diff --git a/SVSim.UnitTests/Wire/ProfileIndexWireShape.cs b/SVSim.UnitTests/Wire/ProfileIndexWireShape.cs
new file mode 100644
index 0000000..03ff9ff
--- /dev/null
+++ b/SVSim.UnitTests/Wire/ProfileIndexWireShape.cs
@@ -0,0 +1,45 @@
+using System.Text.Json;
+using SVSim.EmulatedEntrypoint.Models.Dtos;
+
+namespace SVSim.UnitTests.Wire;
+
+public class ProfileIndexWireShape
+{
+    [Test]
+    public void UserClass_serialization_matches_prod_capture_for_main_class()
+    {
+        // Construct a UserClass mirroring the prod capture's class-1 entry from
+        // data_dumps/captures/traffic_prod_misc_clicking.ndjson line 26.
+        var entry = new UserClass
+        {
+            ClassId = 1,
+            IsAvailable = 1,
+            Level = 5,
+            Exp = 600,
+            IsRandomLeaderSkin = 0,
+            LeaderSkinId = 1,
+            LeaderSkinIds = new List<int> { 1 },
+            DefaultLeaderSkinId = 1,
+        };
+
+        var options = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
+            DefaultIgnoreCondition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull,
+        };
+        var json = JsonSerializer.Serialize(entry, options);
+
+        // Field order matches declaration order in UserClass.cs.
+        var expected =
+            "{\"class_id\":1," +
+            "\"is_available\":1," +
+            "\"level\":5," +
+            "\"exp\":600," +
+            "\"is_random_leader_skin\":0," +
+            "\"leader_skin_id\":1," +
+            "\"leader_skin_id_list\":[1]," +
+            "\"default_leader_skin_id\":1}";
+
+        Assert.That(json, Is.EqualTo(expected));
+    }
+}