diff --git a/ipn/ipnlocal/local.go b/ipn/ipnlocal/local.go
index 53e98e33e..798e43499 100644
--- a/ipn/ipnlocal/local.go
+++ b/ipn/ipnlocal/local.go
@@ -1150,14 +1150,28 @@ func (b *LocalBackend) shouldPauseControlClientLocked(prefs ipn.PrefsView) bool
 	return false
 }
 
-// DisconnectControl shuts down control client. This can be run before node shutdown to force control to consider this ndoe
-// inactive. This can be used to ensure that nodes that are HA subnet router or app connector replicas are shutting
-// down, clients switch over to other replicas whilst the existing connections are kept alive for some period of time.
+// DisconnectControl shuts down the control client. This can be run before
+// node shutdown to force control to consider this node inactive. This can
+// be used to ensure that nodes that are HA subnet router or app connector
+// replicas are shutting down, clients switch over to other replicas whilst
+// the existing connections are kept alive for some period of time.
+//
+// Shutdown of the detached client is synchronous: it cancels the client's
+// in-flight requests and waits for its goroutines to exit, but it does not
+// wait for pending updates to be delivered.
 func (b *LocalBackend) DisconnectControl() {
 	b.mu.Lock()
 	cc := b.resetControlClientLocked()
 	b.mu.Unlock()
 
+	// The Shutdown call must not run while b.mu is held, per the deadlock
+	// history in tailscale/tailscale#18052: controlclient.Auto's
+	// goroutines deliver callbacks into LocalBackend through an execqueue
+	// whose RunSync holds the queue mutex while the callback acquires
+	// b.mu, and Auto.Shutdown acquires that same queue mutex, so calling
+	// it with b.mu held inverts the lock order. A previous attempt to
+	// shut the client down synchronously inside resetControlClientLocked
+	// with b.mu held (#18127) deadlocked and was reverted (#18149).
 	if cc != nil {
 		cc.Shutdown()
 	}
@@ -3004,6 +3018,23 @@ func (b *LocalBackend) controlDebugFlags() []string {
 func (b *LocalBackend) Start(opts ipn.Options) error {
 	defer b.CheckDeadlocks()()
 
+	// Shut down the previous control client, if any, before starting a
+	// new one, so the old client can't race with the new one. Without
+	// this, an in-flight lite map update carrying stale Hostinfo (notably
+	// RequestTags) could be processed by the control plane after the new
+	// client's requests, which made retagging with "tailscale up
+	// --advertise-tags" intermittently look like an invalid tag
+	// transition and log the node out (tailscale/tailscale#20365).
+	//
+	// TODO(bradfitz,nickkhyl): this is still racy if Start is called
+	// concurrently: whichever call loses the race to reacquire b.mu
+	// below then detaches the winner's new control client in startLocked
+	// and shuts it down in a goroutine, without the ordering guarantee
+	// that this call provides. This is a workaround until #18052 is
+	// properly fixed and a control client can be shut down synchronously
+	// with b.mu held.
+	b.DisconnectControl()
+
 	b.mu.Lock()
 	defer b.mu.Unlock()
 	return b.startLocked(opts)
diff --git a/ipn/ipnlocal/state_test.go b/ipn/ipnlocal/state_test.go
index 7ce4f590c..e768c70c5 100644
--- a/ipn/ipnlocal/state_test.go
+++ b/ipn/ipnlocal/state_test.go
@@ -352,6 +352,39 @@ func (b *LocalBackend) nonInteractiveLoginForStateTest() {
 	cc.Login(b.loginFlags | controlclient.LoginInteractive)
 }
 
+// TestStartShutsDownPreviousControlClient verifies that Start waits for the
+// previous control client to fully shut down before creating a new one.
+//
+// If the old client is still alive when the new one starts, its in-flight
+// requests (carrying stale Hostinfo, notably RequestTags) can race with the
+// new client's requests at the control plane. That made retagging a node
+// with "tailscale up --advertise-tags" intermittently log the node out
+// (tailscale/tailscale#20365): a stale RequestTags update processed after
+// the tag transition looks like an invalid transition, so the control
+// server expires the node key.
+func TestStartShutsDownPreviousControlClient(t *testing.T) {
+	const enableLogging = true
+	var cc *mockControl
+	b := newLocalBackendWithTestControl(t, enableLogging, func(tb testing.TB, opts controlclient.Options) controlclient.Client {
+		if cc != nil {
+			select {
+			case <-cc.shutdown:
+			default:
+				t.Errorf("new control client created before the previous one was shut down")
+			}
+		}
+		cc = newClient(t, opts)
+		return cc
+	})
+
+	for i := range 3 {
+		t.Logf("Start %d", i+1)
+		if err := b.Start(ipn.Options{}); err != nil {
+			t.Fatalf("Start: %v", err)
+		}
+	}
+}
+
 // A very precise test of the sequence of function calls generated by
 // ipnlocal.Local into its controlclient instance, and the events it
 // produces upstream into the UI.
diff --git a/tstest/integration/integration_test.go b/tstest/integration/integration_test.go
index cabb50cc7..192ecdd12 100644
--- a/tstest/integration/integration_test.go
+++ b/tstest/integration/integration_test.go
@@ -532,6 +532,138 @@ func TestOneNodeUpAuth(t *testing.T) {
 	}
 }
 
