Don't conflate request and response IDs in Streamable HTTP transports (#475)

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -96,17 +96,17 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         }
 
         var rpcRequest = message as JsonRpcRequest;
-        JsonRpcMessage? rpcResponseCandidate = null;
+        JsonRpcMessageWithId? rpcResponseOrError = null;
 
         if (response.Content.Headers.ContentType?.MediaType == "application/json")
         {
             var responseContent = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
-            rpcResponseCandidate = await ProcessMessageAsync(responseContent, cancellationToken).ConfigureAwait(false);
+            rpcResponseOrError = await ProcessMessageAsync(responseContent, rpcRequest, cancellationToken).ConfigureAwait(false);
         }
         else if (response.Content.Headers.ContentType?.MediaType == "text/event-stream")
         {
             using var responseBodyStream = await response.Content.ReadAsStreamAsync(cancellationToken);
-            rpcResponseCandidate = await ProcessSseResponseAsync(responseBodyStream, rpcRequest, cancellationToken).ConfigureAwait(false);
+            rpcResponseOrError = await ProcessSseResponseAsync(responseBodyStream, rpcRequest, cancellationToken).ConfigureAwait(false);
         }
 
         if (rpcRequest is null)
@@ -114,12 +114,12 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
             return response;
         }
 
-        if (rpcResponseCandidate is not JsonRpcMessageWithId messageWithId || messageWithId.Id != rpcRequest.Id)
+        if (rpcResponseOrError is null)
         {
             throw new McpException($"Streamable HTTP POST response completed without a reply to request with ID: {rpcRequest.Id}");
         }
 
-        if (rpcRequest.Method == RequestMethods.Initialize && rpcResponseCandidate is JsonRpcResponse)
+        if (rpcRequest.Method == RequestMethods.Initialize && rpcResponseOrError is JsonRpcResponse)
         {
             // We've successfully initialized! Copy session-id and start GET request if any.
             if (response.Headers.TryGetValues("mcp-session-id", out var sessionIdValues))
@@ -193,20 +193,20 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
                 continue;
             }
 
-            var message = await ProcessMessageAsync(sseEvent.Data, cancellationToken).ConfigureAwait(false);
+            var rpcResponseOrError = await ProcessMessageAsync(sseEvent.Data, relatedRpcRequest, cancellationToken).ConfigureAwait(false);
 
-            // The server SHOULD end the response here anyway, but we won't leave it to chance. This transport makes
+            // The server SHOULD end the HTTP response body here anyway, but we won't leave it to chance. This transport makes
             // a GET request for any notifications that might need to be sent after the completion of each POST.
-            if (message is JsonRpcMessageWithId messageWithId && relatedRpcRequest?.Id == messageWithId.Id)
+            if (rpcResponseOrError is not null)
             {
-                return messageWithId;
+                return rpcResponseOrError;
             }
         }
 
         return null;
     }
 
-    private async Task<JsonRpcMessage?> ProcessMessageAsync(string data, CancellationToken cancellationToken)
+    private async Task<JsonRpcMessageWithId?> ProcessMessageAsync(string data, JsonRpcRequest? relatedRpcRequest, CancellationToken cancellationToken)
     {
         try
         {
@@ -218,7 +218,12 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
             }
 
             await WriteMessageAsync(message, cancellationToken).ConfigureAwait(false);
-            return message;
+            if (message is JsonRpcResponse or JsonRpcError &&
+                message is JsonRpcMessageWithId rpcResponseOrError &&
+                rpcResponseOrError.Id == relatedRpcRequest?.Id)
+            {
+                return rpcResponseOrError;
+            }
         }
         catch (JsonException ex)
         {

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -60,7 +60,7 @@ internal sealed class StreamableHttpPostTransport(StreamableHttpServerTransport
         {
             yield return message;
 
-            if (message.Data is JsonRpcMessageWithId response && response.Id == _pendingRequest)
+            if (message.Data is JsonRpcResponse or JsonRpcError && ((JsonRpcMessageWithId)message.Data).Id == _pendingRequest)
             {
                 // Complete the SSE response stream now that all pending requests have been processed.
                 break;

tests/Common/Utils/LoggedTest.cs

@@ -12,16 +12,16 @@ public class LoggedTest : IDisposable
         {
             CurrentTestOutputHelper = testOutputHelper,
         };
-        LoggerProvider = new XunitLoggerProvider(_delegatingTestOutputHelper);
+        XunitLoggerProvider = new XunitLoggerProvider(_delegatingTestOutputHelper);
         LoggerFactory = Microsoft.Extensions.Logging.LoggerFactory.Create(builder =>
         {
-            builder.AddProvider(LoggerProvider);
+            builder.AddProvider(XunitLoggerProvider);
         });
     }
 
