diff --git a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/DebugView.razor b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/DebugView.razor
index 8280a50a..7a98209b 100644
--- a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/DebugView.razor
+++ b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/DebugView.razor
@@ -445,6 +445,18 @@
                     });
                 });
 
+            // M2.11: the site returns InstanceNotFound=true when the instance is
+            // not deployed there (e.g. deployment not yet pushed, or wrong site).
+            if (session.InitialSnapshot.InstanceNotFound)
+            {
+                DebugStreamService.StopStream(session.SessionId);
+                _toast.ShowError(
+                    $"Instance is not deployed on that site. " +
+                    $"Deploy it first or choose the correct site.");
+                _connecting = false;
+                return;
+            }
+
             _session = session;
 
             // Populate initial state from snapshot
diff --git a/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/DebugView/DebugViewSnapshot.cs b/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/DebugView/DebugViewSnapshot.cs
index f92a90cd..ba1f8695 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/DebugView/DebugViewSnapshot.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Commons/Messages/DebugView/DebugViewSnapshot.cs
@@ -2,8 +2,38 @@ using ZB.MOM.WW.ScadaBridge.Commons.Messages.Streaming;
 
 namespace ZB.MOM.WW.ScadaBridge.Commons.Messages.DebugView;
 
+/// <summary>
+/// Snapshot of an instance's debug state returned in response to a
+/// <see cref="DebugSnapshotRequest"/> or <see cref="SubscribeDebugViewRequest"/>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Additive-only contract (M2.11):</b> <see cref="InstanceNotFound"/> is an
+/// optional trailing parameter with a default of <see langword="false"/> so every
+/// existing positional constructor call and every existing serialized wire frame
+/// remains valid.  Callers that receive a snapshot with
+/// <c>InstanceNotFound = true</c> know the instance was unknown on the site and
+/// should distinguish that from a deployed-but-empty instance
+/// (<c>InstanceNotFound = false</c>, empty <see cref="AttributeValues"/> and
+/// <see cref="AlarmStates"/>).
+/// </para>
+/// <para>
+/// A new dedicated message type (<c>DebugViewInstanceNotFound</c>) was
+/// considered but rejected: the ClusterClient / ClusterClientReceptionist
+/// channel is typed on the request side and the bridge actor is already
+/// pattern-matching on <c>DebugViewSnapshot</c> for the initial-snapshot TCS
+/// in <c>DebugStreamService</c>.  Introducing a second reply type would require
+/// every consumer to handle an additional <c>Ask</c> result union — more change
+/// for no additive-safety gain.  The defaulted field is strictly additive and
+/// keeps all call sites untouched.
+/// </para>
+/// </remarks>
 public record DebugViewSnapshot(
     string InstanceUniqueName,
     IReadOnlyList<AttributeValueChanged> AttributeValues,
     IReadOnlyList<AlarmStateChanged> AlarmStates,
-    DateTimeOffset SnapshotTimestamp);
+    DateTimeOffset SnapshotTimestamp,
+    // M2.11 — additive field: true when the requested instance is not registered
+    // on this site.  Defaults to false so all existing call sites and wire
+    // frames are unaffected.
+    bool InstanceNotFound = false);
diff --git a/src/ZB.MOM.WW.ScadaBridge.Communication/Actors/DebugStreamBridgeActor.cs b/src/ZB.MOM.WW.ScadaBridge.Communication/Actors/DebugStreamBridgeActor.cs
index c4474739..7ac30fc1 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Communication/Actors/DebugStreamBridgeActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Communication/Actors/DebugStreamBridgeActor.cs
@@ -85,9 +85,23 @@ public class DebugStreamBridgeActor : ReceiveActor, IWithTimers
         _grpcNodeAAddress = grpcNodeAAddress;
         _grpcNodeBAddress = grpcNodeBAddress;
 
-        // Initial snapshot response from the site (via ClusterClient)
+        // Initial snapshot response from the site (via ClusterClient).
+        // M2.11: if the site reports InstanceNotFound=true the instance is not
+        // deployed there.  Forward the snapshot (with InstanceNotFound=true) to
+        // _onEvent so DebugStreamService's TCS resolves and the caller can
+        // inspect the flag; then stop cleanly without opening a gRPC stream.
         Receive<DebugViewSnapshot>(snapshot =>
         {
+            if (snapshot.InstanceNotFound)
+            {
+                _log.Warning("Instance {0} is not deployed on site; terminating debug stream",
+                    _instanceUniqueName);
+                _stopped = true;
+                _onEvent(snapshot); // resolves the snapshot TCS with InstanceNotFound=true
+                Context.Stop(Self);
+                return;
+            }
+
             _log.Info("Received initial snapshot for {0} ({1} attrs, {2} alarms)",
                 _instanceUniqueName, snapshot.AttributeValues.Count, snapshot.AlarmStates.Count);
             _onEvent(snapshot);
diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
index 10a6aae6..d412572e 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
@@ -895,11 +895,14 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
         }
         else
         {
+            // M2.11: set InstanceNotFound=true so the caller can distinguish
+            // "not deployed on this site" from a deployed-but-empty instance.
             _logger.LogWarning(
                 "Debug view subscribe for unknown instance {Instance}", request.InstanceUniqueName);
             Sender.Tell(new DebugViewSnapshot(
                 request.InstanceUniqueName, Array.Empty<Commons.Messages.Streaming.AttributeValueChanged>(),
-                Array.Empty<Commons.Messages.Streaming.AlarmStateChanged>(), DateTimeOffset.UtcNow));
+                Array.Empty<Commons.Messages.Streaming.AlarmStateChanged>(), DateTimeOffset.UtcNow,
+                InstanceNotFound: true));
         }
     }
 
