diff --git a/backend.tests/DeviceRegistryTests.cs b/backend.tests/DeviceRegistryTests.cs
new file mode 100644
index 0000000..327783e
--- /dev/null
+++ b/backend.tests/DeviceRegistryTests.cs
@@ -0,0 +1,25 @@
+using System.Net.WebSockets;
+using backend.DeviceHub;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class DeviceRegistryTests
+{
+    [Fact]
+    public void Register_unregister_round_trip()
+    {
+        var reg = new DeviceRegistry();
+        var id = Guid.NewGuid();
+        var dev = new ActiveDevice(id, new ClientWebSocket());
+        reg.TryRegister(dev).Should().BeTrue();
+        reg.TryRegister(dev).Should().BeFalse();
+        reg.IsOnline(id).Should().BeTrue();
+        reg.OnlineCount.Should().Be(1);
+
+        reg.Unregister(id).Should().BeTrue();
+        reg.IsOnline(id).Should().BeFalse();
+        reg.OnlineCount.Should().Be(0);
+    }
+}
diff --git a/backend.tests/FakeDeviceChannel.cs b/backend.tests/FakeDeviceChannel.cs
index e972ea4..9f92d27 100644
--- a/backend.tests/FakeDeviceChannel.cs
+++ b/backend.tests/FakeDeviceChannel.cs
@@ -1,14 +1,28 @@
 using System.Collections.Concurrent;
 using System.Text.Json;
+using backend.DeviceHub;
 using backend.Tools;
 
 namespace backend.tests;
 
-// Slim version for Tasks 9-15. Task 16 reopens this file to add IDeviceClient + envelope sending.
-public class FakeDeviceChannel : IDeviceChannel
+public class FakeDeviceChannel : IDeviceChannel, IDeviceClient
 {
+    public List<HubEnvelope> SentEnvelopes { get; } = new();
+    public List<byte[]> SentBinary { get; } = new();
     public ConcurrentQueue<JsonElement> PiToolReplies { get; } = new();
 
