diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryProtoMapNode.cs b/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryProtoMapNode.cs
index 900ee4a2..8682a16f 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryProtoMapNode.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryProtoMapNode.cs
@@ -67,17 +67,21 @@ public static class TelemetryProtoMapNode
 
     private static AlarmTransition MapAlarm(AlarmTransitionEvent e)
     {
+        // Required (non-optional) proto string setters throw ArgumentNullException on null, and the
+        // domain records carry no runtime null guard (nullable-ref annotations are compile-time only).
+        // Coalesce every required string to "" so a stray null can never throw out of the pump and kill
+        // the stream. The `optional` fields below keep their null-vs-"" presence guards.
         var msg = new AlarmTransition
         {
-            AlarmId = e.AlarmId,
-            EquipmentPath = e.EquipmentPath,
-            AlarmName = e.AlarmName,
-            TransitionKind = e.TransitionKind,
+            AlarmId = e.AlarmId ?? "",
+            EquipmentPath = e.EquipmentPath ?? "",
+            AlarmName = e.AlarmName ?? "",
+            TransitionKind = e.TransitionKind ?? "",
             Severity = e.Severity,
-            Message = e.Message,
-            User = e.User,
+            Message = e.Message ?? "",
+            User = e.User ?? "",
             TimestampUtc = ToUtcTimestamp(e.TimestampUtc),
-            AlarmTypeName = e.AlarmTypeName,
+            AlarmTypeName = e.AlarmTypeName ?? "",
         };
 
         if (e.Comment is not null)
@@ -93,9 +97,9 @@ public static class TelemetryProtoMapNode
     {
         var msg = new ScriptLog
         {
-            ScriptId = e.ScriptId,
-            Level = e.Level,
-            Message = e.Message,
+            ScriptId = e.ScriptId ?? "",
+            Level = e.Level ?? "",
+            Message = e.Message ?? "",
             TimestampUtc = ToUtcTimestamp(e.TimestampUtc),
         };
 
@@ -113,9 +117,9 @@ public static class TelemetryProtoMapNode
     {
         var msg = new DriverHealth
         {
-            ClusterId = e.ClusterId,
-            DriverInstanceId = e.DriverInstanceId,
-            State = e.State,
+            ClusterId = e.ClusterId ?? "",
+            DriverInstanceId = e.DriverInstanceId ?? "",
+            State = e.State ?? "",
             ErrorCount5Min = e.ErrorCount5Min,
             PublishedUtc = ToUtcTimestamp(e.PublishedUtc),
         };
@@ -132,8 +136,8 @@ public static class TelemetryProtoMapNode
     {
         var msg = new DriverResilienceStatus
         {
-            DriverInstanceId = e.DriverInstanceId,
-            HostName = e.HostName,
+            DriverInstanceId = e.DriverInstanceId ?? "",
+            HostName = e.HostName ?? "",
             BreakerOpen = e.BreakerOpen,
             ConsecutiveFailures = e.ConsecutiveFailures,
             CurrentInFlight = e.CurrentInFlight,
diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryStreamGrpcService.cs b/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryStreamGrpcService.cs
index 20dbc490..b2e7c634 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryStreamGrpcService.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.Host/Grpc/TelemetryStreamGrpcService.cs
@@ -103,6 +103,15 @@ public sealed class TelemetryStreamGrpcService : GeneratedServiceBase
                 // Normal termination: the client disconnected (context token) or the max-lifetime cap
                 // fired (linked token). Neither is an error — swallow and let the stream close cleanly.
             }
+            catch (Exception ex) when (ex is IOException or RpcException)
+            {
+                // Routine mid-stream disconnect: central's TelemetryStreamClient reconnects on ANY
+                // non-Cancelled error, so a broken pipe / connection-reset here is expected churn, not a
+                // fault. Swallow at Debug so grpc-dotnet doesn't log it at Error on every reconnect.
+                // (A genuinely unexpected exception still escapes to surface as an error.)
+                _log.LogDebug(
+                    ex, "Telemetry client disconnected mid-stream (correlationId={CorrelationId})", correlationId);
+            }
 
