto.GetMappingUpdateRequest{
-			ProxyId:   s.ID,
-			Version:   s.Version,
-			StartedAt: timestamppb.New(s.startTime),
-			Address:   s.ProxyURL,
-			Capabilities: &proto.ProxyCapabilities{
-				SupportsCustomPorts:    &s.SupportsCustomPorts,
-				RequireSubdomain:       &s.RequireSubdomain,
-				SupportsCrowdsec:       &supportsCrowdSec,
-				Private:                &privateCapability,
-				SupportsPrivateService: &supportsPrivateService,
-			},
-		})
-		if err != nil {
-			return fmt.Errorf("create mapping stream: %w", err)
+		var streamErr error
+		if syncSupported {
+			streamErr = s.trySyncMappings(ctx, client, &initialSyncDone)
+			if isSyncUnimplemented(streamErr) {
+				syncSupported = false
+				s.Logger.Info("management does not support SyncMappings, falling back to GetMappingUpdate")
+				streamErr = s.tryGetMappingUpdate(ctx, client, &initialSyncDone)
+			}
+		} else {
+			streamErr = s.tryGetMappingUpdate(ctx, client, &initialSyncDone)
 		}
 
-		if s.healthChecker != nil {
-			s.healthChecker.SetManagementConnected(true)
-		}
-		s.Logger.Debug("management mapping stream established")
-
-		// Stream established — reset backoff so the next failure retries quickly.
-		bo.Reset()
-
-		streamErr := s.handleMappingStream(ctx, mappingClient, &initialSyncDone, time.Now())
-
 		if s.healthChecker != nil {
 			s.healthChecker.SetManagementConnected(false)
 		}
 
+		// Stream established — reset backoff so the next failure retries quickly.
+		bo.Reset()
+
 		if streamErr == nil {
 			return fmt.Errorf("stream closed by server")
 		}
@@ -1207,6 +1197,134 @@ func (s *Server) newManagementMappingWorker(ctx context.Context, client proto.Pr
 	}
 }
 
+func (s *Server) proxyCapabilities() *proto.ProxyCapabilities {
+	supportsCrowdSec := s.crowdsecRegistry.Available()
+	privateCapability := s.Private
+	// Always true: this build enforces ProxyMapping.private via the auth middleware.
+	supportsPrivateService := true
+	return &proto.ProxyCapabilities{
+		SupportsCustomPorts:    &s.SupportsCustomPorts,
+		RequireSubdomain:       &s.RequireSubdomain,
+		SupportsCrowdsec:       &supportsCrowdSec,
+		Private:                &privateCapability,
+		SupportsPrivateService: &supportsPrivateService,
+	}
+}
+
+func (s *Server) tryGetMappingUpdate(ctx context.Context, client proto.ProxyServiceClient, initialSyncDone *bool) error {
+	connectTime := time.Now()
+	mappingClient, err := client.GetMappingUpdate(ctx, &proto.GetMappingUpdateRequest{
+		ProxyId:      s.ID,
+		Version:      s.Version,
+		StartedAt:    timestamppb.New(s.startTime),
+		Address:      s.ProxyURL,
+		Capabilities: s.proxyCapabilities(),
+	})
+	if err != nil {
+		return fmt.Errorf("create mapping stream: %w", err)
+	}
+
+	if s.healthChecker != nil {
+		s.healthChecker.SetManagementConnected(true)
+	}
+	s.Logger.Debug("management mapping stream established (GetMappingUpdate)")
+
+	return s.handleMappingStream(ctx, mappingClient, initialSyncDone, connectTime)
+}
+
+func (s *Server) trySyncMappings(ctx context.Context, client proto.ProxyServiceClient, initialSyncDone *bool) error {
+	connectTime := time.Now()
+	stream, err := client.SyncMappings(ctx)
+	if err != nil {
+		return fmt.Errorf("create sync stream: %w", err)
+	}
+
+	if err := stream.Send(&proto.SyncMappingsRequest{
+		Msg: &proto.SyncMappingsRequest_Init{
+			Init: &proto.SyncMappingsInit{
+				ProxyId:      s.ID,
+				Version:      s.Version,
+				StartedAt:    timestamppb.New(s.startTime),
+				Address:      s.ProxyURL,
+				Capabilities: s.proxyCapabilities(),
+			},
+		},
+	}); err != nil {
+		return fmt.Errorf("send sync init: %w", err)
+	}
+
+	if s.healthChecker != nil {
+		s.healthChecker.SetManagementConnected(true)
+	}
+	s.Logger.Debug("management mapping stream established (SyncMappings)")
+
+	return s.handleSyncMappingsStream(ctx, stream, initialSyncDone, connectTime)
+}
+
+func isSyncUnimplemented(err error) bool {
+	if err == nil {
+		return false
+	}
+	st, ok := grpcstatus.FromError(err)
+	return ok && st.Code() == codes.Unimplemented
+}
+
+// handleSyncMappingsStream consumes batches from a bidirectional SyncMappings
+// stream, sending an ack after each batch is fully processed. Management waits
+// for the ack before sending the next batch, providing application-level
+// back-pressure.
