diff --git a/src/AVEVA.Historian.Client/Wcf/HistorianHistoricalWriteProtocol.cs b/src/AVEVA.Historian.Client/Wcf/HistorianHistoricalWriteProtocol.cs
index c252c48..9fbbcef 100644
--- a/src/AVEVA.Historian.Client/Wcf/HistorianHistoricalWriteProtocol.cs
+++ b/src/AVEVA.Historian.Client/Wcf/HistorianHistoricalWriteProtocol.cs
@@ -32,8 +32,9 @@ namespace AVEVA.Historian.Client.Wcf;
 ///                Int8 → Int64(8) · UInt8 → UInt64(8)
 /// </code>
 ///
-/// Live-captured for Float/Double/Int2/Int4/UInt4; UInt1/Int8/UInt8 mirror the captured
-/// Double value layout (8 LE bytes for Int8/UInt8, 1 byte for UInt1) pending live round-trip.
+/// Live-captured for Float/Double/Int2/Int4/UInt4; Int8/UInt8 mirror the captured
+/// Double value layout (8 LE bytes) pending live round-trip. UInt1 is re-gated: the
+/// historian accepts EnsureTags(UInt1) but stores a degenerate tag — writes would fail.
 /// Other tag types have no captured value encoding and are rejected.
 /// </remarks>
 internal static class HistorianHistoricalWriteProtocol
@@ -54,7 +55,8 @@ internal static class HistorianHistoricalWriteProtocol
     /// <paramref name="tagGuid"/> is the per-tag GUID (from the gRPC tag-info read),
     /// <paramref name="dataType"/> is the tag's declared analog type (selects the value width), and
     /// <paramref name="receivedTimeUtc"/> is the storage/received timestamp the orchestrator stamps.
-    /// Throws <see cref="ProtocolEvidenceMissingException"/> for tag types without a captured encoding.
+    /// Throws <see cref="ProtocolEvidenceMissingException"/> for tag types without a captured encoding
+    /// (including UInt1, which is re-gated — the historian creates a degenerate UInt1 analog tag).
     /// Int8/UInt8 carry exact magnitude only up to 2^53 — the value API is <see langword="double"/>;
     /// full 64-bit range is a separate follow-on.
     /// </summary>
@@ -121,10 +123,6 @@ internal static class HistorianHistoricalWriteProtocol
                 BinaryPrimitives.WriteUInt32LittleEndian(b, checked((uint)value));
                 return b;
             }
-            case HistorianDataType.UInt1:
-            {
-                return [checked((byte)value)];
-            }
             case HistorianDataType.Int8:
             {
                 byte[] b = new byte[8];
@@ -140,7 +138,7 @@ internal static class HistorianHistoricalWriteProtocol
             default:
                 throw new ProtocolEvidenceMissingException(
                     $"AddHistoricalValuesAsync has no captured value encoding for tag data type '{dataType}'. " +
-                    "Captured types: Float, Double, Int2, Int4, UInt4, UInt1, Int8, UInt8.");
+                    "Captured types: Float, Double, Int2, Int4, UInt4, Int8, UInt8.");
         }
     }
 
diff --git a/src/AVEVA.Historian.Client/Wcf/HistorianTagWriteProtocol.cs b/src/AVEVA.Historian.Client/Wcf/HistorianTagWriteProtocol.cs
index f5e2aaa..852d25d 100644
--- a/src/AVEVA.Historian.Client/Wcf/HistorianTagWriteProtocol.cs
+++ b/src/AVEVA.Historian.Client/Wcf/HistorianTagWriteProtocol.cs
@@ -60,8 +60,11 @@ internal static class HistorianTagWriteProtocol
     /// <summary>
     /// Native CDataType wire codes per data type — captured by probing every type via
     /// instrument-wcf-writemessage. Matches the codes already documented in
-    /// <see cref="HistorianWcfTagClient"/> MapDataType for the read path; UInt1/Int8/UInt8
-    /// reuse the same read-side codes (0x08/0x19/0x39).
+    /// <see cref="HistorianWcfTagClient"/> MapDataType for the read path; Int8/UInt8
+    /// reuse the same read-side codes (0x19/0x39).
+    /// UInt1 is excluded: live evidence shows the historian accepts EnsureTags(UInt1)
+    /// with success but stores a degenerate tag (null type descriptor) that subsequent
+    /// GetTagInfo cannot parse — not client-fixable on the analog path.
     /// </summary>
     public static byte GetAnalogDataTypeCode(Models.HistorianDataType dataType) => dataType switch
     {
@@ -71,11 +74,10 @@ internal static class HistorianTagWriteProtocol
         Models.HistorianDataType.UInt4 => 0x11,
         Models.HistorianDataType.Int2 => 0x29,
         Models.HistorianDataType.Int4 => 0x31,
-        Models.HistorianDataType.UInt1 => 0x08,
         Models.HistorianDataType.Int8 => 0x19,
         Models.HistorianDataType.UInt8 => 0x39,
         _ => throw new ProtocolEvidenceMissingException(
-            $"EnsureTagAsync data type {dataType} has no captured CTagMetadata wire code; supported: Float, Double, UInt2, UInt4, Int2, Int4, UInt1, Int8, UInt8."),
+            $"EnsureTagAsync data type {dataType} has no captured CTagMetadata wire code; supported: Float, Double, UInt2, UInt4, Int2, Int4, Int8, UInt8."),
     };
 
