diff --git a/infra/lmxfakeproxy/TagMapper.cs b/infra/lmxfakeproxy/TagMapper.cs
new file mode 100644
index 0000000..a41abf9
--- /dev/null
+++ b/infra/lmxfakeproxy/TagMapper.cs
@@ -0,0 +1,43 @@
+using LmxFakeProxy.Grpc;
+
+namespace LmxFakeProxy;
+
+public class TagMapper
+{
+    private readonly string _prefix;
+
+    public TagMapper(string prefix)
+    {
+        _prefix = prefix;
+    }
+
+    public string ToOpcNodeId(string lmxTag) => $"{_prefix}{lmxTag}";
+
+    public static object ParseWriteValue(string value)
+    {
+        if (double.TryParse(value, System.Globalization.NumberStyles.Float,
+            System.Globalization.CultureInfo.InvariantCulture, out var d))
+            return d;
+        if (bool.TryParse(value, out var b))
+            return b;
+        return value;
+    }
+
+    public static string MapQuality(uint statusCode)
+    {
+        if (statusCode == 0) return "Good";
+        if ((statusCode & 0x80000000) != 0) return "Bad";
+        return "Uncertain";
+    }
+
+    public static VtqMessage ToVtqMessage(string tag, object? value, DateTime timestampUtc, uint statusCode)
+    {
+        return new VtqMessage
+        {
+            Tag = tag,
+            Value = value?.ToString() ?? string.Empty,
+            TimestampUtcTicks = timestampUtc.Ticks,
+            Quality = MapQuality(statusCode)
+        };
+    }
+}
diff --git a/infra/lmxfakeproxy/tests/LmxFakeProxy.Tests/TagMappingTests.cs b/infra/lmxfakeproxy/tests/LmxFakeProxy.Tests/TagMappingTests.cs
new file mode 100644
index 0000000..69cf2e6
--- /dev/null
+++ b/infra/lmxfakeproxy/tests/LmxFakeProxy.Tests/TagMappingTests.cs
@@ -0,0 +1,84 @@
+using Xunit;
+
+namespace LmxFakeProxy.Tests;
+
+public class TagMappingTests
+{
+    [Fact]
+    public void ToOpcNodeId_PrependsPrefix()
+    {
+        var mapper = new TagMapper("ns=3;s=");
+        Assert.Equal("ns=3;s=Motor.Speed", mapper.ToOpcNodeId("Motor.Speed"));
+    }
+
+    [Fact]
+    public void ToOpcNodeId_CustomPrefix()
+    {
+        var mapper = new TagMapper("ns=2;s=MyFolder.");
+        Assert.Equal("ns=2;s=MyFolder.Pump.Pressure", mapper.ToOpcNodeId("Pump.Pressure"));
+    }
+
+    [Fact]
+    public void ToOpcNodeId_EmptyPrefix_PassesThrough()
+    {
+        var mapper = new TagMapper("");
+        Assert.Equal("Motor.Speed", mapper.ToOpcNodeId("Motor.Speed"));
+    }
+
+    [Fact]
+    public void ParseWriteValue_Double()
+    {
+        Assert.Equal(42.5, TagMapper.ParseWriteValue("42.5"));
+        Assert.IsType<double>(TagMapper.ParseWriteValue("42.5"));
+    }
+
+    [Fact]
+    public void ParseWriteValue_Bool()
+    {
+        Assert.Equal(true, TagMapper.ParseWriteValue("true"));
+        Assert.Equal(false, TagMapper.ParseWriteValue("False"));
+    }
+
+    [Fact]
+    public void ParseWriteValue_Uint()
+    {
+        // "100" parses as double first (double.TryParse succeeds for integers)
+        var result = TagMapper.ParseWriteValue("100");
+        Assert.IsType<double>(result);
+    }
+
+    [Fact]
+    public void ParseWriteValue_FallsBackToString()
+    {
+        Assert.Equal("hello", TagMapper.ParseWriteValue("hello"));
+        Assert.IsType<string>(TagMapper.ParseWriteValue("hello"));
+    }
+
+    [Fact]
+    public void MapStatusCode_Good()
+    {
+        Assert.Equal("Good", TagMapper.MapQuality(0));
+    }
+
+    [Fact]
+    public void MapStatusCode_Bad()
+    {
+        Assert.Equal("Bad", TagMapper.MapQuality(0x80000000));
+    }
+
+    [Fact]
+    public void MapStatusCode_Uncertain()
+    {
+        Assert.Equal("Uncertain", TagMapper.MapQuality(0x40000000));
+    }
+
+    [Fact]
+    public void ToVtqMessage_ConvertsCorrectly()
+    {
+        var vtq = TagMapper.ToVtqMessage("Motor.Speed", 42.5, DateTime.UtcNow, 0);
+        Assert.Equal("Motor.Speed", vtq.Tag);
+        Assert.Equal("42.5", vtq.Value);
+        Assert.Equal("Good", vtq.Quality);
+        Assert.True(vtq.TimestampUtcTicks > 0);
+    }
+}