Fix iOS background audio pacing

Sleep-paced threads stall for hundreds of milliseconds when iOS coalesces a
backgrounded app's wakeups, and the deadline resets that discarded the deficit
left the capture backlog queued until its ring overflowed: regular dropouts
that worsen the longer the app stays backgrounded. Pace both 20 ms hands-offs
from the AVAudioSourceNode render callback instead (mix via Audio.RunCycle
with deviceClockedAudio, capture handoff from the ring), wake the UDP sender
on a queue signal instead of a 1 ms poll, and let stalled consumers drop their
backlog to the buffer target instead of ratcheting it.

PROGRESS.md

@@ -42,9 +42,11 @@ cap the applied Opus hint at 30%, and feed it back over TLS.
 - Verify iOS remote-user tuning and private-conversation navigation with VoiceOver, including
   multiple streams, users without active streams, and users who disconnect while a view is open.
 - Exercise iOS background/lock, interruption, Bluetooth, route-change, ReplayKit, and iOS 27
-  ScreenCaptureKit paths on devices. Complete a 30-minute iOS call and Wi-Fi/cellular switching
-  with voice restoration, plus extended mono/stereo/voice-chat switching while joined. Verify
-  Windows desktop/per-app stereo sharing.
+  ScreenCaptureKit paths on devices. The background/lock gate keeps a call active for 15+ minutes
+  backgrounded and screen-locked with no periodic glitches and flat `VC_AUDIO` `feedDrops`/`starved`
+  counters (the render callback now paces the mix and the 20 ms capture handoff). Complete a
+  30-minute iOS call and Wi-Fi/cellular switching with voice restoration, plus extended
+  mono/stereo/voice-chat switching while joined. Verify Windows desktop/per-app stereo sharing.
 - Complete Developer ID signing/notarization. The iOS host and ReplayKit extension have been
   distribution-signed and packaged locally; upload the IPA for Apple's server-side validation.
 - Run the published Linux container and a 30-minute-or-longer server soak.

clients/apple/VoiceCat.iOS/AppModel.cs

@@ -99,7 +99,10 @@ internal sealed class AppModel
         explicitDisconnect = false; IsConnecting = true; connectedProfile = profile;
         Status = restoring ? "Reconnecting…" : "Connecting…"; Notify();
         lifetime?.Cancel(); lifetime?.Dispose(); lifetime = new();
-        VoiceCatClient next = new("VoiceCat-iOS", "0.0.1", storage.TofuPath);
+        // The AVAudioSourceNode render callback drives Audio.RunCycle and the 20 ms capture
+        // handoff; a sleep-paced audio worker stalls when iOS coalesces backgrounded wakeups and
+        // the call glitches after several minutes in the background.
+        VoiceCatClient next = new("VoiceCat-iOS", "0.0.1", storage.TofuPath, deviceClockedAudio: true);
         next.ConnectionStateChanged += state =>
         {
             if (state == ClientConnectionState.Disconnected && next.ConnectionFailure is { } failure)

clients/apple/VoiceCat.iOS/IosAudioEngine.cs

@@ -31,7 +31,7 @@ internal sealed class IosAudioEngine
     }
     private MicrophoneRoute? microphone;
     private readonly short[] microphoneFrame = new short[960 * 2];
-    private readonly Thread microphonePump;
+    private int microphoneCredit;
     private AVAudioFormat? microphoneFormat;
     private AVAudioConverter? microphoneConverter;
     private AVAudioPcmBuffer? convertedMicrophone;
@@ -43,12 +43,6 @@ internal sealed class IosAudioEngine
     internal bool IsConnected { get; private set; }
     internal int BufferMilliseconds { get => playbackRing.BufferMilliseconds; set => playbackRing.BufferMilliseconds = value; }
 
-    private IosAudioEngine()
-    {
-        microphonePump = new Thread(PumpMicrophone) { IsBackground = true, Name = "VoiceCat iOS microphone pacer", Priority = ThreadPriority.Highest };
-        microphonePump.Start();
-    }
-
     internal void StartListening(VoiceCatClient owner)
     {
         Stop(); client = owner; IsConnected = true; owner.Audio.MixedPcm += ReceiveMixedPcm; Rebuild();
@@ -183,32 +177,19 @@ internal sealed class IosAudioEngine
         status = AVAudioConverterInputStatus.NoDataNow; return null!;
     }
 