     private static readonly byte[] AnalogPadding16 = new byte[16];
@@ -122,9 +124,10 @@ internal static class HistorianTagWriteProtocol
 
     /// <summary>
     /// Serializes a CTagMetadata payload for an analog tag. Live-verified for Float,
-    /// Double, Int2, Int4, UInt4; UInt1/Int8/UInt8 supported via the read-side type
-    /// codes (pending live round-trip) — see <see cref="GetAnalogDataTypeCode"/> for the
-    /// type-code mapping. Output matches the byte-for-byte capture for the same inputs.
+    /// Double, Int2, Int4, UInt4; Int8/UInt8 supported via the read-side type codes
+    /// (pending live round-trip); UInt1 re-gated (historian creates a degenerate tag) —
+    /// see <see cref="GetAnalogDataTypeCode"/> for the type-code mapping. Output matches
+    /// the byte-for-byte capture for the same inputs.
     /// When MinEU/MaxEU/MinRaw/MaxRaw are all defaults (0/100/0/100) emits the compact
     /// `1A 03` scaling marker; otherwise emits `1F` + 4 doubles in order.
     /// </summary>
diff --git a/tests/AVEVA.Historian.Client.Tests/HistorianTagWriteProtocolTests.cs b/tests/AVEVA.Historian.Client.Tests/HistorianTagWriteProtocolTests.cs
index dba3736..141c7c0 100644
--- a/tests/AVEVA.Historian.Client.Tests/HistorianTagWriteProtocolTests.cs
+++ b/tests/AVEVA.Historian.Client.Tests/HistorianTagWriteProtocolTests.cs
@@ -62,7 +62,7 @@ public sealed class HistorianTagWriteProtocolTests
         + "09120052657465737453646B5772697465496E7432"
         + "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF"
         + "09180053444B2077726974652D52452073616E64626F78207461670904004D44415302010100000001E8030000549B0E36EDDBDC011A030904007465737410270000000000000000F03FFE00")]
-    // UInt1/Int8/UInt8 (H1 un-gate): prefix is the captured Double buffer; only the data-type code byte differs.
+    // Int8/UInt8 (H1 un-gate): prefix is the captured Double buffer; only the data-type code byte differs.
     [InlineData(
         AVEVA.Historian.Client.Models.HistorianDataType.Int8,
         "RetestSdkWriteDouble", 0x01dcdbed24988f3aL,
@@ -77,13 +77,6 @@ public sealed class HistorianTagWriteProtocolTests
         + "09140052657465737453646B5772697465446F75626C65"
         + "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF"
         + "09180053444B2077726974652D52452073616E64626F78207461670904004D44415302010100000001E80300003A8F9824EDDBDC011A030904007465737410270000000000000000F03FFE00")]
-    [InlineData(
-        AVEVA.Historian.Client.Models.HistorianDataType.UInt1,
-        "RetestSdkWriteDouble", 0x01dcdbed24988f3aL,
-        "4E6703000100000004C6020800000000000000000000000000000000"
-        + "09140052657465737453646B5772697465446F75626C65"
-        + "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF"
-        + "09180053444B2077726974652D52452073616E64626F78207461670904004D44415302010100000001E80300003A8F9824EDDBDC011A030904007465737410270000000000000000F03FFE00")]
     public void SerializeAnalogCTagMetadata_PerDataType_MatchesCapturedNativeBytes(
         AVEVA.Historian.Client.Models.HistorianDataType dataType,
         string tagName,
@@ -271,6 +264,9 @@ public sealed class HistorianTagWriteProtocolTests
             () => HistorianTagWriteProtocol.GetAnalogDataTypeCode(AVEVA.Historian.Client.Models.HistorianDataType.SingleByteString));
         Assert.Throws<ProtocolEvidenceMissingException>(
             () => HistorianTagWriteProtocol.GetAnalogDataTypeCode(AVEVA.Historian.Client.Models.HistorianDataType.Int1));
+        // UInt1 re-gated: historian creates a degenerate UInt1 analog tag (null type descriptor) — see pending notes.
+        Assert.Throws<ProtocolEvidenceMissingException>(
+            () => HistorianTagWriteProtocol.GetAnalogDataTypeCode(AVEVA.Historian.Client.Models.HistorianDataType.UInt1));
     }
 
