diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs
index 58412d0f..1bc2cedd 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs
@@ -293,6 +293,9 @@ public sealed class GalaxyDriver
         if (_ownedMxSession is null) return;
         var clientOptions = BuildClientOptions(_options.Gateway);
         await _ownedMxSession.RecreateAsync(clientOptions, cancellationToken).ConfigureAwait(false);
+        // The recreated session invalidates every prior gw item handle; drop the writer's handle/advise
+        // caches so the next write re-AddItems + re-AdviseSupervisory against the fresh session.
+        _dataWriter?.InvalidateHandleCaches();
     }
 
     /// <summary>
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/GatewayGalaxyDataWriter.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/GatewayGalaxyDataWriter.cs
index 2574f1e5..1d74c721 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/GatewayGalaxyDataWriter.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/GatewayGalaxyDataWriter.cs
@@ -42,6 +42,42 @@ public sealed class GatewayGalaxyDataWriter : IGalaxyDataWriter
         _logger = logger ?? NullLogger.Instance;
     }
 
+    /// <inheritdoc />
+    public void InvalidateHandleCaches()
+    {
+        _itemHandles.Clear();
+        _supervisedHandles.Clear();
+    }
+
+    // ===== Test seams (internal — not part of the public contract) =====
+
+    /// <summary>
+    ///     Count of item-handle cache entries. Zero on a fresh instance or immediately after
+    ///     <see cref="InvalidateHandleCaches"/>. Used by unit tests to verify cache state
+    ///     without running a real gRPC round-trip.
+    /// </summary>
+    internal int CachedItemHandleCount => _itemHandles.Count;
+
+    /// <summary>
+    ///     Count of supervisory-advised handle entries. Zero on a fresh instance or immediately
+    ///     after <see cref="InvalidateHandleCaches"/>. Used by unit tests to verify cache state.
+    /// </summary>
+    internal int CachedSupervisedHandleCount => _supervisedHandles.Count;
+
+    /// <summary>
+    ///     Pre-populate both caches as if a write had already occurred. Used by unit tests to
+    ///     simulate the "post-write" state without running a real gRPC gateway session (the SDK
+    ///     session types are sealed + internal-ctor and cannot be faked).
+    /// </summary>
+    /// <param name="fullRef">The tag full reference to add to the item-handle cache.</param>
+    /// <param name="itemHandle">The item handle to cache for that reference.</param>
+    /// <param name="supervised">When true, also records the handle in the supervised-handle cache.</param>
+    internal void SeedHandleCachesForTest(string fullRef, int itemHandle, bool supervised)
+    {
+        _itemHandles[fullRef] = itemHandle;
+        if (supervised) _supervisedHandles.TryAdd(itemHandle, 0);
+    }
+
     /// <summary>Writes values to Galaxy tags through the gateway.</summary>
     /// <param name="writes">The write requests.</param>
     /// <param name="securityResolver">Function to resolve security classification per tag.</param>
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/IGalaxyDataWriter.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/IGalaxyDataWriter.cs
index f91e66e2..5032463c 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/IGalaxyDataWriter.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/IGalaxyDataWriter.cs
@@ -30,4 +30,8 @@ public interface IGalaxyDataWriter
         IReadOnlyList<WriteRequest> writes,
         Func<string, SecurityClassification> securityResolver,
         CancellationToken cancellationToken);
+
+    /// <summary>Drop cached gateway item handles + supervisory-advise state. Call after a session
+    /// reconnect — the prior handles are dead, so the next write must re-AddItem + re-AdviseSupervisory.</summary>
+    void InvalidateHandleCaches();
 }
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/TracedGalaxyDataWriter.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/TracedGalaxyDataWriter.cs
index 4c15b21c..37d745ef 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/TracedGalaxyDataWriter.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/TracedGalaxyDataWriter.cs
@@ -10,6 +10,10 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
 /// </summary>
 internal sealed class TracedGalaxyDataWriter(IGalaxyDataWriter inner, string clientName) : IGalaxyDataWriter
 {
+    /// <inheritdoc />
+    /// <remarks>No span — this is a local cache-clear operation, not a gateway round-trip.</remarks>
+    public void InvalidateHandleCaches() => inner.InvalidateHandleCaches();
+
     /// <summary>Writes data to Galaxy while recording telemetry span.</summary>
     /// <param name="writes">The list of write requests to process.</param>
     /// <param name="securityResolver">Function to resolve security classification for tag references.</param>
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyDriverWriteTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyDriverWriteTests.cs
index 7de25685..61c86d06 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyDriverWriteTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyDriverWriteTests.cs
@@ -94,6 +94,9 @@ public sealed class GalaxyDriverWriteTests
             }
             return Task.FromResult<IReadOnlyList<WriteResult>>(results);
         }
+
+        /// <inheritdoc />
+        public void InvalidateHandleCaches() { /* no-op — this fake has no handle caches */ }
     }
 