+func (s *Server) handleSyncMappingsStream(ctx context.Context, stream proto.ProxyService_SyncMappingsClient, initialSyncDone *bool, _ time.Time) error {
+	select {
+	case <-s.routerReady:
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+
+	var snapshotIDs map[types.ServiceID]struct{}
+	if !*initialSyncDone {
+		snapshotIDs = make(map[types.ServiceID]struct{})
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+			msg, err := stream.Recv()
+			switch {
+			case errors.Is(err, io.EOF):
+				return nil
+			case err != nil:
+				return fmt.Errorf("receive msg: %w", err)
+			}
+			s.Logger.Debug("Received mapping update, starting processing")
+			s.processMappings(ctx, msg.GetMapping())
+			s.Logger.Debug("Processing mapping update completed")
+
+			if !*initialSyncDone {
+				for _, m := range msg.GetMapping() {
+					snapshotIDs[types.ServiceID(m.GetId())] = struct{}{}
+				}
+				if msg.GetInitialSyncComplete() {
+					s.reconcileSnapshot(ctx, snapshotIDs)
+					snapshotIDs = nil
+					if s.healthChecker != nil {
+						s.healthChecker.SetInitialSyncComplete()
+					}
+					*initialSyncDone = true
+					s.Logger.Info("Initial mapping sync complete")
+				}
+			}
+
+			if err := stream.Send(&proto.SyncMappingsRequest{
+				Msg: &proto.SyncMappingsRequest_Ack{Ack: &proto.SyncMappingsAck{}},
+			}); err != nil {
+				return fmt.Errorf("send ack: %w", err)
+			}
+		}
+	}
+}
+
 func (s *Server) handleMappingStream(ctx context.Context, mappingClient proto.ProxyService_GetMappingUpdateClient, initialSyncDone *bool, _ time.Time) error {
 	select {
 	case <-s.routerReady:

proxy/sync_mappings_test.go

@@ -0,0 +1,525 @@
+package proxy
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/credentials/insecure"
+	grpcstatus "google.golang.org/grpc/status"
+
+	"github.com/netbirdio/netbird/management/internals/modules/reverseproxy/service"
+	"github.com/netbirdio/netbird/shared/management/proto"
+)
+
+func TestIntegration_SyncMappings_HappyPath(t *testing.T) {
+	setup := setupIntegrationTest(t)
+	defer setup.cleanup()
+
+	conn, err := grpc.NewClient(setup.grpcAddr, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	defer conn.Close()
+
+	client := proto.NewProxyServiceClient(conn)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	stream, err := client.SyncMappings(ctx)
+	require.NoError(t, err)
+
+	// Send init.
+	err = stream.Send(&proto.SyncMappingsRequest{
+		Msg: &proto.SyncMappingsRequest_Init{
+			Init: &proto.SyncMappingsInit{
+				ProxyId: "sync-proxy-1",
+				Version: "test-v1",
+				Address: "test.proxy.io",
+			},
+		},
+	})
+	require.NoError(t, err)
+
+	mappingsByID := make(map[string]*proto.ProxyMapping)
+	for {
+		msg, err := stream.Recv()
+		require.NoError(t, err)
+		for _, m := range msg.GetMapping() {
+			mappingsByID[m.GetId()] = m
+		}
+
+		// Ack every batch.
