Fix slow shutdown when a Streamable HTTP client is connected (#843)

* Add "Async" suffix to async methods

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.WebUtilities;
+using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using Microsoft.Net.Http.Headers;
@@ -17,8 +18,9 @@ internal sealed class StreamableHttpHandler(
     IOptionsFactory<McpServerOptions> mcpServerOptionsFactory,
     IOptions<HttpServerTransportOptions> httpServerTransportOptions,
     StatefulSessionManager sessionManager,
-    ILoggerFactory loggerFactory,
+    IHostApplicationLifetime hostApplicationLifetime,
-    IServiceProvider applicationServices)
+    IServiceProvider applicationServices,
+    ILoggerFactory loggerFactory)
 {
     private const string McpSessionIdHeaderName = "Mcp-Session-Id";
 
@@ -60,7 +62,7 @@ internal sealed class StreamableHttpHandler(
         }
 
         InitializeSseResponse(context);
-        var wroteResponse = await session.Transport.HandlePostRequest(message, context.Response.Body, context.RequestAborted);
+        var wroteResponse = await session.Transport.HandlePostRequestAsync(message, context.Response.Body, context.RequestAborted);
         if (!wroteResponse)
         {
             // We wound up writing nothing, so there should be no Content-Type response header.
@@ -94,14 +96,28 @@ internal sealed class StreamableHttpHandler(
             return;
         }
 
-        await using var _ = await session.AcquireReferenceAsync(context.RequestAborted);
+        // Link the GET request to both RequestAborted and ApplicationStopping.
-        InitializeSseResponse(context);
+        // The GET request should complete immediately during graceful shutdown without waiting for
+        // in-flight POST requests to complete. This prevents slow shutdown when clients are still connected.
+        using var sseCts = CancellationTokenSource.CreateLinkedTokenSource(context.RequestAborted, hostApplicationLifetime.ApplicationStopping);
+        var cancellationToken = sseCts.Token;
 
-        // We should flush headers to indicate a 200 success quickly, because the initialization response
+        try
-        // will be sent in response to a different POST request. It might be a while before we send a message
+        {
-        // over this response body.
+            await using var _ = await session.AcquireReferenceAsync(cancellationToken);
-        await context.Response.Body.FlushAsync(context.RequestAborted);
+            InitializeSseResponse(context);
-        await session.Transport.HandleGetRequest(context.Response.Body, context.RequestAborted);
+
+            // We should flush headers to indicate a 200 success quickly, because the initialization response
+            // will be sent in response to a different POST request. It might be a while before we send a message
+            // over this response body.
+            await context.Response.Body.FlushAsync(cancellationToken);
+            await session.Transport.HandleGetRequestAsync(context.Response.Body, cancellationToken);
+        }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            // RequestAborted always triggers when the client disconnects before a complete response body is written,
+            // but this is how SSE connections are typically closed.
+        }
     }
 
     public async Task HandleDeleteRequestAsync(HttpContext context)

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -175,16 +175,27 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         request.Headers.Accept.Add(s_textEventStreamMediaType);
         CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
 
-        using var response = await _httpClient.SendAsync(request, message: null, _connectionCts.Token).ConfigureAwait(false);
+        // Server support for the GET request is optional. If it fails, we don't care. It just means we won't receive unsolicited messages.
-
+        HttpResponseMessage response;
-        if (!response.IsSuccessStatusCode)
+        try
+        {
+            response = await _httpClient.SendAsync(request, message: null, _connectionCts.Token).ConfigureAwait(false);
+        }
+        catch (HttpRequestException)
         {
-            // Server support for the GET request is optional. If it fails, we don't care. It just means we won't receive unsolicited messages.
             return;
         }
 
-        using var responseStream = await response.Content.ReadAsStreamAsync(_connectionCts.Token).ConfigureAwait(false);
+        using (response)
-        await ProcessSseResponseAsync(responseStream, relatedRpcRequest: null, _connectionCts.Token).ConfigureAwait(false);
+        {
+            if (!response.IsSuccessStatusCode)
+            {
+                return;
+            }
+
+            using var responseStream = await response.Content.ReadAsStreamAsync(_connectionCts.Token).ConfigureAwait(false);
+            await ProcessSseResponseAsync(responseStream, relatedRpcRequest: null, _connectionCts.Token).ConfigureAwait(false);
+        }
     }
 
     private async Task<JsonRpcMessageWithId?> ProcessSseResponseAsync(Stream responseStream, JsonRpcRequest? relatedRpcRequest, CancellationToken cancellationToken)

src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

@@ -40,7 +40,7 @@ public abstract class JsonRpcMessage
     /// <remarks>
     /// This property should only be set when implementing a custom <see cref="ITransport"/>
     /// that needs to pass additional per-message context or to pass a <see cref="JsonRpcMessageContext.User"/>
-    /// to <see cref="StreamableHttpServerTransport.HandlePostRequest(JsonRpcMessage, Stream, CancellationToken)"/>
+    /// to <see cref="StreamableHttpServerTransport.HandlePostRequestAsync(JsonRpcMessage, Stream, CancellationToken)"/>
     /// or <see cref="SseResponseStreamTransport.OnMessageReceivedAsync(JsonRpcMessage, CancellationToken)"/> .
     /// </remarks>
     [JsonIgnore]

