diff --git a/src/NetEscapades.EnumGenerators.Generators/Diagnostics/AnalyzerHelpers.cs b/src/NetEscapades.EnumGenerators.Generators/Diagnostics/AnalyzerHelpers.cs
index d706d48..6be407c 100644
--- a/src/NetEscapades.EnumGenerators.Generators/Diagnostics/AnalyzerHelpers.cs
+++ b/src/NetEscapades.EnumGenerators.Generators/Diagnostics/AnalyzerHelpers.cs
@@ -22,15 +22,13 @@ public static (INamedTypeSymbol? enumExtensionsAttr, ExternalEnumDictionary? ext
         var externalEnumTypes = new ExternalEnumDictionary(SymbolEqualityComparer.Default);
         if (externalEnumExtensionsAttr is not null)
         {
-            foreach (var attribute in compilation.Assembly.GetAttributes())
+            // Check current assembly
+            ScanAssemblyAttributes(compilation.Assembly, externalEnumExtensionsAttr, externalEnumTypes);
+
+            // Check referenced assemblies
+            foreach (var referencedAssembly in compilation.SourceModule.ReferencedAssemblySymbols)
             {
-                if (attribute.AttributeClass is { IsGenericType: true } attrClass &&
-                    SymbolEqualityComparer.Default.Equals(attrClass.ConstructedFrom, externalEnumExtensionsAttr) &&
-                    attrClass.TypeArguments is [INamedTypeSymbol { TypeKind: TypeKind.Enum } enumType])
-                {
-                    var details = ExtractExtensionClassDetails(enumType, attribute);
-                    externalEnumTypes.Add(enumType, details);
-                }
+                ScanAssemblyAttributes(referencedAssembly, externalEnumExtensionsAttr, externalEnumTypes);
             }
         }
 
@@ -67,6 +65,23 @@ public static bool IsEnumWithExtensions(
         return false;
     }
 
+    private static void ScanAssemblyAttributes(
+        IAssemblySymbol assembly,
+        INamedTypeSymbol externalEnumExtensionsAttr,
+        ExternalEnumDictionary externalEnumTypes)
+    {
+        foreach (var attribute in assembly.GetAttributes())
+        {
+            if (attribute.AttributeClass is { IsGenericType: true } attrClass &&
+                SymbolEqualityComparer.Default.Equals(attrClass.ConstructedFrom, externalEnumExtensionsAttr) &&
+                attrClass.TypeArguments is [INamedTypeSymbol { TypeKind: TypeKind.Enum } enumType])
+            {
+                var details = ExtractExtensionClassDetails(enumType, attribute);
+                externalEnumTypes[enumType] = details;
+            }
+        }
+    }
+
     private static string ExtractExtensionClassDetails(
         ITypeSymbol receiverType,
         AttributeData attributeData)
diff --git a/tests/NetEscapades.EnumGenerators.Tests/IsDefinedAnalyzerTests.cs b/tests/NetEscapades.EnumGenerators.Tests/IsDefinedAnalyzerTests.cs
index 04c99ee..845f504 100644
--- a/tests/NetEscapades.EnumGenerators.Tests/IsDefinedAnalyzerTests.cs
+++ b/tests/NetEscapades.EnumGenerators.Tests/IsDefinedAnalyzerTests.cs
@@ -1,5 +1,8 @@
 using System;
 using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Testing;
+using Microsoft.CodeAnalysis.Testing;
 using NetEscapades.EnumGenerators.Diagnostics.UsageAnalyzers;
 using Xunit;
 
@@ -571,6 +574,70 @@ public void TestMethod()
         await VerifyAnalyzerAsync(test, EnableState.Disabled);
     }
 
+    [Fact]
+    public async Task IsDefinedOnEnumFromReferencedAssemblyWithAssemblyLevelEnumExtensionsShouldHaveDiagnostic()
+    {
+        var mainCode = """
+            using System;
+            using ExternalNamespace;
+
+            namespace ConsoleApplication1
+            {
+                public class TestClass
+                {
+                    public void TestMethod()
+                    {
+                        var value = ExternalEnum.Value1;
+                        var isDefined = {|NEEG006:Enum.IsDefined(typeof(ExternalEnum), value)|};
+                    }
+                }
+            }
+
+            namespace ExternalNamespace
+            {
+                // This code would be generated, just hacked in here for simplicity
+                public static class ExternalEnumExtensions
+                {
+                    public static bool IsDefined(ExternalEnum value) => true;
+                }
+            }
+            """;
+
+        var externalCode = $$"""
+            [assembly: NetEscapades.EnumGenerators.EnumExtensions<ExternalNamespace.ExternalEnum>()]
+
+            namespace ExternalNamespace
+            {
+                public enum ExternalEnum
+                {
+                    Value1 = 1,
+                    Value2 = 2,
+                }
+            }
+
+            {{TestHelpers.LoadEmbeddedAttribute()}}
+            {{TestHelpers.LoadEmbeddedMetadataSource()}}
+            """;
+
+        var test = new CSharpAnalyzerTest<IsDefinedAnalyzer, DefaultVerifier>
+        {
+            TestCode = mainCode,
+        };
+
+        test.TestState.AdditionalProjects.Add("ExternalProject", new ProjectState("ExternalProject", LanguageNames.CSharp, "external_", ".cs"));
+        test.TestState.AdditionalProjects["ExternalProject"].Sources.Add(("external_Code.cs", externalCode));
+        test.TestState.AdditionalProjectReferences.Add("ExternalProject");
+
+        test.TestState.AnalyzerConfigFiles.Add(
+            ("/.editorconfig",
+                $"""
+                 is_global = true
+                 {UsageAnalyzerConfig.EnableKey} = true
+                 """));
+
+        await test.RunAsync();
+    }
+
     private static string GetTestCodeWithExternalEnum(string testCode) => $$"""
         using System;
         using System.Collections.Generic;