yep

Shogi.Contracts/Socket/PlayerHasMovedMessage.cs

@@ -4,15 +4,17 @@ namespace Shogi.Contracts.Socket;
 
 public class PlayerHasMovedMessage : ISocketResponse
 {
-  public SocketAction Action { get; }
-  public string SessionName { get; set; }
-  /// <summary>
-  /// The player that made the move.
-  /// </summary>
-  public string PlayerName { get; set; }
+    public SocketAction Action { get; }
+    public string SessionName { get; set; }
+    /// <summary>
+    /// The player that made the move.
+    /// </summary>
+    public string PlayerName { get; set; }
 
-  public PlayerHasMovedMessage()
-  {
-    Action = SocketAction.PieceMoved;
-  }
+    public PlayerHasMovedMessage()
+    {
+        Action = SocketAction.PieceMoved;
+        SessionName = string.Empty;
+        PlayerName = string.Empty;
+    }
 }

Shogi.Contracts/Socket/SessionCreatedSocketMessage.cs

@@ -4,10 +4,5 @@ namespace Shogi.Contracts.Socket;
 
 public class SessionCreatedSocketMessage : ISocketResponse
 {
-  public SocketAction Action { get; }
-  
-  public SessionCreatedSocketMessage()
-  {
-    Action = SocketAction.SessionCreated;
-  }
+    public SocketAction Action => SocketAction.SessionCreated;
 }

Shogi.Contracts/Socket/SessionJoinedByPlayerSocketMessage.cs

@@ -0,0 +1,9 @@
+using Shogi.Contracts.Types;
+
+namespace Shogi.Contracts.Socket
+{
+    public class SessionJoinedByPlayerSocketMessage : ISocketResponse
+    {
+        public SocketAction Action => SocketAction.SessionJoined;
+    }
+}