Add diagnostics for messages dropped on the GET SSE stream (#1609)

src/ModelContextProtocol.Core/Server/McpServer.Methods.cs

@@ -54,9 +54,18 @@ public abstract partial class McpServer : McpSession
     /// <exception cref="InvalidOperationException">The client does not support sampling.</exception>
     /// <exception cref="McpException">The request failed or the client returned an error response.</exception>
     /// <remarks>
+    /// <para>
+    /// When the server is using the Streamable HTTP transport, prefer calling this method on the
+    /// <see cref="McpServer"/> instance available via <c>RequestContext</c> from inside a tool, prompt,
+    /// or resource handler. That routes the request through the originating POST response stream via
+    /// <see cref="JsonRpcMessageContext.RelatedTransport"/>, which is always open for the duration of
+    /// the request, rather than relying on the optional standalone GET SSE stream.
+    /// </para>
+    /// <para>
     /// When called during task-augmented tool execution, this method automatically updates the task
     /// status to <see cref="McpTaskStatus.InputRequired"/> while waiting for the client response,
     /// then returns to <see cref="McpTaskStatus.Working"/> when the response is received.
+    /// </para>
     /// </remarks>
     public async ValueTask<CreateMessageResult> SampleAsync(
         CreateMessageRequestParams requestParams,
@@ -252,6 +261,13 @@ public abstract partial class McpServer : McpSession
     /// <param name="serializerOptions">The <see cref="JsonSerializerOptions"/> to use for serialization. If <see langword="null"/>, <see cref="McpJsonUtilities.DefaultOptions"/> is used.</param>
     /// <returns>The <see cref="IChatClient"/> that can be used to issue sampling requests to the client.</returns>
     /// <exception cref="InvalidOperationException">The client does not support sampling.</exception>
+    /// <remarks>
+    /// When the server is using the Streamable HTTP transport, prefer obtaining this chat client from the
+    /// <see cref="McpServer"/> instance available via <c>RequestContext</c> from inside a tool, prompt,
+    /// or resource handler. That routes sampling requests through the originating POST response stream via
+    /// <see cref="JsonRpcMessageContext.RelatedTransport"/>, which is always open for the duration of
+    /// the request, rather than relying on the optional standalone GET SSE stream.
+    /// </remarks>
     public IChatClient AsSamplingChatClient(JsonSerializerOptions? serializerOptions = null)
     {
         ThrowIfSamplingUnsupported();
@@ -273,6 +289,13 @@ public abstract partial class McpServer : McpSession
     /// <exception cref="ArgumentNullException"><paramref name="requestParams"/> is <see langword="null"/>.</exception>
     /// <exception cref="InvalidOperationException">The client does not support roots.</exception>
     /// <exception cref="McpException">The request failed or the client returned an error response.</exception>
+    /// <remarks>
+    /// When the server is using the Streamable HTTP transport, prefer calling this method on the
+    /// <see cref="McpServer"/> instance available via <c>RequestContext</c> from inside a tool, prompt,
+    /// or resource handler. That routes the request through the originating POST response stream via
+    /// <see cref="JsonRpcMessageContext.RelatedTransport"/>, which is always open for the duration of
+    /// the request, rather than relying on the optional standalone GET SSE stream.
+    /// </remarks>
     public ValueTask<ListRootsResult> RequestRootsAsync(
         ListRootsRequestParams requestParams,
         CancellationToken cancellationToken = default)
@@ -298,9 +321,18 @@ public abstract partial class McpServer : McpSession
     /// <exception cref="InvalidOperationException">The client does not support elicitation.</exception>
     /// <exception cref="McpException">The request failed or the client returned an error response.</exception>
     /// <remarks>
+    /// <para>
+    /// When the server is using the Streamable HTTP transport, prefer calling this method on the
+    /// <see cref="McpServer"/> instance available via <c>RequestContext</c> from inside a tool, prompt,
+    /// or resource handler. That routes the request through the originating POST response stream via
+    /// <see cref="JsonRpcMessageContext.RelatedTransport"/>, which is always open for the duration of
+    /// the request, rather than relying on the optional standalone GET SSE stream.
+    /// </para>
+    /// <para>
     /// When called during task-augmented tool execution, this method automatically updates the task
     /// status to <see cref="McpTaskStatus.InputRequired"/> while waiting for user input,
     /// then returns to <see cref="McpTaskStatus.Working"/> when the response is received.
+    /// </para>
     /// </remarks>
     public async ValueTask<ElicitResult> ElicitAsync(
         ElicitRequestParams requestParams, 

src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs

@@ -221,6 +221,34 @@ public sealed partial class StreamableHttpServerTransport : ITransport
     }
 
