diff --git a/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/ReadCommand.cs b/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/ReadCommand.cs
index a2378a55..3b48f4d1 100644
--- a/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/ReadCommand.cs
+++ b/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/ReadCommand.cs
@@ -57,6 +57,14 @@ public sealed class ReadCommand : ModbusCommandBase
         ValidateEndpoint();
         var ct = console.RegisterCancellationHandler();
 
+        // The driver's tag factory refuses an incompatible region/type pair (the tag would read
+        // BadNodeIdUnknown) — say why up front instead.
+        if (!ModbusTagDefinitionFactory.IsRegionTypeCompatible(Region, DataType))
+            throw new CliFx.Exceptions.CommandException(
+                Region is ModbusRegion.Coils or ModbusRegion.DiscreteInputs
+                    ? $"Region '{Region}' only carries Bool values (--type Bool); got '{DataType}'."
+                    : $"Region '{Region}' carries register types; use --type BitInRegister for a bit of a register, not '{DataType}'.");
+
         var tagName = SynthesiseTagName(Region, Address, DataType);
         var tag = new ModbusTagDefinition(
             Name: tagName,
diff --git a/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/WriteCommand.cs b/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/WriteCommand.cs
index 07bb3bf7..e1876a49 100644
--- a/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/WriteCommand.cs
+++ b/src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/WriteCommand.cs
@@ -74,6 +74,9 @@ public sealed class WriteCommand : ModbusCommandBase
             throw new CliFx.Exceptions.CommandException(
                 $"Region 'Coils' only supports boolean values (--type Bool). " +
                 $"Type '{DataType}' cannot represent a single-bit coil write.");
+        if (!ModbusTagDefinitionFactory.IsRegionTypeCompatible(Region, DataType))
+            throw new CliFx.Exceptions.CommandException(
+                $"Region '{Region}' carries register types; use --type BitInRegister for a bit of a register, not '{DataType}'.");
 
         var tagName = ReadCommand.SynthesiseTagName(Region, Address, DataType);
         var tag = new ModbusTagDefinition(
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusTagDefinitionFactory.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusTagDefinitionFactory.cs
index 02e2ff34..86b0a4e9 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusTagDefinitionFactory.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusTagDefinitionFactory.cs
@@ -50,7 +50,13 @@ public static class ModbusTagDefinitionFactory
             // Strict enum reads: a present-but-invalid (typo'd) enum rejects the tag
             // (→ BadNodeIdUnknown) instead of silently defaulting to a wrong-width Good.
             if (!TagConfigJson.TryReadEnumStrict(root, "region", ModbusRegion.HoldingRegisters, out var region)) return false;
-            if (!TagConfigJson.TryReadEnumStrict(root, "dataType", ModbusDataType.Int16, out var dataType)) return false;
+            // An absent dataType defaults per region: a bit region (Coils / DiscreteInputs) only carries
+            // Bool, so defaulting it to Int16 would reject every bit tag authored without the key.
+            if (!TagConfigJson.TryReadEnumStrict(root, "dataType", DefaultDataType(region), out var dataType)) return false;
+            // Region/type compatibility: a bit region carries only Bool, a register region never Bool.
+            // A mismatch used to throw out of the coalescing planner (killing every tag's poll) or
+            // silently return a bool into an Int32 node.
+            if (!IsRegionTypeCompatible(region, dataType)) return false;
             if (!TagConfigJson.TryReadEnumStrict(root, "byteOrder", ModbusByteOrder.BigEndian, out var byteOrder)) return false;
             if (!TagConfigJson.TryReadEnumStrict(root, "stringByteOrder", ModbusStringByteOrder.HighByteFirst, out var stringByteOrder)) return false;
             var bitIndex = (byte)ReadInt(root, "bitIndex");
@@ -94,6 +100,25 @@ public static class ModbusTagDefinitionFactory
         catch (InvalidOperationException) { return false; }
     }
 
+    /// <summary>The data type an authored tag gets when its <c>dataType</c> key is absent.</summary>
+    /// <param name="region">The tag's region.</param>
+    /// <returns><c>Bool</c> for Coils / DiscreteInputs, <c>Int16</c> for the register regions.</returns>
+    public static ModbusDataType DefaultDataType(ModbusRegion region)
+        => region is ModbusRegion.Coils or ModbusRegion.DiscreteInputs ? ModbusDataType.Bool : ModbusDataType.Int16;
+
+    /// <summary>
+    ///     Region/type compatibility, the one rule every authoring path and the driver share: Coils and
+    ///     DiscreteInputs carry only <c>Bool</c>; HoldingRegisters and InputRegisters carry every register
+    ///     type (including <c>BitInRegister</c>) but never <c>Bool</c>.
