diff --git a/backend/internal/api/handlers_test.go b/backend/internal/api/handlers_test.go
index 1a4437d..a361c21 100644
--- a/backend/internal/api/handlers_test.go
+++ b/backend/internal/api/handlers_test.go
@@ -505,7 +505,10 @@ func TestApproveReviewPublishesToSubscriber(t *testing.T) {
 	}
 
 	// Fake SDK subscriber.
-	ch, dereg := hub.Register(sessID)
+	ch, dereg, err := hub.Register(sessID, "test-owner")
+	if err != nil {
+		t.Fatalf("Register: %v", err)
+	}
 	defer dereg()
 
 	resp := postJSON(t, srv, cookie, "/api/reviews/"+queueID+"/approve", map[string]any{})
diff --git a/backend/internal/ws/frames.go b/backend/internal/ws/frames.go
index f7ae604..23c1e91 100644
--- a/backend/internal/ws/frames.go
+++ b/backend/internal/ws/frames.go
@@ -21,6 +21,7 @@ const schemaVersion = 2
 // plus our application-specific 4xxx codes.
 const (
 	closeProtocolError     = 1002
+	closePolicyViolation   = 1008
 	closeInternalServerErr = 1011
 	closeQuotaExhausted    = 4003
 )
diff --git a/backend/internal/ws/handler.go b/backend/internal/ws/handler.go
index 29a16a5..6db577d 100644
--- a/backend/internal/ws/handler.go
+++ b/backend/internal/ws/handler.go
@@ -99,7 +99,25 @@ func serve(ctx context.Context, conn *websocket.Conn, sess *session, st *store.S
 	// verdicts and HITL resolutions land here, drained by the writer
 	// goroutine. The LoginAck is written directly inside handleLogin
 	// (single goroutine, pre-register, no concurrency to serialise).
-	hubCh, deregister := hub.Register(sess.sessionID)
+	hubCh, deregister, err := hub.Register(sess.sessionID, sess.routeOwner())
+	if err != nil {
+		if errors.Is(err, ErrSessionOwnerConflict) {
+			slog.WarnContext(ctx, "ws.session_owner_conflict",
+				"session_id", sess.sessionID,
+				"api_key_id", sess.apiKey.ID,
+				"route_owner", sess.routeOwner(),
+			)
+			closeWith(conn, closePolicyViolation, "session_id already active for another owner")
+			return
+		}
+		slog.ErrorContext(ctx, "ws.session_register_failed",
+			"error", err,
+			"session_id", sess.sessionID,
+			"api_key_id", sess.apiKey.ID,
+		)
+		closeWith(conn, closeInternalServerErr, "internal error")
+		return
+	}
 	writerDone := make(chan struct{})
 	go func() {
 		defer close(writerDone)
diff --git a/backend/internal/ws/handler_test.go b/backend/internal/ws/handler_test.go
index a4f9142..4cdf544 100644
--- a/backend/internal/ws/handler_test.go
+++ b/backend/internal/ws/handler_test.go
@@ -235,6 +235,97 @@ func TestAlertModeNoFanOut(t *testing.T) {
 	}
 }
 
