diff --git a/apps/daemon/internal/gateway/gateway_test.go b/apps/daemon/internal/gateway/gateway_test.go
index b2f319c9..266b978b 100644
--- a/apps/daemon/internal/gateway/gateway_test.go
+++ b/apps/daemon/internal/gateway/gateway_test.go
@@ -117,7 +117,7 @@ func startSandbox(t *testing.T) *sandbox {
 		sandboxlink.Serve(ctx, sandboxlink.ServeConfig{URL: srv.URL, TLS: srv.TLS, Credential: []byte("serve credential"),
 			Resource: resource, ServerInstanceID: sandboxwire.NewID(),
 			Services: []sandboxlink.ServiceHandler{{Service: sandboxlink.ServiceNetwork, Version: sandboxnet.Version,
-				Serve: func(ctx context.Context, b sandboxlink.Bind, s sandboxlink.Stream) {
+				Serve: func(ctx context.Context, b sandboxlink.Bind, _ uint64, s sandboxlink.Stream) {
 					sandboxnet.Serve(ctx, s, b.Egress, sb)
 				}}},
 			OnConnected: func(sandboxlink.HelloAccepted) {
diff --git a/apps/sandboxio/internal/netservice/service.go b/apps/sandboxio/internal/netservice/service.go
index 75a8a433..9c980382 100644
--- a/apps/sandboxio/internal/netservice/service.go
+++ b/apps/sandboxio/internal/netservice/service.go
@@ -32,7 +32,7 @@ func New() *Service { return &Service{resolver: net.DefaultResolver} }
 
 // Handle serves one Network stream under the egress its Bind carries. It is
 // the Serve function of the sandboxlink.ServiceNetwork handler.
-func (s *Service) Handle(ctx context.Context, b sandboxlink.Bind, st sandboxlink.Stream) {
+func (s *Service) Handle(ctx context.Context, b sandboxlink.Bind, _ uint64, st sandboxlink.Stream) {
 	sandboxnet.Serve(ctx, st, b.Egress, s)
 }
 
diff --git a/apps/sandboxio/internal/netservice/service_test.go b/apps/sandboxio/internal/netservice/service_test.go
index d7df57c3..1925315c 100644
--- a/apps/sandboxio/internal/netservice/service_test.go
+++ b/apps/sandboxio/internal/netservice/service_test.go
@@ -78,7 +78,7 @@ func newFixture(t *testing.T) *fixture {
 		sandboxlink.Serve(ctx, sandboxlink.ServeConfig{URL: f.srv.URL, TLS: f.srv.TLS, Credential: []byte("serve credential"),
 			Resource: resource, ServerInstanceID: sandboxwire.NewID(),
 			Services: []sandboxlink.ServiceHandler{{Service: sandboxlink.ServiceNetwork, Version: sandboxnet.Version,
-				Serve: func(ctx context.Context, b sandboxlink.Bind, s sandboxlink.Stream) {
+				Serve: func(ctx context.Context, b sandboxlink.Bind, _ uint64, s sandboxlink.Stream) {
 					f.served <- sandboxnet.Serve(ctx, s, b.Egress, svc)
 				}}},
 			OnConnected: func(sandboxlink.HelloAccepted) {
diff --git a/apps/sandboxio/internal/sandboxio/sandboxio.go b/apps/sandboxio/internal/sandboxio/sandboxio.go
index 3a7dfe6f..75e8cfe4 100644
--- a/apps/sandboxio/internal/sandboxio/sandboxio.go
+++ b/apps/sandboxio/internal/sandboxio/sandboxio.go
@@ -98,10 +98,10 @@ func run(ctx context.Context, bootstrapPath string, opt options) error {
 		// incarnation, so the link announces that one.
 		ServerInstanceID: files.InstanceID(),
 		Services: []sandboxlink.ServiceHandler{
-			{Service: sandboxlink.ServiceFile, Version: sandboxfs.Version, Serve: func(ctx context.Context, b sandboxlink.Bind, s sandboxlink.Stream) {
+			{Service: sandboxlink.ServiceFile, Version: sandboxfs.Version, Serve: func(ctx context.Context, b sandboxlink.Bind, _ uint64, s sandboxlink.Stream) {
 				sandboxfs.Serve(ctx, s, files, sandboxfs.Attachment{ID: b.AttachmentID, ServerInstanceID: b.ExpectedServerInstanceID, Lease: ctx, Exports: b.Exports})
 			}},
-			{Service: sandboxlink.ServiceProcess, Version: sandboxprocess.Version, Serve: func(ctx context.Context, b sandboxlink.Bind, s sandboxlink.Stream) {
+			{Service: sandboxlink.ServiceProcess, Version: sandboxprocess.Version, Serve: func(ctx context.Context, b sandboxlink.Bind, _ uint64, s sandboxlink.Stream) {
 				sandboxprocess.Serve(ctx, s, sandboxprocess.Attachment{ID: b.AttachmentID}, procs)
 			}},
 			{Service: sandboxlink.ServiceNetwork, Version: sandboxnet.Version, Serve: netservice.New().Handle},
diff --git a/docs/sandbox-link-protocol.md b/docs/sandbox-link-protocol.md
index b89a27b7..47177bf5 100644
--- a/docs/sandbox-link-protocol.md
+++ b/docs/sandbox-link-protocol.md
@@ -19,7 +19,7 @@ A stream ends in one of two ways, and the relay keeps them apart end to end. An
 The Sandbox I/O service runs `sandboxlink.Serve` with a `ServeConfig`:
 