-    private void PumpMicrophone()
+    // One paced 20 ms handoff from the capture ring into the encoder. The render callback calls
+    // this at its demand rate: the ring absorbs RemoteIO's burst pattern and the small capture vs
+    // output clock difference, so the sender sees a steady 20 ms feed without a sleep-paced pacer
+    // thread to stall when iOS coalesces a backgrounded app's wakeups.
+    private void PumpMicrophoneChunk(VoiceCatClient owner)
     {
-        long deadline = System.Diagnostics.Stopwatch.GetTimestamp();
-        while (true)
-        {
-            MicrophoneRoute? route = Volatile.Read(ref microphone);
-            VoiceCatClient? owner = client;
-            if (route is not null && owner is not null)
-            {
-                int required = 960 * route.Channels;
-                if (route.Ring.Read(microphoneFrame.AsSpan(0, required)) == required &&
-                    ReferenceEquals(route, Volatile.Read(ref microphone)) &&
-                    !owner.Audio.FeedPcm(route.StreamId, microphoneFrame.AsSpan(0, required), route.Channels))
-                    Interlocked.Increment(ref rejectedFeeds);
-            }
-            deadline += System.Diagnostics.Stopwatch.Frequency / 50;
-            while (true)
-            {
-                long remaining = deadline - System.Diagnostics.Stopwatch.GetTimestamp();
-                if (remaining <= 0) break;
-                double milliseconds = remaining * 1000.0 / System.Diagnostics.Stopwatch.Frequency;
-                if (milliseconds > 2) Thread.Sleep(Math.Max(1, (int)milliseconds - 1)); else Thread.SpinWait(64);
-            }
-            if ((deadline - System.Diagnostics.Stopwatch.GetTimestamp()) * 1000.0 / System.Diagnostics.Stopwatch.Frequency < -100)
-                deadline = System.Diagnostics.Stopwatch.GetTimestamp();
-        }
+        MicrophoneRoute? route = Volatile.Read(ref microphone);
+        if (route is null) return;
+        int required = 960 * route.Channels;
+        if (route.Ring.Read(microphoneFrame.AsSpan(0, required)) == required &&
+            ReferenceEquals(route, Volatile.Read(ref microphone)) &&
+            !owner.Audio.FeedPcm(route.StreamId, microphoneFrame.AsSpan(0, required), route.Channels))
+            Interlocked.Increment(ref rejectedFeeds);
     }
 
     private void ReceiveMixedPcm(ReadOnlySpan<short> pcm) => playbackRing.TryWrite(pcm);
@@ -217,6 +198,20 @@ internal sealed class IosAudioEngine
     {
         int frames = checked((int)frameCount), requested = checked(frames * 2);
         if (requested > renderScratch.Length) return -1;
+        // This callback is the cadence iOS keeps exact while the app is backgrounded or the device
+        // is locked, so it owns both managed 20 ms hands-offs: capture into the sender and one mix
+        // cycle per 20 ms of render demand. Both run allocation-free and without locks or I/O.
+        VoiceCatClient? owner = Volatile.Read(ref client);
+        if (owner is null || !IsConnected) microphoneCredit = 0;
+        else
+        {
+            microphoneCredit += frames;
+            while (microphoneCredit >= 960) { microphoneCredit -= 960; PumpMicrophoneChunk(owner); }
+            // Top the mix ring up to cover this callback plus its configured target so the read
+            // below never starves and the buffer keeps its chosen buffering latency.
+            int deficit = frames + playbackRing.TargetFrames - playbackRing.CountFrames;
+            for (int produced = 0; produced < deficit; produced += 960) owner.Audio.RunCycle();
+        }
         Span<short> input = renderScratch.AsSpan(0, requested);
         int read = playbackRing.Read(input); input[read..].Clear();
         int count = Marshal.ReadInt32(outputData), first = IntPtr.Size == 8 ? 8 : 4, stride = IntPtr.Size == 8 ? 16 : 12;