     public ITestOutputHelper TestOutputHelper => _delegatingTestOutputHelper;
-    public ILoggerFactory LoggerFactory { get; }
+    public ILoggerFactory LoggerFactory { get; set; }
-    public ILoggerProvider LoggerProvider { get; }
+    public ILoggerProvider XunitLoggerProvider { get; }
 
     public virtual void Dispose()
     {

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpSseTests.cs

@@ -52,7 +52,7 @@ public class MapMcpSseTests(ITestOutputHelper outputHelper) : MapMcpTests(output
 
         await app.StartAsync(TestContext.Current.CancellationToken);
 
-        var mcpClient = await ConnectAsync(requestPath);
+        await using var mcpClient = await ConnectAsync(requestPath);
 
         Assert.Equal("TestCustomRouteServer", mcpClient.ServerInfo.Name);
     }

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs

@@ -31,7 +31,7 @@ public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpT
 
         await app.StartAsync(TestContext.Current.CancellationToken);
 
-        var mcpClient = await ConnectAsync(requestPath);
+        await using var mcpClient = await ConnectAsync(requestPath);
 
         Assert.Equal("TestCustomRouteServer", mcpClient.ServerInfo.Name);
     }
@@ -135,7 +135,7 @@ public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpT
 
         await app.StartAsync(TestContext.Current.CancellationToken);
 
-        await using var mcpClient = await ConnectAsync(options: new()
+        await using var mcpClient = await ConnectAsync(transportOptions: new()
         {
             Endpoint = new Uri("http://localhost/sse"),
             TransportMode = HttpTransportMode.Sse

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -1,9 +1,12 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
 using ModelContextProtocol.AspNetCore.Tests.Utils;
 using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
+using ModelContextProtocol.Tests.Utils;
 using System.ComponentModel;
 using System.Net;
 using System.Security.Claims;
@@ -20,18 +23,21 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
         options.Stateless = Stateless;
     }
 
-    protected async Task<IMcpClient> ConnectAsync(string? path = null, SseClientTransportOptions? options = null)
+    protected async Task<IMcpClient> ConnectAsync(
+        string? path = null,
+        SseClientTransportOptions? transportOptions = null,
+        McpClientOptions? clientOptions = null)
     {
         // Default behavior when no options are provided
         path ??= UseStreamableHttp ? "/" : "/sse";
 
-        await using var transport = new SseClientTransport(options ?? new SseClientTransportOptions()
+        await using var transport = new SseClientTransport(transportOptions ?? new SseClientTransportOptions()
         {
             Endpoint = new Uri($"http://localhost{path}"),
             TransportMode = UseStreamableHttp ? HttpTransportMode.StreamableHttp : HttpTransportMode.Sse,
         }, HttpClient, LoggerFactory);
 
-        return await McpClientFactory.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
+        return await McpClientFactory.CreateAsync(transport, clientOptions, LoggerFactory, TestContext.Current.CancellationToken);
     }
 
     [Fact]
@@ -71,7 +77,7 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
 
         await app.StartAsync(TestContext.Current.CancellationToken);
 
-        var mcpClient = await ConnectAsync();
+        await using var mcpClient = await ConnectAsync();
 
         var response = await mcpClient.CallToolAsync(
             "EchoWithUserName",
@@ -111,13 +117,90 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
         Assert.Equal(HttpStatusCode.Forbidden, httpRequestException.StatusCode);
     }
 
