Implement SEP-2663 Tasks Extension (#1579)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/ModelContextProtocol.Tests/Server/McpServerTasksNoStoreTests.cs

@@ -0,0 +1,70 @@
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+using System.Runtime.InteropServices;
+
+#pragma warning disable MCPEXP001
+
+namespace ModelContextProtocol.Tests.Server;
+
+/// <summary>
+/// Pins the behavior when a client signals the SEP-2575 tasks opt-in via <c>_meta</c> but the server
+/// has neither an <see cref="McpServerOptions.TaskStore"/> nor a <see cref="McpServerHandlers.CallToolWithTaskHandler"/>
+/// configured. The expected behavior is a silent synchronous fallback: the server returns the normal
+/// <see cref="CallToolResult"/> with no <c>Task</c> envelope and no exception.
+/// </summary>
+public class McpServerTasksNoStoreTests : ClientServerTestBase
+{
+    public McpServerTasksNoStoreTests(ITestOutputHelper testOutputHelper) : base(testOutputHelper)
+    {
+#if !NET
+        Assert.SkipWhen(RuntimeInformation.IsOSPlatform(OSPlatform.Windows), "https://github.com/modelcontextprotocol/csharp-sdk/issues/587");
+#endif
+    }
+
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder)
+    {
+        // Intentionally do NOT configure TaskStore or CallToolWithTaskHandler.
+        mcpServerBuilder.WithTools<NoStoreTools>();
+    }
+
+    [Fact]
+    public async Task ClientOptIn_NoTaskStore_FallsBackToSyncResult()
+    {
+        await using var client = await CreateMcpClientForServer();
+        var ct = TestContext.Current.CancellationToken;
+
+        // CallToolRawAsync always writes the SEP-2575 tasks opt-in into _meta.
+        var augmented = await client.CallToolRawAsync(
+            new CallToolRequestParams { Name = "sync-tool" }, ct);
+
+        // With no task store configured, the server must complete synchronously and return
+        // the standard CallToolResult — not a task envelope.
+        Assert.False(augmented.IsTask);
+        Assert.NotNull(augmented.Result);
+        Assert.Equal("sync result", Assert.IsType<TextContentBlock>(augmented.Result!.Content[0]).Text);
+    }
+
+    [Fact]
+    public async Task ClientOptIn_NoTaskStore_CallToolAsync_StillReturnsResult()
+    {
+        // CallToolAsync (the higher-level convenience) must also work in the no-store case,
+        // delegating to the underlying sync path without throwing.
+        await using var client = await CreateMcpClientForServer();
+        var ct = TestContext.Current.CancellationToken;
+
+        var result = await client.CallToolAsync(
+            new CallToolRequestParams { Name = "sync-tool" }, ct);
+
+        Assert.NotNull(result);
+        Assert.Equal("sync result", Assert.IsType<TextContentBlock>(result.Content[0]).Text);
+    }
+
+    [McpServerToolType]
+    private sealed class NoStoreTools
+    {
+        [McpServerTool(Name = "sync-tool"), System.ComponentModel.Description("A plain sync tool")]
+        public static string SyncTool() => "sync result";
+    }
+}