diff --git a/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json b/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json
index b154d0d3..d3016198 100644
--- a/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json
+++ b/archreview/plans/R2-09-driver-fleet-batch-plan.md.tasks.json
@@ -155,7 +155,7 @@
     {
       "id": "B5.1",
       "subject": "B5: failing ResolveHost tests in AbCip/AbLegacy/TwinCAT/FOCAS (equipment ref for device B must key B; fallback regression pins) \u2014 CONV-4 repro",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": []
     },
     {
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipResolveHostTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipResolveHostTests.cs
new file mode 100644
index 00000000..fd305b8f
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipResolveHostTests.cs
@@ -0,0 +1,43 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;
+
+/// <summary>
+///     CONV-4 — <see cref="AbCipDriver.ResolveHost"/> keys per-host resilience (bulkhead / circuit
+///     breaker). It must resolve an equipment-tag reference (raw TagConfig JSON) to its OWN device
+///     host, not fall back to the first device — otherwise a broken device B trips device A's
+///     breaker. Authored tags and unknown refs keep the current fallback.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbCipResolveHostTests
+{
+    private const string DeviceA = "ab://10.0.0.5/1,0";
+    private const string DeviceB = "ab://10.0.0.6/1,0";
+
+    private static AbCipDriver NewDriver() => new(
+        new AbCipDriverOptions
+        {
+            Devices = [new AbCipDeviceOptions(DeviceA), new AbCipDeviceOptions(DeviceB)],
+            Probe = new AbCipProbeOptions { Enabled = false },
+        },
+        "abcip-1", new FakeAbCipTagFactory());
+
+    /// <summary>An equipment-tag ref naming device B resolves to device B's host, not the first device.</summary>
+    [Fact]
+    public void ResolveHost_EquipmentTagRef_ReturnsItsOwnDeviceHost()
+    {
+        var drv = NewDriver();
+        var json = $$"""{"deviceHostAddress":"{{DeviceB}}","tagPath":"Motor1.Speed","dataType":"DInt"}""";
+        drv.ResolveHost(json).ShouldBe(DeviceB);
+    }
+
+    /// <summary>An unresolvable ref keeps the current first-device fallback.</summary>
+    [Fact]
+    public void ResolveHost_UnknownRef_KeepsCurrentFallback()
+    {
+        var drv = NewDriver();
+        drv.ResolveHost("totally-unknown").ShouldBe(DeviceA);
+    }
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyResolveHostTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyResolveHostTests.cs
new file mode 100644
index 00000000..52109f9d
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyResolveHostTests.cs
@@ -0,0 +1,42 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
+
+/// <summary>
+///     CONV-4 — <see cref="AbLegacyDriver.ResolveHost"/> must resolve an equipment-tag reference
+///     (raw TagConfig JSON) to its OWN device host so per-host breaker keys are correct for
+///     equipment tags, not the first-device fallback.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbLegacyResolveHostTests
+{
+    private const string DeviceA = "ab://10.0.0.5/1,0";
+    private const string DeviceB = "ab://10.0.0.6/1,0";
+
+    private static AbLegacyDriver NewDriver() => new(
+        new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions(DeviceA), new AbLegacyDeviceOptions(DeviceB)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        },
+        "ablegacy-1", new FakeAbLegacyTagFactory());
+
+    /// <summary>An equipment-tag ref naming device B resolves to device B's host, not the first device.</summary>
+    [Fact]
+    public void ResolveHost_EquipmentTagRef_ReturnsItsOwnDeviceHost()
+    {
+        var drv = NewDriver();
+        var json = $$"""{"deviceHostAddress":"{{DeviceB}}","address":"N7:0","dataType":"Int"}""";
+        drv.ResolveHost(json).ShouldBe(DeviceB);
+    }
+
+    /// <summary>An unresolvable ref keeps the current first-device fallback.</summary>
+    [Fact]
+    public void ResolveHost_UnknownRef_KeepsCurrentFallback()
+    {
+        var drv = NewDriver();
+        drv.ResolveHost("totally-unknown").ShouldBe(DeviceA);
