diff --git a/backend.tests/FakeDeviceChannel.cs b/backend.tests/FakeDeviceChannel.cs
new file mode 100644
index 0000000..e972ea4
--- /dev/null
+++ b/backend.tests/FakeDeviceChannel.cs
@@ -0,0 +1,18 @@
+using System.Collections.Concurrent;
+using System.Text.Json;
+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 ConcurrentQueue<JsonElement> PiToolReplies { get; } = new();
+
+    public Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct)
+    {
+        if (PiToolReplies.TryDequeue(out var reply))
+            return Task.FromResult(reply);
+        return Task.FromResult(JsonDocument.Parse("""{"ok":true}""").RootElement);
+    }
+}
diff --git a/backend.tests/ScriptedRealtimeUpstream.cs b/backend.tests/ScriptedRealtimeUpstream.cs
new file mode 100644
index 0000000..5a332f4
--- /dev/null
+++ b/backend.tests/ScriptedRealtimeUpstream.cs
@@ -0,0 +1,53 @@
+using System.Text.Json.Nodes;
+using System.Threading.Channels;
+using backend.Realtime;
+
+namespace backend.tests;
+
+public class ScriptedRealtimeUpstream : IRealtimeUpstream
+{
+    private readonly Channel<JsonObject> _toRelay =
+        Channel.CreateUnbounded<JsonObject>(new UnboundedChannelOptions { SingleReader = true });
+
+    private readonly Channel<JsonObject> _fromRelay =
+        Channel.CreateUnbounded<JsonObject>(new UnboundedChannelOptions { SingleWriter = true });
+
+    public IList<JsonObject> Sent { get; } = new List<JsonObject>();
+
+    public Task SendJsonAsync(JsonObject envelope, CancellationToken ct)
+    {
+        Sent.Add(envelope);
+        return _fromRelay.Writer.WriteAsync(envelope, ct).AsTask();
+    }
+
+    public async Task<JsonObject?> ReceiveJsonAsync(CancellationToken ct)
+    {
+        try
+        {
+            return await _toRelay.Reader.ReadAsync(ct);
+        }
+        catch (ChannelClosedException)
+        {
+            return null;
+        }
+    }
+
+    public void Push(JsonObject evt) => _toRelay.Writer.TryWrite(evt);
+    public void CloseUpstream() => _toRelay.Writer.TryComplete();
+
+    public async Task<JsonObject> WaitForSentAsync(string type, CancellationToken ct)
+    {
+        await foreach (var item in _fromRelay.Reader.ReadAllAsync(ct))
+        {
+            if ((string?)item["type"] == type) return item;
+        }
+        throw new TimeoutException($"never saw outbound {type}");
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        _toRelay.Writer.TryComplete();
+        _fromRelay.Writer.TryComplete();
+        return ValueTask.CompletedTask;
+    }
+}