+		err = stream.Send(&proto.SyncMappingsRequest{
+			Msg: &proto.SyncMappingsRequest_Ack{Ack: &proto.SyncMappingsAck{}},
+		})
+		require.NoError(t, err)
+
+		if msg.GetInitialSyncComplete() {
+			break
+		}
+	}
+
+	assert.Len(t, mappingsByID, 2, "Should receive 2 mappings")
+
+	rp1 := mappingsByID["rp-1"]
+	require.NotNil(t, rp1)
+	assert.Equal(t, "app1.test.proxy.io", rp1.GetDomain())
+	assert.Equal(t, "test-account-1", rp1.GetAccountId())
+	assert.Equal(t, proto.ProxyMappingUpdateType_UPDATE_TYPE_CREATED, rp1.GetType())
+	assert.NotEmpty(t, rp1.GetAuthToken(), "Should have auth token")
+
+	rp2 := mappingsByID["rp-2"]
+	require.NotNil(t, rp2)
+	assert.Equal(t, "app2.test.proxy.io", rp2.GetDomain())
+}
+
+func TestIntegration_SyncMappings_BackPressure(t *testing.T) {
+	setup := setupIntegrationTest(t)
+	defer setup.cleanup()
+
+	// Add enough services to guarantee multiple batches (default batch size 500).
+	addServicesToStore(t, setup, 600, "test.proxy.io")
+
+	conn, err := grpc.NewClient(setup.grpcAddr, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	defer conn.Close()
+
+	client := proto.NewProxyServiceClient(conn)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	stream, err := client.SyncMappings(ctx)
+	require.NoError(t, err)
+
+	err = stream.Send(&proto.SyncMappingsRequest{
+		Msg: &proto.SyncMappingsRequest_Init{
+			Init: &proto.SyncMappingsInit{
+				ProxyId: "sync-proxy-backpressure",
+				Version: "test-v1",
+				Address: "test.proxy.io",
+			},
+		},
+	})
+	require.NoError(t, err)
+
+	// Strategy: receive batch 1, then hold for a significant delay before
+	// acking. If back-pressure works, batch 2 cannot arrive until after
+	// the ack is sent — so its receive timestamp must be >= the ack
+	// timestamp. If management were fire-and-forget, all batches would
+	// already be buffered in the gRPC transport and batch 2 would arrive
+	// well before the ack time.
+	const ackDelay = 300 * time.Millisecond
+
+	type batchEvent struct {
+		recvAt time.Time
+		ackAt  time.Time
+		count  int
+	}
+	var batches []batchEvent
+	var totalMappings int
+
+	for {
+		msg, err := stream.Recv()
+		require.NoError(t, err)
+
+		recvAt := time.Now()
+		totalMappings += len(msg.GetMapping())
+
+		// Delay the ack on non-final batches to create a measurable gap.
+		if !msg.GetInitialSyncComplete() {
+			time.Sleep(ackDelay)
+		}
+
+		ackAt := time.Now()
+		batches = append(batches, batchEvent{
+			recvAt: recvAt,
+			ackAt:  ackAt,
+			count:  len(msg.GetMapping()),
+		})
+
+		err = stream.Send(&proto.SyncMappingsRequest{
+			Msg: &proto.SyncMappingsRequest_Ack{Ack: &proto.SyncMappingsAck{}},
+		})
+		require.NoError(t, err)
+
+		if msg.GetInitialSyncComplete() {
+			break
+		}
+	}
+
+	// 2 original + 600 added = 602 services total.
+	assert.Equal(t, 602, totalMappings, "should receive all 602 mappings")
+	require.GreaterOrEqual(t, len(batches), 2, "need at least 2 batches to verify back-pressure")
+
+	// For every batch after the first, its receive time must be after the
+	// previous batch's ack time. This proves management waited for the ack
+	// before sending the next batch.