-    protected ClaimsPrincipal CreateUser(string name)
+    [Fact]
+    public async Task Sampling_DoesNotCloseStream_Prematurely()
+    {
+        Assert.SkipWhen(Stateless, "Sampling is not supported in stateless mode.");
+
+        Builder.Services.AddMcpServer().WithHttpTransport(ConfigureStateless).WithTools<SamplingRegressionTools>();
+
+        var mockLoggerProvider = new MockLoggerProvider();
+        Builder.Logging.AddProvider(mockLoggerProvider);
+        Builder.Logging.SetMinimumLevel(LogLevel.Debug);
+
+        await using var app = Builder.Build();
+
+        // Reset the LoggerFactory used by the client to use the MockLoggerProvider as well.
+        LoggerFactory = app.Services.GetRequiredService<ILoggerFactory>();
+
+        app.MapMcp();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        var sampleCount = 0;
+        var clientOptions = new McpClientOptions
+        {
+            Capabilities = new()
+            {
+                Sampling = new()
+                {
+                    SamplingHandler = async (parameters, _, _) =>
+                    {
+                        Assert.NotNull(parameters?.Messages);
+                        var message = Assert.Single(parameters.Messages);
+                        Assert.Equal(Role.User, message.Role);
+                        Assert.Equal("text", message.Content.Type);
+                        Assert.Equal("Test prompt for sampling", message.Content.Text);
+
+                        sampleCount++;
+                        return new CreateMessageResult
+                        {
+                            Model = "test-model",
+                            Role = Role.Assistant,
+                            Content = new Content
+                            {
+                                Type = "text",
+                                Text = "Sampling response from client"
+                            }
+                        };
+                    },
+                },
+            },
+        };
+
+        await using var mcpClient = await ConnectAsync(clientOptions: clientOptions);
+
+        var result = await mcpClient.CallToolAsync("sampling-tool", new Dictionary<string, object?>
+        {
+            ["prompt"] = "Test prompt for sampling"
+        }, cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.NotNull(result);
+        Assert.False(result.IsError);
+        var textContent = Assert.Single(result.Content);
+        Assert.Equal("text", textContent.Type);
+        Assert.Equal("Sampling completed successfully. Client responded: Sampling response from client", textContent.Text);
+
+        Assert.Equal(2, sampleCount);
+
+        // Verify that the tool call and the sampling request both used the same ID to ensure we cover against regressions.
+        // https://github.com/modelcontextprotocol/csharp-sdk/issues/464
+        Assert.Single(mockLoggerProvider.LogMessages, m =>
+            m.Category == "ModelContextProtocol.Client.McpClient" &&
+            m.Message.Contains("request '2' for method 'tools/call'"));
+
+        Assert.Single(mockLoggerProvider.LogMessages, m =>
+            m.Category == "ModelContextProtocol.Server.McpServer" &&
+            m.Message.Contains("request '2' for method 'sampling/createMessage'"));
+    }
+
+    private ClaimsPrincipal CreateUser(string name)
         => new ClaimsPrincipal(new ClaimsIdentity(
             [new Claim("name", name), new Claim(ClaimTypes.NameIdentifier, name)],
             "TestAuthType", "name", "role"));
 
     [McpServerToolType]
-    protected class EchoHttpContextUserTools(IHttpContextAccessor contextAccessor)
+    private class EchoHttpContextUserTools(IHttpContextAccessor contextAccessor)
     {
         [McpServerTool, Description("Echoes the input back to the client with their user name.")]
         public string EchoWithUserName(string message)
@@ -127,4 +210,37 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
             return $"{userName}: {message}";
         }
     }
+
+    [McpServerToolType]
+    private class SamplingRegressionTools
+    {
+        [McpServerTool(Name = "sampling-tool")]
+        public static async Task<string> SamplingToolAsync(IMcpServer server, string prompt, CancellationToken cancellationToken)
+        {
+            // This tool reproduces the scenario described in https://github.com/modelcontextprotocol/csharp-sdk/issues/464
+            // 1. The client calls tool with request ID 2, because it's the first request after the initialize request.
+            // 2. This tool makes two sampling requests which use IDs 1 and 2.
+            // 3. In the old buggy Streamable HTTP transport code, this would close the SSE response stream,
+            //    because the second sampling request used an ID matching the tool call.
+            var samplingRequest = new CreateMessageRequestParams
+            {
+                Messages = [
+                    new SamplingMessage
+                    {
+                        Role = Role.User,
+                        Content = new Content
+                        {
+                            Type = "text",
+                            Text = prompt
+                        },
+                    }
+                ],
+            };
+
+            await server.SampleAsync(samplingRequest, cancellationToken);
+            var samplingResult = await server.SampleAsync(samplingRequest, cancellationToken);
+
+            return $"Sampling completed successfully. Client responded: {samplingResult.Content.Text}";
+        }
+    }
 }

tests/ModelContextProtocol.AspNetCore.Tests/Utils/KestrelInMemoryTest.cs

@@ -18,7 +18,7 @@ public class KestrelInMemoryTest : LoggedTest
         Builder = WebApplication.CreateSlimBuilder();
         Builder.Services.RemoveAll<IConnectionListenerFactory>();
         Builder.Services.AddSingleton<IConnectionListenerFactory>(_inMemoryTransport);
-        Builder.Services.AddSingleton(LoggerProvider);
+        Builder.Services.AddSingleton(XunitLoggerProvider);
 
         HttpClient = new HttpClient(new SocketsHttpHandler()
         {