diff --git a/Libraries/Microsoft.Teams.Apps/App.cs b/Libraries/Microsoft.Teams.Apps/App.cs
index 4d20690a..12009245 100644
--- a/Libraries/Microsoft.Teams.Apps/App.cs
+++ b/Libraries/Microsoft.Teams.Apps/App.cs
@@ -41,7 +41,7 @@ public partial class App
     internal IServiceProvider? Provider { get; set; }
     internal IContainer Container { get; set; }
 
-    private readonly IEnumerable<string>? _additionalAllowedDomains;
+    private readonly IReadOnlyList<string>? _additionalAllowedDomains;
     private readonly CloudEnvironment _cloud;
     internal string UserAgent
     {
@@ -63,7 +63,9 @@ public App(AppOptions? options = null)
         Plugins = options?.Plugins ?? [];
         OAuth = options?.OAuth ?? new OAuthSettings();
         Provider = options?.Provider;
-        _additionalAllowedDomains = options?.AdditionalAllowedDomains;
+        // Defensive copy so a caller-provided list can't mutate validator behavior
+        // after construction (IEnumerable may be lazy or mutable).
+        _additionalAllowedDomains = options?.AdditionalAllowedDomains?.ToList();
 
         if (_additionalAllowedDomains?.Contains("*") == true)
         {
diff --git a/Libraries/Microsoft.Teams.Apps/AppOptions.cs b/Libraries/Microsoft.Teams.Apps/AppOptions.cs
index e887cf12..3c0f909c 100644
--- a/Libraries/Microsoft.Teams.Apps/AppOptions.cs
+++ b/Libraries/Microsoft.Teams.Apps/AppOptions.cs
@@ -19,9 +19,12 @@ public class AppOptions
     public CloudEnvironment? Cloud { get; set; }
 
     /// <summary>
-    /// Additional allowed service URL hostnames beyond the built-in defaults.
-    /// Use this if your bot receives activities from non-standard channels.
+    /// Additional service URL hostnames accepted beyond the cloud preset.
+    /// Entries must be bare hostnames matched exactly (case-insensitive)
+    /// wildcard patterns like <c>"*.example.com"</c>, URL suffixes, or full URLs are NOT supported.
+    /// Pass <c>["*"]</c> as the sole wildcard to accept any hostname (disables service-URL validation).
     /// </summary>
+    /// <example>new[] { "api.my-custom-channel.com" }</example>
     public IEnumerable<string>? AdditionalAllowedDomains { get; set; }
 
     public AppOptions()
diff --git a/Tests/Microsoft.Teams.Apps.Tests/AppTests.cs b/Tests/Microsoft.Teams.Apps.Tests/AppTests.cs
index 5ce7a061..24e5087f 100644
--- a/Tests/Microsoft.Teams.Apps.Tests/AppTests.cs
+++ b/Tests/Microsoft.Teams.Apps.Tests/AppTests.cs
@@ -367,4 +367,39 @@ await Assert.ThrowsAsync<ArgumentException>(() =>
             app.Reply("19:abc@thread.skype", "not-a-number", new MessageActivity("Hello")));
     }
 
+    [Fact]
+    public void Test_App_AdditionalAllowedDomains_NullConstructionSucceeds()
+    {
+        var options = new AppOptions { AdditionalAllowedDomains = null };
+        var exception = Record.Exception(() => new App(options));
+        Assert.Null(exception);
+    }
+
+    [Fact]
+    public void Test_App_AdditionalAllowedDomains_EmptyConstructionSucceeds()
+    {
+        var options = new AppOptions { AdditionalAllowedDomains = Array.Empty<string>() };
+        var exception = Record.Exception(() => new App(options));
+        Assert.Null(exception);
+    }
+
+    [Fact]
+    public void Test_App_AdditionalAllowedDomains_MaterializesLazyEnumerable()
+    {
+        // A lazy IEnumerable must be materialized at construction (not held and
+        // re-enumerated later), otherwise mutation between iterations could change
+        // validator behavior. Asserts the caller's lazy is invoked exactly once.
+        var callCount = 0;
+        IEnumerable<string> Lazy()
+        {
+            callCount++;
+            yield return "api.custom-channel.com";
+        }
+
+        var options = new AppOptions { AdditionalAllowedDomains = Lazy() };
+        _ = new App(options);
+
+        Assert.Equal(1, callCount);
+    }
+
 }
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