src/VoiceCat.Audio/AdaptivePcmBuffer.cs

@@ -23,6 +23,7 @@ public sealed class AdaptivePcmBuffer
 
     public int Channels => channels;
     public int CountFrames => unchecked(Volatile.Read(ref writtenFrame) - Volatile.Read(ref readFrame));
+    public int TargetFrames => Volatile.Read(ref targetFrames);
     public int BufferMilliseconds
     {
         get => Volatile.Read(ref targetFrames) * 1000 / SampleRate;
@@ -50,6 +51,19 @@ public sealed class AdaptivePcmBuffer
         finally { Volatile.Write(ref producer, 0); }
     }
 
+    // A consumer that stalls leaves its producer backlog queued at a fixed offset forever: feed
+    // and drain rates are equal in steady state, so the correction above only sheds roughly half
+    // a percent at a time and the queue ratchets toward its hard edge one stall at a time. The
+    // consumer calls this after a detected stall so the backlog becomes one bounded gap instead
+    // of accumulating; queued audio beyond the target is dropped and the next read re-primes.
+    public void Resynchronize()
+    {
+        int written = Volatile.Read(ref writtenFrame), available = unchecked(written - Volatile.Read(ref readFrame));
+        int target = Volatile.Read(ref targetFrames);
+        if (available > target) Volatile.Write(ref readFrame, unchecked(written - target));
+        primed = false; phase = 0;
+    }
+
     // Returns interleaved samples written. A zero return means the caller should treat the
     // already-cleared destination as silence. Once primed, short scheduling stalls re-prime
     // instead of repeatedly clicking at the ring edge.

src/VoiceCat.Audio/AudioEngine.cs

@@ -166,6 +166,17 @@ public sealed class AudioEngine : IDisposable
         Volatile.Write(ref completedEpoch, current.Epoch);
     }
 
+    // Device-clocked pull entry for platforms whose audio callback is the only reliable cadence:
+    // iOS keeps render callbacks running in the background while sleep-paced threads coalesce
+    // there. The device callback owns this call, so failures are contained instead of thrown and
+    // later cycles stay inert. Constructed with startWorker: false, the caller alone paces it.
+    public void RunCycle()
+    {
+        if (Failure is not null || Volatile.Read(ref disposed) != 0) return;
+        try { ProcessCycle(); }
+        catch (Exception exception) { Failure = exception; stop.Cancel(); }
+    }
+
     private void Work()
     {
         long deadline = Stopwatch.GetTimestamp();
@@ -177,7 +188,14 @@ public sealed class AudioEngine : IDisposable
                 deadline += Stopwatch.Frequency / 50;
                 WaitUntil(deadline);
                 if ((deadline - Stopwatch.GetTimestamp()) * 1000.0 / Stopwatch.Frequency < -100)
+                {
+                    // A stall this long cannot be caught up in place. Discarding only the schedule
+                    // deficit would leave the producer backlog queued at a fixed offset forever and
+                    // eventually overflow its ring, so drop back to the buffer targets instead: one
+                    // bounded gap per stall rather than growing latency and feed drops.
                     deadline = Stopwatch.GetTimestamp();
+                    foreach (LocalStream stream in Volatile.Read(ref routes).Local) stream.Input.Resynchronize();
+                }
             }
         }
         catch (Exception exception) { Failure = exception; stop.Cancel(); }

src/VoiceCat.Core/ClientMediaTransport.cs

@@ -19,6 +19,8 @@ internal sealed class ClientMediaTransport : IAsyncDisposable
     private readonly Thread sending;
     private readonly Task receiving;
     private readonly TaskCompletionSource bound = new(TaskCreationOptions.RunContinuationsAsynchronously);
+    private readonly AutoResetEvent sendReady = new(false);
+    private int senderNapping;
     internal event EncodedVoiceHandler? Received;
     internal Task Bound => bound.Task;
 
@@ -38,7 +40,14 @@ internal sealed class ClientMediaTransport : IAsyncDisposable
         sending.Start();
     }
 