+// TestRetagStaleMapRequestRace reproduces tailscale/tailscale#20365: a node
+// tagged tag:tag1, where tag:tag1 owns tag:tag2, is retagged with "tailscale
+// up --advertise-tags=tag:tag2". This should always succeed, but sometimes
+// the machine is logged out instead.
+//
+// The cause is a race: "tailscale up" makes LocalBackend.Start shut down the
+// old control client asynchronously while the new one starts, so a lite map
+// update carrying the old Hostinfo.RequestTags can still be in flight when
+// the new client's requests retag the node. If control processes the stale
+// update after the tag transition, it looks like a request to change the
+// node's tags from tag:tag2 back to tag:tag1. That fails the tag ownership
+// check (tag:tag2 doesn't own tag:tag1), and control expires the node's key
+// to force reauthentication, logging the machine out.
+//
+// The test recreates that interleaving deterministically: it triggers a lite
+// map update carrying the old tags (any routine hostinfo change does that),
+// holds it at the server, retags the node, and only then lets the held
+// update be processed.
+func TestRetagStaleMapRequestRace(t *testing.T) {
+	tstest.Parallel(t)
+
+	var (
+		holdStale    atomic.Bool
+		staleHeld    = make(chan struct{}, 1)
+		staleRelease = make(chan struct{})
+		staleDone    = make(chan struct{}, 1)
+	)
+	env := NewTestEnv(t, ConfigureControl(func(control *testcontrol.Server) {
+		control.TagOwners = map[string][]string{
+			"tag:tag1": nil,
+			"tag:tag2": {"tag:tag1"},
+		}
+		control.HoldMapRequest = func(req *tailcfg.MapRequest) (done func()) {
+			if req.Stream || req.Hostinfo == nil || !holdStale.Load() {
+				return nil
+			}
+			if !slices.Equal(req.Hostinfo.RequestTags, []string{"tag:tag1"}) {
+				return nil
+			}
+			select {
+			case staleHeld <- struct{}{}:
+			default:
+			}
+			<-staleRelease
+			return func() {
+				select {
+				case staleDone <- struct{}{}:
+				default:
+				}
+			}
+		}
+	}))
+
+	n1 := NewTestNode(t, env)
+	d1 := n1.StartDaemon()
+	defer d1.MustCleanShutdown(t)
+	n1.AwaitResponding()
+	n1.MustUp("--advertise-tags=tag:tag1")
+	n1.AwaitRunning()
+
+	nodes := env.Control.AllNodes()
+	if len(nodes) != 1 {
+		t.Fatalf("expected 1 node, got %d", len(nodes))
+	}
+	origKey := nodes[0].Key
+	if got, want := nodes[0].Tags, []string{"tag:tag1"}; !slices.Equal(got, want) {
+		t.Fatalf("node tags = %v; want %v", got, want)
+	}
+
+	// Make the current control client send a lite map update carrying the
+	// old RequestTags, as any routine hostinfo change does. Control holds
+	// it (per HoldMapRequest above) so that it's still outstanding when
+	// the node is retagged below. Shutting down that control client
+	// cancels the request but can't unsend it; control still has it.
+	holdStale.Store(true)
+	if out, err := n1.Tailscale("set", "--hostname=retag-race-test").CombinedOutput(); err != nil {
+		t.Fatalf("tailscale set: %v, %s", err, out)
+	}
+	select {
+	case <-staleHeld:
+	case <-time.After(10 * time.Second):
+		t.Fatal("timed out waiting for the stale map update to reach control")
+	}
+
+	// Retag the node while the stale update is still outstanding. This
+	// doesn't block on the held update: the new control client's requests
+	// are processed while it is held.
+	n1.MustUp("--advertise-tags=tag:tag2")
+	if err := tstest.WaitFor(10*time.Second, func() error {
+		n := env.Control.Node(origKey)
+		if n == nil {
+			return fmt.Errorf("node %v not found in control", origKey.ShortString())
+		}
+		if !slices.Equal(n.Tags, []string{"tag:tag2"}) {
+			return fmt.Errorf("node tags = %v; want [tag:tag2]", n.Tags)
+		}
+		return nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	// Let control process the stale update, now that the retag has been
+	// committed, and wait for it to finish.
+	close(staleRelease)
+	select {
+	case <-staleDone:
+	case <-time.After(10 * time.Second):
+		t.Fatal("timed out waiting for control to process the stale map update")
+	}
+
+	// The retag should stick, and the machine should stay logged in with
+	// the same node key. With the bug present, control instead expires the
+	// node key when it processes the stale update.
+	n := env.Control.Node(origKey)
+	if n == nil {
+		t.Fatalf("node %v disappeared from control", origKey.ShortString())
+	}
+	if !n.KeyExpiry.IsZero() {
+		t.Fatalf("control expired the node key after processing a stale map update; the machine was logged out (issue 20365)")
+	}
+	if got, want := n.Tags, []string{"tag:tag2"}; !slices.Equal(got, want) {
+		t.Fatalf("node tags = %v; want %v", got, want)
+	}
+	st := n1.MustStatus()
+	if st.BackendState != "Running" {
+		t.Errorf("BackendState = %q; want Running", st.BackendState)
+	}
+	if st.Self.PublicKey != origKey {
+		t.Errorf("node key changed from %v to %v; the machine was logged out and re-registered", origKey.ShortString(), st.Self.PublicKey.ShortString())
+	}
+}
+
 // Returns true if the error returned by [exec.Run] fails with a non-zero
 // exit code, false otherwise.
 func isNonZeroExitCode(err error) bool {
diff --git a/tstest/integration/testcontrol/testcontrol.go b/tstest/integration/testcontrol/testcontrol.go
index 0ad335ae3..4545f23ec 100644
--- a/tstest/integration/testcontrol/testcontrol.go
+++ b/tstest/integration/testcontrol/testcontrol.go
@@ -70,6 +70,29 @@ type Server struct {
 	// belong to the same user.
 	AllNodesSameUser bool
 