+func TestSessionIDReuseDifferentOwnerDoesNotStealVerdicts(t *testing.T) {
+	db := openInMemoryDB(t)
+	t.Cleanup(func() { _ = db.Close() })
+
+	st := store.New(db)
+	plaintextKeyA := "adr_local_test_key_owner_a"
+	plaintextKeyB := "adr_local_test_key_owner_b"
+	insertAPIKeyWithProfile(t, db, sha256Hex(plaintextKeyA), "agent-profile-a")
+	insertAPIKeyWithProfile(t, db, sha256Hex(plaintextKeyB), "agent-profile-b")
+
+	if _, err := db.Exec(`UPDATE policies SET mode = 'block' WHERE id = 1`); err != nil {
+		t.Fatalf("set mode=block: %v", err)
+	}
+
+	mux := http.NewServeMux()
+	mux.Handle("/ws", ws.AuthMiddleware(st)(ws.NewHandler(st, &fakeClassifier{}, ws.NewHub(), nil, nil)))
+	srv := httptest.NewServer(mux)
+	t.Cleanup(srv.Close)
+
+	wsURL := "ws" + strings.TrimPrefix(srv.URL, "http") + "/ws"
+	connA, _, err := websocket.DefaultDialer.Dial(wsURL, http.Header{
+		"Authorization": {"Bearer " + plaintextKeyA},
+	})
+	if err != nil {
+		t.Fatalf("dial client A: %v", err)
+	}
+	t.Cleanup(func() { _ = connA.Close() })
+
+	const sessionID = "shared-session-takeover-test"
+	if err := writeProto(connA, &bpb.ClientFrame{
+		Frame: &bpb.ClientFrame_Login{Login: &bpb.SessionLogin{
+			SessionId: sessionID, SchemaVersion: 2,
+		}},
+	}); err != nil {
+		t.Fatalf("send login client A: %v", err)
+	}
+	if _, err := readServerFrame(connA); err != nil {
+		t.Fatalf("read login_ack client A: %v", err)
+	}
+
+	connB, _, err := websocket.DefaultDialer.Dial(wsURL, http.Header{
+		"Authorization": {"Bearer " + plaintextKeyB},
+	})
+	if err != nil {
+		t.Fatalf("dial client B: %v", err)
+	}
+	t.Cleanup(func() { _ = connB.Close() })
+	if err := writeProto(connB, &bpb.ClientFrame{
+		Frame: &bpb.ClientFrame_Login{Login: &bpb.SessionLogin{
+			SessionId: sessionID, SchemaVersion: 2,
+		}},
+	}); err != nil {
+		t.Fatalf("send login client B: %v", err)
+	}
+	if _, err := readServerFrame(connB); err != nil {
+		t.Fatalf("read login_ack client B: %v", err)
+	}
+	if err := connB.SetReadDeadline(time.Now().Add(2 * time.Second)); err != nil {
+		t.Fatalf("set client B deadline: %v", err)
+	}
+	if _, _, err := connB.ReadMessage(); err == nil {
+		t.Fatal("expected conflicting client B to be closed")
+	} else if closeErr, ok := err.(*websocket.CloseError); !ok || closeErr.Code != websocket.ClosePolicyViolation {
+		t.Fatalf("client B close err = %v, want close code %d", err, websocket.ClosePolicyViolation)
+	}
+
+	eventID := uuid.NewString()
+	if err := writeProto(connA, &bpb.ClientFrame{
+		Frame: &bpb.ClientFrame_PairedBatch{PairedBatch: &bpb.PairedEventBatch{
+			Events: []*bpb.PairedEvent{{
+				EventId: eventID, SessionId: sessionID,
+				PairType: bpb.PairType_PAIR_TYPE_TOOL,
+				Agent:    &bpb.AgentContext{AgentId: "owner-a-agent"},
+				Data: &bpb.PairedEvent_Tool{Tool: &bpb.ToolPairData{
+					ToolName: "noop", ToolCallId: "tc-owner-a", Input: "{}", Output: "ok",
+				}},
+			}},
+		}},
+	}); err != nil {
+		t.Fatalf("send paired_batch client A: %v", err)
+	}
+
+	verdict, err := readServerFrame(connA)
+	if err != nil {
+		t.Fatalf("read verdict client A: %v", err)
+	}
+	if got := verdict.GetVerdict(); got == nil || got.EventId != eventID {
+		t.Fatalf("client A verdict = %+v, want event_id %q", got, eventID)
+	}
+}
+
 // TestRevokeKicksLiveWS asserts the security guarantee: an open WS
 // authenticated with key X gets terminated within seconds when X is
 // revoked, not at next-disconnect-whenever. Drives the path the REST
@@ -403,6 +494,17 @@ func insertAPIKey(t *testing.T, db *sql.DB, hashHex string) {
 	}
 }
 
+func insertAPIKeyWithProfile(t *testing.T, db *sql.DB, hashHex, agentProfileID string) {
+	t.Helper()
+	_, err := db.Exec(
+		`INSERT INTO api_keys (id, key_hash, prefix, label, agent_profile_id) VALUES (?, ?, ?, ?, ?)`,
+		uuid.NewString(), hashHex, "adr_local_te", "test", agentProfileID,
+	)
+	if err != nil {
+		t.Fatalf("insert api_keys: %v", err)
+	}
+}
+
 func writeProto(conn *websocket.Conn, msg proto.Message) error {
 	buf, err := proto.Marshal(msg)
 	if err != nil {
diff --git a/backend/internal/ws/hub.go b/backend/internal/ws/hub.go
index fe2c129..7bf5d86 100644
--- a/backend/internal/ws/hub.go
+++ b/backend/internal/ws/hub.go
@@ -4,6 +4,7 @@
 package ws
 
 import (
+	"errors"
 	"sync"
 