+    public Task SendEnvelopeAsync<T>(T envelope, CancellationToken ct) where T : HubEnvelope
+    {
+        SentEnvelopes.Add(envelope);
+        return Task.CompletedTask;
+    }
+
+    public Task SendBinaryAsync(ReadOnlyMemory<byte> bytes, CancellationToken ct)
+    {
+        SentBinary.Add(bytes.ToArray());
+        return Task.CompletedTask;
+    }
+
     public Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct)
     {
         if (PiToolReplies.TryDequeue(out var reply))
diff --git a/backend/DeviceHub/ActiveDevice.cs b/backend/DeviceHub/ActiveDevice.cs
new file mode 100644
index 0000000..e0e791f
--- /dev/null
+++ b/backend/DeviceHub/ActiveDevice.cs
@@ -0,0 +1,112 @@
+using System.Collections.Concurrent;
+using System.Net.WebSockets;
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using backend.Realtime;
+using backend.Tools;
+
+namespace backend.DeviceHub;
+
+public interface IDeviceClient
+{
+    Task SendEnvelopeAsync<T>(T envelope, CancellationToken ct) where T : HubEnvelope;
+    Task SendBinaryAsync(ReadOnlyMemory<byte> bytes, CancellationToken ct);
+}
+
+public class ActiveDevice : IDeviceChannel, IDeviceClient, ISessionOutput, IAsyncDisposable
+{
+    public Guid DeviceId { get; }
+    public WebSocket Ws => _ws;
+    public RealtimeSession? CurrentSession { get; set; }
+    public CancellationTokenSource? SessionCts { get; set; }
+
+    private readonly WebSocket _ws;
+    private readonly SemaphoreSlim _sendLock = new(1, 1);
+    private readonly ConcurrentDictionary<string, TaskCompletionSource<JsonElement>> _pendingTools = new();
+
+    public ActiveDevice(Guid deviceId, WebSocket ws)
+    {
+        DeviceId = deviceId;
+        _ws = ws;
+    }
+
+    public async Task SendEnvelopeAsync<T>(T envelope, CancellationToken ct) where T : HubEnvelope
+    {
+        var json = JsonSerializer.Serialize(envelope, envelope.GetType());
+        await _sendLock.WaitAsync(ct);
+        try
+        {
+            await _ws.SendAsync(Encoding.UTF8.GetBytes(json),
+                WebSocketMessageType.Text, endOfMessage: true, ct);
+        }
+        finally { _sendLock.Release(); }
+    }
+
+    public async Task SendBinaryAsync(ReadOnlyMemory<byte> bytes, CancellationToken ct)
+    {
+        await _sendLock.WaitAsync(ct);
+        try
+        {
+            await _ws.SendAsync(bytes, WebSocketMessageType.Binary, endOfMessage: true, ct);
+        }
+        finally { _sendLock.Release(); }
+    }
+
+    public Task WriteEnvelopeAsync(JsonObject envelope, CancellationToken ct)
+    {
+        var json = envelope.ToJsonString();
+        return WriteRawTextAsync(json, ct);
+    }
+
+    private async Task WriteRawTextAsync(string json, CancellationToken ct)
+    {
+        await _sendLock.WaitAsync(ct);
+        try
+        {
+            await _ws.SendAsync(Encoding.UTF8.GetBytes(json),
+                WebSocketMessageType.Text, endOfMessage: true, ct);
+        }
+        finally { _sendLock.Release(); }
+    }
+
+    public Task WriteBinaryAsync(ReadOnlyMemory<byte> bytes, CancellationToken ct)
+        => SendBinaryAsync(bytes, ct);
+
+    public Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct)
+    {
+        var callId = "srv_" + Guid.NewGuid().ToString("N")[..12];
+        var tcs = new TaskCompletionSource<JsonElement>(TaskCreationOptions.RunContinuationsAsynchronously);
+        _pendingTools[callId] = tcs;
+
+        var envelope = new ToolCallEnvelope("tool_call", callId, name, args);
+        _ = SendEnvelopeAsync(envelope, ct);
+
+        ct.Register(() => tcs.TrySetCanceled());
+        return tcs.Task;
+    }
+
+    public void CompleteToolResult(ToolResultEnvelope env)
+    {
+        if (!_pendingTools.TryRemove(env.CallId, out var tcs)) return;
+        if (env.Ok && env.Result is JsonElement r) tcs.TrySetResult(r);
+        else
+        {
+            var msg = (env.Error ?? "unknown").Replace("\\", "\\\\").Replace("\"", "\\\"");
+            var doc = JsonDocument.Parse($$"""{"ok":false,"error":"{{msg}}"}""");
+            tcs.TrySetResult(doc.RootElement.Clone());
+        }
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        try
+        {
+            if (_ws.State == WebSocketState.Open)
+                await _ws.CloseAsync(WebSocketCloseStatus.NormalClosure, "bye", CancellationToken.None);
+        }
+        catch { }
+        _ws.Dispose();
+        _sendLock.Dispose();
+    }
+}
diff --git a/backend/DeviceHub/DeviceRegistry.cs b/backend/DeviceHub/DeviceRegistry.cs
new file mode 100644
index 0000000..c7db098
--- /dev/null
+++ b/backend/DeviceHub/DeviceRegistry.cs
@@ -0,0 +1,21 @@
+using System.Collections.Concurrent;
+
+namespace backend.DeviceHub;
+
+public class DeviceRegistry
+{
+    private readonly ConcurrentDictionary<Guid, ActiveDevice> _online = new();
+
+    public bool TryRegister(ActiveDevice dev)
+        => _online.TryAdd(dev.DeviceId, dev);
+
+    public bool Unregister(Guid deviceId)
+        => _online.TryRemove(deviceId, out _);
+
+    public ActiveDevice? Get(Guid deviceId)
+        => _online.TryGetValue(deviceId, out var d) ? d : null;
+
+    public bool IsOnline(Guid deviceId) => _online.ContainsKey(deviceId);
+
+    public int OnlineCount => _online.Count;
+}
diff --git a/backend/DeviceHub/HubEnvelopes.cs b/backend/DeviceHub/HubEnvelopes.cs
new file mode 100644
index 0000000..b7da0c6
--- /dev/null
+++ b/backend/DeviceHub/HubEnvelopes.cs
@@ -0,0 +1,42 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace backend.DeviceHub;
+
+public record HubEnvelope([property: JsonPropertyName("type")] string Type);
+
+public record HelloEnvelope(string Type,
+    [property: JsonPropertyName("device_id")] Guid DeviceId,
+    [property: JsonPropertyName("client_version")] string ClientVersion) : HubEnvelope(Type);
+
+public record HelloAckEnvelope(string Type,
+    [property: JsonPropertyName("config")] JsonElement Config) : HubEnvelope(Type);
+
+public record PongEnvelope(string Type) : HubEnvelope(Type);
+
+public record WakeEnvelope(string Type,
+    [property: JsonPropertyName("at")] long? At) : HubEnvelope(Type);
+
+public record SessionStartedEnvelope(string Type,
+    [property: JsonPropertyName("conversation_id")] Guid ConversationId) : HubEnvelope(Type);
+
+public record SessionEndedEnvelope(string Type,
+    [property: JsonPropertyName("reason")] string Reason) : HubEnvelope(Type);
+
+public record AssistantDoneEnvelope(string Type) : HubEnvelope(Type);
+
+public record ToolCallEnvelope(string Type,
+    [property: JsonPropertyName("call_id")] string CallId,
+    [property: JsonPropertyName("name")] string Name,
+    [property: JsonPropertyName("arguments")] JsonElement Arguments) : HubEnvelope(Type);
+
+public record ToolResultEnvelope(string Type,
+    [property: JsonPropertyName("call_id")] string CallId,
+    [property: JsonPropertyName("ok")] bool Ok,
+    [property: JsonPropertyName("result")] JsonElement? Result,
+    [property: JsonPropertyName("error")] string? Error) : HubEnvelope(Type);
+
+public record ErrorEnvelope(string Type,
+    [property: JsonPropertyName("code")] string Code,
+    [property: JsonPropertyName("message")] string Message,
+    [property: JsonPropertyName("fatal")] bool? Fatal) : HubEnvelope(Type);
diff --git a/backend/Program.cs b/backend/Program.cs
index 3af97c9..02fa7ca 100644
--- a/backend/Program.cs
+++ b/backend/Program.cs
@@ -40,6 +40,7 @@ builder.Services.AddSingleton<backend.Tools.ITool, backend.Tools.EndSessionTool>
 builder.Services.AddSingleton<backend.Tools.ITool, backend.Tools.SetVolumeTool>();
 builder.Services.AddSingleton<backend.Tools.ToolRegistry>();
 builder.Services.AddScoped<backend.Realtime.RealtimeSessionFactory>();
+builder.Services.AddSingleton<backend.DeviceHub.DeviceRegistry>();
 
 var app = builder.Build();