diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Recorder/GatewayHistorianValueWriterTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Recorder/GatewayHistorianValueWriterTests.cs
new file mode 100644
index 00000000..3367cfa2
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Recorder/GatewayHistorianValueWriterTests.cs
@@ -0,0 +1,149 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Xunit;
+using ZB.MOM.WW.HistorianGateway.Contracts.Grpc;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Recorder;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests.Recorder;
+
+/// <summary>
+/// Driver.Historian.Gateway-003 (offline unit coverage for the continuous-historization write path):
+/// before this file, <see cref="GatewayHistorianValueWriter"/> — the recorder's <c>WriteLiveValues</c>
+/// adapter, in production the write path CLAUDE.md's #491/#537/#538 live-gate history bit hard — was
+/// exercised only by the env-gated <c>Category=LiveIntegration</c> suite, which skips cleanly offline.
+/// These tests use the same <see cref="FakeHistorianGatewayClient"/> the rest of the module's offline
+/// tests share.
+/// </summary>
+public sealed class GatewayHistorianValueWriterTests
+{
+    private static GatewayHistorianValueWriter Writer(FakeHistorianGatewayClient fake) =>
+        new(fake, NullLogger<GatewayHistorianValueWriter>.Instance);
+
+    [Fact]
+    public async Task Success_ack_returns_true()
+    {
+        var fake = new FakeHistorianGatewayClient { WriteLiveValuesResult = new WriteAck { Success = true } };
+        var values = new[] { new HistorizationValue(DateTime.UtcNow, 42.0, 192) };
+
+        var ok = await Writer(fake).WriteLiveValuesAsync("T", values, TestContext.Current.CancellationToken);
+
+        Assert.True(ok);
+        Assert.Equal(1, fake.WriteLiveValuesCallCount);
+        Assert.Equal("T", fake.LastWriteLiveTag);
+    }
+
+    [Fact]
+    public async Task Queued_ack_is_treated_as_success()
+    {
+        // A value the gateway durably store-forward-queued must not be re-drained by the recorder.
+        var fake = new FakeHistorianGatewayClient
+        {
+            WriteLiveValuesResult = new WriteAck { Success = false, Queued = true },
+        };
+        var values = new[] { new HistorizationValue(DateTime.UtcNow, 1.0, 192) };
+
+        var ok = await Writer(fake).WriteLiveValuesAsync("T", values, TestContext.Current.CancellationToken);
+
+        Assert.True(ok);
+    }
+
+    [Fact]
+    public async Task Soft_failure_ack_returns_false_for_retry()
+    {
+        var fake = new FakeHistorianGatewayClient
+        {
+            WriteLiveValuesResult = new WriteAck { Success = false, Queued = false },
+        };
+        var values = new[] { new HistorizationValue(DateTime.UtcNow, 1.0, 192) };
+
+        var ok = await Writer(fake).WriteLiveValuesAsync("T", values, TestContext.Current.CancellationToken);
+
+        Assert.False(ok);
+    }
+
+    [Fact]
+    public async Task Gateway_exception_is_swallowed_and_returns_false()
+    {
+        // NEVER throw out of the writer — the recorder's drain loop expects a plain bool.
+        var fake = new FakeHistorianGatewayClient { WriteLiveValuesThrows = new InvalidOperationException("boom") };
+        var values = new[] { new HistorizationValue(DateTime.UtcNow, 1.0, 192) };
+
+        var ok = await Writer(fake).WriteLiveValuesAsync("T", values, TestContext.Current.CancellationToken);
+
+        Assert.False(ok);
+    }
+
+    [Fact]
+    public async Task Empty_batch_is_trivially_acked_without_calling_the_gateway()
+    {
+        var fake = new FakeHistorianGatewayClient();
+
+        var ok = await Writer(fake).WriteLiveValuesAsync(
+            "T", Array.Empty<HistorizationValue>(), TestContext.Current.CancellationToken);
+
+        Assert.True(ok);
+        Assert.Equal(0, fake.WriteLiveValuesCallCount);
+    }
+
+    [Fact]
+    public async Task Present_timestamp_is_sent_and_absent_timestamp_leaves_the_proto_field_unset()
+    {
+        var fake = new FakeHistorianGatewayClient();
+        var stamped = new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        var values = new[]
+        {
+            new HistorizationValue(stamped, 1.0, 192),
+            new HistorizationValue(null, 2.0, 192),
+        };
+
+        await Writer(fake).WriteLiveValuesAsync("T", values, TestContext.Current.CancellationToken);
+
+        var sent = fake.LastWriteLiveValues!;
+        Assert.Equal(2, sent.Count);
+        Assert.Equal(stamped, sent[0].Timestamp!.ToDateTime());
+        Assert.Null(sent[1].Timestamp); // absent -> unset proto field -> gateway server-stamps now
+    }
+
+    [Fact]
+    public async Task Quality_and_numeric_value_are_forwarded_unchanged()
+    {
