diff --git a/SVSim.BattleNode/Protocol/Bodies/BattleStartBody.cs b/SVSim.BattleNode/Protocol/Bodies/BattleStartBody.cs
new file mode 100644
index 0000000..31a0239
--- /dev/null
+++ b/SVSim.BattleNode/Protocol/Bodies/BattleStartBody.cs
@@ -0,0 +1,28 @@
+using System.Text.Json.Serialization;
+
+namespace SVSim.BattleNode.Protocol.Bodies;
+
+public sealed record BattleStartBody(
+    [property: JsonPropertyName("turnState")] int TurnState,
+    [property: JsonPropertyName("battleType")] int BattleType,
+    [property: JsonPropertyName("selfInfo")] BattleStartSelfInfo SelfInfo,
+    [property: JsonPropertyName("oppoInfo")] BattleStartOppoInfo OppoInfo,
+    [property: JsonPropertyName("resultCode")] int ResultCode = 1) : IMsgBody;
+
+public sealed record BattleStartSelfInfo(
+    [property: JsonPropertyName("rank")] string Rank,
+    [property: JsonPropertyName("battlePoint")] string BattlePoint,
+    [property: JsonPropertyName("classId")] string ClassId,
+    [property: JsonPropertyName("charaId")] string CharaId,
+    [property: JsonPropertyName("cardMasterName")] string CardMasterName);
+
+// Note: BattlePoint is int on the wire here (not string as on self) — matches the
+// captured prod frame at data_dumps/captures/battle-traffic_tk2_regular.ndjson.
+public sealed record BattleStartOppoInfo(
+    [property: JsonPropertyName("rank")] string Rank,
+    [property: JsonPropertyName("isMasterRank")] string IsMasterRank,
+    [property: JsonPropertyName("battlePoint")] int BattlePoint,
+    [property: JsonPropertyName("masterPoint")] string MasterPoint,
+    [property: JsonPropertyName("classId")] string ClassId,
+    [property: JsonPropertyName("charaId")] string CharaId,
+    [property: JsonPropertyName("cardMasterName")] string CardMasterName);
diff --git a/SVSim.UnitTests/BattleNode/Protocol/Bodies/BattleStartBodyTests.cs b/SVSim.UnitTests/BattleNode/Protocol/Bodies/BattleStartBodyTests.cs
new file mode 100644
index 0000000..21b7620
--- /dev/null
+++ b/SVSim.UnitTests/BattleNode/Protocol/Bodies/BattleStartBodyTests.cs
@@ -0,0 +1,54 @@
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using NUnit.Framework;
+using SVSim.BattleNode.Protocol.Bodies;
+
+namespace SVSim.UnitTests.BattleNode.Protocol.Bodies;
+
+[TestFixture]
+public class BattleStartBodyTests
+{
+    [Test]
+    public void Serializes_TopLevelFields_WithCorrectWireKeys()
+    {
+        var body = new BattleStartBody(
+            TurnState: 0, BattleType: 11,
+            SelfInfo: new BattleStartSelfInfo("10", "6270", "1", "1", "card_master_node_10015"),
+            OppoInfo: new BattleStartOppoInfo("1", "0", 0, "0", "8", "8", "card_master_node_10015"));
+
+        var node = (JsonObject)JsonSerializer.SerializeToNode(body)!;
+
+        Assert.That(node["turnState"]!.GetValue<int>(), Is.EqualTo(0));
+        Assert.That(node["battleType"]!.GetValue<int>(), Is.EqualTo(11));
+        Assert.That(node["resultCode"]!.GetValue<int>(), Is.EqualTo(1));   // default
+    }
+
+    [Test]
+    public void SelfInfo_BattlePoint_IsString_OnTheWire()
+    {
+        var body = new BattleStartBody(0, 11,
+            new BattleStartSelfInfo("10", "6270", "1", "1", "cm"),
+            new BattleStartOppoInfo("1", "0", 0, "0", "8", "8", "cm"));
+
+        var node = (JsonObject)JsonSerializer.SerializeToNode(body)!;
+        var selfInfo = (JsonObject)node["selfInfo"]!;
+
+        // Wire shape: self.battlePoint is a string, oppo.battlePoint is an int. Preserved verbatim.
+        Assert.That(selfInfo["battlePoint"]!.GetValue<string>(), Is.EqualTo("6270"));
+    }
+
+    [Test]
+    public void OppoInfo_BattlePoint_IsInt_OnTheWire()
+    {
+        var body = new BattleStartBody(0, 11,
+            new BattleStartSelfInfo("10", "6270", "1", "1", "cm"),
+            new BattleStartOppoInfo("1", "0", 0, "0", "8", "8", "cm"));
+
+        var node = (JsonObject)JsonSerializer.SerializeToNode(body)!;
+        var oppoInfo = (JsonObject)node["oppoInfo"]!;
+
+        Assert.That(oppoInfo["battlePoint"]!.GetValue<int>(), Is.EqualTo(0));
+        Assert.That(oppoInfo["isMasterRank"]!.GetValue<string>(), Is.EqualTo("0"));
+        Assert.That(oppoInfo["masterPoint"]!.GetValue<string>(), Is.EqualTo("0"));
+    }
+}