 - `URL`, `Credential` and `Resource` come from the [bootstrap input](sandbox-bootstrap.md). The credential identifies the service, so the Hello carries no peer ID. `ServerInstanceID` is a new ID whenever the service starts without its operation and handle registries.
-- `Services` holds one handler per offered service and version. A handler receives the `Bind`, which carries the authorized binding including a File stream's exports, and the stream. It owns the stream and returns when it is done with it. Its context ends when the attachment closes or `Serve` returns.
+- `Services` holds one handler per offered service and version. A handler receives the `Bind`, which carries the authorized binding including a File stream's exports, a bind sequence and the stream. It owns the stream and returns when it is done with it. Its context ends when the attachment closes or `Serve` returns. Bind order is the order in which `Serve` assigns bind sequences, under the lock that tracks attachments and before it answers `Bound`; concurrent `Bound` and `Opened` replies can reach the opener in another order, and a handler that runs late keeps its stream's place. The sequence belongs to one `Serve` call and survives reconnects; it increases strictly but has gaps, since all attachments share it, and a service may rely on it to fence succession.
 - `Serve` reconnects with jittered exponential backoff, sending the same `ServerInstanceID`, whenever the link drops. It returns when its context ends or when the relay refuses the Hello with a failure that is not [retryable](#failures), for example `AuthenticationFailed` after the credential is withdrawn or `StaleGeneration` after a newer sandbox took over the resource. Before returning it cancels every handler's context and waits for the handlers.
 - `OnAttachmentLost` fires when an attachment's last open stream ends while the attachment is still open, such as when the link drops. `OnAttachmentRestored` fires when a stream binds a lost attachment again. `OnAttachmentClosed` fires with the reason when the relay reports `AttachmentClosed`. Losing a socket is not closing an attachment: the service keeps an attachment's state until it is closed.
 - Closing is final. A `Bind` the relay sent before a close can arrive after the `AttachmentClosed`, so `Serve` refuses a `Bind` for an attachment closed within the last `sandboxlink.HandshakeTimeout` with `LeaseExpired`. A stream of a closed attachment that ends later never marks another attachment lost.
diff --git a/internal/sandboxlink/relay/relay_test.go b/internal/sandboxlink/relay/relay_test.go
index 2972697d..58bfa82f 100644
--- a/internal/sandboxlink/relay/relay_test.go
+++ b/internal/sandboxlink/relay/relay_test.go
@@ -113,7 +113,8 @@ func newFixture(t *testing.T) *fixture {
 }
 