+        var fake = new FakeHistorianGatewayClient();
+        var values = new[] { new HistorizationValue(null, 123.5, 192) };
+
+        await Writer(fake).WriteLiveValuesAsync("T", values, TestContext.Current.CancellationToken);
+
+        var sent = fake.LastWriteLiveValues!;
+        Assert.Equal(123.5, sent[0].NumericValue);
+        Assert.Equal(192u, sent[0].Quality);
+    }
+
+    [Fact]
+    public async Task Null_tag_throws()
+    {
+        var fake = new FakeHistorianGatewayClient();
+        var values = new[] { new HistorizationValue(null, 1.0, 192) };
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() =>
+            Writer(fake).WriteLiveValuesAsync(null!, values, TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task Null_values_throws()
+    {
+        var fake = new FakeHistorianGatewayClient();
+
+        await Assert.ThrowsAsync<ArgumentNullException>(() =>
+            Writer(fake).WriteLiveValuesAsync("T", null!, TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task DisposeAsync_disposes_underlying_client()
+    {
+        var fake = new FakeHistorianGatewayClient();
+        var writer = Writer(fake);
+
+        await writer.DisposeAsync();
+
+        Assert.Equal(1, fake.DisposeCallCount);
+    }
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Recorder/HistorizationOutboxEntrySerializerTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Recorder/HistorizationOutboxEntrySerializerTests.cs
new file mode 100644
index 00000000..0efdc6ae
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Recorder/HistorizationOutboxEntrySerializerTests.cs
@@ -0,0 +1,73 @@
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Recorder;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests.Recorder;
+
+/// <summary>
+/// Driver.Historian.Gateway-003 (offline unit coverage): <see cref="FasterLogHistorizationOutboxTests"/>
+/// exercises <see cref="HistorizationOutboxEntrySerializer"/> only indirectly, and only with short
+/// ASCII tag names ("A"/"B"/"C"). The hand-rolled little-endian layout computes its tag byte-length
+/// from <c>Encoding.UTF8.GetByteCount</c> before writing, so a multi-byte tag exercises the one part
+/// of the layout arithmetic the indirect coverage never reaches. This asserts the (de)serializer
+/// directly, in isolation from FasterLog.
+/// </summary>
+public sealed class HistorizationOutboxEntrySerializerTests
+{
+    [Fact]
+    public void Round_trips_all_fields()
+    {
+        var entry = new HistorizationOutboxEntry(
+            Guid.NewGuid(), "Pump1.Temp", 123.456, 192,
+            new DateTime(2026, 1, 1, 12, 34, 56, DateTimeKind.Utc));
+
+        var bytes = HistorizationOutboxEntrySerializer.Serialize(entry);
+        var round = HistorizationOutboxEntrySerializer.Deserialize(bytes);
+
+        Assert.Equal(entry.Id, round.Id);
+        Assert.Equal(entry.Tag, round.Tag);
+        Assert.Equal(entry.NumericValue, round.NumericValue);
+        Assert.Equal(entry.Quality, round.Quality);
+        Assert.Equal(entry.TimestampUtc, round.TimestampUtc);
+        Assert.Equal(entry.TimestampUtc.Kind, round.TimestampUtc.Kind); // ToBinary/FromBinary round-trips Kind
+    }
+
+    [Fact]
+    public void Round_trips_a_multi_byte_utf8_tag()
+    {
+        // The tag length prefix is a UTF-8 BYTE count, not a char count — a tag whose char count and
+        // byte count diverge (e.g. Ω is 2 bytes, 日 is 3) is the one case that would desync the layout
+        // if GetByteCount/GetBytes ever disagreed with what Deserialize slices out.
+        var entry = new HistorizationOutboxEntry(
+            Guid.NewGuid(), "Pumpe_Ω_日本語_Temp", -1.5, 0, DateTime.UtcNow);
+
+        var round = HistorizationOutboxEntrySerializer.Deserialize(
+            HistorizationOutboxEntrySerializer.Serialize(entry));
+
+        Assert.Equal(entry.Tag, round.Tag);
+        Assert.Equal(entry.NumericValue, round.NumericValue);
+    }
+
+    [Fact]
+    public void Round_trips_an_empty_tag()
+    {
+        var entry = new HistorizationOutboxEntry(Guid.NewGuid(), string.Empty, 0.0, 0, DateTime.UtcNow);
+
+        var round = HistorizationOutboxEntrySerializer.Deserialize(
+            HistorizationOutboxEntrySerializer.Serialize(entry));
+
+        Assert.Equal(string.Empty, round.Tag);
+    }
+
+    [Fact]
+    public void Round_trips_boundary_quality_and_negative_value()
+    {
+        var entry = new HistorizationOutboxEntry(Guid.NewGuid(), "T", double.MinValue, ushort.MaxValue, DateTime.UtcNow);
+
+        var round = HistorizationOutboxEntrySerializer.Deserialize(
+            HistorizationOutboxEntrySerializer.Serialize(entry));
+
+        Assert.Equal(ushort.MaxValue, round.Quality);
+        Assert.Equal(double.MinValue, round.NumericValue);
+    }
+}