-    internal bool TrySend(VoiceFrameHeader header, ReadOnlySpan<byte> payload) => packets.TryWrite(header, payload);
+    internal bool TrySend(VoiceFrameHeader header, ReadOnlySpan<byte> payload)
+    {
+        if (!packets.TryWrite(header, payload)) return false;
+        // Wake the sender only when it is actually waiting. The flag is raised before its final
+        // queue check, so a write can never slip between that check and the wait unnoticed.
+        if (Volatile.Read(ref senderNapping) != 0) sendReady.Set();
+        return true;
+    }
 
     private void Send()
     {
@@ -48,6 +57,7 @@ internal sealed class ClientMediaTransport : IAsyncDisposable
         {
             while (!stop.IsCancellationRequested)
             {
+                bool drained = false;
                 try
                 {
                     if (Environment.TickCount64 >= nextKeepalive)
@@ -58,6 +68,7 @@ internal sealed class ClientMediaTransport : IAsyncDisposable
                     }
                     while (packets.TryRead(plain, out VoiceFrameHeader header, out int length))
                     {
+                        drained = true;
                         int size = crypto.Encryptor.Encrypt(header, plain.AsSpan(0, length), packet);
                         socket.Send(packet.AsSpan(0, size), SocketFlags.None);
                     }
@@ -69,7 +80,17 @@ internal sealed class ClientMediaTransport : IAsyncDisposable
                     nextKeepalive = 0;
                     Thread.Sleep(100);
                 }
-                Thread.Sleep(1);
+                if (drained) continue;
+                // Wait on the queue's signal instead of polling every millisecond. A backgrounded
+                // iOS app coalesces timed sleeps, and a polling sender then flushes queued voice in
+                // bursts; a signalled wake is immediate. The keepalive deadline bounds the wait.
+                Volatile.Write(ref senderNapping, 1);
+                try
+                {
+                    if (packets.IsEmpty && Environment.TickCount64 < nextKeepalive)
+                        sendReady.WaitOne(checked((int)Math.Clamp(nextKeepalive - Environment.TickCount64, 1, 5000)));
+                }
+                finally { Volatile.Write(ref senderNapping, 0); }
             }
         }
         catch (Exception exception) when (exception is SocketException or ObjectDisposedException)
@@ -107,9 +128,9 @@ internal sealed class ClientMediaTransport : IAsyncDisposable
 
     public async ValueTask DisposeAsync()
     {
-        stop.Cancel(); socket.Dispose();
+        stop.Cancel(); socket.Dispose(); sendReady.Set();
         try { sending.Join(); await receiving.ConfigureAwait(false); }
-        finally { System.Security.Cryptography.CryptographicOperations.ZeroMemory(binding); stop.Dispose(); }
+        finally { System.Security.Cryptography.CryptographicOperations.ZeroMemory(binding); stop.Dispose(); sendReady.Dispose(); }
     }
 
     // A bounded, allocation-free packet handoff. A contending producer drops instead of
@@ -120,6 +141,7 @@ internal sealed class ClientMediaTransport : IAsyncDisposable
         private readonly VoiceFrameHeader[] headers = new VoiceFrameHeader[64];
         private readonly int[] lengths = new int[64];
         private int read, written, producer;
+        internal bool IsEmpty => read == Volatile.Read(ref written);
         internal bool TryWrite(VoiceFrameHeader header, ReadOnlySpan<byte> payload)
         {
             if (payload.Length is < 1 or > 1275 || Interlocked.CompareExchange(ref producer, 1, 0) != 0) return false;

src/VoiceCat.Core/VoiceCatClient.cs

@@ -55,12 +55,15 @@ public sealed partial class VoiceCatClient : IAsyncDisposable
     public bool TryReadEvent(out Envelope? envelope) => events.Reader.TryRead(out envelope);
     public IAsyncEnumerable<Envelope> ReadEventsAsync(CancellationToken cancellationToken = default) => events.Reader.ReadAllAsync(cancellationToken);
 
