diff --git a/SVSim.BattleNode/Wire/SocketIoFrame.cs b/SVSim.BattleNode/Wire/SocketIoFrame.cs
new file mode 100644
index 0000000..7001598
--- /dev/null
+++ b/SVSim.BattleNode/Wire/SocketIoFrame.cs
@@ -0,0 +1,171 @@
+using System.Text;
+using System.Text.Json;
+
+namespace SVSim.BattleNode.Wire;
+
+/// <summary>
+/// Socket.IO v2 packet. Wire form: <c>&lt;type&gt;&lt;N&gt;-&lt;ackId?&gt;[json-args]</c> where
+/// <c>&lt;N&gt;-</c> appears only on binary types (5/6). For binary events/acks, the JSON contains
+/// placeholders <c>{"_placeholder":true,"num":N}</c> that index into <see cref="BinaryAttachments"/>.
+/// </summary>
+public sealed class SocketIoFrame
+{
+    public SocketIoPacketType Type { get; }
+    public int? AckId { get; }
+    public int AttachmentCount { get; }
+    public string? EventName { get; }
+    public JsonElement[] RawArgs { get; }
+    public IReadOnlyList<byte[]> BinaryAttachments { get; }
+
+    public SocketIoFrame(
+        SocketIoPacketType type,
+        int? ackId,
+        int attachmentCount,
+        string? eventName,
+        JsonElement[] rawArgs,
+        IReadOnlyList<byte[]> binaryAttachments)
+    {
+        Type = type;
+        AckId = ackId;
+        AttachmentCount = attachmentCount;
+        EventName = eventName;
+        RawArgs = rawArgs;
+        BinaryAttachments = binaryAttachments;
+    }
+
+    /// <summary>
+    /// Parse the text portion of a SIO frame. For binary events the attachments arrive as separate
+    /// WS frames after the text — the caller wires them up via <see cref="WithAttachments"/>.
+    /// </summary>
+    public static SocketIoFrame Parse(string raw)
+    {
+        if (string.IsNullOrEmpty(raw))
+            throw new ArgumentException("Empty SIO payload", nameof(raw));
+
+        var type = (SocketIoPacketType)(raw[0] - '0');
+        var cursor = 1;
+
+        var attachmentCount = 0;
+        if (type is SocketIoPacketType.BinaryEvent or SocketIoPacketType.BinaryAck)
+        {
+            var dashIdx = raw.IndexOf('-', cursor);
+            if (dashIdx < 0)
+                throw new ArgumentException("Binary frame missing '-' separator", nameof(raw));
+            if (!int.TryParse(raw.AsSpan(cursor, dashIdx - cursor), out attachmentCount))
+                throw new ArgumentException("Binary frame attachment count not parseable", nameof(raw));
+            cursor = dashIdx + 1;
+        }
+
+        // Skip namespace (only present if a '/' starts here, terminated by ',').
+        if (cursor < raw.Length && raw[cursor] == '/')
+        {
+            var commaIdx = raw.IndexOf(',', cursor);
+            cursor = commaIdx >= 0 ? commaIdx + 1 : raw.Length;
+        }
+
+        int? ackId = null;
+        if (cursor < raw.Length && char.IsDigit(raw[cursor]))
+        {
+            var start = cursor;
+            while (cursor < raw.Length && char.IsDigit(raw[cursor])) cursor++;
+            ackId = int.Parse(raw.AsSpan(start, cursor - start));
+        }
+
+        var argsJson = cursor < raw.Length ? raw.Substring(cursor) : string.Empty;
+        var allElements = string.IsNullOrEmpty(argsJson)
+            ? Array.Empty<JsonElement>()
+            : JsonDocument.Parse(argsJson).RootElement.EnumerateArray().ToArray();
+
+        string? eventName = null;
+        JsonElement[] rawArgs;
+        if (type is SocketIoPacketType.Event or SocketIoPacketType.BinaryEvent && allElements.Length > 0)
+        {
+            eventName = allElements[0].GetString();
+            // RawArgs excludes the leading event-name element so callers index args from 0.
+            rawArgs = allElements.Length > 1 ? allElements[1..] : Array.Empty<JsonElement>();
+        }
+        else
+        {
+            rawArgs = allElements;
+        }
+
+        return new SocketIoFrame(type, ackId, attachmentCount, eventName, rawArgs, Array.Empty<byte[]>());
+    }
+
+    /// <summary>
+    /// Return a new frame with the given binary attachments attached. Throws if the count doesn't
+    /// match the header's declared attachment count.