+	for i := 1; i < len(batches); i++ {
+		prevAckAt := batches[i-1].ackAt
+		thisRecvAt := batches[i].recvAt
+		assert.True(t, !thisRecvAt.Before(prevAckAt),
+			"batch %d received at %v, but batch %d was acked at %v — "+
+				"management sent the next batch before receiving the ack",
+			i, thisRecvAt, i-1, prevAckAt)
+	}
+}
+
+func TestIntegration_SyncMappings_IncrementalUpdate(t *testing.T) {
+	setup := setupIntegrationTest(t)
+	defer setup.cleanup()
+
+	conn, err := grpc.NewClient(setup.grpcAddr, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	defer conn.Close()
+
+	client := proto.NewProxyServiceClient(conn)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	stream, err := client.SyncMappings(ctx)
+	require.NoError(t, err)
+
+	err = stream.Send(&proto.SyncMappingsRequest{
+		Msg: &proto.SyncMappingsRequest_Init{
+			Init: &proto.SyncMappingsInit{
+				ProxyId: "sync-proxy-incremental",
+				Version: "test-v1",
+				Address: "test.proxy.io",
+			},
+		},
+	})
+	require.NoError(t, err)
+
+	// Drain initial snapshot.
+	for {
+		msg, err := stream.Recv()
+		require.NoError(t, err)
+
+		err = stream.Send(&proto.SyncMappingsRequest{
+			Msg: &proto.SyncMappingsRequest_Ack{Ack: &proto.SyncMappingsAck{}},
+		})
+		require.NoError(t, err)
+
+		if msg.GetInitialSyncComplete() {
+			break
+		}
+	}
+
+	// Now send an incremental update via the management server.
+	setup.proxyService.SendServiceUpdate(&proto.GetMappingUpdateResponse{
+		Mapping: []*proto.ProxyMapping{{
+			Type:      proto.ProxyMappingUpdateType_UPDATE_TYPE_REMOVED,
+			Id:        "rp-1",
+			AccountId: "test-account-1",
+			Domain:    "app1.test.proxy.io",
+		}},
+	})
+
+	// Receive the incremental update on the sync stream.
+	msg, err := stream.Recv()
+	require.NoError(t, err)
+	require.NotEmpty(t, msg.GetMapping())
+	assert.Equal(t, "rp-1", msg.GetMapping()[0].GetId())
+	assert.Equal(t, proto.ProxyMappingUpdateType_UPDATE_TYPE_REMOVED, msg.GetMapping()[0].GetType())
+}
+
+func TestIntegration_SyncMappings_MixedProxyVersions(t *testing.T) {
+	setup := setupIntegrationTest(t)
+	defer setup.cleanup()
+
+	conn, err := grpc.NewClient(setup.grpcAddr, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	defer conn.Close()
+
+	client := proto.NewProxyServiceClient(conn)
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// Old proxy uses GetMappingUpdate.
+	legacyStream, err := client.GetMappingUpdate(ctx, &proto.GetMappingUpdateRequest{
+		ProxyId: "legacy-proxy",
+		Version: "old-v1",
+		Address: "test.proxy.io",
+	})
+	require.NoError(t, err)
+
+	var legacyMappings []*proto.ProxyMapping
+	for {
+		msg, err := legacyStream.Recv()
+		require.NoError(t, err)
+		legacyMappings = append(legacyMappings, msg.GetMapping()...)
+		if msg.GetInitialSyncComplete() {
+			break
+		}
+	}
+
+	// New proxy uses SyncMappings.
+	syncStream, err := client.SyncMappings(ctx)
+	require.NoError(t, err)
+
+	err = syncStream.Send(&proto.SyncMappingsRequest{
+		Msg: &proto.SyncMappingsRequest_Init{
+			Init: &proto.SyncMappingsInit{
+				ProxyId: "new-proxy",
+				Version: "new-v2",
+				Address: "test.proxy.io",
+			},
+		},
+	})
+	require.NoError(t, err)
+
+	var syncMappings []*proto.ProxyMapping
+	for {
+		msg, err := syncStream.Recv()
+		require.NoError(t, err)
+		syncMappings = append(syncMappings, msg.GetMapping()...)
+
+		err = syncStream.Send(&proto.SyncMappingsRequest{
+			Msg: &proto.SyncMappingsRequest_Ack{Ack: &proto.SyncMappingsAck{}},
+		})
+		require.NoError(t, err)
+
+		if msg.GetInitialSyncComplete() {
+			break
+		}
+	}
+
+	// Both should receive the same set of mappings.
