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 d3016198..ef0a502d 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
@@ -161,7 +161,7 @@
     {
       "id": "B5.2",
       "subject": "B5: rewrite the four multi-device ResolveHost bodies via EquipmentTagRefResolver + empty-host guard",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": [
         "B5.1"
       ]
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs
index e276bf53..ca7e045f 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs
@@ -501,10 +501,19 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
 
     // ---- IPerCallHostResolver ----
 
-    /// <inheritdoc />
+    /// <summary>
+    ///     Resolve a read/write/subscribe reference to the device host that keys its per-host
+    ///     resilience (bulkhead / circuit breaker). CONV-4: routes through
+    ///     <see cref="EquipmentTagRefResolver{TDef}"/> so an equipment-tag reference (raw TagConfig
+    ///     JSON) resolves to its OWN authored device rather than the first-device fallback — else a
+    ///     broken device B would trip device A's breaker. The resolver caches parses, so this adds
+    ///     no per-call cost after first use.
+    /// </summary>
+    /// <param name="fullReference">The tag reference (authored name or equipment-tag JSON).</param>
+    /// <returns>The device host key.</returns>
     public string ResolveHost(string fullReference)
     {
-        if (_tagsByName.TryGetValue(fullReference, out var def))
+        if (_resolver.TryResolve(fullReference, out var def) && !string.IsNullOrEmpty(def.DeviceHostAddress))
             return def.DeviceHostAddress;
         return _options.Devices.FirstOrDefault()?.HostAddress ?? DriverInstanceId;
     }
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
index a45cd80a..b5f21f9c 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
@@ -511,10 +511,18 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
 
     // ---- IPerCallHostResolver ----
 
-    /// <inheritdoc />
+    /// <summary>
+    ///     Resolve a reference to the device host that keys its per-host resilience
+    ///     (bulkhead / circuit breaker). CONV-4: routes through
+    ///     <see cref="EquipmentTagRefResolver{TDef}"/> so an equipment-tag reference (raw TagConfig
+    ///     JSON) resolves to its OWN authored device rather than the first-device fallback. The
+    ///     resolver caches parses, so this adds no per-call cost after first use.
+    /// </summary>
+    /// <param name="fullReference">The tag reference (authored name or equipment-tag JSON).</param>
+    /// <returns>The device host key.</returns>
     public string ResolveHost(string fullReference)
     {
-        if (_tagsByName.TryGetValue(fullReference, out var def))
+        if (_resolver.TryResolve(fullReference, out var def) && !string.IsNullOrEmpty(def.DeviceHostAddress))
             return def.DeviceHostAddress;
         return _options.Devices.FirstOrDefault()?.HostAddress ?? DriverInstanceId;
     }
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs
index 25386b28..9fca75f6 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs
@@ -1114,10 +1114,20 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
 
     // ---- IPerCallHostResolver ----
 
-    /// <inheritdoc />
+    /// <summary>
+    ///     Resolve a reference to the device host that keys its per-host resilience
+    ///     (bulkhead / circuit breaker). CONV-4: routes through
+    ///     <see cref="EquipmentTagRefResolver{TDef}"/> so an equipment-tag reference (raw TagConfig
+    ///     JSON) resolves to its OWN authored device rather than the first-device fallback. The
+    ///     empty-host guard covers FOCAS's <c>?? ""</c> default so an address-less equipment tag
+    ///     falls back rather than keying an empty host. The resolver caches parses, so this adds no
+    ///     per-call cost after first use.
+    /// </summary>
+    /// <param name="fullReference">The tag reference (authored name or equipment-tag JSON).</param>
+    /// <returns>The device host key.</returns>
     public string ResolveHost(string fullReference)
     {
-        if (_tagsByName.TryGetValue(fullReference, out var def))
+        if (_resolver.TryResolve(fullReference, out var def) && !string.IsNullOrEmpty(def.DeviceHostAddress))
             return def.DeviceHostAddress;
         return _options.Devices.FirstOrDefault()?.HostAddress ?? DriverInstanceId;
     }
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs
index 831e0c9a..a538dc16 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs
@@ -706,10 +706,20 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
     /// </summary>
     public const string UnresolvedHostSentinel = "";
 
-    /// <inheritdoc />
+    /// <summary>
+    ///     Resolve a reference to the device host that keys its per-host resilience
+    ///     (bulkhead / circuit breaker). CONV-4: routes through
+    ///     <see cref="EquipmentTagRefResolver{TDef}"/> so an equipment-tag reference (raw TagConfig
+    ///     JSON) resolves to its OWN authored device rather than the first-device fallback. The
+    ///     empty-host guard keeps an address-less equipment tag on the fallback rather than keying
+    ///     the <see cref="UnresolvedHostSentinel"/>. The resolver caches parses, so this adds no
+    ///     per-call cost after first use.
+    /// </summary>
+    /// <param name="fullReference">The tag reference (authored name or equipment-tag JSON).</param>
+    /// <returns>The device host key.</returns>
     public string ResolveHost(string fullReference)
     {
-        if (_tagsByName.TryGetValue(fullReference, out var def))
+        if (_resolver.TryResolve(fullReference, out var def) && !string.IsNullOrEmpty(def.DeviceHostAddress))
             return def.DeviceHostAddress;
         // First device's HostAddress when one exists; otherwise the unresolved sentinel —
         // intentionally NOT DriverInstanceId, which is a config-DB key, not a host address.
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
index c4827c9c..1fe06c2c 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasResolveHostTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasResolveHostTests.cs
@@ -16,37 +16,45 @@ 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());
+    // FOCAS's equipment-ref resolver validates against the initialized Devices map, so — unlike
+    // the other drivers — the driver must be initialized (as it always is when ResolveHost runs at
+    // runtime) before an equipment ref resolves. Probe off / no connect: init just parses devices.
+    private static async Task<FocasDriver> NewDriverAsync()
+    {
+        var drv = new FocasDriver(
+            new FocasDriverOptions
+            {
+                Devices = [new FocasDeviceOptions(DeviceA), new FocasDeviceOptions(DeviceB)],
+                Probe = new FocasProbeOptions { Enabled = false },
+            },
+            "focas-1", new FakeFocasClientFactory());
+        await drv.InitializeAsync("{}", CancellationToken.None);
+        return drv;
+    }
 
     /// <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()
+    public async Task ResolveHost_EquipmentTagRef_ReturnsItsOwnDeviceHost()
     {
-        var drv = NewDriver();
+        var drv = await NewDriverAsync();
         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()
+    public async Task ResolveHost_EquipmentTagRef_WithoutDeviceHost_FallsBack()
     {
-        var drv = NewDriver();
+        var drv = await NewDriverAsync();
         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()
+    public async Task ResolveHost_UnknownRef_KeepsCurrentFallback()
     {
-        var drv = NewDriver();
+        var drv = await NewDriverAsync();
         drv.ResolveHost("totally-unknown").ShouldBe(DeviceA);
     }
 }