-    public VoiceCatClient(string clientName = "VoiceCat .NET", string clientVersion = "0.1.0", string? tofuStorePath = null)
+    // deviceClockedAudio: the platform's audio callback drives Audio.RunCycle() itself (iOS
+    // keeps render callbacks running in the background while sleep-paced threads coalesce there),
+    // so the audio engine must not start its own pacing worker.
+    public VoiceCatClient(string clientName = "VoiceCat .NET", string clientVersion = "0.1.0", string? tofuStorePath = null, bool deviceClockedAudio = false)
     {
         this.clientName = clientName;
         this.clientVersion = clientVersion;
         pins = new(tofuStorePath ?? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), "VoiceCat", "tofu.txt"));
-        Audio = new(TrySendEncodedVoice);
+        Audio = new(TrySendEncodedVoice, startWorker: !deviceClockedAudio);
         VoiceReceived += Audio.Receive;
     }
 

tests/VoiceCat.Tests/AudioEngineTests.cs

@@ -1,6 +1,11 @@
+using System.Net;
+using System.Net.Sockets;
 using VoiceCat.Audio;
 using VoiceCat.Codec;
+using VoiceCat.Core;
+using VoiceCat.Crypto;
 using VoiceCat.Protocol;
+using VoiceCat.Transport;
 using Voicecat.V1;
 
 namespace VoiceCat.Tests;
@@ -28,6 +33,63 @@ public class AudioEngineTests
         Assert.DoesNotContain("SendAsync(packet.AsMemory", source);
     }
 
+    [Fact]
+    public void MediaSenderWakesOnQueueSignalInsteadOfTimedPolling()
+    {
+        string source = File.ReadAllText(Path.Combine(FindRoot(), "src", "VoiceCat.Core", "ClientMediaTransport.cs"));
+
+        Assert.Contains("sendReady.WaitOne(", source);
+        Assert.Contains("senderNapping", source);
+        Assert.DoesNotContain("Thread.Sleep(1);", source);
+    }
+
+    [Fact]
+    public async Task MediaSenderDeliversQueuedVoicePromptlyWithoutTimedPolling()
+    {
+        // A loopback "relay" answers the binding so the sender's own keepalive pass falls back to
+        // its five second schedule: only the queue signal can then deliver queued voice promptly,
+        // so a lost wake or a polling loop shows up here as a multi-second delay.
+        using var relay = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
+        relay.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        byte[] key = new byte[32]; Random.Shared.NextBytes(key);
+        using var crypto = new MediaSessionCrypto(new MediaEncryptor(key), new MediaDecryptor(key));
+        await using var transport = new ClientMediaTransport((IPEndPoint)relay.LocalEndPoint!, new byte[16], crypto, CancellationToken.None);
+
+        byte[] datagram = new byte[512];
+        using (var startup = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
+        {
+            SocketReceiveFromResult probe = await relay.ReceiveFromAsync(datagram, SocketFlags.None, new IPEndPoint(IPAddress.Any, 0), startup.Token);
+            Assert.Equal(VoiceFrameHeader.Size + 16, probe.ReceivedBytes);
+            byte[] keepalive = new byte[VoiceFrameHeader.Size];
+            new VoiceFrameHeader(MediaFrameType.Keepalive, 0, 0, 0, 0, 0).Write(keepalive);
+            await relay.SendToAsync(keepalive, SocketFlags.None, probe.RemoteEndPoint);
+        }
+        await transport.Bound.WaitAsync(TimeSpan.FromSeconds(5));
+
+        var stopwatch = System.Diagnostics.Stopwatch.StartNew();
+        Assert.True(transport.TrySend(new(MediaFrameType.Voice, VoiceFrameFlags.None, 0, 7, 0, 0), new byte[80]));
+        await ReceiveVoiceAsync(relay, datagram, 1, TimeSpan.FromSeconds(2));
+        Assert.True(stopwatch.Elapsed < TimeSpan.FromSeconds(2));
+
+        // A burst queued while the sender naps must drain in one wake, not one keepalive pass.
+        stopwatch.Restart();
+        for (uint i = 1; i <= 30; i++)
+            Assert.True(transport.TrySend(new(MediaFrameType.Voice, VoiceFrameFlags.None, 0, 7, 0, i * 960), new byte[80]));
+        await ReceiveVoiceAsync(relay, datagram, 30, TimeSpan.FromSeconds(2));
+        Assert.True(stopwatch.Elapsed < TimeSpan.FromSeconds(2));
+    }
+
+    private static async Task ReceiveVoiceAsync(Socket relay, byte[] datagram, int count, TimeSpan timeout)
+    {
+        int voice = VoiceFrameHeader.Size + 80 + MediaEncryptor.TagSize;
+        using var deadline = new CancellationTokenSource(timeout);
+        while (count > 0)
+        {
+            SocketReceiveFromResult packet = await relay.ReceiveFromAsync(datagram, SocketFlags.None, new IPEndPoint(IPAddress.Any, 0), deadline.Token);
+            if (packet.ReceivedBytes == voice) count--;
+        }
+    }
+
     [Theory]
     [InlineData(-1000)]
     [InlineData(1000)]
@@ -124,6 +186,31 @@ public class AudioEngineTests
         Assert.Equal(0, lateStarvation);
     }
 