src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs

@@ -43,14 +43,14 @@ public sealed class StreamableHttpServerTransport : ITransport
     /// <summary>
     /// Configures whether the transport should be in stateless mode that does not require all requests for a given session
     /// to arrive to the same ASP.NET Core application process. Unsolicited server-to-client messages are not supported in this mode,
-    /// so calling <see cref="HandleGetRequest(Stream, CancellationToken)"/> results in an <see cref="InvalidOperationException"/>.
+    /// so calling <see cref="HandleGetRequestAsync(Stream, CancellationToken)"/> results in an <see cref="InvalidOperationException"/>.
     /// Server-to-client requests are also unsupported, because the responses may arrive at another ASP.NET Core application process.
     /// Client sampling and roots capabilities are also disabled in stateless mode, because the server cannot make requests.
     /// </summary>
     public bool Stateless { get; init; }
 
     /// <summary>
-    /// Gets a value indicating whether the execution context should flow from the calls to <see cref="HandlePostRequest(JsonRpcMessage, Stream, CancellationToken)"/>
+    /// Gets a value indicating whether the execution context should flow from the calls to <see cref="HandlePostRequestAsync(JsonRpcMessage, Stream, CancellationToken)"/>
     /// to the corresponding <see cref="JsonRpcMessageContext.ExecutionContext"/> property contained in the <see cref="JsonRpcMessage"/> instances returned by the <see cref="MessageReader"/>.
     /// </summary>
     /// <remarks>
@@ -76,7 +76,7 @@ public sealed class StreamableHttpServerTransport : ITransport
     /// <param name="sseResponseStream">The response stream to write MCP JSON-RPC messages as SSE events to.</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A task representing the send loop that writes JSON-RPC messages to the SSE response stream.</returns>
-    public async Task HandleGetRequest(Stream sseResponseStream, CancellationToken cancellationToken = default)
+    public async Task HandleGetRequestAsync(Stream sseResponseStream, CancellationToken cancellationToken = default)
     {
         Throw.IfNull(sseResponseStream);
 
@@ -111,7 +111,7 @@ public sealed class StreamableHttpServerTransport : ITransport
     /// If 's an authenticated <see cref="ClaimsPrincipal"/> sent the message, that can be included in the <see cref="JsonRpcMessage.Context"/>.
     /// No other part of the context should be set.
     /// </para>
-    public async Task<bool> HandlePostRequest(JsonRpcMessage message, Stream responseStream, CancellationToken cancellationToken = default)
+    public async Task<bool> HandlePostRequestAsync(JsonRpcMessage message, Stream responseStream, CancellationToken cancellationToken = default)
     {
         Throw.IfNull(message);
         Throw.IfNull(responseStream);

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs

@@ -181,9 +181,10 @@ public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpT
 
         await mcpClient.DisposeAsync();
 
-        // The header should be included in the GET request, the initialized notification, the tools/list call, and the delete request.
+        // The GET request might not have started in time, and the DELETE request won't be sent in
-        // The DELETE request won't be sent for Stateless mode due to the lack of an Mcp-Session-Id.
+        // Stateless mode due to the lack of an Mcp-Session-Id, but the header should be included in the
-        Assert.Equal(Stateless ? 3 : 4, protocolVersionHeaderValues.Count);
+        // initialized notification and the tools/list call at a minimum.
+        Assert.True(protocolVersionHeaderValues.Count > 1);
         Assert.All(protocolVersionHeaderValues, v => Assert.Equal("2025-03-26", v));
     }
 }

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -8,6 +8,7 @@ using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
 using ModelContextProtocol.Tests.Utils;
 using System.ComponentModel;
+using System.Diagnostics;
 using System.Net;
 using System.Security.Claims;
 
@@ -114,10 +115,9 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
     }
 
     [Fact]
-    public async Task ClaimsPrincipal_CanBeInjectedIntoToolMethod()
+    public async Task ClaimsPrincipal_CanBeInjected_IntoToolMethod()
     {
         Builder.Services.AddMcpServer().WithHttpTransport(ConfigureStateless).WithTools<ClaimsPrincipalTools>();
-        Builder.Services.AddHttpContextAccessor();
 
         await using var app = Builder.Build();
 
@@ -211,6 +211,35 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
             m.Message.Contains("request '2' for method 'sampling/createMessage'"));
     }
 
