diff --git a/backend/src/Taskdeck.Application/Services/LlmIntentClassifier.cs b/backend/src/Taskdeck.Application/Services/LlmIntentClassifier.cs
index 29372d56b..4ff0e1c83 100644
--- a/backend/src/Taskdeck.Application/Services/LlmIntentClassifier.cs
+++ b/backend/src/Taskdeck.Application/Services/LlmIntentClassifier.cs
@@ -4,6 +4,9 @@ public static class LlmIntentClassifier
 {
     public static (bool IsActionable, string? ActionIntent) Classify(string message)
     {
+        if (string.IsNullOrWhiteSpace(message))
+            return (false, null);
+
         var lower = message.ToLowerInvariant();
 
         // Card creation — explicit commands and natural language
diff --git a/backend/tests/Taskdeck.Application.Tests/Services/LlmIntentClassifierTests.cs b/backend/tests/Taskdeck.Application.Tests/Services/LlmIntentClassifierTests.cs
index d4011bce3..f4577031d 100644
--- a/backend/tests/Taskdeck.Application.Tests/Services/LlmIntentClassifierTests.cs
+++ b/backend/tests/Taskdeck.Application.Tests/Services/LlmIntentClassifierTests.cs
@@ -142,6 +142,84 @@ public void Classify_NonActionable_ShouldReturnFalse(string message)
 
     #endregion
 
+    #region Edge Cases — Input Extremes
+
+    [Fact]
+    public void Classify_NullInput_ReturnsNotActionable()
+    {
+        var (isActionable, actionIntent) = LlmIntentClassifier.Classify(null!);
+
+        isActionable.Should().BeFalse();
+        actionIntent.Should().BeNull();
+    }
+
+    [Fact]
+    public void Classify_VeryLongString_ReturnsNotActionable()
+    {
+        var longMessage = new string('x', 50_000);
+
+        var (isActionable, actionIntent) = LlmIntentClassifier.Classify(longMessage);
+
+        isActionable.Should().BeFalse();
+        actionIntent.Should().BeNull();
+    }
+
+    [Fact]
+    public void Classify_VeryLongStringContainingPattern_StillMatches()
+    {
+        var longMessage = new string('x', 25_000) + " create card for testing " + new string('x', 25_000);
+
+        var (isActionable, actionIntent) = LlmIntentClassifier.Classify(longMessage);
+
+        isActionable.Should().BeTrue();
+        actionIntent.Should().Be("card.create");
+    }
+
+    [Theory]
+    [InlineData("   ")]
+    [InlineData("\t\t")]
+    [InlineData("\n\n\n")]
+    public void Classify_WhitespaceOnly_ReturnsNotActionable(string message)
+    {
+        var (isActionable, actionIntent) = LlmIntentClassifier.Classify(message);
+
+        isActionable.Should().BeFalse();
+        actionIntent.Should().BeNull();
+    }
+
+    [Theory]
+    [InlineData("Hello! @#$%^&*() special chars")]
+    [InlineData("Unicode: \u00e9\u00e8\u00ea\u00eb\u00fc\u00f6\u00e4")]
+    [InlineData("<script>alert('xss')</script>")]
+    [InlineData("SELECT * FROM cards; DROP TABLE boards;")]
+    public void Classify_SpecialCharacters_WithoutPattern_ReturnsNotActionable(string message)
+    {
+        var (isActionable, actionIntent) = LlmIntentClassifier.Classify(message);
+
+        isActionable.Should().BeFalse();
+        actionIntent.Should().BeNull();
+    }
+
+    [Fact]
+    public void Classify_PatternWithSpecialCharsSurrounding_StillMatches()
+    {
+        var (isActionable, actionIntent) = LlmIntentClassifier.Classify("!!! create card !!! @#$ testing");
+
+        isActionable.Should().BeTrue();
+        actionIntent.Should().Be("card.create");
+    }
+
+    [Fact]
+    public void Classify_PatternWithNewlines_StillMatches()
+    {
+        var (isActionable, actionIntent) = LlmIntentClassifier.Classify("line 1\ncreate card for testing\nline 3");
+
+        isActionable.Should().BeTrue();
+        actionIntent.Should().Be("card.create");
+    }
+
+    #endregion
+
     #region Known Gaps — Natural Language Misses (Documents #570/#571)
 
     /// <summary>