diff --git a/SVSim.BattleNode/Sessions/BattleSession.cs b/SVSim.BattleNode/Sessions/BattleSession.cs
new file mode 100644
index 0000000..a7ef774
--- /dev/null
+++ b/SVSim.BattleNode/Sessions/BattleSession.cs
@@ -0,0 +1,119 @@
+using System.Net.WebSockets;
+using System.Text;
+using Microsoft.Extensions.Logging;
+using SVSim.BattleNode.Protocol;
+using SVSim.BattleNode.Reliability;
+using SVSim.BattleNode.Wire;
+
+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>
+/// One per connected client. Owns the WebSocket + reliability ledgers + lifecycle phase.
+/// </summary>
+public sealed class BattleSession
+{
+    private readonly WebSocket _ws;
+    private readonly ILogger<BattleSession> _log;
+
+    public string BattleId { get; }
+    public long ViewerId { get; }
+    public BattleSessionPhase Phase { get; internal set; } = BattleSessionPhase.AwaitingInitNetwork;
+    public InboundTracker Inbound { get; } = new();
+    public OutboundSequencer Outbound { get; } = new();
+
+    public BattleSession(WebSocket ws, string battleId, long viewerId, ILogger<BattleSession> log)
+    {
+        _ws = ws;
+        _log = log;
+        BattleId = battleId;
+        ViewerId = viewerId;
+    }
+
+    /// <summary>
+    /// Send the EIO3 open handshake then enter the read loop. Returns when the WS closes.
+    /// Dispatch is a no-op in this task; Task 13 fills it in.
+    /// </summary>
+    public async Task RunAsync(CancellationToken cancellation)
+    {
+        await SendEioOpenAsync(cancellation);
+
+        var buffer = new byte[8192];
+        var pendingAttachments = new List<byte[]>();
+        SocketIoFrame? pendingFrame = null;
+
+        while (_ws.State == WebSocketState.Open && !cancellation.IsCancellationRequested)
+        {
+            var msg = await ReadCompleteMessageAsync(buffer, cancellation);
+            if (msg is null) break;
+
+            if (msg.Value.IsText)
+            {
+                var text = Encoding.UTF8.GetString(msg.Value.Bytes);
+                if (text.Length == 0) continue;
+                var eio = EngineIoFrame.Parse(text);
+                if (eio.Type == EngineIoPacketType.Ping)
+                {
+                    await SendTextAsync("3", cancellation);  // EIO3 pong
+                    continue;
+                }
+                if (eio.Type != EngineIoPacketType.Message) continue;
+
+                // SIO inside the message payload.
+                var sio = SocketIoFrame.Parse(eio.Payload);
+                if (sio.AttachmentCount > 0)
+                {
+                    pendingFrame = sio;
+                    pendingAttachments.Clear();
+                    continue;
+                }
+                DispatchSocketIo(sio);
+            }
+            else
+            {
+                // Binary frame — an attachment for a pending binary event.
+                pendingAttachments.Add(msg.Value.Bytes);
+                if (pendingFrame is not null && pendingAttachments.Count == pendingFrame.AttachmentCount)
+                {
+                    var assembled = pendingFrame.WithAttachments(pendingAttachments.ToArray());
+                    pendingFrame = null;
+                    DispatchSocketIo(assembled);
+                }
+            }
+        }
+    }
+
+    private void DispatchSocketIo(SocketIoFrame frame)
+    {
+        // v1 task 11 stub: log and drop. Task 13 wires the lifecycle dispatch in.
+        _log.LogDebug("BattleSession {Bid}: received SIO {Type} event={Event}",
+            BattleId, frame.Type, frame.EventName);
+    }
+
+    private async Task SendEioOpenAsync(CancellationToken ct)
+    {
+        var sid = Guid.NewGuid().ToString("N").Substring(0, 16);
+        var handshake = new EngineIoHandshake(sid, Array.Empty<string>(), 25000, 60000).ToJson();
+        await SendTextAsync($"0{handshake}", ct);
+    }
+
+    private Task SendTextAsync(string text, CancellationToken ct)
+    {
+        var bytes = Encoding.UTF8.GetBytes(text);
+        return _ws.SendAsync(bytes, WebSocketMessageType.Text, endOfMessage: true, ct);
+    }
+
+    private async Task<(byte[] Bytes, bool IsText)?> ReadCompleteMessageAsync(byte[] buffer, CancellationToken ct)
+    {
+        using var ms = new MemoryStream();
+        WebSocketReceiveResult result;
+        do
+        {
+            try { result = await _ws.ReceiveAsync(buffer, ct); }
+            catch (OperationCanceledException) { return null; }
+            catch (WebSocketException) { return null; }
+            if (result.MessageType == WebSocketMessageType.Close) return null;
+            ms.Write(buffer, 0, result.Count);
+        } while (!result.EndOfMessage);
+        return (ms.ToArray(), result.MessageType == WebSocketMessageType.Text);
+    }
+}
diff --git a/SVSim.BattleNode/Sessions/BattleSessionPhase.cs b/SVSim.BattleNode/Sessions/BattleSessionPhase.cs
new file mode 100644
index 0000000..8e4abe1
--- /dev/null
+++ b/SVSim.BattleNode/Sessions/BattleSessionPhase.cs
@@ -0,0 +1,14 @@
+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>
+/// Where we are in the v1 scripted lifecycle. Drives which scripted frames the session pushes
+/// in response to inbound emits.
+/// </summary>
+public enum BattleSessionPhase
+{
+    AwaitingInitNetwork,
+    AwaitingLoaded,
+    AwaitingSwap,
+    AfterReady,
+    Terminal,
+}