Activate iOS audio session before stereo input selection

PROGRESS.md

@@ -54,6 +54,12 @@ no longer held mapped through suspension. A TestFlight report from build 2026092
 `0xdead10cc` shared-file-lock termination; physical-device validation of that mitigation remains
 open.
 
+Stereo capture now selects the preferred input only after activating the audio session and no
+longer calls `SetInputDataSource` with a source from a port that may not yet be current. A Debug
+build installed on an iPhone 16 Pro Max connected with the StereoMicrophone preset and recorded
+a two-channel input graph with distinct left/right PCM and encoded packets. The simulator's mono
+input also completed stereo-preset login, voice join, disconnect, and reconnect.
+
 Voice-chat capture is not yet reliable on the first attempt: a graph with voice processing still
 sometimes fails to start and is only recovered by a watchdog rebuild, which costs seconds before
 audio appears. Speaker output is an explicit choice that no longer changes the preset. Every

clients/apple/VoiceCat.iOS/IosAudioRouter.cs

@@ -147,8 +147,9 @@ internal sealed class IosAudioRouter
                 : BluetoothMode == IosBluetoothMode.BuiltInMicA2dp ? AVAudioSessionMode.VideoRecording : AVAudioSessionMode.VoiceChat;
             if (!session.SetCategory(AVAudioSessionCategory.PlayAndRecord, mode, options, out NSError? categoryError)) throw new InvalidOperationException(categoryError?.LocalizedDescription ?? "Could not configure the iOS audio session.");
             session.SetPreferredSampleRate(48_000, out _); session.SetPreferredIOBufferDuration(0.02, out _);
-            if (configureInput) ApplyInputSelection(session);
             if (!session.SetActive(true, AVAudioSessionSetActiveOptions.NotifyOthersOnDeactivation, out NSError? activeError)) throw new InvalidOperationException(activeError?.LocalizedDescription ?? "Could not activate the iOS audio session.");
+            // Apple requires an active session before selecting a preferred input or data source.
+            if (configureInput) ApplyInputSelection(session);
             Console.Error.WriteLine($"VC_ROUTE preset={Preset} requestedCh={CaptureChannels} sessionCh={session.InputNumberOfChannels} " +
                 $"preferred={session.PreferredInput?.PortName ?? "default"} dataSource={session.InputDataSource?.DataSourceName ?? "default"} " +
                 $"pattern={session.InputDataSource?.SelectedPolarPattern.ToString() ?? "default"}");
@@ -172,6 +173,7 @@ internal sealed class IosAudioRouter
         AVAudioSessionPortDescription? port = session.AvailableInputs?.FirstOrDefault(value => value.UID == SelectedInputId);
         if (CaptureChannels == 2) port ??= session.AvailableInputs?.FirstOrDefault(value => value.PortType == AVAudioSession.PortBuiltInMic);
         if (port is null) { if (CaptureChannels == 1 && stereoApplied) ClearStereo(session); return; }
+        if (!session.SetPreferredInput(port, out NSError? inputError)) throw new InvalidOperationException(inputError?.LocalizedDescription ?? "Could not select the microphone input.");
         if (CaptureChannels == 2)
         {
             // Polar-pattern discovery alone is insufficient on current iPhones: until a stereo
@@ -195,13 +197,11 @@ internal sealed class IosAudioRouter
                 !source.SetPreferredPolarPattern(SelectedPolarPattern, out NSError? patternError))
                 throw new InvalidOperationException(patternError?.LocalizedDescription ?? "Could not select the microphone polar pattern.");
         }
-        if (!session.SetPreferredInput(port, out NSError? inputError)) throw new InvalidOperationException(inputError?.LocalizedDescription ?? "Could not select the microphone input.");
         // The working Swift client deliberately does not call
         // SetPreferredInputNumberOfChannels: doing so disrupts stereo + A2DP routing. The stereo
         // capsule and polar pattern above cause the input node to expose its two-channel format.
-        if (CaptureChannels == 2 && source is not null &&
+        // A port preference is enough. SetInputDataSource only accepts a member of the *current*
-            !session.SetInputDataSource(source, out NSError? sourceError))
+        // port's InputDataSources, which can still be a different route after SetPreferredInput.
-            throw new InvalidOperationException(sourceError?.LocalizedDescription ?? "Could not activate the stereo microphone source.");
         Console.Error.WriteLine($"VC_ROUTE_SELECT port={port.PortName} source={source?.DataSourceName ?? "none"} " +
             $"stereoPattern={source is not null && SupportsStereoPolarPattern(source)} " +
             $"patterns={string.Join(',', source?.SupportedPolarPatterns?.Select(value => value.ToString()) ?? [])}");

clients/apple/VoiceCat.iOS/SimulatorSmoke.cs

@@ -32,6 +32,8 @@ internal static class SimulatorSmoke
                     model.UpsertProfile(profile, parts[2]);
                 }
                 model.Load();
+                if (Environment.GetEnvironmentVariable("VOICECAT_SIM_SMOKE_MODE") == "stereo")
+                    IosAudioRouter.Shared.SelectPreset(IosAudioPreset.StereoMicrophone);
                 profile = model.Profiles.Single(value => value.Id == id);
                 Console.Error.WriteLine("VC_SIM phase=connect");
                 await model.ConnectAsync(profile);

docs/ios-deploy.md

@@ -163,6 +163,8 @@ synthetic screen audio with the microphone off and during a voice call, voice jo
 and reconnect. Look for
 `VC_SIM phase=screenSharePassed` and `VC_SIM phase=passed`. Terminate the app, then launch
 again with mode `resume` to verify the saved account after a process restart. The simulator
+gate also accepts mode `stereo` to select the StereoMicrophone preset before login and voice join;
+the simulator exposes only a mono microphone, so check stereo capture on an iPhone. It also
 does not validate Bluetooth hardware, the iOS 27 ScreenCaptureKit picker, or suspension on
 a physical device.
 

tests/VoiceCat.Tests/PublishServerScriptTests.cs

@@ -207,8 +207,11 @@ public class PublishServerScriptTests
         string router = await File.ReadAllTextAsync(Path.Combine(root, "clients", "apple", "VoiceCat.iOS", "IosAudioRouter.cs"));
         string engine = await File.ReadAllTextAsync(Path.Combine(root, "clients", "apple", "VoiceCat.iOS", "IosAudioEngine.cs"));
 
-        Assert.True(router.IndexOf("ApplyInputSelection(session)", StringComparison.Ordinal) <
+        Assert.True(router.IndexOf("SetActive(true", StringComparison.Ordinal) <
-            router.IndexOf("SetActive(true", StringComparison.Ordinal));
+            router.IndexOf("ApplyInputSelection(session)", StringComparison.Ordinal));
+        Assert.True(router.IndexOf("session.SetPreferredInput(port", StringComparison.Ordinal) <
+            router.IndexOf("port.SetPreferredDataSource(source", StringComparison.Ordinal));
+        Assert.DoesNotContain("session.SetInputDataSource(source", router);
         Assert.DoesNotContain("session.SetPreferredInputNumberOfChannels", router);
         Assert.DoesNotContain("MaximumInputNumberOfChannels", router);
         Assert.Contains("SetPreferredInputOrientation(AVAudioStereoOrientation.Portrait", router);