+    [Theory]
+    [InlineData(20)]
+    [InlineData(40)]
+    [InlineData(120)]
+    public void AdaptivePcmBufferResynchronizeDropsStallBacklogToTheTarget(int bufferMilliseconds)
+    {
+        var buffer = new AdaptivePcmBuffer(1, bufferMilliseconds, 65_536);
+        short[] chunk = new short[960], output = new short[960];
+        chunk.AsSpan().Fill(1234);
+
+        // A stalled consumer leaves every chunk queued: feed and drain rates are equal in steady
+        // state, so this backlog would otherwise sit at a fixed offset until overflow drops begin.
+        for (int i = 0; i < 50; i++) Assert.True(buffer.TryWrite(chunk));
+        Assert.Equal(50 * 960, buffer.CountFrames);
+
+        buffer.Resynchronize();
+        Assert.Equal(bufferMilliseconds * 48, buffer.CountFrames);
+        Assert.Equal(960, buffer.Read(output));
+        Assert.All(output, value => Assert.Equal(1234, value));
+
+        // At or below the target the resynchronization drops nothing.
+        buffer.Resynchronize();
+        Assert.Equal(bufferMilliseconds * 48 - 960, buffer.CountFrames);
+    }
+
     [Fact]
     public void OneCaptureMissDoesNotRestartTalkspurtButSustainedStarvationDoes()
     {
@@ -282,6 +369,83 @@ public class AudioEngineTests
         Assert.Equal(0, GC.GetAllocatedBytesForCurrentThread() - before);
     }
 
