Prevent cancellation of initialize request per MCP spec (#1350)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

tests/ModelContextProtocol.Tests/Protocol/CancellationTests.cs

@@ -1,6 +1,10 @@
 using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
+using ModelContextProtocol.Tests.Utils;
+using System.IO.Pipelines;
+using System.Text;
 
 namespace ModelContextProtocol.Tests;
 
@@ -65,4 +69,37 @@ public class CancellationTests : ClientServerTestBase
         cts.Cancel();
         await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await waitTask);
     }
+
+    [Fact]
+    public async Task InitializeTimeout_DoesNotSendCancellationNotification()
+    {
+        // Arrange: Create a transport where the server never responds, so the client will time out.
+        var serverInput = new MemoryStream();
+        var serverOutputPipe = new Pipe();
+
+        var clientTransport = new StreamClientTransport(
+            serverInput: serverInput,
+            serverOutputPipe.Reader.AsStream(),
+            LoggerFactory);
+
+        var clientOptions = new McpClientOptions
+        {
+            InitializationTimeout = TimeSpan.FromMilliseconds(500),
+        };
+
+        // Act: Client will send initialize, then time out since no response comes.
+        // Per spec, "The initialize request MUST NOT be cancelled by clients",
+        // so no cancellation notification should be sent.
+        await Assert.ThrowsAsync<TimeoutException>(async () =>
+        {
+            await McpClient.CreateAsync(clientTransport, clientOptions: clientOptions, loggerFactory: LoggerFactory,
+                cancellationToken: TestContext.Current.CancellationToken);
+        });
+
+        // Assert: Read what was written to serverInput.
+        // The only message should be the initialize request, NOT a cancellation notification.
+        var content = Encoding.UTF8.GetString(serverInput.ToArray());
+        Assert.Contains("\"method\":\"initialize\"", content);
+        Assert.DoesNotContain("notifications/cancelled", content);
+    }
 }