+    [Fact]
+    public async Task Server_ShutsDownQuickly_WhenClientIsConnected()
+    {
+        Builder.Services.AddMcpServer().WithHttpTransport().WithTools<ClaimsPrincipalTools>();
+
+        await using var app = Builder.Build();
+        app.MapMcp();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        // Connect a client which will open a long-running GET request (SSE or Streamable HTTP)
+        await using var mcpClient = await ConnectAsync();
+
+        // Verify the client is connected
+        var tools = await mcpClient.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken);
+        Assert.NotEmpty(tools);
+
+        // Now measure how long it takes to stop the server
+        var stopwatch = Stopwatch.StartNew();
+        await app.StopAsync(TestContext.Current.CancellationToken);
+        stopwatch.Stop();
+
+        // The server should shut down quickly (within a few seconds). We use 5 seconds as a generous threshold.
+        // This is much less than the default HostOptions.ShutdownTimeout of 30 seconds.
+        Assert.True(stopwatch.Elapsed < TimeSpan.FromSeconds(5),
+            $"Server took {stopwatch.Elapsed.TotalSeconds:F2} seconds to shut down with a connected client. " +
+            "This suggests the GET request is not respecting ApplicationStopping token.");
+    }
+
     private ClaimsPrincipal CreateUser(string name)
         => new ClaimsPrincipal(new ClaimsIdentity(
             [new Claim("name", name), new Claim(ClaimTypes.NameIdentifier, name)],

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

@@ -6,28 +6,29 @@ namespace ModelContextProtocol.AspNetCore.Tests.Utils;
 
 public sealed class KestrelInMemoryConnection : ConnectionContext
 {
-    private readonly Pipe _clientToServerPipe = new();
-    private readonly Pipe _serverToClientPipe = new();
     private readonly CancellationTokenSource _connectionClosedCts = new();
     private readonly FeatureCollection _features = new();
 
     public KestrelInMemoryConnection()
     {
+        Pipe clientToServerPipe = new();
+        Pipe serverToClientPipe = new();
+
         ConnectionClosed = _connectionClosedCts.Token;
         Transport = new DuplexPipe
         {
-            Input = _clientToServerPipe.Reader,
+            Input = clientToServerPipe.Reader,
-            Output = _serverToClientPipe.Writer,
+            Output = serverToClientPipe.Writer,
         };
-        Application = new DuplexPipe
+        ClientPipe = new DuplexPipe
         {
-            Input = _serverToClientPipe.Reader,
+            Input = serverToClientPipe.Reader,
-            Output = _clientToServerPipe.Writer,
+            Output = clientToServerPipe.Writer,
         };
-        ClientStream = new DuplexStream(Application, _connectionClosedCts);
+        ClientStream = new DuplexStream(ClientPipe, _connectionClosedCts);
     }
 
-    public IDuplexPipe Application { get; }
+    public IDuplexPipe ClientPipe { get; }
     public Stream ClientStream { get; }
 
     public override IDuplexPipe Transport { get; set; }
@@ -41,8 +42,8 @@ public sealed class KestrelInMemoryConnection : ConnectionContext
     {
         // This is called by Kestrel. The client should dispose the DuplexStream which
         // completes the other half of these pipes.
-        await _serverToClientPipe.Writer.CompleteAsync();
+        await Transport.Input.CompleteAsync();
-        await _serverToClientPipe.Reader.CompleteAsync();
+        await Transport.Output.CompleteAsync();
 
         // Don't bother disposing the _connectionClosedCts, since this is just for testing,
         // and it's annoying to synchronize with DuplexStream.

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

@@ -13,7 +13,11 @@ public sealed class KestrelInMemoryTransport : IConnectionListenerFactory
     public KestrelInMemoryConnection CreateConnection(EndPoint endpoint)
     {
         var connection = new KestrelInMemoryConnection();
-        GetAcceptQueue(endpoint).Writer.TryWrite(connection);
+        if (!GetAcceptQueue(endpoint).Writer.TryWrite(connection))
+        {
+            throw new IOException("The KestrelInMemoryTransport has been shut down.");
+        };
+
         return connection;
     }
 
@@ -37,12 +41,9 @@ public sealed class KestrelInMemoryTransport : IConnectionListenerFactory
 
         public async ValueTask<ConnectionContext?> AcceptAsync(CancellationToken cancellationToken = default)
         {
-            if (await acceptQueue.Reader.WaitToReadAsync(cancellationToken))
+            await foreach (var item in acceptQueue.Reader.ReadAllAsync(cancellationToken))
             {
-                while (acceptQueue.Reader.TryRead(out var item))
+                return item;
-                {
-                    return item;
-                }
             }
 
             return null;

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -370,6 +370,7 @@ public class Program
         var serviceCollection = new ServiceCollection();
         serviceCollection.AddLogging();
         serviceCollection.AddSingleton(app.ApplicationServices.GetRequiredService<ILoggerFactory>());
+        serviceCollection.AddSingleton(app.ApplicationServices.GetRequiredService<IHostApplicationLifetime>());
         serviceCollection.AddSingleton(app.ApplicationServices.GetRequiredService<DiagnosticListener>());
         serviceCollection.AddRoutingCore();