             _log.LogDebug("Telemetry stream closed (correlationId={CorrelationId})", correlationId);
         }
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/Grpc/TelemetryStreamGrpcServiceTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/Grpc/TelemetryStreamGrpcServiceTests.cs
index f98e9c58..b3b60e21 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/Grpc/TelemetryStreamGrpcServiceTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/Grpc/TelemetryStreamGrpcServiceTests.cs
@@ -210,6 +210,62 @@ public sealed class TelemetryStreamGrpcServiceTests
         evt.DriverHealth.LastSuccessfulReadUtc.ToDateTime().ShouldBe(expectedUtc);
     }
 
+    [Fact]
+    public async Task Client_disconnect_mid_stream_ends_cleanly_without_leaking_a_slot()
+    {
+        // A broken pipe surfaces as IOException out of WriteAsync. It must be swallowed as a routine
+        // disconnect (not escape), and the finally must still decrement the counter. Looping well past
+        // the cap proves no leak: a single leaked slot per pass would trip ResourceExhausted by pass 101.
+        var hub = new TelemetryLocalHub();
+        var service = NewService(hub);
+
+        // Cached snapshot ⇒ every fresh subscription is deterministically handed one item to write.
+        hub.Emit(new TelemetryItem.Health(new DriverHealthChanged(
+            "cluster-a", "drv-1", "Healthy", null, null, 0, DateTime.UtcNow)));
+
+        for (var i = 0; i < 150; i++)
+        {
+            using var cts = new CancellationTokenSource();
+            var pump = service.Subscribe(
+                new TelemetryStreamRequest { CorrelationId = $"disc-{i}" },
+                new ThrowingStreamWriter(new IOException("connection reset")),
+                Ctx(cts.Token));
+
+            // No exception escapes Subscribe, and no ResourceExhausted ever fires (proves the slot is freed).
+            await Should.NotThrowAsync(() => pump);
+        }
+
+        // Counter is back at its prior value: a normal stream still opens and pumps.
+        using var okCts = new CancellationTokenSource();
+        var writer = new CapturingStreamWriter();
+        var okPump = service.Subscribe(
+            new TelemetryStreamRequest { CorrelationId = "disc-after" }, writer, Ctx(okCts.Token));
+        await WaitUntilAsync(() => writer.Count >= 1, "the cached snapshot to stream after the disconnect loop");
+        okCts.Cancel();
+        await okPump;
+    }
+
+    [Fact]
+    public void ToProto_null_required_string_does_not_throw_and_maps_to_empty()
+    {
+        // Required proto string setters throw on null; the domain records carry no runtime guard.
+        var alarm = new AlarmTransitionEvent(
+            AlarmId: null!, EquipmentPath: null!, AlarmName: null!, TransitionKind: null!,
+            Severity: 100, Message: null!, User: null!, TimestampUtc: DateTime.UtcNow,
+            AlarmTypeName: null!, Comment: null, HistorizeToAveva: null, ReferencingEquipmentPaths: null);
+
+        AlarmTransition msg = null!;
+        Should.NotThrow(() => msg = TelemetryProtoMapNode.ToProto(new TelemetryItem.Alarm(alarm), "c").AlarmTransition);
+
+        msg.AlarmId.ShouldBe("");
+        msg.EquipmentPath.ShouldBe("");
+        msg.AlarmName.ShouldBe("");
+        msg.TransitionKind.ShouldBe("");
+        msg.Message.ShouldBe("");
+        msg.User.ShouldBe("");
+        msg.AlarmTypeName.ShouldBe("");
+    }
+
     [Fact]
     public void ToProto_alarm_omits_absent_nullables()
     {
@@ -284,4 +340,18 @@ public sealed class TelemetryStreamGrpcServiceTests
                 return [.. _items];
         }
     }
+
+    /// <summary>A stream writer that always throws the given exception — models a broken client pipe.</summary>
+    private sealed class ThrowingStreamWriter(Exception toThrow) : IServerStreamWriter<TelemetryEvent>
+    {
+        public WriteOptions? WriteOptions { get; set; }
+
+        public Task WriteAsync(TelemetryEvent message) => throw toThrow;
+
+        public Task WriteAsync(TelemetryEvent message, CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            return WriteAsync(message);
+        }
+    }
 }