diff --git a/SVSim.BattleNode/Protocol/MsgEnvelope.cs b/SVSim.BattleNode/Protocol/MsgEnvelope.cs
index 7baa7a6..0d505dd 100644
--- a/SVSim.BattleNode/Protocol/MsgEnvelope.cs
+++ b/SVSim.BattleNode/Protocol/MsgEnvelope.cs
@@ -24,9 +24,25 @@ public sealed record MsgEnvelope(
     // EngineIoHandshake). Every wire key here is explicit via the manual ToJson layering below.
     private static readonly JsonSerializerOptions Options = WireJsonOptions.CamelCase;
 
+    /// <summary>The fixed envelope wire keys, single-sourced. <see cref="ReservedEnvelopeKeys"/>,
+    /// the <see cref="ToJson"/> writes, and the <see cref="FromJson"/> reads all draw from here, so
+    /// the three encodings can't drift — adding a key in one place but not another (which would let a
+    /// body key silently shadow an envelope field) is no longer possible.</summary>
+    private static class Keys
+    {
+        public const string Uri = "uri";
+        public const string ViewerId = "viewerId";
+        public const string Uuid = "uuid";
+        public const string Bid = "bid";
+        public const string Try = "try";
+        public const string Cat = "cat";
+        public const string PubSeq = "pubSeq";
+        public const string PlaySeq = "playSeq";
+    }
+
     private static readonly HashSet<string> ReservedEnvelopeKeys = new()
     {
-        "uri", "viewerId", "uuid", "bid", "try", "cat", "pubSeq", "playSeq",
+        Keys.Uri, Keys.ViewerId, Keys.Uuid, Keys.Bid, Keys.Try, Keys.Cat, Keys.PubSeq, Keys.PlaySeq,
     };
 
     public static string ToJson(MsgEnvelope env)
@@ -37,14 +53,14 @@ public sealed record MsgEnvelope(
         // field processed before "uri" is wiped before Matching.StartBattleLoad reads
         // it back. The prod wire emits envelope keys first; we must too.
         var result = new JsonObject();
-        result["uri"] = env.Uri.ToString();
-        result["viewerId"] = env.ViewerId;
-        result["uuid"] = env.Uuid;
-        result["try"] = env.RetryAttempt;
-        result["cat"] = (int)env.Cat;
-        if (env.Bid is not null) result["bid"] = env.Bid;
-        if (env.PubSeq.HasValue) result["pubSeq"] = env.PubSeq.Value;
-        if (env.PlaySeq.HasValue) result["playSeq"] = env.PlaySeq.Value;
+        result[Keys.Uri] = env.Uri.ToString();
+        result[Keys.ViewerId] = env.ViewerId;
+        result[Keys.Uuid] = env.Uuid;
+        result[Keys.Try] = env.RetryAttempt;
+        result[Keys.Cat] = (int)env.Cat;
+        if (env.Bid is not null) result[Keys.Bid] = env.Bid;
+        if (env.PubSeq.HasValue) result[Keys.PubSeq] = env.PubSeq.Value;
+        if (env.PlaySeq.HasValue) result[Keys.PlaySeq] = env.PlaySeq.Value;
 
         if (env.Body is RawBody raw)
         {
@@ -118,14 +134,14 @@ public sealed record MsgEnvelope(
         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 retryAttempt = 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 uri = Enum.Parse<NetworkBattleUri>(root.GetProperty(Keys.Uri).GetString()!);
+        var viewerId = root.GetProperty(Keys.ViewerId).GetInt64();
+        var uuid = root.GetProperty(Keys.Uuid).GetString()!;
+        var bid = root.TryGetProperty(Keys.Bid, out var bidEl) ? bidEl.GetString() : null;
+        var retryAttempt = root.TryGetProperty(Keys.Try, out var tryEl) ? tryEl.GetInt32() : 0;
+        var cat = root.TryGetProperty(Keys.Cat, out var catEl) ? (EmitCategory)catEl.GetInt32() : EmitCategory.Battle;
+        var pubSeq = root.TryGetProperty(Keys.PubSeq, out var psEl) ? psEl.GetInt64() : (long?)null;
+        var playSeq = root.TryGetProperty(Keys.PlaySeq, out var plsEl) ? plsEl.GetInt64() : (long?)null;
 
         var bodyDict = new Dictionary<string, object?>();
         foreach (var prop in root.EnumerateObject())