diff --git a/SVSim.BattleNode/Protocol/EmitCategory.cs b/SVSim.BattleNode/Protocol/EmitCategory.cs
new file mode 100644
index 0000000..87477c3
--- /dev/null
+++ b/SVSim.BattleNode/Protocol/EmitCategory.cs
@@ -0,0 +1,11 @@
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>The "cat" field on the msg envelope.</summary>
+public enum EmitCategory
+{
+    Battle = 1,
+    Matching = 2,
+    Room = 3,
+    Watch = 11,
+    General = 99,
+}
diff --git a/SVSim.BattleNode/Protocol/MsgEnvelope.cs b/SVSim.BattleNode/Protocol/MsgEnvelope.cs
new file mode 100644
index 0000000..a45d665
--- /dev/null
+++ b/SVSim.BattleNode/Protocol/MsgEnvelope.cs
@@ -0,0 +1,89 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>
+/// The shared envelope on every encrypted msg / synchronize frame. Body is opaque
+/// (Dictionary&lt;string, object?&gt;) because shape varies per Uri — typed register-action
+/// models come in a later slice.
+/// </summary>
+public sealed record MsgEnvelope(
+    NetworkBattleUri Uri,
+    long ViewerId,
+    string Uuid,
+    string? Bid,
+    int Try,
+    EmitCategory Cat,
+    long? PubSeq,
+    long? PlaySeq,
+    Dictionary<string, object?> Body)
+{
+    private static readonly JsonSerializerOptions Options = CreateOptions();
+
+    private static JsonSerializerOptions CreateOptions()
+    {
+        var opt = new JsonSerializerOptions
+        {
+            // Wire-key casing here is bare camelCase — NOT EmulatedEntrypoint's snake_case policy.
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        };
+        opt.Converters.Add(new JsonStringEnumConverter());
+        return opt;
+    }
+
+    public static string ToJson(MsgEnvelope env)
+    {
+        var doc = new Dictionary<string, object?>
+        {
+            ["uri"] = env.Uri.ToString(),
+            ["viewerId"] = env.ViewerId,
+            ["uuid"] = env.Uuid,
+            ["try"] = env.Try,
+            ["cat"] = (int)env.Cat,
+        };
+        if (env.Bid is not null) doc["bid"] = env.Bid;
+        if (env.PubSeq.HasValue) doc["pubSeq"] = env.PubSeq.Value;
+        if (env.PlaySeq.HasValue) doc["playSeq"] = env.PlaySeq.Value;
+        foreach (var (k, v) in env.Body) doc[k] = v;
+        return JsonSerializer.Serialize(doc, Options);
+    }
+
+    public static MsgEnvelope FromJson(string json)
+    {
+        using var doc = JsonDocument.Parse(json);
+        var root = doc.RootElement;
+
+        var uri = Enum.Parse<NetworkBattleUri>(root.GetProperty("uri").GetString()!);
+        var viewerId = root.GetProperty("viewerId").GetInt64();
+        var uuid = root.GetProperty("uuid").GetString()!;
+        var bid = root.TryGetProperty("bid", out var bidEl) ? bidEl.GetString() : null;
+        var @try = root.TryGetProperty("try", out var tryEl) ? tryEl.GetInt32() : 0;
+        var cat = root.TryGetProperty("cat", out var catEl) ? (EmitCategory)catEl.GetInt32() : EmitCategory.Battle;
+        var pubSeq = root.TryGetProperty("pubSeq", out var psEl) ? psEl.GetInt64() : (long?)null;
+        var playSeq = root.TryGetProperty("playSeq", out var plsEl) ? plsEl.GetInt64() : (long?)null;
+
+        var body = new Dictionary<string, object?>();
+        var reserved = new HashSet<string> { "uri", "viewerId", "uuid", "bid", "try", "cat", "pubSeq", "playSeq" };
+        foreach (var prop in root.EnumerateObject())
+        {
+            if (reserved.Contains(prop.Name)) continue;
+            body[prop.Name] = ToObject(prop.Value);
+        }
+
+        return new MsgEnvelope(uri, viewerId, uuid, bid, @try, cat, pubSeq, playSeq, body);
+    }
+
+    private static object? ToObject(JsonElement el) => el.ValueKind switch
+    {
+        JsonValueKind.String => el.GetString(),
+        JsonValueKind.Number => el.TryGetInt64(out var l) ? l : el.GetDouble(),
+        JsonValueKind.True => true,
+        JsonValueKind.False => false,
+        JsonValueKind.Null => null,
+        JsonValueKind.Array => el.EnumerateArray().Select(ToObject).ToList(),
+        JsonValueKind.Object => el.EnumerateObject().ToDictionary(p => p.Name, p => ToObject(p.Value)),
+        _ => el.GetRawText(),
+    };
+}
diff --git a/SVSim.BattleNode/Protocol/NetworkBattleUri.cs b/SVSim.BattleNode/Protocol/NetworkBattleUri.cs
new file mode 100644
index 0000000..b5420e5
--- /dev/null
+++ b/SVSim.BattleNode/Protocol/NetworkBattleUri.cs
@@ -0,0 +1,46 @@
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>
+/// Discriminator for every msg/synchronize envelope. Wire form is the bare member name
+/// (case-sensitive). See docs/api-spec/in-battle/enums.md.
