Add private protected constructors to McpClient and McpServer to prevent external derivation

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -7,6 +7,10 @@ namespace ModelContextProtocol.Client;
 /// </summary>
 public abstract partial class McpClient : McpSession
 {
+    /// <summary>Initializes a new instance of the <see cref="McpClient"/> class.</summary>
+    private protected McpClient()
+    {
+    }
     /// <summary>
     /// Gets the capabilities supported by the connected server.
     /// </summary>

src/ModelContextProtocol.Core/Server/McpServer.cs

@@ -7,6 +7,10 @@ namespace ModelContextProtocol.Server;
 /// </summary>
 public abstract partial class McpServer : McpSession
 {
+    /// <summary>Initializes a new instance of the <see cref="McpServer"/> class.</summary>
+    private protected McpServer()
+    {
+    }
     /// <summary>
     /// Gets the capabilities supported by the client.
     /// </summary>

tests/Common/Utils/TestServerTransport.cs

@@ -93,10 +93,11 @@ await WriteMessageAsync(new JsonRpcResponse
         else
         {
             // Return a normal sampling response
+            var result = MockSamplingResult ?? new CreateMessageResult { Content = [new TextContentBlock { Text = "" }], Model = "model" };
             await WriteMessageAsync(new JsonRpcResponse
             {
                 Id = request.Id,
-                Result = JsonSerializer.SerializeToNode(new CreateMessageResult { Content = [new TextContentBlock { Text = "" }], Model = "model" }, McpJsonUtilities.DefaultOptions),
+                Result = JsonSerializer.SerializeToNode(result, McpJsonUtilities.DefaultOptions),
             }, cancellationToken);
         }
     }
@@ -125,6 +126,12 @@ await WriteMessageAsync(new JsonRpcResponse
         }
     }
 
+    /// <summary>
+    /// Gets or sets the sampling result to return from sampling/createMessage requests.
+    /// When null, a default sampling response is returned.
+    /// </summary>
+    public CreateMessageResult? MockSamplingResult { get; set; }
+
     /// <summary>
     /// Gets or sets the task to return from tasks/get requests.
     /// </summary>

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -807,15 +807,36 @@ private async Task Succeeds_Even_If_No_Handler_Assigned(ServerCapabilities serve
     [Fact]
     public async Task AsSamplingChatClient_NoSamplingSupport_Throws()
     {
-        await using var server = new TestServerForIChatClient(supportsSampling: false);
+        await using var transport = new TestServerTransport();
+        await using var server = McpServer.Create(transport, _options, LoggerFactory);
 
         Assert.Throws<InvalidOperationException>(() => server.AsSamplingChatClient());
     }
 
     [Fact]
     public async Task AsSamplingChatClient_HandlesRequestResponse()
     {
-        await using var server = new TestServerForIChatClient(supportsSampling: true);
+        await using var transport = new TestServerTransport();
+        transport.MockSamplingResult = new CreateMessageResult
+        {
+            Content = [new TextContentBlock { Text = "The Eiffel Tower." }],
+            Model = "amazingmodel",
+            Role = Role.Assistant,
+            StopReason = "endTurn",
+        };
+        await using var server = McpServer.Create(transport, _options, LoggerFactory);
+        var runTask = server.RunAsync(TestContext.Current.CancellationToken);
+        await InitializeServerAsync(transport, new ClientCapabilities { Sampling = new SamplingCapability() }, TestContext.Current.CancellationToken);
+
+        // Capture the sampling request for validation
+        CreateMessageRequestParams? capturedParams = null;
+        transport.OnMessageSent = (message) =>
+        {
+            if (message is JsonRpcRequest request && request.Method == RequestMethods.SamplingCreateMessage)
+            {
+                capturedParams = JsonSerializer.Deserialize<CreateMessageRequestParams>(request.Params, McpJsonUtilities.DefaultOptions);
+            }
+        };
 
         IChatClient client = server.AsSamplingChatClient();
 
