diff --git a/tests/ProcessBus.Tests/ArstackM3hSvInteroperabilityTests.cs b/tests/ProcessBus.Tests/ArstackM3hSvInteroperabilityTests.cs
new file mode 100644
index 00000000..10282aab
--- /dev/null
+++ b/tests/ProcessBus.Tests/ArstackM3hSvInteroperabilityTests.cs
@@ -0,0 +1,77 @@
+using System.Buffers.Binary;
+using ProcessBus.Iec61850.Raw.Analysis;
+using ProcessBus.Iec61850.Raw.Decoding;
+using Xunit;
+
+namespace ProcessBus.Tests;
+
+/// <summary>
+/// Regression for the 24 Oct 2026 ARStack ESP32-P4 M3.H 4I+4V reference
+/// profile. Independent BER / Ethernet fixture, not a DigSub-generated profile.
+/// This proves decoder acceptance, not packet arrival through a real NIC.
+/// </summary>
+public sealed class ArstackM3hSvInteroperabilityTests
+{
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void M3hReference_SvWithPriorityTaggedVlanZeroAndUntagged_Decodes(bool tagged)
+    {
+        var frame = MakeM3hReferenceFrame(tagged, sampleCount: 3999);
+        Assert.True(RawProcessBusDecoder.TryDecode(frame, out var result));
+        Assert.True(result.HasDecodedApdu);
+        Assert.NotNull(result.SampledValues);
+        Assert.Equal((ushort)0x4000, result.Frame.AppId);
+        if (tagged)
+        {
+            Assert.Equal((ushort)0, result.Frame.Ethernet.Vlan?.VlanId);
+            Assert.Equal((byte)4, result.Frame.Ethernet.Vlan?.PriorityCodePoint);
+        }
+        else
+        {
+            Assert.Null(result.Frame.Ethernet.Vlan);
+        }
+
+        var asdu = Assert.Single(result.SampledValues!.Asdus);
+        Assert.Equal("MU01_SV1", asdu.SvId);
+        Assert.Equal((ushort)3999, asdu.SmpCnt);
+        Assert.Equal((uint)1, asdu.ConfRev);
+        Assert.Equal((byte)2, asdu.SmpSynch);
+        Assert.Equal(64, asdu.SamplePayload.Length);
+        Assert.Equal(2500, BinaryPrimitives.ReadInt32BigEndian(asdu.SamplePayload.Span[..4]));
+        Assert.Equal(0u, BinaryPrimitives.ReadUInt32BigEndian(asdu.SamplePayload.Span.Slice(4, 4)));
+
+        var analyzer = new RawProcessBusAnalyzer();
+        analyzer.ObserveFrame(frame);
+        var snapshot = analyzer.GetAnalyzerSnapshot();
+        var stream = Assert.Single(snapshot.Streams);
+        Assert.Equal("MU01_SV1", stream.SvId);
+    }
+
+    private static byte[] MakeM3hReferenceFrame(bool tagged, ushort sampleCount)
+    {
+        var values = new[] { 2500, -1250, -1250, 0, 330000, -165000, -165000, 0 };
+        var payload = new byte[64];
+        for (var i = 0; i < values.Length; i++)
+            BinaryPrimitives.WriteInt32BigEndian(payload.AsSpan(i * 8, 4), values[i]);
+
+        // M3.H prebuilt template uses fixed 2-byte smpCnt and 4-byte confRev.
+        // Optional gmIdentity uses context-specific tag 9.
+        var asdu = GoldenFrames.Tlv(0x30,
+            GoldenFrames.Tlv(0x80, GoldenFrames.Ascii("MU01_SV1")),
+            GoldenFrames.Tlv(0x82, GoldenFrames.U16(sampleCount)),
+            GoldenFrames.Tlv(0x83, new byte[] { 0x00, 0x00, 0x00, 0x01 }),
+            GoldenFrames.Tlv(0x85, new byte[] { 0x02 }),
+            GoldenFrames.Tlv(0x87, payload),
+            GoldenFrames.Tlv(0x89, new byte[] { 0x02, 0x11, 0x22, 0xFF, 0xFE, 0x33, 0x44, 0x55 }));
+
+        var apdu = GoldenFrames.Tlv(0x60,
+            GoldenFrames.Tlv(0x80, new byte[] { 0x01 }),
+            GoldenFrames.Tlv(0xA2, asdu));
+
+        return GoldenFrames.EthernetFrame(
+            GoldenFrames.SvEtherType,
+            GoldenFrames.ProcessBusPayload(0x4000, apdu),
+            tagged ? (ushort)0x8000 : null);
+    }
+}