+/// </summary>
+public enum NetworkBattleUri
+{
+    None,
+    Resume,
+    Retry,
+    InitNetwork,
+    InitBattle,
+    InitRoomBattle,
+    Matched,
+    Loaded,
+    Deal,
+    Swap,
+    Ready,
+    TurnStart,
+    TurnEndActions,
+    TurnEnd,
+    TurnEndFinal,
+    PlayActions,
+    BattleStart,
+    BattleFinish,
+    ChatStamp,
+    Gungnir,
+    Echo,
+    Retire,
+    OppoDisconnect,
+    End,
+    Judge,
+    Touch,
+    SelectSkill,
+    SelectObject,
+    SlideObject,
+    TurnEndReady,
+    RecoveryStart,
+    RecoveryEnd,
+    JudgeResult,
+    Maintenance,
+    ReplayFinish,
+    Kill,
+    Watch,
+}
diff --git a/SVSim.BattleNode/Protocol/ReceiveNodeResultCode.cs b/SVSim.BattleNode/Protocol/ReceiveNodeResultCode.cs
new file mode 100644
index 0000000..2c1a3a9
--- /dev/null
+++ b/SVSim.BattleNode/Protocol/ReceiveNodeResultCode.cs
@@ -0,0 +1,16 @@
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>The "resultCode" field on synchronize pushes. 1=Success, else error.</summary>
+public enum ReceiveNodeResultCode
+{
+    None = 0,
+    Success = 1,
+    Different_UUID = 30001,
+    RedisReplyError = 30002,
+    UnexistUserinfoError = 30003,
+    MatchingTimeOut = 30201,
+    UnmatchedError = 30211,
+    CurrentBattleError = 30212,
+    UnexpectedPhaseError = 30213,
+    // Other codes from spec/enums.md added as needed; v1 only triggers the four above.
+}
diff --git a/SVSim.UnitTests/BattleNode/Protocol/MsgEnvelopeTests.cs b/SVSim.UnitTests/BattleNode/Protocol/MsgEnvelopeTests.cs
new file mode 100644
index 0000000..04e66e9
--- /dev/null
+++ b/SVSim.UnitTests/BattleNode/Protocol/MsgEnvelopeTests.cs
@@ -0,0 +1,66 @@
+using System.Text.Json;
+using NUnit.Framework;
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.UnitTests.BattleNode.Protocol;
+
+[TestFixture]
+public class MsgEnvelopeTests
+{
+    [Test]
+    public void Roundtrip_PreservesEnvelopeAndBody()
+    {
+        var env = new MsgEnvelope(
+            Uri: NetworkBattleUri.InitNetwork,
+            ViewerId: 906243102,
+            Uuid: "udid-1234",
+            Bid: "597830888107",
+            Try: 0,
+            Cat: EmitCategory.General,
+            PubSeq: null,
+            PlaySeq: null,
+            Body: new Dictionary<string, object?> { ["foo"] = 42 });
+
+        var json = MsgEnvelope.ToJson(env);
+        var back = MsgEnvelope.FromJson(json);
+
+        Assert.That(back.Uri, Is.EqualTo(NetworkBattleUri.InitNetwork));
+        Assert.That(back.ViewerId, Is.EqualTo(906243102));
+        Assert.That(back.Uuid, Is.EqualTo("udid-1234"));
+        Assert.That(back.Bid, Is.EqualTo("597830888107"));
+        Assert.That(back.Cat, Is.EqualTo(EmitCategory.General));
+        Assert.That(back.Body["foo"], Is.EqualTo(42L));   // JsonElement → int64
+    }
+
+    [Test]
+    public void ToJson_OmitsNullEnvelopeFields()
+    {
+        var env = new MsgEnvelope(
+            Uri: NetworkBattleUri.Ready,
+            ViewerId: 1,
+            Uuid: "u",
+            Bid: null,
+            Try: 0,
+            Cat: EmitCategory.Battle,
+            PubSeq: null,
+            PlaySeq: 5,
+            Body: new Dictionary<string, object?>());
+
+        var json = MsgEnvelope.ToJson(env);
+
+        Assert.That(json, Does.Not.Contain("\"bid\""));
+        Assert.That(json, Does.Not.Contain("\"pubSeq\""));
+        Assert.That(json, Does.Contain("\"playSeq\":5"));
+    }
+
+    [Test]
+    public void FromJson_DispatchesUriToEnum()
+    {
+        const string json = "{\"uri\":\"PlayActions\",\"viewerId\":1,\"uuid\":\"u\",\"try\":0,\"cat\":1}";
+
+        var env = MsgEnvelope.FromJson(json);
+
+        Assert.That(env.Uri, Is.EqualTo(NetworkBattleUri.PlayActions));
+        Assert.That(env.Cat, Is.EqualTo(EmitCategory.Battle));
+    }
+}