 // servePeer is a serve peer whose File handler echoes until EOF and whose
-// Process handler resets its stream after one byte.
+// Process handler reports its bind sequence after one byte and then resets its
+// stream.
 type servePeer struct {
 	instance  sandboxwire.ID
 	connected chan sandboxlink.HelloAccepted
@@ -123,6 +124,7 @@ type servePeer struct {
 	closed    chan sandboxlink.CloseReason
 	binds     chan sandboxlink.Bind
 	echoed    chan error
+	seqs      chan uint64
 	done      chan struct{}
 	err       error // Serve's result, once done is closed
 }
@@ -146,8 +148,8 @@ func (f *fixture) startServe(generation uint64) *servePeer {
 	f.auth.AddServe(credential, sandboxlink.ServePeer{PeerID: sandboxwire.NewID(), Resource: resource(generation)})
 	p := &servePeer{instance: sandboxwire.NewID(), connected: make(chan sandboxlink.HelloAccepted, 16), conns: make(chan net.Conn, 16),
 		lost: make(chan sandboxwire.ID, 16), restored: make(chan sandboxwire.ID, 16), closed: make(chan sandboxlink.CloseReason, 128),
-		binds: make(chan sandboxlink.Bind, 16), echoed: make(chan error, 16), done: make(chan struct{})}
-	echo := func(_ context.Context, b sandboxlink.Bind, s sandboxlink.Stream) {
+		binds: make(chan sandboxlink.Bind, 16), echoed: make(chan error, 16), seqs: make(chan uint64, 16), done: make(chan struct{})}
+	echo := func(_ context.Context, b sandboxlink.Bind, _ uint64, s sandboxlink.Stream) {
 		put(p.binds, b)
 		_, err := io.Copy(s, s)
 		if err == nil {
@@ -155,8 +157,9 @@ func (f *fixture) startServe(generation uint64) *servePeer {
 		}
 		put(p.echoed, err)
 	}
-	resetAfterOne := func(_ context.Context, _ sandboxlink.Bind, s sandboxlink.Stream) {
+	resetAfterOne := func(_ context.Context, _ sandboxlink.Bind, seq uint64, s sandboxlink.Stream) {
 		s.Read(make([]byte, 1))
+		put(p.seqs, seq)
 		s.Reset()
 	}
 	cfg := sandboxlink.ServeConfig{
@@ -487,3 +490,31 @@ func TestServeReconnect(t *testing.T) {
 		t.Fatalf("restored %s, want %s", id, attachment)
 	}
 }
+
+// Handlers receive bind sequences in bind order however late they run, and the
+// sequence keeps increasing after the serve peer reconnects.
+func TestBindSequence(t *testing.T) {
+	f := newFixture(t)
+	p := f.serve(1)
+	attachment := sandboxwire.NewID()
+	// A Process handler reports its sequence once its stream's first byte
+	// arrives, so the test decides which handler runs first.
+	seq := func(s sandboxlink.Stream) uint64 {
+		t.Helper()
+		if _, err := s.Write([]byte{0}); err != nil {
+			t.Fatal(err)
+		}
+		return recv(t, p.seqs)
+	}
+	first, _ := f.mustOpen(sandboxlink.ServiceProcess, attachment, 1)
+	second, _ := f.mustOpen(sandboxlink.ServiceProcess, attachment, 1)
+	later := seq(second)
+	earlier := seq(first)
+	recv(t, p.conns).Close()
+	recv(t, p.connected)
+	third, _ := f.mustOpen(sandboxlink.ServiceProcess, attachment, 1)
+	reconnected := seq(third)
+	if !(0 < earlier && earlier < later && later < reconnected) {
+		t.Fatalf("bind sequences %d, %d, then %d after a reconnect; want them to increase from above zero", earlier, later, reconnected)
+	}
+}
diff --git a/internal/sandboxlink/serve.go b/internal/sandboxlink/serve.go
index 98964e07..726a008c 100644
--- a/internal/sandboxlink/serve.go
+++ b/internal/sandboxlink/serve.go
@@ -15,10 +15,17 @@ import (
 // ServiceHandler serves one service. Serve owns the stream and returns when
 // it is done with it. ctx ends when the attachment closes or Serve returns.
 // The Bind carries the authorized binding, including a File stream's exports.
+// seq is the stream's bind sequence. Bind order is the order in which the
+// serve peer assigns it, under the lock that tracks attachments and before it
+// answers Bound; concurrent Bound and Opened replies may reach the opener in
+// another order, and a handler that runs late keeps its stream's place. The
+// sequence belongs to one Serve call and survives reconnects; it increases
+// strictly but has gaps, since all attachments share it. A service may rely
+// on it to fence a stream's successor.
 type ServiceHandler struct {
 	Service Service
 	Version uint16
-	Serve   func(ctx context.Context, b Bind, s Stream)
+	Serve   func(ctx context.Context, b Bind, seq uint64, s Stream)
 }
 