     /// <inheritdoc/>
+    /// <remarks>
+    /// <para>
+    /// This method sends server-to-client messages via the standalone SSE stream opened by an
+    /// optional HTTP GET request (see <see cref="HandleGetRequestAsync(Stream, CancellationToken)"/>).
+    /// </para>
+    /// <para>
+    /// <strong>This is generally the wrong channel for server-to-client requests.</strong> Requests
+    /// sent via the GET stream depend on the client keeping a long-lived GET open, have no per-request
+    /// correlation to a caller, and race with GET startup and teardown. When called from inside a
+    /// tool, prompt, or resource handler, use the <see cref="McpServer"/> instance available via
+    /// <c>RequestContext</c> instead — it routes through the originating POST response stream via
+    /// <see cref="JsonRpcMessageContext.RelatedTransport"/>, which is always open for the duration of
+    /// the request. A <see cref="LogLevel.Warning"/> diagnostic is emitted whenever a
+    /// <see cref="JsonRpcRequest"/> is sent through this method.
+    /// </para>
+    /// <para>
+    /// If no GET SSE stream has yet been opened on this session, behavior depends on the message kind:
+    /// <see cref="JsonRpcRequest"/> messages throw <see cref="InvalidOperationException"/> because the
+    /// awaiting caller has no way to receive a response; <see cref="JsonRpcNotification"/> messages are
+    /// dropped (notifications are best-effort and the spec does not require clients to issue a GET)
+    /// and a <see cref="LogLevel.Debug"/> diagnostic is logged; other messages are dropped and a
+    /// <see cref="LogLevel.Warning"/> diagnostic is logged.
+    /// </para>
+    /// </remarks>
+    /// <exception cref="InvalidOperationException">
+    /// <see cref="Stateless"/> is <see langword="true"/>, or <paramref name="message"/> is a
+    /// <see cref="JsonRpcRequest"/> and no GET SSE stream has been opened on this session.
+    /// </exception>
     public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
         Throw.IfNull(message);
@@ -234,9 +262,32 @@ public sealed partial class StreamableHttpServerTransport : ITransport
 
         if (!_getHttpRequestStarted)
         {
-            // Clients are not required to make a GET request for unsolicited messages.
+            switch (message)
-            // If no GET request has been made, drop the message.
+            {
-            return;
+                case JsonRpcRequest request:
+                    throw new InvalidOperationException(
+                        $"Cannot send server-to-client JSON-RPC request '{request.Method}' because no GET SSE stream has been opened on this session " +
+                        $"(SessionId: '{SessionId}'). " +
+                        "Inside a tool, prompt, or resource handler, use the IMcpServer instance from RequestContext (or any IMcpServer obtained via DI from a request-scoped service provider) so the request is routed through the originating POST response stream via JsonRpcMessageContext.RelatedTransport. " +
+                        "The standalone GET SSE stream is optional for clients and is not a reliable channel for server-to-client requests.");
+
+                case JsonRpcNotification notification:
+                    // Clients are not required to make a GET request for unsolicited messages.
+                    // If no GET request has been made, drop the notification (best-effort).
+                    LogNotificationDroppedNoGetStream(notification.Method, SessionId ?? string.Empty);
+                    return;
+
+                default:
+                    // JsonRpcResponse / JsonRpcError generally flow through the originating POST response
+                    // stream, so receiving one here without a GET is unexpected. Log loudly and drop.
+                    LogMessageDroppedNoGetStream(message.GetType().Name, GetMessageId(message), SessionId ?? string.Empty);
+                    return;
+            }
+        }
+
+        if (message is JsonRpcRequest openRequest)
+        {
+            LogServerRequestOverGetStream(openRequest.Method, SessionId ?? string.Empty);
         }
 
         Debug.Assert(_httpResponseTcs is not null);
@@ -263,6 +314,9 @@ public sealed partial class StreamableHttpServerTransport : ITransport
         }
     }
 
+    private static string GetMessageId(JsonRpcMessage message) =>
+        message is JsonRpcMessageWithId withId ? withId.Id.ToString() : string.Empty;
+
     /// <inheritdoc/>
     public async ValueTask DisposeAsync()
     {
@@ -323,4 +377,18 @@ public sealed partial class StreamableHttpServerTransport : ITransport
 