 	"google.golang.org/protobuf/proto"
@@ -11,40 +12,53 @@ import (
 	pb "github.com/secureagentics/Adrian/backend/internal/proto"
 )
 
+// ErrSessionOwnerConflict is returned when another logical client owner
+// already holds the subscriber slot for a session_id.
+var ErrSessionOwnerConflict = errors.New("session_id already registered by another owner")
+
+type subscriber struct {
+	owner string
+	ch    chan []byte
+}
+
 // Hub is a process-local pub/sub keyed by session_id. The WS handler
 // for each connected SDK registers a write channel; the REST review
 // approve/reject path publishes a HITL-resolution Verdict frame to it.
 //
-// Single subscriber per session_id, re-Register replaces any prior
-// channel (covers SDK reconnect during a hold). On disconnect the WS
-// handler must call deregister to free the slot.
+// Single subscriber per session_id. Re-register by the same server-derived
+// owner replaces the prior channel (SDK reconnect / key rotation). A
+// different owner claiming the same session_id is rejected so it cannot steal
+// verdict or HITL routing.
 type Hub struct {
 	mu   sync.Mutex
-	subs map[string]chan []byte
+	subs map[string]subscriber
 }
 
 // NewHub returns a fresh hub.
 func NewHub() *Hub {
-	return &Hub{subs: make(map[string]chan []byte)}
+	return &Hub{subs: make(map[string]subscriber)}
 }
 
-// Register adds a subscriber for sessionID and returns its write
+// Register adds a subscriber for sessionID + owner and returns its write
 // channel plus a deregister callback. The caller spawns a writer
 // goroutine that drains the channel and calls conn.WriteMessage.
 //
-// If a prior subscriber exists for the same session_id, its channel
-// is closed so its writer goroutine exits cleanly. (Concurrent
-// connections under one session_id are not a normal case; an SDK
-// reconnect during a hold is the realistic path.)
-func (h *Hub) Register(sessionID string) (<-chan []byte, func()) {
+// If a prior subscriber exists for the same session_id and owner, its channel
+// is closed so its writer goroutine exits cleanly. If the existing subscriber
+// belongs to another owner, registration fails and the old channel remains
+// active.
+func (h *Hub) Register(sessionID, owner string) (<-chan []byte, func(), error) {
 	h.mu.Lock()
 	defer h.mu.Unlock()
 
 	if old, ok := h.subs[sessionID]; ok {
-		close(old)
+		if old.owner != owner {
+			return nil, nil, ErrSessionOwnerConflict
+		}
+		close(old.ch)
 	}
 	ch := make(chan []byte, 8)
-	h.subs[sessionID] = ch
+	h.subs[sessionID] = subscriber{owner: owner, ch: ch}
 
 	deregister := func() {
 		h.mu.Lock()
@@ -52,12 +66,12 @@ func (h *Hub) Register(sessionID string) (<-chan []byte, func()) {
 		// Only delete + close when the entry is still ours; a later
 		// Register may have replaced it and already closed the prior
 		// channel.
-		if cur, ok := h.subs[sessionID]; ok && cur == ch {
+		if cur, ok := h.subs[sessionID]; ok && cur.ch == ch {
 			delete(h.subs, sessionID)
 			close(ch)
 		}
 	}
-	return ch, deregister
+	return ch, deregister, nil
 }
 
 // Publish marshals and pushes a frame to the subscriber for sessionID.
@@ -70,13 +84,13 @@ func (h *Hub) Publish(sessionID string, frame *pb.ServerFrame) bool {
 		return false
 	}
 	h.mu.Lock()
-	ch, ok := h.subs[sessionID]
-	h.mu.Unlock()
+	defer h.mu.Unlock()
+	sub, ok := h.subs[sessionID]
 	if !ok {
 		return false
 	}
 	select {
-	case ch <- buf:
+	case sub.ch <- buf:
 		return true
 	default:
 		return false
diff --git a/backend/internal/ws/hub_test.go b/backend/internal/ws/hub_test.go
index 3c33a39..17a5c18 100644
--- a/backend/internal/ws/hub_test.go
+++ b/backend/internal/ws/hub_test.go
@@ -4,6 +4,7 @@
 package ws
 
 import (
+	"errors"
 	"testing"
 
