diff --git a/PolyPilot.Tests/VolatileFieldGuardTests.cs b/PolyPilot.Tests/VolatileFieldGuardTests.cs
new file mode 100644
index 0000000000..9593de28db
--- /dev/null
+++ b/PolyPilot.Tests/VolatileFieldGuardTests.cs
@@ -0,0 +1,37 @@
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using PolyPilot.Services;
+
+namespace PolyPilot.Tests;
+
+/// <summary>
+/// Guards that fields accessed from multiple threads are declared volatile.
+/// Prevents regressions where someone removes the volatile modifier.
+/// </summary>
+public class VolatileFieldGuardTests
+{
+    [Fact]
+    public void ActiveSessionName_IsDeclaredVolatile_OnCopilotService()
+    {
+        // _activeSessionName is read by WsBridge background threads (SyncRemoteSessions),
+        // restore background threads, and written by UI thread (SetActiveSession, CloseSession).
+        // Must be volatile for cross-thread visibility on ARM (iOS/Android).
+        var field = typeof(CopilotService)
+            .GetField("_activeSessionName", BindingFlags.NonPublic | BindingFlags.Instance)!;
+        Assert.NotNull(field);
+        Assert.True(
+            field.GetRequiredCustomModifiers().Any(m => m == typeof(IsVolatile)),
+            "_activeSessionName must be declared volatile for cross-thread visibility");
+    }
+
+    [Fact]
+    public void ActiveSessionName_IsDeclaredVolatile_OnDemoService()
+    {
+        var field = typeof(DemoService)
+            .GetField("_activeSessionName", BindingFlags.NonPublic | BindingFlags.Instance)!;
+        Assert.NotNull(field);
+        Assert.True(
+            field.GetRequiredCustomModifiers().Any(m => m == typeof(IsVolatile)),
+            "DemoService._activeSessionName must be declared volatile for consistency");
+    }
+}
diff --git a/PolyPilot/Services/CopilotService.cs b/PolyPilot/Services/CopilotService.cs
index 9bdb631d78..ffaf8fc8b2 100644
--- a/PolyPilot/Services/CopilotService.cs
+++ b/PolyPilot/Services/CopilotService.cs
@@ -51,7 +51,7 @@ public partial class CopilotService : IAsyncDisposable
     // Cached dotfiles status — checked once when first SetupRequired state is encountered
     private CodespaceService.DotfilesStatus? _dotfilesStatus;
     private ConnectionSettings? _currentSettings;
-    private string? _activeSessionName;
+    private volatile string? _activeSessionName;
     private SynchronizationContext? _syncContext;
     // Serializes the IsConnectionError reconnect path so concurrent workers
     // don't destroy each other's freshly-created client (thundering herd fix).
diff --git a/PolyPilot/Services/DemoService.cs b/PolyPilot/Services/DemoService.cs
index 2e4fa81d33..7958f5b73e 100644
--- a/PolyPilot/Services/DemoService.cs
+++ b/PolyPilot/Services/DemoService.cs
@@ -10,7 +10,7 @@ namespace PolyPilot.Services;
 public class DemoService : IDemoService
 {
     private readonly ConcurrentDictionary<string, AgentSessionInfo> _sessions = new();
-    private string? _activeSessionName;
+    private volatile string? _activeSessionName;
     private int _sessionCounter;
 
     public event Action? OnStateChanged;