+    /// </summary>
+    /// <param name="region">The tag's region.</param>
+    /// <param name="dataType">The tag's data type.</param>
+    /// <returns><see langword="true"/> when the driver can read the pair.</returns>
+    public static bool IsRegionTypeCompatible(ModbusRegion region, ModbusDataType dataType)
+        => region is ModbusRegion.Coils or ModbusRegion.DiscreteInputs
+            ? dataType == ModbusDataType.Bool
+            : dataType != ModbusDataType.Bool;
+
     /// <summary>
     ///     Inverse of <see cref="FromTagConfig"/>: serialises a typed definition back to the camelCase
     ///     <c>TagConfig</c> JSON the mapper reads. Used by authoring paths (the Client CLI) that construct a
@@ -247,7 +272,17 @@ public static class ModbusTagDefinitionFactory
         // Type-dependent guards. The byte/ushort NARROWING is part of the rule, not an accident of the
         // mapper: it reads `(byte)bitIndex` / `(ushort)stringLength`, so 256 is bit 0 and is ACCEPTED.
         // Describing the un-narrowed value here would call a blob invalid that the driver serves.
-        if (!TagConfigJson.TryReadEnumStrict(root, "dataType", ModbusDataType.Int16, out var dataType)) return;
+        var region = TagConfigJson.TryReadEnumStrict(root, "region", ModbusRegion.HoldingRegisters, out var r)
+            ? r : ModbusRegion.HoldingRegisters;
+        if (!TagConfigJson.TryReadEnumStrict(root, "dataType", DefaultDataType(region), out var dataType)) return;
+
+        if (root.TryGetProperty("region", out _) && !IsRegionTypeCompatible(region, dataType))
+        {
+            warnings.Add(region is ModbusRegion.Coils or ModbusRegion.DiscreteInputs
+                ? $"Modbus region {region} only carries Bool tags; got {dataType} — the tag is rejected (BadNodeIdUnknown)."
+                : $"Modbus Bool tags only live in Coils / DiscreteInputs; a {region} tag must use a register type "
+                  + "(use BitInRegister for a bit of a register) — the tag is rejected (BadNodeIdUnknown).");
+        }
 
         if (dataType == ModbusDataType.String && (ushort)ReadInt(root, "stringLength") < 1)
         {
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 648e4c99..eedfb70b 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs
@@ -1186,6 +1186,9 @@ public sealed class ModbusDriver
         {
             if (!table.TryGetValue(fullReferences[i], out var tag)) continue;
             if (tag.CoalesceProhibited) continue;
+            // Defensive: the factory refuses incompatible pairs, but the planner's register math must never
+            // throw for a tag that slipped through — one bad tag would otherwise fail the whole batch.