@@ -919,11 +922,14 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
         }
         else
         {
+            // M2.11: set InstanceNotFound=true so the caller can distinguish
+            // "not deployed on this site" from a deployed-but-empty instance.
             _logger.LogWarning(
                 "Debug snapshot for unknown instance {Instance}", request.InstanceUniqueName);
             Sender.Tell(new DebugViewSnapshot(
                 request.InstanceUniqueName, Array.Empty<Commons.Messages.Streaming.AttributeValueChanged>(),
-                Array.Empty<Commons.Messages.Streaming.AlarmStateChanged>(), DateTimeOffset.UtcNow));
+                Array.Empty<Commons.Messages.Streaming.AlarmStateChanged>(), DateTimeOffset.UtcNow,
+                InstanceNotFound: true));
         }
     }
 
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs
index 6b57e936..062592af 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs
@@ -3,6 +3,7 @@ using Akka.TestKit.Xunit2;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.Deployment;
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.DebugView;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.InboundApi;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.Lifecycle;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
@@ -387,4 +388,64 @@ public class DeploymentManagerActorTests : TestKit, IDisposable
         Assert.Equal("route-corr-2", response.CorrelationId);
         Assert.True(response.Success, $"Routed call failed: {response.ErrorMessage}");
     }
+
+    // ── M2.11: Debug-view routing — unknown-instance not-found signal ──
+
+    [Fact]
+    public async Task RouteDebugSnapshot_UnknownInstance_SetsInstanceNotFound()
+    {
+        // An instance that was never deployed to this site must return a
+        // DebugViewSnapshot with InstanceNotFound=true so the caller can
+        // distinguish "not deployed here" from a deployed-but-empty instance.
+        var actor = CreateDeploymentManager();
+        await Task.Delay(500);
+
+        actor.Tell(new DebugSnapshotRequest("NeverDeployed", "snap-corr-1"));
+
+        var snapshot = ExpectMsg<DebugViewSnapshot>(TimeSpan.FromSeconds(5));
+        Assert.Equal("NeverDeployed", snapshot.InstanceUniqueName);
+        Assert.True(snapshot.InstanceNotFound,
+            "Expected InstanceNotFound=true for an instance not registered on this site.");
+        Assert.Empty(snapshot.AttributeValues);
+        Assert.Empty(snapshot.AlarmStates);
+    }
+
+    [Fact]
+    public async Task RouteDebugViewSubscribe_UnknownInstance_SetsInstanceNotFound()
+    {
+        // Same contract for the subscribe path: unknown instance → InstanceNotFound=true.
+        var actor = CreateDeploymentManager();
+        await Task.Delay(500);
+
+        actor.Tell(new SubscribeDebugViewRequest("NeverDeployed2", "sub-corr-1"));
+
+        var snapshot = ExpectMsg<DebugViewSnapshot>(TimeSpan.FromSeconds(5));
+        Assert.Equal("NeverDeployed2", snapshot.InstanceUniqueName);
+        Assert.True(snapshot.InstanceNotFound,
+            "Expected InstanceNotFound=true for an instance not registered on this site.");
+        Assert.Empty(snapshot.AttributeValues);
+        Assert.Empty(snapshot.AlarmStates);
+    }
+
+    [Fact]
+    public async Task RouteDebugSnapshot_KnownButEmptyInstance_DoesNotSetInstanceNotFound()
+    {
+        // A deployed instance with no runtime data yet must return an empty
+        // snapshot with InstanceNotFound=false — the known-empty path is unchanged.
+        var actor = CreateDeploymentManager();
+        await Task.Delay(500);
+
+        actor.Tell(new DeployInstanceCommand(
+            "dep-dbg", "EmptyPump", "sha256:dbg",
+            MakeConfigJson("EmptyPump"), "admin", DateTimeOffset.UtcNow));
+        ExpectMsg<DeploymentStatusResponse>(TimeSpan.FromSeconds(5));
+        await Task.Delay(800); // let InstanceActor spin up
+
+        actor.Tell(new DebugSnapshotRequest("EmptyPump", "snap-corr-2"));
+
+        var snapshot = ExpectMsg<DebugViewSnapshot>(TimeSpan.FromSeconds(5));
+        Assert.Equal("EmptyPump", snapshot.InstanceUniqueName);
+        Assert.False(snapshot.InstanceNotFound,
+            "A deployed (but empty) instance must NOT set InstanceNotFound.");
+    }
 }