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 ef0a502d..9198b710 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
@@ -169,7 +169,7 @@
     {
       "id": "B5.3",
       "subject": "B5: Modbus \u2014 optional unitId in ModbusEquipmentTagParser + ResolveHost via resolver (per-unit breaker key); scope-note vs R2-11",
-      "status": "pending",
+      "status": "completed",
       "blockedBy": [
         "B5.1"
       ]
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusEquipmentTagParser.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusEquipmentTagParser.cs
index a9ce79a7..65b16e29 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusEquipmentTagParser.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusEquipmentTagParser.cs
@@ -55,10 +55,14 @@ public static class ModbusEquipmentTagParser
             // "writable" defaults to true when absent (today's value) — makes read-only equipment tags
             // authorable (UNDER-6). Node-level authz remains the effective write gate.
             var writable = TagConfigJson.ReadWritable(root);
+            // CONV-4: optional per-tag unitId (0–255). Absent ⇒ null ⇒ the driver-level UnitId via
+            // ModbusDriver.ResolveUnitId, so multi-unit equipment tags key their own per-host breaker.
+            // Scope: unitId ONLY — the remaining parser-strictness gaps are R2-11's.
+            var unitId = ReadOptionalByte(root, "unitId");
             def = new ModbusTagDefinition(
                 Name: reference, Region: region, Address: (ushort)address, DataType: dataType,
                 Writable: writable, ByteOrder: byteOrder, BitIndex: bitIndex, StringLength: stringLength,
-                ArrayCount: arrayCount);
+                ArrayCount: arrayCount, UnitId: unitId);
             return true;
         }
         catch (JsonException) { return false; }
@@ -109,6 +113,10 @@ public static class ModbusEquipmentTagParser
         => o.TryGetProperty(name, out var e) && e.ValueKind == JsonValueKind.Number
            && e.TryGetInt32(out var v) ? v : 0;
 
+    private static byte? ReadOptionalByte(JsonElement o, string name)
+        => o.TryGetProperty(name, out var e) && e.ValueKind == JsonValueKind.Number
+           && e.TryGetInt32(out var v) && v is >= 0 and <= 255 ? (byte)v : null;
+
     private static bool ReadBool(JsonElement o, string name)
         => o.TryGetProperty(name, out var e) && e.ValueKind == JsonValueKind.True;
 }
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs
index 227b1abc..e522b3ef 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs
@@ -140,10 +140,19 @@ public sealed class ModbusDriver
             WriteHealth(new DriverHealth(DriverState.Degraded, current.LastSuccessfulRead, ex.Message));
     }
 
-    /// <inheritdoc />
+    /// <summary>
+    ///     Resolve a reference to the per-slave host key (<c>host:port/unitN</c>) 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) keys its OWN authored unit — via the tag's optional <c>unitId</c> or the driver
+    ///     default — rather than the single-slave 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 per-slave host key.</returns>
     public string ResolveHost(string fullReference)
     {
-        if (_tagsByName.TryGetValue(fullReference, out var tag))
+        if (_resolver.TryResolve(fullReference, out var tag))
             return BuildSlaveHostName(ResolveUnitId(tag));
         // Unknown reference — fall back to driver-instance host (single-slave behaviour).
         return HostName;
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusUnitIdResolveHostTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusUnitIdResolveHostTests.cs
new file mode 100644
index 00000000..933a201f
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusUnitIdResolveHostTests.cs
@@ -0,0 +1,63 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+/// <summary>
+///     CONV-4 (Modbus leg) — Modbus keys per-host resilience per SLAVE UNIT
+///     (<c>host:port/unitN</c>). An equipment tag must carry its own <c>unitId</c> so multi-unit
+///     equipment tags key their own breaker; <see cref="ModbusDriver.ResolveHost"/> must resolve
+///     the equipment ref through the resolver rather than only matching authored names. Scope:
+///     <c>unitId</c> ONLY — the parser's other strictness gaps are R2-11's.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class ModbusUnitIdResolveHostTests
+{
+    private sealed class NoopTransport : IModbusTransport
+    {
+        public Task ConnectAsync(CancellationToken ct) => Task.CompletedTask;
+        public Task<byte[]> SendAsync(byte unitId, byte[] pdu, CancellationToken ct)
+            => Task.FromResult(new byte[] { 0x03, 0x00 });
+        public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+    }
+
+    private static ModbusDriver NewDriver() => new(
+        new ModbusDriverOptions { Host = "10.0.0.1", Port = 502, UnitId = 1, Probe = new ModbusProbeOptions { Enabled = false } },
+        "modbus-1", _ => new NoopTransport());
+
+    /// <summary>The parser reads an optional unitId into the transient definition.</summary>
+    [Fact]
+    public void Parser_reads_optional_unitId()
+    {
+        var json = """{"region":"HoldingRegisters","address":0,"dataType":"Int16","unitId":7}""";
+        ModbusEquipmentTagParser.TryParse(json, out var def).ShouldBeTrue();
+        def.UnitId.ShouldBe((byte)7);
+    }
+
+    /// <summary>An absent unitId leaves the definition on the driver-level default (null override).</summary>
+    [Fact]
+    public void Parser_absent_unitId_is_null()
+    {
+        var json = """{"region":"HoldingRegisters","address":0,"dataType":"Int16"}""";
+        ModbusEquipmentTagParser.TryParse(json, out var def).ShouldBeTrue();
+        def.UnitId.ShouldBeNull();
+    }
+
+    /// <summary>An equipment ref carrying a unitId keys its own per-unit host name.</summary>
+    [Fact]
+    public void EquipmentTag_WithUnitId_ResolvesPerUnitHostName()
+    {
+        var drv = NewDriver();
+        var json = """{"region":"HoldingRegisters","address":0,"dataType":"Int16","unitId":7}""";
+        drv.ResolveHost(json).ShouldBe("10.0.0.1:502/unit7");
+    }
+
+    /// <summary>An equipment ref without a unitId keys the driver-level unit host name.</summary>
+    [Fact]
+    public void EquipmentTag_WithoutUnitId_KeysDriverDefaultUnit()
+    {
+        var drv = NewDriver();
+        var json = """{"region":"HoldingRegisters","address":0,"dataType":"Int16"}""";
+        drv.ResolveHost(json).ShouldBe("10.0.0.1:502/unit1");
+    }
+}