+    }
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasResolveHostTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasResolveHostTests.cs
new file mode 100644
index 00000000..c4827c9c
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasResolveHostTests.cs
@@ -0,0 +1,52 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests;
+
+/// <summary>
+///     CONV-4 — <see cref="FocasDriver.ResolveHost"/> must resolve an equipment-tag reference (raw
+///     TagConfig JSON) to its OWN device host so per-host breaker keys are correct for equipment
+///     tags. An address-less equipment ref (FOCAS coalesces a missing <c>deviceHostAddress</c> to
+///     <c>""</c>) must fall back rather than key an empty host.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class FocasResolveHostTests
+{
+    private const string DeviceA = "focas://10.0.0.5:8193";
+    private const string DeviceB = "focas://10.0.0.6:8193";
+
+    private static FocasDriver NewDriver() => new(
+        new FocasDriverOptions
+        {
+            Devices = [new FocasDeviceOptions(DeviceA), new FocasDeviceOptions(DeviceB)],
+            Probe = new FocasProbeOptions { Enabled = false },
+        },
+        "focas-1", new FakeFocasClientFactory());
+
+    /// <summary>An equipment-tag ref naming device B resolves to device B's host, not the first device.</summary>
+    [Fact]
+    public void ResolveHost_EquipmentTagRef_ReturnsItsOwnDeviceHost()
+    {
+        var drv = NewDriver();
+        var json = $$"""{"deviceHostAddress":"{{DeviceB}}","address":"R100","dataType":"Byte"}""";
+        drv.ResolveHost(json).ShouldBe(DeviceB);
+    }
+
+    /// <summary>An equipment ref with no deviceHostAddress falls back to the first device, not an empty host.</summary>
+    [Fact]
+    public void ResolveHost_EquipmentTagRef_WithoutDeviceHost_FallsBack()
+    {
+        var drv = NewDriver();
+        var json = """{"address":"R100","dataType":"Byte"}""";
+        drv.ResolveHost(json).ShouldBe(DeviceA);
+    }
+
+    /// <summary>An unresolvable ref keeps the current first-device fallback.</summary>
+    [Fact]
+    public void ResolveHost_UnknownRef_KeepsCurrentFallback()
+    {
+        var drv = NewDriver();
+        drv.ResolveHost("totally-unknown").ShouldBe(DeviceA);
+    }
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATResolveHostTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATResolveHostTests.cs
new file mode 100644
index 00000000..c7a52c07
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/TwinCATResolveHostTests.cs
@@ -0,0 +1,43 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.TwinCAT;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests;
+
+/// <summary>
+///     CONV-4 — <see cref="TwinCATDriver.ResolveHost"/> must resolve an equipment-tag reference
+///     (raw TagConfig JSON) to its OWN device host so per-host breaker keys are correct for
+///     equipment tags, not the first-device fallback.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class TwinCATResolveHostTests
+{
+    private const string DeviceA = "ads://5.23.91.23.1.1:851";
+    private const string DeviceB = "ads://5.23.91.23.1.2:851";
+
+    private static TwinCATDriver NewDriver() => new(
+        new TwinCATDriverOptions
+        {
+            Devices = [new TwinCATDeviceOptions(DeviceA), new TwinCATDeviceOptions(DeviceB)],
+            Probe = new TwinCATProbeOptions { Enabled = false },
+            EnableControllerBrowse = false,
+        },
+        "twincat-1", new FakeTwinCATClientFactory());
+
+    /// <summary>An equipment-tag ref naming device B resolves to device B's host, not the first device.</summary>
+    [Fact]
+    public void ResolveHost_EquipmentTagRef_ReturnsItsOwnDeviceHost()
+    {
+        var drv = NewDriver();
+        var json = $$"""{"deviceHostAddress":"{{DeviceB}}","symbolPath":"MAIN.Speed","dataType":"DInt"}""";
+        drv.ResolveHost(json).ShouldBe(DeviceB);
+    }
+
+    /// <summary>An unresolvable ref keeps the current first-device fallback.</summary>
+    [Fact]
+    public void ResolveHost_UnknownRef_KeepsCurrentFallback()
+    {
+        var drv = NewDriver();
+        drv.ResolveHost("totally-unknown").ShouldBe(DeviceA);
+    }
+}