+	// TagOwners, if non-nil, enables modeling of the production control
+	// server's tag transition handling. Map keys are tags (e.g. "tag:foo")
+	// and values are the tags whose nodes are allowed to assign the key's
+	// tag. A node registering with Hostinfo.RequestTags gets those tags
+	// (signup-time ownership checks are not modeled). A later non-streaming
+	// map request whose Hostinfo.RequestTags differ from both the node's
+	// stored Hostinfo's RequestTags and its current tags is treated as a
+	// tag transition request: if the node's current tags own each requested
+	// tag, the node is retagged; otherwise its node key is expired to force
+	// reauthentication, as the production control server does.
+	//
+	// If nil, RequestTags in map requests are ignored.
+	TagOwners map[string][]string
+
+	// HoldMapRequest, if non-nil, is called with each incoming MapRequest
+	// before the server starts processing it. It may block to delay
+	// processing, letting tests control the order in which concurrent map
+	// requests are handled. If it returns a non-nil func, the server calls
+	// it when it finishes handling the request. For streaming requests
+	// that is when the poll ends, so returning a done func is mostly
+	// useful for non-streaming requests.
+	HoldMapRequest func(*tailcfg.MapRequest) (done func())
+
 	// AllOnline, if true, marks every peer entry in MapResponses as
 	// Online=true. This is a coarse stand-in for the per-node
 	// online/offline tracking that production control servers do based
@@ -1027,6 +1050,11 @@ func (s *Server) serveRegister(w http.ResponseWriter, r *http.Request, mkey key.
 			CapMap:            capMap,
 			Capabilities:      slices.Collect(maps.Keys(capMap)),
 		}
+		if s.TagOwners != nil && req.Hostinfo != nil {
+			// Trust the requested tags at signup; ownership checks
+			// against the registering user are not modeled.
+			node.Tags = slices.Clone(req.Hostinfo.RequestTags)
+		}
 		if s.MagicDNSDomain != "" {
 			node.Name = node.Name + "." + s.MagicDNSDomain + "."
 		}
@@ -1258,6 +1286,71 @@ func (s *Server) incrInServeMap(delta int) {
 	s.inServeMap += delta
 }
 
