diff --git a/src/TeamsISO.Engine/Discovery/DiscoveryEvent.cs b/src/TeamsISO.Engine/Discovery/DiscoveryEvent.cs
new file mode 100644
index 0000000..eb3fd2b
--- /dev/null
+++ b/src/TeamsISO.Engine/Discovery/DiscoveryEvent.cs
@@ -0,0 +1,9 @@
+using TeamsISO.Engine.Domain;
+
+namespace TeamsISO.Engine.Discovery;
+
+public abstract record DiscoveryEvent
+{
+    public sealed record Added(NdiSource Source) : DiscoveryEvent;
+    public sealed record Removed(NdiSource Source) : DiscoveryEvent;
+}
diff --git a/src/tests/TeamsISO.Engine.Tests/Fakes/FakeFrameClock.cs b/src/tests/TeamsISO.Engine.Tests/Fakes/FakeFrameClock.cs
new file mode 100644
index 0000000..e28c94c
--- /dev/null
+++ b/src/tests/TeamsISO.Engine.Tests/Fakes/FakeFrameClock.cs
@@ -0,0 +1,39 @@
+using TeamsISO.Engine.Pipeline;
+
+namespace TeamsISO.Engine.Tests.Fakes;
+
+/// <summary>
+/// Manual-tick clock. Tests advance the clock and trigger the awaiter explicitly.
+/// </summary>
+public sealed class FakeFrameClock : IFrameClock
+{
+    private long _nowTicks;
+    private TaskCompletionSource<bool>? _pendingTick;
+    private readonly object _gate = new();
+
+    public long NowTicks => Interlocked.Read(ref _nowTicks);
+
+    public void Advance(TimeSpan by)
+    {
+        Interlocked.Add(ref _nowTicks, by.Ticks);
+        TaskCompletionSource<bool>? toSignal;
+        lock (_gate)
+        {
+            toSignal = _pendingTick;
+            _pendingTick = null;
+        }
+        toSignal?.TrySetResult(true);
+    }
+
+    public ValueTask<bool> WaitForNextTickAsync(CancellationToken cancellationToken)
+    {
+        TaskCompletionSource<bool> tcs;
+        lock (_gate)
+        {
+            _pendingTick ??= new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+            tcs = _pendingTick;
+        }
+        cancellationToken.Register(() => tcs.TrySetResult(false));
+        return new ValueTask<bool>(tcs.Task);
+    }
+}
diff --git a/src/tests/TeamsISO.Engine.Tests/Fakes/FakeNdiInterop.cs b/src/tests/TeamsISO.Engine.Tests/Fakes/FakeNdiInterop.cs
new file mode 100644
index 0000000..35ae09c
--- /dev/null
+++ b/src/tests/TeamsISO.Engine.Tests/Fakes/FakeNdiInterop.cs
@@ -0,0 +1,71 @@
+using System.Collections.Concurrent;
+using TeamsISO.Engine.Interop;
+using TeamsISO.Engine.Pipeline;
+
+namespace TeamsISO.Engine.Tests.Fakes;
+
+/// <summary>
+/// In-memory test double for <see cref="INdiInterop"/>. Tests configure source lists and frame
+/// queues; the fake feeds those into engine code as if a real NDI runtime were present.
+/// </summary>
+public sealed class FakeNdiInterop : INdiInterop
+{
+    public List<string> Sources { get; } = new();
+    public ConcurrentDictionary<string, ConcurrentQueue<RawFrame>> ReceiverFrames { get; } = new();
+    public ConcurrentDictionary<string, List<ProcessedFrame>> SentFrames { get; } = new();
+    public string RuntimeVersion { get; set; } = "6.0.0";
+    public Dictionary<string, int> ReceiverCreatedCount { get; } = new();
+    public Dictionary<string, int> SenderCreatedCount { get; } = new();
+
+    public NdiFindHandle CreateFinder() => new FakeFindHandle();
+    public IReadOnlyList<string> GetCurrentSources(NdiFindHandle finder) => Sources.ToArray();
+
+    public NdiReceiverHandle CreateReceiver(string sourceFullName)
+    {
+        ReceiverCreatedCount[sourceFullName] = ReceiverCreatedCount.GetValueOrDefault(sourceFullName) + 1;
+        ReceiverFrames.GetOrAdd(sourceFullName, _ => new ConcurrentQueue<RawFrame>());
+        return new FakeReceiverHandle(sourceFullName);
+    }
+
+    public RawFrame? CaptureFrame(NdiReceiverHandle receiver, int timeoutMs)
+    {
+        var key = ((FakeReceiverHandle)receiver).Source;
+        if (ReceiverFrames.TryGetValue(key, out var q) && q.TryDequeue(out var frame))
+            return frame;
+        return null; // simulate timeout
+    }
+
+    public NdiSenderHandle CreateSender(string outputName)
+    {
+        SenderCreatedCount[outputName] = SenderCreatedCount.GetValueOrDefault(outputName) + 1;
+        SentFrames.GetOrAdd(outputName, _ => new List<ProcessedFrame>());
+        return new FakeSenderHandle(outputName);
+    }
+
+    public void SendFrame(NdiSenderHandle sender, ProcessedFrame frame)
+    {
+        var key = ((FakeSenderHandle)sender).Output;
+        SentFrames[key].Add(frame);
+    }
+
+    public string GetRuntimeVersion() => RuntimeVersion;
+
+    private sealed class FakeFindHandle : NdiFindHandle
+    {
+        public override void Dispose() { }
+    }
+
+    private sealed class FakeReceiverHandle : NdiReceiverHandle
+    {
+        public string Source { get; }
+        public FakeReceiverHandle(string source) => Source = source;
+        public override void Dispose() { }
+    }
+
+    private sealed class FakeSenderHandle : NdiSenderHandle
+    {
+        public string Output { get; }
+        public FakeSenderHandle(string output) => Output = output;
+        public override void Dispose() { }
+    }
+}