+    /// </summary>
+    public SocketIoFrame WithAttachments(IReadOnlyList<byte[]> attachments)
+    {
+        if (attachments.Count != AttachmentCount)
+            throw new ArgumentException(
+                $"Attachment count mismatch: header says {AttachmentCount}, got {attachments.Count}");
+        return new SocketIoFrame(Type, AckId, AttachmentCount, EventName, RawArgs, attachments);
+    }
+
+    /// <summary>
+    /// Build a binary event frame for the given event name + binary attachments.
+    /// The JSON args become <c>[eventName, {_placeholder:true,num:0}, {_placeholder:true,num:1}, ...]</c>.
+    /// </summary>
+    public static SocketIoFrame BinaryEventWithAttachments(string eventName, IReadOnlyList<byte[]> attachments)
+    {
+        // Build the placeholder-only portion (without the event name) — event name is stored separately.
+        var sb = new StringBuilder();
+        sb.Append('[');
+        for (var i = 0; i < attachments.Count; i++)
+        {
+            if (i > 0) sb.Append(',');
+            sb.Append("{\"_placeholder\":true,\"num\":").Append(i).Append('}');
+        }
+        sb.Append(']');
+        var args = JsonDocument.Parse(sb.ToString()).RootElement.EnumerateArray().ToArray();
+
+        return new SocketIoFrame(
+            SocketIoPacketType.BinaryEvent,
+            ackId: null,
+            attachmentCount: attachments.Count,
+            eventName: eventName,
+            rawArgs: args,
+            binaryAttachments: attachments);
+    }
+
+    /// <summary>Build an ack response with a single int argument (the spec's pubSeq echo).</summary>
+    public static SocketIoFrame AckResponse(int ackId, int arg)
+    {
+        var args = JsonDocument.Parse($"[{arg}]").RootElement.EnumerateArray().ToArray();
+        return new SocketIoFrame(
+            SocketIoPacketType.Ack, ackId, 0, null, args, Array.Empty<byte[]>());
+    }
+
+    /// <summary>
+    /// Encode to the wire form: (text payload, ordered list of binary attachments).
+    /// The caller is responsible for sending the text frame first then each binary attachment frame.
+    /// </summary>
+    public (string Text, IReadOnlyList<byte[]> Binaries) Encode()
+    {
+        var sb = new StringBuilder();
+        sb.Append((int)Type);
+        if (Type is SocketIoPacketType.BinaryEvent or SocketIoPacketType.BinaryAck)
+        {
+            sb.Append(AttachmentCount).Append('-');
+        }
+        if (AckId.HasValue) sb.Append(AckId.Value);
+        // Re-serialize args — for event/binary-event types, re-prepend the event name.
+        sb.Append('[');
+        var prependEventName = Type is SocketIoPacketType.Event or SocketIoPacketType.BinaryEvent
+                               && EventName is not null;
+        if (prependEventName)
+        {
+            sb.Append('"').Append(EventName).Append('"');
+            if (RawArgs.Length > 0) sb.Append(',');
+        }
+        for (var i = 0; i < RawArgs.Length; i++)
+        {
+            if (i > 0) sb.Append(',');
+            sb.Append(RawArgs[i].GetRawText());
+        }
+        sb.Append(']');
+        return (sb.ToString(), BinaryAttachments);
+    }
+}
diff --git a/SVSim.BattleNode/Wire/SocketIoPacketType.cs b/SVSim.BattleNode/Wire/SocketIoPacketType.cs
new file mode 100644
index 0000000..11394ad
--- /dev/null
+++ b/SVSim.BattleNode/Wire/SocketIoPacketType.cs
@@ -0,0 +1,12 @@
+namespace SVSim.BattleNode.Wire;
+
+public enum SocketIoPacketType
+{
+    Connect = 0,
+    Disconnect = 1,
+    Event = 2,
+    Ack = 3,
+    Error = 4,