+// handleTagTransitionLocked models the production control server's handling
+// of tag changes requested via Hostinfo.RequestTags in non-streaming map
+// requests (see updateTags in the control server). A request whose
+// RequestTags differ from both the node's stored Hostinfo's RequestTags and
+// the node's current tags is a tag transition request. If the transition is
+// permitted by s.TagOwners, the node is retagged; otherwise its node key is
+// expired to force reauthentication.
+//
+// hi is the Hostinfo from the incoming request; node.Hostinfo is the
+// previously stored one. s.mu must be held.
+func (s *Server) handleTagTransitionLocked(node *tailcfg.Node, hi tailcfg.HostinfoView) {
+	var oldReqTags []string
+	if node.Hostinfo.Valid() {
+		oldReqTags = node.Hostinfo.RequestTags().AsSlice()
+	}
+	newReqTags := hi.RequestTags().AsSlice()
+	if tagsEqualAnyOrder(oldReqTags, newReqTags) || tagsEqualAnyOrder(newReqTags, node.Tags) {
+		return
+	}
+	if s.validTagTransition(node.Tags, newReqTags) {
+		s.logf("testcontrol: retagging node %v: %v -> %v", node.ID, node.Tags, newReqTags)
+		node.Tags = newReqTags
+	} else {
+		s.logf("testcontrol: invalid tag transition %v -> %v for node %v; expiring its node key", node.Tags, newReqTags, node.ID)
+		node.KeyExpiry = time.Now().Add(-time.Minute)
+	}
+}
+
+// validTagTransition reports whether a node currently tagged cur may retag
+// itself as want, per s.TagOwners: every requested tag must be owned by one
+// of the node's current tags. An untagged node may claim any defined tag,
+// standing in for the production server's checks against the requesting
+// user, which this server doesn't model. Removing all tags is not allowed,
+// matching the production server.
+func (s *Server) validTagTransition(cur, want []string) bool {
+	if len(want) == 0 {
+		return len(cur) == 0
+	}
+	for _, tag := range want {
+		owners, ok := s.TagOwners[tag]
+		if !ok {
+			return false
+		}
+		if len(cur) == 0 {
+			continue
+		}
+		owned := slices.ContainsFunc(cur, func(c string) bool { return slices.Contains(owners, c) })
+		if !owned {
+			return false
+		}
+	}
+	return true
+}
+
+// tagsEqualAnyOrder reports whether a and b contain the same tags, ignoring order.
+func tagsEqualAnyOrder(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	as, bs := slices.Clone(a), slices.Clone(b)
+	slices.Sort(as)
+	slices.Sort(bs)
+	return slices.Equal(as, bs)
+}
+
 // InServeMap returns the number of clients currently in a MapRequest HTTP handler.
 func (s *Server) InServeMap() int {
 	s.mu.Lock()
@@ -1289,6 +1382,12 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 	}
 	s.mu.Unlock()
 
+	if s.HoldMapRequest != nil {
+		if done := s.HoldMapRequest(req); done != nil {
+			defer done()
+		}
+	}
+
 	if s.AltMapStream != nil {
 		// The caller takes over the stream entirely; it must handle
 		// keeping the HTTP response alive until ctx is done.
@@ -1320,6 +1419,16 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 	streamingNonUpdate := req.Stream && req.Version >= 68
 	var peersToUpdate []tailcfg.NodeID
 	if !req.ReadOnly && !streamingNonUpdate {
+		if ctx.Err() != nil {
+			// The client canceled the request (say, its control client
+			// was shut down mid-request when "tailscale up" or a
+			// profile switch created a new one), so its contents may
+			// predate newer requests that were already processed.
+			// Don't apply its Hostinfo/endpoints; they may be stale.
+			s.logf("testcontrol: dropping canceled map update from %v", req.NodeKey.ShortString())
+			http.Error(w, "request canceled", 400)
+			return
+		}
 		endpoints := filterInvalidIPv6Endpoints(req.Endpoints)
 		var hi tailcfg.HostinfoView
 		var newDERP int
@@ -1339,6 +1448,9 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 			live.DiscoKey = req.DiscoKey
 			live.Cap = req.Version
 			if hi.Valid() {
+				if s.TagOwners != nil {
+					s.handleTagTransitionLocked(live, hi)
+				}
 				live.Hostinfo = hi
 				if newDERP != 0 {
 					live.HomeDERP = newDERP