     [Fact]
diff --git a/tests/AVEVA.Historian.Client.Tests/WcfHistoricalWriteProtocolTests.cs b/tests/AVEVA.Historian.Client.Tests/WcfHistoricalWriteProtocolTests.cs
index 98b902c..0a68062 100644
--- a/tests/AVEVA.Historian.Client.Tests/WcfHistoricalWriteProtocolTests.cs
+++ b/tests/AVEVA.Historian.Client.Tests/WcfHistoricalWriteProtocolTests.cs
@@ -21,13 +21,11 @@ public sealed class WcfHistoricalWriteProtocolTests
         "4f4e0100380000002e00" + "ca9735f7f841b244b56f9c14ccfeac32" + "b09bc72fd701dd01" + "c000" + "c0100100" + "104b59f3e701dd01" + "0000000039300000")]
     [InlineData(HistorianDataType.UInt4, 305419896d,
         "4f4e0100380000002e00" + "e7ae22d8e4cc65439ebd8bcb09402974" + "602d6663d701dd01" + "c000" + "c0100100" + "498af726e801dd01" + "0000000078563412")]
-    // UInt1/Int8/UInt8 (H1 un-gate): prefix is the captured Double buffer; only the data-type code / value bytes differ. Live round-trip confirms (gateway HistorianTypeRoundTripTests).
+    // Int8/UInt8 (H1 un-gate): prefix is the captured Double buffer; only the value bytes differ. Live round-trip confirms (gateway HistorianTypeRoundTripTests).
     [InlineData(HistorianDataType.Int8, 100000d,
         "4f4e01003c0000003200" + "7f970bb0b5a7344bb7bf6899ff06d027" + "c07d5f23d701dd01" + "c000" + "c0100100" + "08eff1e6e701dd01" + "00000000a086010000000000")]
     [InlineData(HistorianDataType.UInt8, 100001d,
         "4f4e01003c0000003200" + "7f970bb0b5a7344bb7bf6899ff06d027" + "c07d5f23d701dd01" + "c000" + "c0100100" + "08eff1e6e701dd01" + "00000000a186010000000000")]
-    [InlineData(HistorianDataType.UInt1, 7d,
-        "4f4e0100" + "35000000" + "2b00" + "7f970bb0b5a7344bb7bf6899ff06d027" + "c07d5f23d701dd01" + "c000" + "c0100100" + "08eff1e6e701dd01" + "00000000" + "07")]
     public void SerializeAddStreamValuesBuffer_MatchesCapturedNativeBuffer(HistorianDataType dataType, double value, string capturedHex)
     {
         byte[] captured = Convert.FromHexString(capturedHex);
@@ -54,6 +52,10 @@ public sealed class WcfHistoricalWriteProtocolTests
         Assert.Throws<ProtocolEvidenceMissingException>(() =>
             HistorianHistoricalWriteProtocol.SerializeAddStreamValuesBuffer(
                 Guid.NewGuid(), HistorianDataType.SingleByteString, DateTime.UtcNow, 1.0, DateTime.UtcNow));
+        // UInt1 re-gated: historian creates a degenerate UInt1 analog tag (null type descriptor) — see pending notes.
+        Assert.Throws<ProtocolEvidenceMissingException>(() =>
+            HistorianHistoricalWriteProtocol.SerializeAddStreamValuesBuffer(
+                Guid.NewGuid(), HistorianDataType.UInt1, DateTime.UtcNow, 1.0, DateTime.UtcNow));
     }
 
     [Fact]