@@ -839,6 +860,22 @@ public async Task AsSamplingChatClient_HandlesRequestResponse()
         Assert.Single(response.Messages);
         Assert.Equal("The Eiffel Tower.", response.Text);
         Assert.Equal(ChatRole.Assistant, response.Messages[0].Role);
+
+        // Validate the request parameters
+        Assert.NotNull(capturedParams);
+        Assert.Equal(0.75f, capturedParams.Temperature);
+        Assert.Equal(42, capturedParams.MaxTokens);
+        Assert.Equal(["."], capturedParams.StopSequences);
+        Assert.Null(capturedParams.IncludeContext);
+        Assert.Null(capturedParams.Metadata);
+        Assert.Null(capturedParams.ModelPreferences);
+        Assert.Equal($"You are a helpful assistant.{Environment.NewLine}More system stuff.", capturedParams.SystemPrompt);
+        Assert.Equal(2, capturedParams.Messages.Count);
+        Assert.Equal("I am going to France.", Assert.IsType<TextContentBlock>(Assert.Single(capturedParams.Messages[0].Content)).Text);
+        Assert.Equal("What is the most famous tower in Paris?", Assert.IsType<TextContentBlock>(Assert.Single(capturedParams.Messages[1].Content)).Text);
+
+        await transport.DisposeAsync();
+        await runTask;
     }
 
     [Fact]
@@ -890,62 +927,6 @@ private static async Task InitializeServerAsync(TestServerTransport transport, C
         await tcs.Task.WaitAsync(TestConstants.DefaultTimeout, cancellationToken);
     }
 
-    private sealed class TestServerForIChatClient(bool supportsSampling) : McpServer
-    {
-        public override ClientCapabilities? ClientCapabilities =>
-            supportsSampling ? new ClientCapabilities { Sampling = new SamplingCapability() } :
-            null;
-
-        public override McpServerOptions ServerOptions => new();
-
-        public override Task<JsonRpcResponse> SendRequestAsync(JsonRpcRequest request, CancellationToken cancellationToken)
-        {
-            CreateMessageRequestParams? rp = JsonSerializer.Deserialize<CreateMessageRequestParams>(request.Params, McpJsonUtilities.DefaultOptions);
-
-            Assert.NotNull(rp);
-            Assert.Equal(0.75f, rp.Temperature);
-            Assert.Equal(42, rp.MaxTokens);
-            Assert.Equal(["."], rp.StopSequences);
-            Assert.Null(rp.IncludeContext);
-            Assert.Null(rp.Metadata);
-            Assert.Null(rp.ModelPreferences);
-
-            Assert.Equal($"You are a helpful assistant.{Environment.NewLine}More system stuff.", rp.SystemPrompt);
-
-            Assert.Equal(2, rp.Messages.Count);
-            Assert.Equal("I am going to France.", Assert.IsType<TextContentBlock>(Assert.Single(rp.Messages[0].Content)).Text);
-            Assert.Equal("What is the most famous tower in Paris?", Assert.IsType<TextContentBlock>(Assert.Single(rp.Messages[1].Content)).Text);
-
-            CreateMessageResult result = new()
-            {
-                Content = [new TextContentBlock { Text = "The Eiffel Tower." }],
-                Model = "amazingmodel",
-                Role = Role.Assistant,
-                StopReason = "endTurn",
-            };
-
-            return Task.FromResult(new JsonRpcResponse
-            {
-                Id = new RequestId("0"),
-                Result = JsonSerializer.SerializeToNode(result, McpJsonUtilities.DefaultOptions),
-            });
-        }
-
-        public override ValueTask DisposeAsync() => default;
-
-        public override string? SessionId => throw new NotImplementedException();
-        public override string? NegotiatedProtocolVersion => throw new NotImplementedException();
-        public override Implementation? ClientInfo => throw new NotImplementedException();
-        public override IServiceProvider? Services => throw new NotImplementedException();
-        public override LoggingLevel? LoggingLevel => throw new NotImplementedException();
-        public override Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default) =>
-            throw new NotImplementedException();
-        public override Task RunAsync(CancellationToken cancellationToken = default) =>
-            throw new NotImplementedException();
-        public override IAsyncDisposable RegisterNotificationHandler(string method, Func<JsonRpcNotification, CancellationToken, ValueTask> handler) =>
-            throw new NotImplementedException();
-    }
-
     [Fact]
     public async Task NotifyProgress_Should_Be_Handled()
     {