Skip external models from mocking

Author: rhurey

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelFactoryProvider.cs

@@ -488,6 +488,16 @@ private static IReadOnlyList<ValueExpression> GetCtorArgs(
 
         private static ModelProvider? GetModelToInstantiateForFactoryMethod(ModelProvider modelProvider)
         {
+            // Externally-linked models are reused from another shipped package (e.g. the OpenAI .NET
+            // SDK), so this library does not own or emit their constructors. Their real constructor
+            // surface (accessibility and parameter set) is unknown here; emitting a
+            // `new ExternalType(...)` mocking factory produces calls that may be inaccessible
+            // (protected/internal) or have a non-matching arity. Skip factory generation for them.
+            if (modelProvider.IsExternal)
+            {
+                return null;
+            }
+
             var fullConstructor = modelProvider.FullConstructor;
             if (modelProvider.DeclarationModifiers.HasFlag(TypeSignatureModifiers.Internal)
                 || fullConstructor.Signature.Parameters.Any(p => !p.Type.IsPublic && !IsEnumDiscriminator(p)))

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs

@@ -77,6 +77,10 @@ public ModelProvider(InputModelType inputModel) : base(inputModel)
 
         public bool IsUnknownDiscriminatorModel => _inputModel.IsUnknownDiscriminatorModel;
 
+        // Whether this model is reused from another shipped package (linked via an `external` block)
+        // rather than emitted by this library. Such a type's constructor surface is owned elsewhere.
+        internal bool IsExternal => _inputModel.External is not null;
+
         public string? DiscriminatorValue => _inputModel.DiscriminatorValue;
 
         private ValueExpression? _discriminatorValueExpression;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/ModelFactoryProviderTests.cs

@@ -32,6 +32,26 @@ public void SkipInternalModels()
             Assert.AreEqual(ModelList.Length - ModelList.Where(m => m.Access == "internal").Count(), modelFactory.Methods.Count);
         }
 
+        [Test]
+        public void SkipExternalModels()
+        {
+            var external = new InputExternalTypeMetadata("OpenAI.Responses.ResponseTool", "OpenAI", "2.11.0");
+            var externalModel = InputFactory.Model("ExternalTool", external: external);
+            var localModel = InputFactory.Model("LocalThing");
+            var instance = MockHelpers.LoadMockGenerator(
+                inputNamespaceName: "Sample.Namespace",
+                inputModelTypes: [externalModel, localModel]).Object;
+
+            var modelFactory = instance.OutputLibrary.ModelFactory.Value;
+
+            Assert.IsNull(
+                modelFactory.Methods.FirstOrDefault(m => m.Signature.Name == "ExternalTool"),
+                "Externally-linked models must not get a model factory method.");
+            Assert.IsNotNull(
+                modelFactory.Methods.FirstOrDefault(m => m.Signature.Name == "LocalThing"),
+                "Local models must still get a model factory method.");
+        }
+
         [Test]
         public void ListParamShape()
         {