diff --git a/src/Analyzers/Orchestration/OrchestrationAnalyzer.cs b/src/Analyzers/Orchestration/OrchestrationAnalyzer.cs
index a299d5777..342b1e6c0 100644
--- a/src/Analyzers/Orchestration/OrchestrationAnalyzer.cs
+++ b/src/Analyzers/Orchestration/OrchestrationAnalyzer.cs
@@ -141,7 +141,14 @@ public override void Initialize(AnalysisContext context)
                     case IMethodReferenceOperation methodReferenceOperation:
                         // use the method reference as the method symbol
                         methodSymbol = methodReferenceOperation.Method;
-                        methodSyntax = methodReferenceOperation.Method.DeclaringSyntaxReferences.First().GetSyntax();
+
+                        // Only get syntax for methods in the current project (skip external assemblies)
+                        // If IsEmpty (external method), methodSyntax stays null and we skip analysis below
+                        if (!methodReferenceOperation.Method.DeclaringSyntaxReferences.IsEmpty)
+                        {
+                            methodSyntax = methodReferenceOperation.Method.DeclaringSyntaxReferences.First().GetSyntax();
+                        }
+
                         break;
                     default:
                         break;
@@ -316,6 +323,13 @@ void FindInvokedMethods(
             IEnumerable<MethodDeclarationSyntax> calleeSyntaxes = calleeMethodSymbol.GetSyntaxNodes();
             foreach (MethodDeclarationSyntax calleeSyntax in calleeSyntaxes)
             {
+                // Check if the syntax tree is part of the current compilation before trying to get its semantic model.
+                // Extension methods from external assemblies won't be in the compilation, so skip them.
+                if (!semanticModel.Compilation.ContainsSyntaxTree(calleeSyntax.SyntaxTree))
+                {
+                    continue;
+                }
+
                 // Since the syntax tree of the callee method might be different from the caller method, we need to get the correct semantic model,
                 // avoiding the exception 'Syntax node is not within syntax tree'.
                 SemanticModel sm = semanticModel.SyntaxTree == calleeSyntax.SyntaxTree ?
diff --git a/src/Analyzers/RoslynExtensions.cs b/src/Analyzers/RoslynExtensions.cs
index 10339f7a4..597013334 100644
--- a/src/Analyzers/RoslynExtensions.cs
+++ b/src/Analyzers/RoslynExtensions.cs
@@ -115,6 +115,13 @@ public static bool BaseTypeIsConstructedFrom(this INamedTypeSymbol symbol, IType
     /// <returns>The collection of syntax nodes of a given method symbol.</returns>
     public static IEnumerable<MethodDeclarationSyntax> GetSyntaxNodes(this IMethodSymbol methodSymbol)
     {
+        // If the method has no syntax references (e.g., extension methods from external assemblies),
+        // return empty to skip analysis rather than throwing ArgumentException.
+        if (methodSymbol.DeclaringSyntaxReferences.IsEmpty)
+        {
+            return Enumerable.Empty<MethodDeclarationSyntax>();
+        }
+
         return methodSymbol.DeclaringSyntaxReferences.Select(r => r.GetSyntax()).OfType<MethodDeclarationSyntax>();
     }