+	assert.Equal(t, len(legacyMappings), len(syncMappings),
+		"legacy and sync proxies should receive the same number of mappings")
+
+	legacyIDs := make(map[string]bool)
+	for _, m := range legacyMappings {
+		legacyIDs[m.GetId()] = true
+	}
+	for _, m := range syncMappings {
+		assert.True(t, legacyIDs[m.GetId()],
+			"mapping %s should be present in both streams", m.GetId())
+	}
+
+	// Both proxies should be connected.
+	proxies := setup.proxyService.GetConnectedProxies()
+	assert.Contains(t, proxies, "legacy-proxy")
+	assert.Contains(t, proxies, "new-proxy")
+
+	// Both should receive incremental updates.
+	setup.proxyService.SendServiceUpdate(&proto.GetMappingUpdateResponse{
+		Mapping: []*proto.ProxyMapping{{
+			Type:      proto.ProxyMappingUpdateType_UPDATE_TYPE_REMOVED,
+			Id:        "rp-1",
+			AccountId: "test-account-1",
+			Domain:    "app1.test.proxy.io",
+		}},
+	})
+
+	// Legacy proxy receives via GetMappingUpdateResponse.
+	legacyMsg, err := legacyStream.Recv()
+	require.NoError(t, err)
+	assert.Equal(t, "rp-1", legacyMsg.GetMapping()[0].GetId())
+
+	// Sync proxy receives via SyncMappingsResponse.
+	syncMsg, err := syncStream.Recv()
+	require.NoError(t, err)
+	assert.Equal(t, "rp-1", syncMsg.GetMapping()[0].GetId())
+}
+
+func TestIntegration_SyncMappings_Reconnect(t *testing.T) {
+	setup := setupIntegrationTest(t)
+	defer setup.cleanup()
+
+	conn, err := grpc.NewClient(setup.grpcAddr, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	defer conn.Close()
+
+	client := proto.NewProxyServiceClient(conn)
+	proxyID := "sync-proxy-reconnect"
+
+	receiveMappings := func() []*proto.ProxyMapping {
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+
+		stream, err := client.SyncMappings(ctx)
+		require.NoError(t, err)
+
+		err = stream.Send(&proto.SyncMappingsRequest{
+			Msg: &proto.SyncMappingsRequest_Init{
+				Init: &proto.SyncMappingsInit{
+					ProxyId: proxyID,
+					Version: "test-v1",
+					Address: "test.proxy.io",
+				},
+			},
+		})
+		require.NoError(t, err)
+
+		var mappings []*proto.ProxyMapping
+		for {
+			msg, err := stream.Recv()
+			require.NoError(t, err)
+			mappings = append(mappings, msg.GetMapping()...)
+
+			err = stream.Send(&proto.SyncMappingsRequest{
+				Msg: &proto.SyncMappingsRequest_Ack{Ack: &proto.SyncMappingsAck{}},
+			})
+			require.NoError(t, err)
+
+			if msg.GetInitialSyncComplete() {
+				break
+			}
+		}
+		return mappings
+	}
+
+	first := receiveMappings()
+	time.Sleep(100 * time.Millisecond)
+	second := receiveMappings()
+
+	assert.Equal(t, len(first), len(second),
+		"should receive same mappings on reconnect")
+
+	firstIDs := make(map[string]bool)
+	for _, m := range first {
+		firstIDs[m.GetId()] = true
+	}
+	for _, m := range second {
+		assert.True(t, firstIDs[m.GetId()],
+			"mapping %s should be present in both connections", m.GetId())
+	}
+}
+
+// --- Fallback tests: old management returns Unimplemented ---
+
+// unimplementedProxyServer embeds UnimplementedProxyServiceServer so
+// SyncMappings returns codes.Unimplemented while GetMappingUpdate works.
+type unimplementedSyncServer struct {
+	proto.UnimplementedProxyServiceServer
+	getMappingCalls atomic.Int32
+}
+
+func (s *unimplementedSyncServer) GetMappingUpdate(_ *proto.GetMappingUpdateRequest, stream proto.ProxyService_GetMappingUpdateServer) error {
+	s.getMappingCalls.Add(1)
+	return stream.Send(&proto.GetMappingUpdateResponse{
+		Mapping:             []*proto.ProxyMapping{{Id: "svc-1", AccountId: "acct-1", Domain: "example.com"}},
+		InitialSyncComplete: true,
+	})
+}
+
+func TestIntegration_FallbackToGetMappingUpdate(t *testing.T) {
+	// Start a gRPC server that does NOT implement SyncMappings.