 // ServeConfig configures a serve peer. Dial replaces the WebSocket dial to URL
@@ -119,6 +126,7 @@ type server struct {
 
 	mu          sync.Mutex
 	attachments map[sandboxwire.ID]*served
+	binds       uint64 // bind sequence of the last stream bound
 	// closedIDs holds recently closed attachment IDs until the time given. A
 	// Bind the relay sent before a close can reach bind after the close.
 	closedIDs map[sandboxwire.ID]time.Time
@@ -199,6 +207,7 @@ func (s *server) bind(ctx context.Context, st *yamux.Stream) {
 	}
 	var refuse Code
 	var a *served
+	var seq uint64
 	switch {
 	case err != nil || !ok:
 		refuse = ProtocolViolation
@@ -209,7 +218,7 @@ func (s *server) bind(ctx context.Context, st *yamux.Stream) {
 	case b.ExpectedServerInstanceID != s.cfg.ServerInstanceID:
 		refuse = InstanceChanged
 	default:
-		if a = s.track(ctx, b.AttachmentID); a == nil {
+		if a, seq = s.track(ctx, b.AttachmentID); a == nil {
 			refuse = LeaseExpired
 		}
 	}
@@ -228,16 +237,16 @@ func (s *server) bind(ctx context.Context, st *yamux.Stream) {
 		return
 	}
 	st.SetDeadline(time.Time{})
-	h.Serve(a.ctx, b, st)
+	h.Serve(a.ctx, b, seq, st)
 }
 
-// track counts a bound stream of attachment id. It returns nil for a recently
-// closed attachment.
-func (s *server) track(ctx context.Context, id sandboxwire.ID) *served {
+// track counts a bound stream of attachment id and returns its bind sequence.
+// It returns nil for a recently closed attachment.
+func (s *server) track(ctx context.Context, id sandboxwire.ID) (*served, uint64) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	if until, closed := s.closedIDs[id]; closed && time.Now().Before(until) {
-		return nil
+		return nil, 0
 	}
 	a := s.attachments[id]
 	if a == nil {
@@ -252,7 +261,8 @@ func (s *server) track(ctx context.Context, id sandboxwire.ID) *served {
 			s.cfg.OnAttachmentRestored(id)
 		}
 	}
-	return a
+	s.binds++
+	return a, s.binds
 }
 
 // release ends one bound stream of a. Only the current attachment of its ID
diff --git a/internal/sandboxlink/serve_test.go b/internal/sandboxlink/serve_test.go
index 70c23cdf..5c0d40bd 100644
--- a/internal/sandboxlink/serve_test.go
+++ b/internal/sandboxlink/serve_test.go
@@ -19,7 +19,7 @@ func TestCloseBeforeBind(t *testing.T) {
 	s := testServer(&lost)
 	id := sandboxwire.NewID()
 	s.closed(AttachmentClosed{AttachmentID: id, Reason: CloseRevoked})
-	if a := s.track(context.Background(), id); a != nil {
+	if a, _ := s.track(context.Background(), id); a != nil {
 		t.Fatal("a Bind after AttachmentClosed was tracked")
 	}
 }
@@ -30,10 +30,13 @@ func TestReleaseOfClosedAttachment(t *testing.T) {
 	var lost []sandboxwire.ID
 	s := testServer(&lost)
 	id := sandboxwire.NewID()
-	old := s.track(context.Background(), id)
+	old, oldSeq := s.track(context.Background(), id)
 	s.closed(AttachmentClosed{AttachmentID: id, Reason: CloseRequested})
 	s.closedIDs[id] = time.Now() // let the tombstone lapse
-	current := s.track(context.Background(), id)
+	current, seq := s.track(context.Background(), id)
+	if seq <= oldSeq {
+		t.Fatalf("bind sequence %d after %d; want it to increase", seq, oldSeq)
+	}
 	s.release(old)
 	if current.streams != 1 || len(lost) != 0 {
 		t.Fatalf("after the old stream ended: %d streams, lost %v; want 1 stream and none lost", current.streams, lost)