Use dedicated "plugins" property in package.json for plugin loading

docs/samples/client/csharp/SampleService/package.json

@@ -1,6 +1,8 @@
 {
   "dependencies": {
-    "@typespec/http-client-csharp": "1.0.0-alpha.20250923.2",
+    "@typespec/http-client-csharp": "1.0.0-alpha.20250923.2"
+  },
+  "plugins": {
     "logging-plugin": "file:../plugins/logging"
   }
 }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/StartUp/GeneratorHandler.cs

@@ -102,21 +102,28 @@ internal static IList<string> GetOrderedPluginDlls(string pluginDirectoryStart)
             }
 
             using var doc = JsonDocument.Parse(File.ReadAllText(packagePath));
-            if (!doc.RootElement.TryGetProperty("dependencies", out var deps))
+            if (!doc.RootElement.TryGetProperty("plugins", out var plugins))
             {
                 return dllPathsInOrder;
             }
 
-            var packageNamesInOrder = deps.EnumerateObject().Select(p => p.Name).ToList();
+            var packageNamesInOrder = plugins.EnumerateObject().Select(p => p.Name).ToList();
 
             foreach (var package in packageNamesInOrder)
             {
                 var packageDistPath = Path.Combine(rootDirectory, NodeModulesDir, package, "dist");
-                if (Directory.Exists(packageDistPath))
+                if (!Directory.Exists(packageDistPath))
                 {
-                    var dlls = Directory.EnumerateFiles(packageDistPath, "*.dll", SearchOption.AllDirectories);
-                    dllPathsInOrder.AddRange(dlls);
+                    throw new InvalidOperationException($"Plugin '{package}' specified in package.json but its dist directory was not found at '{packageDistPath}'.");
                 }
+
+                var dlls = Directory.EnumerateFiles(packageDistPath, "*.dll", SearchOption.AllDirectories).ToList();
+                if (dlls.Count == 0)
+                {
+                    throw new InvalidOperationException($"Plugin '{package}' specified in package.json but no DLL files were found in '{packageDistPath}'.");
+                }
+
+                dllPathsInOrder.AddRange(dlls);
             }
 
             return dllPathsInOrder;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/StartUp/GeneratorHandlerTests.cs

@@ -167,7 +167,7 @@ public void GetOrderedPluginDlls()
                     ""name"": ""dummy-project"",
                     ""version"": ""1.0.0"",
                     ""description"": ""Dummy project for testing purposes."",
-                    ""dependencies"": { ""plugin-1"": ""^1.0.0"", ""plugin-2"": ""^2.0.0"" }
+                    ""plugins"": { ""plugin-1"": ""^1.0.0"", ""plugin-2"": ""^2.0.0"" }
                 }");
 
                 File.WriteAllText(Path.Combine(plugin1Directory, "Plugin1.dll"), "Dummy DLL content");
@@ -184,5 +184,97 @@ public void GetOrderedPluginDlls()
                 File.Delete(Path.Combine(plugin2Directory, "Plugin2.dll"));
             }
         }
+
+        [Test]
+        public void GetOrderedPluginDlls_ThrowsIfPluginDirectoryNotFound()
+        {
+            var testRoot = TestContext.CurrentContext.TestDirectory;
+            var pluginDirectory = Path.Combine(testRoot, "node_modules", "missing-plugin", "dist");
+            try
+            {
+                File.WriteAllText(Path.Combine(testRoot, "package.json"), @"{
+                    ""name"": ""dummy-project"",
+                    ""version"": ""1.0.0"",
+                    ""description"": ""Dummy project for testing purposes."",
+                    ""plugins"": { ""missing-plugin"": ""^1.0.0"" }
+                }");
+
+                var ex = Assert.Throws<InvalidOperationException>(() =>
+                    GeneratorHandler.GetOrderedPluginDlls(pluginDirectory));
+
+                Assert.That(ex!.Message, Does.Contain("missing-plugin"));
+                Assert.That(ex.Message, Does.Contain("dist directory was not found"));
+            }
+            finally
+            {
+                // Cleanup
+                var packageJsonPath = Path.Combine(testRoot, "package.json");
+                if (File.Exists(packageJsonPath))
+                {
+                    File.Delete(packageJsonPath);
+                }
+            }
+        }
+
+        [Test]
+        public void GetOrderedPluginDlls_ThrowsIfPluginDllNotFound()
+        {
+            var testRoot = TestContext.CurrentContext.TestDirectory;
+            var pluginDirectory = Path.Combine(testRoot, "node_modules", "empty-plugin", "dist");
+            try
+            {
+                Directory.CreateDirectory(pluginDirectory);
+
+                File.WriteAllText(Path.Combine(testRoot, "package.json"), @"{
+                    ""name"": ""dummy-project"",
+                    ""version"": ""1.0.0"",
+                    ""description"": ""Dummy project for testing purposes."",
+                    ""plugins"": { ""empty-plugin"": ""^1.0.0"" }
+                }");
+
+                var ex = Assert.Throws<InvalidOperationException>(() =>
+                    GeneratorHandler.GetOrderedPluginDlls(pluginDirectory));
+
+                Assert.That(ex!.Message, Does.Contain("empty-plugin"));
+                Assert.That(ex.Message, Does.Contain("no DLL files were found"));
+            }
+            finally
+            {
+                // Cleanup
+                var packageJsonPath = Path.Combine(testRoot, "package.json");
+                if (File.Exists(packageJsonPath))
+                {
+                    File.Delete(packageJsonPath);
+                }
+            }
+        }
+
+        [Test]
+        public void GetOrderedPluginDlls_ReturnsEmptyListWhenNoPluginsProperty()
+        {
+            var testRoot = TestContext.CurrentContext.TestDirectory;
+            try
+            {
+                File.WriteAllText(Path.Combine(testRoot, "package.json"), @"{
+                    ""name"": ""dummy-project"",
+                    ""version"": ""1.0.0"",
+                    ""description"": ""Dummy project for testing purposes."",
+                    ""dependencies"": { ""some-dep"": ""^1.0.0"" }
+                }");
+
+                var dlls = GeneratorHandler.GetOrderedPluginDlls(testRoot);
+
+                Assert.AreEqual(0, dlls.Count);
+            }
+            finally
+            {
+                // Cleanup
+                var packageJsonPath = Path.Combine(testRoot, "package.json");
+                if (File.Exists(packageJsonPath))
+                {
+                    File.Delete(packageJsonPath);
+                }
+            }
+        }
     }
 }