Multi Round-Trip Requests (MRTR) (#1458)

tests/ModelContextProtocol.Tests/Server/MrtrInputRequiredExceptionTests.cs

@@ -0,0 +1,61 @@
+using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
+using ModelContextProtocol.Server;
+using ModelContextProtocol.Tests.Utils;
+
+namespace ModelContextProtocol.Tests.Server;
+
+/// <summary>
+/// Tests for the MRTR server API - IsMrtrSupported, InputRequiredException,
+/// and client auto-retry of incomplete results.
+/// </summary>
+public class MrtrInputRequiredExceptionTests : ClientServerTestBase
+{
+    private readonly ServerMessageTracker _messageTracker = new();
+
+    public MrtrInputRequiredExceptionTests(ITestOutputHelper testOutputHelper)
+        : base(testOutputHelper, startServer: false)
+    {
+    }
+
+    protected override void ConfigureServices(ServiceCollection services, IMcpServerBuilder mcpServerBuilder)
+    {
+        services.Configure<McpServerOptions>(options =>
+        {
+            options.ProtocolVersion = "DRAFT-2026-v1";
+            _messageTracker.AddFilters(options.Filters.Message);
+        });
+
+        mcpServerBuilder.WithTools([
+            McpServerTool.Create(
+                static string (McpServer server) =>
+                {
+                    throw new InputRequiredException(requestState: "should-not-work");
+                },
+                new McpServerToolCreateOptions
+                {
+                    Name = "always-incomplete",
+                    Description = "Tool that always throws InputRequiredException"
+                }),
+        ]);
+    }
+
+    [Fact]
+    public async Task InputRequiredException_WithoutInputRequests_ExhaustsRetries()
+    {
+        StartServer();
+        var clientOptions = new McpClientOptions();
+
+        await using var client = await CreateMcpClientForServer(clientOptions);
+
+        // The always-incomplete tool throws InputRequiredException with only requestState
+        // and no inputRequests. The client has nothing to dispatch, so it keeps retrying
+        // with the same requestState until the retry budget is exhausted.
+        var exception = await Assert.ThrowsAsync<McpException>(() =>
+            client.CallToolAsync("always-incomplete",
+                cancellationToken: TestContext.Current.CancellationToken).AsTask());
+
+        Assert.Contains("more than", exception.Message);
+    }
+}