+	lis, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+
+	srv := &unimplementedSyncServer{}
+	grpcServer := grpc.NewServer()
+	proto.RegisterProxyServiceServer(grpcServer, srv)
+	go func() { _ = grpcServer.Serve(lis) }()
+	defer grpcServer.GracefulStop()
+
+	conn, err := grpc.NewClient(lis.Addr().String(), grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	defer conn.Close()
+
+	client := proto.NewProxyServiceClient(conn)
+
+	// Try SyncMappings — should get Unimplemented.
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+
+	stream, err := client.SyncMappings(ctx)
+	require.NoError(t, err)
+
+	err = stream.Send(&proto.SyncMappingsRequest{
+		Msg: &proto.SyncMappingsRequest_Init{
+			Init: &proto.SyncMappingsInit{
+				ProxyId: "test-proxy",
+				Address: "test.example.com",
+			},
+		},
+	})
+	require.NoError(t, err)
+
+	_, err = stream.Recv()
+	require.Error(t, err)
+	st, ok := grpcstatus.FromError(err)
+	require.True(t, ok)
+	assert.Equal(t, codes.Unimplemented, st.Code(),
+		"unimplemented SyncMappings should return Unimplemented code")
+
+	// isSyncUnimplemented should detect this.
+	assert.True(t, isSyncUnimplemented(err))
+
+	// The actual fallback: GetMappingUpdate should work.
+	legacyStream, err := client.GetMappingUpdate(ctx, &proto.GetMappingUpdateRequest{
+		ProxyId: "test-proxy",
+		Address: "test.example.com",
+	})
+	require.NoError(t, err)
+
+	msg, err := legacyStream.Recv()
+	require.NoError(t, err)
+	assert.True(t, msg.GetInitialSyncComplete())
+	assert.Len(t, msg.GetMapping(), 1)
+	assert.Equal(t, int32(1), srv.getMappingCalls.Load())
+}
+
+func TestIsSyncUnimplemented(t *testing.T) {
+	tests := []struct {
+		name string
+		err  error
+		want bool
+	}{
+		{"nil error", nil, false},
+		{"non-grpc error", errors.New("random"), false},
+		{"grpc internal", grpcstatus.Error(codes.Internal, "fail"), false},
+		{"grpc unavailable", grpcstatus.Error(codes.Unavailable, "fail"), false},
+		{"grpc unimplemented", grpcstatus.Error(codes.Unimplemented, "method not found"), true},
+		{
+			"wrapped unimplemented",
+			fmt.Errorf("create sync stream: %w", grpcstatus.Error(codes.Unimplemented, "nope")),
+			// grpc/status.FromError unwraps in recent versions of grpc-go.
+			true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, isSyncUnimplemented(tt.err))
+		})
+	}
+}
+
+// addServicesToStore adds n extra services to the test store for the given cluster.
+func addServicesToStore(t *testing.T, setup *integrationTestSetup, n int, cluster string) {
+	t.Helper()
+	ctx := context.Background()
+	for i := 0; i < n; i++ {
+		svc := &service.Service{
+			ID:           fmt.Sprintf("extra-svc-%d", i),
+			AccountID:    "test-account-1",
+			Name:         fmt.Sprintf("Extra Service %d", i),
+			Domain:       fmt.Sprintf("extra-%d.test.proxy.io", i),
+			ProxyCluster: cluster,
+			Enabled:      true,
+			Targets: []*service.Target{{
+				Path:       strPtr("/"),
+				Host:       fmt.Sprintf("10.0.1.%d", i%256),
+				Port:       8080,
+				Protocol:   "http",
+				TargetId:   fmt.Sprintf("peer-extra-%d", i),
+				TargetType: "peer",
+				Enabled:    true,
+			}},
+		}
+		require.NoError(t, setup.store.CreateService(ctx, svc))
+	}
+}