+            if (!ModbusTagDefinitionFactory.IsRegionTypeCompatible(tag.Region, tag.DataType)) continue;
             if (tag.ArrayCount.HasValue) continue;
             if (tag.Region is not (ModbusRegion.HoldingRegisters or ModbusRegion.InputRegisters)) continue;
             if (tag.DataType is ModbusDataType.String or ModbusDataType.BitInRegister) continue;
diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Uns/TagEditors/ModbusTagConfigEditor.razor b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Uns/TagEditors/ModbusTagConfigEditor.razor
index 116f9bd5..a3b38dd4 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Uns/TagEditors/ModbusTagConfigEditor.razor
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Uns/TagEditors/ModbusTagConfigEditor.razor
@@ -5,7 +5,7 @@
 
 <div class="row g-2">
     <div class="col-md-4"><label class="form-label">Region</label>
-        <select class="form-select form-select-sm" value="@_m.Region" @onchange="@(e => Update(() => _m.Region = ParseEnum(e.Value, ModbusRegion.HoldingRegisters)))">
+        <select class="form-select form-select-sm" value="@_m.Region" @onchange="@(e => Update(() => _m.SetRegion(ParseEnum(e.Value, ModbusRegion.HoldingRegisters))))">
             @foreach (var v in Enum.GetValues<ModbusRegion>()) { <option value="@v">@v</option> }
         </select></div>
     <div class="col-md-2"><label class="form-label">Address</label>
diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/TagEditors/ModbusTagConfigModel.cs b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/TagEditors/ModbusTagConfigModel.cs
index 323830ab..29050fc4 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/TagEditors/ModbusTagConfigModel.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/TagEditors/ModbusTagConfigModel.cs
@@ -30,6 +30,19 @@ public sealed class ModbusTagConfigModel
     /// key stays absent.</summary>
     public bool? Writable { get; set; }
 
+    /// <summary>
+    ///     Changes the region and keeps the data type compatible with it (the driver refuses a bit region
+    ///     with a non-Bool type, and a register region with Bool): switching to Coils / DiscreteInputs selects
+    ///     Bool, switching from them back to a register region selects Int16.
+    /// </summary>
+    /// <param name="region">The newly selected region.</param>
+    public void SetRegion(ModbusRegion region)
+    {
+        Region = region;
+        if (!ModbusTagDefinitionFactory.IsRegionTypeCompatible(region, DataType))
+            DataType = ModbusTagDefinitionFactory.DefaultDataType(region);
+    }
+
     private JsonObject _bag = new();
 
     // The blob this model was LOADED FROM, or null when it was built by object initialiser rather than
@@ -49,7 +62,9 @@ public sealed class ModbusTagConfigModel
             _source = json ?? "",
             Region = TagConfigJson.GetEnum(o, "region", ModbusRegion.HoldingRegisters),
             Address = TagConfigJson.GetInt(o, "address"),
-            DataType = TagConfigJson.GetEnum(o, "dataType", ModbusDataType.Int16),
+            // Absent dataType defaults per region, exactly as the driver's factory does (Bool for a bit region).
+            DataType = TagConfigJson.GetEnum(o, "dataType",
+                ModbusTagDefinitionFactory.DefaultDataType(TagConfigJson.GetEnum(o, "region", ModbusRegion.HoldingRegisters))),
             ByteOrder = TagConfigJson.GetEnum(o, "byteOrder", ModbusByteOrder.BigEndian),
             BitIndex = TagConfigJson.GetInt(o, "bitIndex"),
             StringLength = TagConfigJson.GetInt(o, "stringLength"),
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusArrayWriteSupportParityTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusArrayWriteSupportParityTests.cs
index a47a721a..6d2363b1 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusArrayWriteSupportParityTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusArrayWriteSupportParityTests.cs
@@ -25,10 +25,43 @@ public sealed class ModbusArrayWriteSupportParityTests
         return data;
     }
 
+    /// <summary>
+    ///     Shapes where the driver has moved ahead of the server-side table (review 07 P6 / U3, Modbus lane) and
+    ///     <see cref="DriverArrayWriteSupport"/> — owned outside the Modbus lane — has not followed yet
+    ///     (follow-up F1): the tag factory now REFUSES a Coils tag with a non-Bool type (so the driver serves no
+    ///     such tag), and the driver now encodes String[] and BitInRegister[] holding-register writes.
+    ///     <see cref="Pending_F1_divergences_still_exist"/> fails the moment the table follows, so this list
+    ///     is removed with it rather than left to rot.