+    [Fact]
+    public void DeviceClockedRunCyclePacesMixAndCaptureWithNoWorkerThread()
+    {
+        var sent = new List<(uint Timestamp, VoiceFrameFlags Flags)>(512);
+        StreamInfo stream = Stream();
+        using var receive = new AudioEngine((_, _, _, _) => true, false);
+        receive.SetRemoteStreams([new() { Id = 2, ChannelId = 1, Streams = { stream } }], 1, 1);
+        using var send = new AudioEngine((ssrc, timestamp, payload, flags) =>
+        {
+            sent.Add((timestamp, flags));
+            receive.Receive(new(MediaFrameType.Voice, flags, 0, ssrc, 0, timestamp), payload);
+            return true;
+        }, false) { InputMode = AudioInputMode.AlwaysOn, DeviceBufferMilliseconds = 20 };
+        send.AddLocalStream(stream);
+        short[] tone = Tone(); long energy = 0;
+        receive.MixedPcm += pcm => { foreach (short sample in pcm) energy += Math.Abs((int)sample); };
+
+        // Each iteration is one 20 ms render demand quantum: feed the capture handoff, then pull
+        // both mixes, exactly as the iOS render callback drives its device-clocked engine.
+        for (int i = 0; i < 30; i++) { Assert.True(send.FeedPcm(1, tone, 1)); send.RunCycle(); receive.RunCycle(); }
+        Assert.Equal(30, send.GetLocalDiagnostics(1).Cycles);
+        Assert.Equal(0, send.GetLocalDiagnostics(1).StarvedCycles);
+        Assert.Equal(30, sent.Count);
+        Assert.True(energy > 100000);
+
+        long before = GC.GetAllocatedBytesForCurrentThread();
+        for (int i = 0; i < 100; i++) { send.FeedPcm(1, tone, 1); send.RunCycle(); receive.RunCycle(); }
+        Assert.Equal(0, GC.GetAllocatedBytesForCurrentThread() - before);
+    }
+
+    [Fact]
+    public void RunCycleContainsFailuresInsteadOfThrowingIntoRealtimeCallbacks()
+    {
+        using var engine = new AudioEngine((_, _, _, _) => throw new InvalidOperationException("boom"), false)
+            { InputMode = AudioInputMode.AlwaysOn, DeviceBufferMilliseconds = 20 };
+        engine.AddLocalStream(Stream());
+        Assert.True(engine.FeedPcm(1, Tone(), 1));
+        engine.RunCycle();
+        Assert.IsType<InvalidOperationException>(engine.Failure);
+        engine.RunCycle();
+        Assert.Equal(1, engine.GetLocalDiagnostics(1).Cycles);
+    }
+
+    [Fact]
+    public async Task DeviceClockedClientLeavesMixCadenceToTheAudioCallback()
+    {
+        string directory = Path.Combine(Path.GetTempPath(), $"voicecat-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(directory);
+        try
+        {
+            await using var paced = new VoiceCatClient(tofuStorePath: Path.Combine(directory, "paced.txt"));
+            await using var pulled = new VoiceCatClient(tofuStorePath: Path.Combine(directory, "pulled.txt"), deviceClockedAudio: true);
+            paced.Audio.AddLocalStream(Stream());
+            pulled.Audio.AddLocalStream(Stream());
+            await Task.Delay(250);
+            Assert.True(paced.Audio.GetLocalDiagnostics(1).Cycles > 0);
+            Assert.Equal(0, pulled.Audio.GetLocalDiagnostics(1).Cycles);
+            pulled.Audio.RunCycle();
+            Assert.Equal(1, pulled.Audio.GetLocalDiagnostics(1).Cycles);
+        }
+        finally
+        {
+            try { Directory.Delete(directory, true); }
+            catch (IOException) { } catch (UnauthorizedAccessException) { }
+        }
+    }
+
+    [Fact]
+    public void ManagedAudioWorkerResynchronizesProducerBacklogAfterStalls()
+    {
+        string source = File.ReadAllText(Path.Combine(FindRoot(), "src", "VoiceCat.Audio", "AudioEngine.cs"));
+        string buffer = File.ReadAllText(Path.Combine(FindRoot(), "src", "VoiceCat.Audio", "AdaptivePcmBuffer.cs"));
+
+        Assert.Contains("stream.Input.Resynchronize()", source);
+        Assert.Contains("public void Resynchronize()", buffer);
+    }
+
     [Fact]
     public void RemotePlaybackSettingsAreIndependentForEachUserStream()
     {

tests/VoiceCat.Tests/PublishServerScriptTests.cs

@@ -132,6 +132,16 @@ public class PublishServerScriptTests
         Assert.DoesNotContain("Task.Delay(10", screen);
         Assert.Contains("MaximumCaptureCallbackFrames = 16_384", microphone);
         Assert.DoesNotContain("Math.Ceiling(4_096 * 48_000", microphone);
+
+        // Both 20 ms hands-offs run from the AVAudioSourceNode render callback: sleep-paced
+        // threads coalesce when the app is backgrounded and glitch the call after several minutes.
+        string model = await File.ReadAllTextAsync(Path.Combine(
+            root, "clients", "apple", "VoiceCat.iOS", "AppModel.cs"));
+        Assert.DoesNotContain("new Thread(", microphone);
+        Assert.DoesNotContain("Thread.Sleep", microphone);
+        Assert.Contains("owner.Audio.RunCycle()", microphone);
+        Assert.Contains("while (microphoneCredit >= 960)", microphone);
+        Assert.Contains("deviceClockedAudio: true", model);
     }
 
     [Fact]