+    BinaryEvent = 5,
+    BinaryAck = 6,
+}
diff --git a/SVSim.UnitTests/BattleNode/Wire/SocketIoFrameTests.cs b/SVSim.UnitTests/BattleNode/Wire/SocketIoFrameTests.cs
new file mode 100644
index 0000000..841d731
--- /dev/null
+++ b/SVSim.UnitTests/BattleNode/Wire/SocketIoFrameTests.cs
@@ -0,0 +1,88 @@
+using System.Text.Json;
+using NUnit.Framework;
+using SVSim.BattleNode.Wire;
+
+namespace SVSim.UnitTests.BattleNode.Wire;
+
+[TestFixture]
+public class SocketIoFrameTests
+{
+    [Test]
+    public void Parse_ConnectPacket_HasNoPayload()
+    {
+        var frame = SocketIoFrame.Parse("0");
+        Assert.That(frame.Type, Is.EqualTo(SocketIoPacketType.Connect));
+        Assert.That(frame.AckId, Is.Null);
+        Assert.That(frame.AttachmentCount, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Parse_EventWithAck_ExtractsAckIdAndArgs()
+    {
+        var frame = SocketIoFrame.Parse("27[\"msg\",42]");
+        Assert.That(frame.Type, Is.EqualTo(SocketIoPacketType.Event));
+        Assert.That(frame.AckId, Is.EqualTo(7));
+        Assert.That(frame.EventName, Is.EqualTo("msg"));
+        Assert.That(frame.RawArgs[0].GetInt32(), Is.EqualTo(42));
+    }
+
+    [Test]
+    public void Parse_BinaryEvent_RecordsAttachmentCount_WithAttachments_Assembles()
+    {
+        var attachment = new byte[] { 0x01, 0x02, 0x03 };
+        var header = SocketIoFrame.Parse("51-[\"msg\",{\"_placeholder\":true,\"num\":0}]");
+        var assembled = header.WithAttachments(new[] { attachment });
+
+        Assert.That(assembled.Type, Is.EqualTo(SocketIoPacketType.BinaryEvent));
+        Assert.That(assembled.AckId, Is.Null);
+        Assert.That(assembled.AttachmentCount, Is.EqualTo(1));
+        Assert.That(assembled.BinaryAttachments[0], Is.EqualTo(attachment));
+        Assert.That(assembled.EventName, Is.EqualTo("msg"));
+    }
+
+    [Test]
+    public void Parse_BinaryEventWithAckId_ExtractsBoth()
+    {
+        var frame = SocketIoFrame.Parse("51-3[\"msg\",{\"_placeholder\":true,\"num\":0}]");
+        Assert.That(frame.Type, Is.EqualTo(SocketIoPacketType.BinaryEvent));
+        Assert.That(frame.AckId, Is.EqualTo(3));
+        Assert.That(frame.AttachmentCount, Is.EqualTo(1));
+    }
+
+    [Test]
+    public void Parse_AckResponse_ExtractsIdAndIntArg()
+    {
+        var frame = SocketIoFrame.Parse("37[123]");
+        Assert.That(frame.Type, Is.EqualTo(SocketIoPacketType.Ack));
+        Assert.That(frame.AckId, Is.EqualTo(7));
+        Assert.That(frame.RawArgs[0].GetInt32(), Is.EqualTo(123));
+    }
+
+    [Test]
+    public void Encode_BinaryEventWithAttachment_EmitsCountDashAndPlaceholder()
+    {
+        var attachment = new byte[] { 0xff };
+        var frame = SocketIoFrame.BinaryEventWithAttachments("synchronize", new[] { attachment });
+
+        var (text, bins) = frame.Encode();
+        Assert.That(text, Is.EqualTo("51-[\"synchronize\",{\"_placeholder\":true,\"num\":0}]"));
+        Assert.That(bins.Single(), Is.EqualTo(attachment));
+    }
+
+    [Test]
+    public void Encode_AckResponse_IsTypeIdAndArrayOfArgs()
+    {
+        var frame = SocketIoFrame.AckResponse(ackId: 7, arg: 123);
+        var (text, bins) = frame.Encode();
+
+        Assert.That(text, Is.EqualTo("37[123]"));
+        Assert.That(bins, Is.Empty);
+    }
+
+    [Test]
+    public void WithAttachments_CountMismatch_Throws()
+    {
+        var header = SocketIoFrame.Parse("51-[\"msg\",{\"_placeholder\":true,\"num\":0}]");
+        Assert.Throws<ArgumentException>(() => header.WithAttachments(Array.Empty<byte[]>()));
+    }
+}