+    /// </summary>
+    private static bool PendingF1(ModbusRegion region, ModbusDataType dataType) =>
+        (region == ModbusRegion.Coils && dataType != ModbusDataType.Bool)
+        || (region == ModbusRegion.HoldingRegisters && dataType is ModbusDataType.String or ModbusDataType.BitInRegister && PendingU3);
+
+    // Flipped to true by the U3 commit (String[] / BitInRegister[] encoders).
+    private const bool PendingU3 = false;
+
+    public static TheoryData<ModbusRegion, ModbusDataType> PendingShapes()
+    {
+        var data = new TheoryData<ModbusRegion, ModbusDataType>();
+        foreach (var (region, dt) in Shapes().Where(s => PendingF1(s.Region, s.DataType))) data.Add(region, dt);
+        return data;
+    }
+
+    [Theory]
+    [MemberData(nameof(PendingShapes))]
+    public void Pending_F1_divergences_still_exist(ModbusRegion region, ModbusDataType dataType)
+    {
+        var json = $$"""{"region":"{{region}}","address":10,"dataType":"{{dataType}}","stringLength":4,"isArray":true,"arrayLength":2}""";
+        var driverAccepts = region == ModbusRegion.HoldingRegisters; // String[] / BitInRegister[] encode; mistyped Coils are refused
+        DriverArrayWriteSupport.Supports(DriverTypeNames.Modbus, "Int16", json).ShouldNotBe(driverAccepts,
+            "F1 landed — DriverArrayWriteSupport now agrees with the driver for this shape; delete it from PendingF1");
+    }
+
     [Theory]
     [MemberData(nameof(AllShapes))]
     public async Task Table_agrees_with_the_driver_write_path(ModbusRegion region, ModbusDataType dataType)
     {
+        if (PendingF1(region, dataType)) return; // covered by Pending_F1_divergences_still_exist
         var json = $$"""{"region":"{{region}}","address":10,"dataType":"{{dataType}}","stringLength":4,"isArray":true,"arrayLength":2}""";
         var fake = new ModbusDriverTests.FakeTransport();
         var drv = new ModbusDriver(
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusRegionTypeValidationTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusRegionTypeValidationTests.cs
new file mode 100644
index 00000000..b4acdaee
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusRegionTypeValidationTests.cs
@@ -0,0 +1,70 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+/// <summary>
+///     Review 07 P6: region/type compatibility is enforced at the tag factory (and so at the deploy gate
+///     and the AdminUI parse consult). A <c>HoldingRegisters</c> + <c>Bool</c> tag used to throw out of the
+///     coalescing planner and kill the whole subscription's poll cycle; <c>Coils</c> + <c>Int16</c> silently
+///     returned a bool into an Int32 node.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class ModbusRegionTypeValidationTests
+{
+    [Theory]
+    [InlineData("""{"region":"Coils","address":1,"dataType":"Int16"}""")]
+    [InlineData("""{"region":"DiscreteInputs","address":1,"dataType":"Float32"}""")]
+    [InlineData("""{"region":"HoldingRegisters","address":1,"dataType":"Bool"}""")]
+    [InlineData("""{"region":"InputRegisters","address":1,"dataType":"Bool"}""")]
+    public void Incompatible_region_and_type_is_rejected(string json)
+    {
+        ModbusTagDefinitionFactory.FromTagConfig(json, "T", out _).ShouldBeFalse();
+        ModbusTagDefinitionFactory.Inspect(json).ShouldContain(w => w.Contains("only") || w.Contains("register"));
+    }
+
+    [Theory]
+    [InlineData("""{"region":"Coils","address":1,"dataType":"Bool"}""", ModbusDataType.Bool)]
+    [InlineData("""{"region":"Coils","address":1}""", ModbusDataType.Bool)]
+    [InlineData("""{"region":"DiscreteInputs","address":1}""", ModbusDataType.Bool)]
+    [InlineData("""{"region":"InputRegisters","address":1,"dataType":"BitInRegister","bitIndex":3}""", ModbusDataType.BitInRegister)]
+    [InlineData("""{"region":"HoldingRegisters","address":1}""", ModbusDataType.Int16)]
+    public void Compatible_region_and_type_is_accepted(string json, ModbusDataType expected)
+    {
+        // Positive control for the rejection theory, including the absent-dataType default per region.