     private static GalaxyAttribute Attr(string name, int sec)
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyTelemetryTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyTelemetryTests.cs
index a1017e1a..bfef6df2 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyTelemetryTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GalaxyTelemetryTests.cs
@@ -178,6 +178,9 @@ public sealed class GalaxyTelemetryTests
             CancellationToken cancellationToken)
             => Task.FromResult<IReadOnlyList<WriteResult>>(
                 writes.Select(_ => new WriteResult(0u)).ToList());
+
+        /// <inheritdoc />
+        public void InvalidateHandleCaches() { /* no-op — this fake has no handle caches */ }
     }
 
     private sealed class FakeHierarchy : IGalaxyHierarchySource
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GatewayGalaxyDataWriterTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GatewayGalaxyDataWriterTests.cs
new file mode 100644
index 00000000..5d159665
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Runtime/GatewayGalaxyDataWriterTests.cs
@@ -0,0 +1,91 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.Runtime;
+
+/// <summary>
+///     Tests for <see cref="GatewayGalaxyDataWriter.InvalidateHandleCaches"/>.
+///     The SDK session types are sealed with internal ctors and cannot be faked, so we
+///     drive the cache-seeding path through
+///     <see cref="GatewayGalaxyDataWriter.SeedHandleCachesForTest"/> and verify the
+///     handle-count seams — the contract under test is purely that
+///     <see cref="GatewayGalaxyDataWriter.InvalidateHandleCaches"/> zeroes both dictionaries
+///     so the next write is forced to re-AddItem + re-AdviseSupervisory.
+/// </summary>
+public sealed class GatewayGalaxyDataWriterTests
+{
+    private static GalaxyMxSession MinimalSession()
+        => new(new GalaxyMxAccessOptions(ClientName: "OtOpcUa-Test"));
+
+    /// <summary>
+    ///     Approach (b): seed the item-handle cache directly via the internal test seam,
+    ///     confirm the count is positive, call <see cref="GatewayGalaxyDataWriter.InvalidateHandleCaches"/>,
+    ///     and confirm both caches are cleared.
+    ///     The next write (not simulated here — needs a live gw) would therefore be forced
+    ///     to re-AddItem because the cache is empty.
+    /// </summary>
+    [Fact]
+    public void InvalidateHandleCaches_clears_item_and_supervised_handle_caches()
+    {
+        var session = MinimalSession();
+        var writer = new GatewayGalaxyDataWriter(session, writeUserId: 0);
+
+        // Pre-seed both caches via the internal test seam so we can assert the
+        // "after a write" state without spinning up a real gRPC gateway session.
+        writer.SeedHandleCachesForTest("TestMachine_001.TestAttr", itemHandle: 42, supervised: true);
+
+        writer.CachedItemHandleCount.ShouldBe(1);
+        writer.CachedSupervisedHandleCount.ShouldBe(1);
+
+        writer.InvalidateHandleCaches();
+
+        writer.CachedItemHandleCount.ShouldBe(0);
+        writer.CachedSupervisedHandleCount.ShouldBe(0);
+    }
+
+    /// <summary>
+    ///     A second seed + invalidate cycle proves the method isn't one-shot — a reconnect
+    ///     followed by writes followed by another reconnect must also start fresh.
+    /// </summary>
+    [Fact]
+    public void InvalidateHandleCaches_is_repeatable_across_multiple_reconnects()
+    {
+        var session = MinimalSession();
+        var writer = new GatewayGalaxyDataWriter(session, writeUserId: 0);
+
+        // First session cycle
+        writer.SeedHandleCachesForTest("Tag.A", itemHandle: 1, supervised: false);
+        writer.SeedHandleCachesForTest("Tag.B", itemHandle: 2, supervised: true);
+        writer.CachedItemHandleCount.ShouldBe(2);
+        writer.InvalidateHandleCaches();
+        writer.CachedItemHandleCount.ShouldBe(0);
+        writer.CachedSupervisedHandleCount.ShouldBe(0);
+
+        // Second session cycle — handles re-populated after the reconnect's replay
+        writer.SeedHandleCachesForTest("Tag.A", itemHandle: 99, supervised: true);
+        writer.CachedItemHandleCount.ShouldBe(1);
+        writer.InvalidateHandleCaches();
+        writer.CachedItemHandleCount.ShouldBe(0);
+    }
+
+    /// <summary>
+    ///     <see cref="GatewayGalaxyDataWriter.InvalidateHandleCaches"/> on a fresh (never-used)
+    ///     writer must be a no-op rather than throwing — the reconnect supervisor may call it
+    ///     before any write has occurred.
+    /// </summary>
+    [Fact]
+    public void InvalidateHandleCaches_on_empty_caches_is_a_noop()
+    {
+        var session = MinimalSession();
+        var writer = new GatewayGalaxyDataWriter(session, writeUserId: 0);
+
+        // Caches are empty — must not throw.
+        writer.CachedItemHandleCount.ShouldBe(0);
+        writer.CachedSupervisedHandleCount.ShouldBe(0);
+        writer.InvalidateHandleCaches();
+        writer.CachedItemHandleCount.ShouldBe(0);
+        writer.CachedSupervisedHandleCount.ShouldBe(0);
+    }
+}