 	"google.golang.org/protobuf/proto"
@@ -13,7 +14,10 @@ import (
 
 func TestHubPublishDeliversToSubscriber(t *testing.T) {
 	h := NewHub()
-	ch, dereg := h.Register("sess-1")
+	ch, dereg, err := h.Register("sess-1", "owner-1")
+	if err != nil {
+		t.Fatalf("Register: %v", err)
+	}
 	defer dereg()
 
 	frame := &pb.ServerFrame{Frame: &pb.ServerFrame_Verdict{Verdict: &pb.Verdict{
@@ -45,11 +49,17 @@ func TestHubPublishNoSubscriberReturnsFalse(t *testing.T) {
 
 func TestHubReRegisterClosesPriorChannel(t *testing.T) {
 	h := NewHub()
-	first, _ := h.Register("sess-x")
+	first, _, err := h.Register("sess-x", "owner-1")
+	if err != nil {
+		t.Fatalf("first Register: %v", err)
+	}
 
 	// New register replaces the slot; the old channel must close so a
 	// writer goroutine reading from it exits cleanly.
-	second, dereg := h.Register("sess-x")
+	second, dereg, err := h.Register("sess-x", "owner-1")
+	if err != nil {
+		t.Fatalf("second Register: %v", err)
+	}
 	defer dereg()
 
 	if _, ok := <-first; ok {
@@ -67,9 +77,39 @@ func TestHubReRegisterClosesPriorChannel(t *testing.T) {
 	}
 }
 
+func TestHubRejectsReRegisterFromDifferentOwner(t *testing.T) {
+	h := NewHub()
+	first, dereg, err := h.Register("sess-x", "owner-1")
+	if err != nil {
+		t.Fatalf("first Register: %v", err)
+	}
+	defer dereg()
+
+	second, secondDereg, err := h.Register("sess-x", "owner-2")
+	if !errors.Is(err, ErrSessionOwnerConflict) {
+		t.Fatalf("Register err = %v, want ErrSessionOwnerConflict", err)
+	}
+	if second != nil || secondDereg != nil {
+		t.Fatal("conflicting Register returned a subscriber")
+	}
+
+	if !h.Publish("sess-x", &pb.ServerFrame{
+		Frame: &pb.ServerFrame_Verdict{Verdict: &pb.Verdict{EventId: "ev-y"}},
+	}) {
+		t.Fatal("Publish to original subscriber should still succeed")
+	}
+	got := <-first
+	if len(got) == 0 {
+		t.Fatal("expected non-empty frame delivered to original subscriber")
+	}
+}
+
 func TestHubDeregisterRemovesEntry(t *testing.T) {
 	h := NewHub()
-	_, dereg := h.Register("sess-d")
+	_, dereg, err := h.Register("sess-d", "owner-1")
+	if err != nil {
+		t.Fatalf("Register: %v", err)
+	}
 	dereg()
 	if h.Publish("sess-d", &pb.ServerFrame{
 		Frame: &pb.ServerFrame_Verdict{Verdict: &pb.Verdict{}},
diff --git a/backend/internal/ws/session.go b/backend/internal/ws/session.go
index f10ed69..cb251a3 100644
--- a/backend/internal/ws/session.go
+++ b/backend/internal/ws/session.go
@@ -25,3 +25,16 @@ func (s *session) agentProfileID() *string {
 	}
 	return s.apiKey.AgentProfileID
 }
+
+// routeOwner returns the server-authenticated logical owner for hub routing.
+// Agent-profile keys may rotate, so profile ownership is preferred over raw
+// key ID to preserve reconnect continuity. Unprofiled keys fall back to key ID.
+func (s *session) routeOwner() string {
+	if s.apiKey == nil {
+		return ""
+	}
+	if s.apiKey.AgentProfileID != nil && *s.apiKey.AgentProfileID != "" {
+		return "agent_profile:" + *s.apiKey.AgentProfileID
+	}
+	return "api_key:" + s.apiKey.ID
+}