+        ModbusTagDefinitionFactory.FromTagConfig(json, "T", out var def).ShouldBeTrue();
+        def.DataType.ShouldBe(expected);
+        ModbusTagDefinitionFactory.Inspect(json).ShouldBeEmpty();
+    }
+
+    [Fact]
+    public async Task A_mis_typed_tag_no_longer_kills_the_coalesced_poll_of_its_neighbours()
+    {
+        var fake = new ModbusDriverTests.FakeTransport();
+        fake.HoldingRegisters[100] = 7;
+        fake.HoldingRegisters[102] = 9;
+        var good1 = new ModbusTagDefinition("G1", ModbusRegion.HoldingRegisters, 100, ModbusDataType.Int16);
+        var good2 = new ModbusTagDefinition("G2", ModbusRegion.HoldingRegisters, 102, ModbusDataType.Int16);
+        var opts = new ModbusDriverOptions
+        {
+            Host = "f",
+            MaxReadGap = 5,
+            RawTags =
+            [
+                ModbusRawTags.Entry(good1),
+                new RawTagEntry("Bad", """{"region":"HoldingRegisters","address":101,"dataType":"Bool"}""", false),
+                ModbusRawTags.Entry(good2),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var drv = new ModbusDriver(opts, "m1", _ => fake);
+        await drv.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var r = await drv.ReadAsync(["G1", "Bad", "G2"], TestContext.Current.CancellationToken);
+        r[0].Value.ShouldBe((short)7);
+        r[1].StatusCode.ShouldBe(0x80340000u); // BadNodeIdUnknown — skipped at init, reported
+        r[2].Value.ShouldBe((short)9);
+        drv.GetSkippedTags()!.Sample.ShouldContain(s => s.RawPath == "Bad");
+    }
+}
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/ModbusTagConfigModelTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/ModbusTagConfigModelTests.cs
index 2bb03383..e1afd0c1 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/ModbusTagConfigModelTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/ModbusTagConfigModelTests.cs
@@ -121,4 +121,24 @@ public sealed class ModbusTagConfigModelTests
     [InlineData("""{"region":3,"address":1,"dataType":true}""")]
     public void Validate_accepts_every_enum_value_the_driver_accepts(string json)
         => ModbusTagConfigModel.FromJson(json).Validate().ShouldBeNull();
+
+    /// <summary>Review 07 P6: switching region keeps the data type one the driver accepts for it.</summary>
+    [Theory]
+    [InlineData(ModbusDataType.Int16, ModbusRegion.Coils, ModbusDataType.Bool)]
+    [InlineData(ModbusDataType.Float32, ModbusRegion.DiscreteInputs, ModbusDataType.Bool)]
+    [InlineData(ModbusDataType.Bool, ModbusRegion.HoldingRegisters, ModbusDataType.Int16)]
+    [InlineData(ModbusDataType.Float32, ModbusRegion.InputRegisters, ModbusDataType.Float32)]
+    public void SetRegion_keeps_the_type_compatible(ModbusDataType from, ModbusRegion region, ModbusDataType expected)
+    {
+        var m = new ModbusTagConfigModel { DataType = from };
+        m.SetRegion(region);
+        m.Region.ShouldBe(region);
+        m.DataType.ShouldBe(expected);
+        ModbusTagConfigModel.FromJson(m.ToJson()).Validate().ShouldBeNull();
+    }
+
+    /// <summary>A Coils + Int16 blob (CSV / API) is refused at authoring, matching the driver.</summary>
+    [Fact]
+    public void Validate_refuses_an_incompatible_region_and_type()
+        => ModbusTagConfigModel.FromJson("""{"region":"Coils","address":1,"dataType":"Int16"}""").Validate().ShouldNotBeNull();
 }