         return sseEventStreamWriter;
     }
+
+    [LoggerMessage(Level = LogLevel.Warning,
+        Message = "Sending server-to-client JSON-RPC request '{Method}' over the standalone GET SSE stream (SessionId: '{SessionId}'). " +
+            "Consider using the IMcpServer instance from RequestContext inside a tool, prompt, or resource handler so the request is routed through the originating POST response stream via JsonRpcMessageContext.RelatedTransport, which is more reliable than the optional GET SSE stream.")]
+    private partial void LogServerRequestOverGetStream(string method, string sessionId);
+
+    [LoggerMessage(Level = LogLevel.Debug,
+        Message = "Dropping server-to-client JSON-RPC notification '{Method}' because no GET SSE stream has been opened on this session (SessionId: '{SessionId}').")]
+    private partial void LogNotificationDroppedNoGetStream(string method, string sessionId);
+
+    [LoggerMessage(Level = LogLevel.Warning,
+        Message = "Dropping unexpected server-to-client {MessageType} (Id: '{MessageId}') because no GET SSE stream has been opened on this session (SessionId: '{SessionId}'). " +
+            "Responses normally flow through the originating POST response stream via JsonRpcMessageContext.RelatedTransport.")]
+    private partial void LogMessageDroppedNoGetStream(string messageType, string messageId, string sessionId);
 }

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs

@@ -384,7 +384,8 @@ public class StreamableHttpServerConformanceTests(ITestOutputHelper outputHelper
     public async Task SendNotificationAsync_DoesNotThrow_WhenNoGetRequestHasBeenMade()
     {
         // Clients are not required to make a GET request for unsolicited messages.
-        // If no GET request has been made, the messages should be dropped rather than throwing.
+        // If no GET request has been made, the messages should be dropped rather than throwing,
+        // and the drop should be visible as a Debug-level log so it can be diagnosed.
         McpServer? server = null;
 
         Builder.Services.AddMcpServer()
@@ -409,6 +410,156 @@ public class StreamableHttpServerConformanceTests(ITestOutputHelper outputHelper
         var exception = await Record.ExceptionAsync(() =>
             server.SendNotificationAsync("test-method", TestContext.Current.CancellationToken));
         Assert.Null(exception);
+
+        Assert.Contains(MockLoggerProvider.LogMessages, log =>
+            log.Category == typeof(StreamableHttpServerTransport).FullName &&
+            log.LogLevel == LogLevel.Debug &&
+            log.Message.Contains("test-method") &&
+            log.Message.Contains("no GET SSE stream"));
+    }
+
+    [Fact]
+    public async Task SendRequestAsync_Throws_WhenNoGetRequestHasBeenMade()
+    {
+        // A server-to-client request sent before any GET SSE stream is opened can never
+        // receive a response, so the transport should fail fast with InvalidOperationException
+        // instead of silently dropping the message and leaving the caller hanging on the TCS
+        // registered by SendRequestAsync.
+        McpServer? server = null;
+
+        Builder.Services.AddMcpServer()
+            .WithHttpTransport(options =>
+            {
+#pragma warning disable MCPEXP002 // RunSessionHandler is experimental
+                options.RunSessionHandler = (httpContext, mcpServer, cancellationToken) =>
+                {
+                    server = mcpServer;
+                    return mcpServer.RunAsync(cancellationToken);
+                };
+#pragma warning restore MCPEXP002
+            });
+
+        await StartAsync();
+
+        await CallInitializeAndValidateAsync();
+        Assert.NotNull(server);
+
+        var request = new JsonRpcRequest
+        {
+            Method = "roots/list",
+            Id = new RequestId(42),
+        };
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            server.SendRequestAsync(request, TestContext.Current.CancellationToken));
+
+        Assert.Contains("roots/list", ex.Message);
+        Assert.Contains("no GET SSE stream", ex.Message);
+        Assert.Contains("RequestContext", ex.Message);
+        Assert.Contains("RelatedTransport", ex.Message);
+    }
+
+    [Fact]
+    public async Task SendMessageAsync_LogsWarning_OnUnexpectedResponse_WhenNoGetRequestHasBeenMade()
+    {
+        // Responses normally ride the originating POST response stream via RelatedTransport, so
+        // receiving one through the GET path without an open GET is unexpected. The message is
+        // dropped (preserving best-effort semantics) but a warning is logged so the situation is
+        // visible.
+        McpServer? server = null;
+
+        Builder.Services.AddMcpServer()
+            .WithHttpTransport(options =>
+            {
+#pragma warning disable MCPEXP002 // RunSessionHandler is experimental
+                options.RunSessionHandler = (httpContext, mcpServer, cancellationToken) =>
+                {
+                    server = mcpServer;
+                    return mcpServer.RunAsync(cancellationToken);
+                };
+#pragma warning restore MCPEXP002
+            });
+
+        await StartAsync();
+
+        await CallInitializeAndValidateAsync();
+        Assert.NotNull(server);
+
+        var response = new JsonRpcResponse
+        {
+            Id = new RequestId(7),
+            Result = new JsonObject(),
+        };
+
+        var exception = await Record.ExceptionAsync(() =>
+            server.SendMessageAsync(response, TestContext.Current.CancellationToken));
+        Assert.Null(exception);
+
+        Assert.Contains(MockLoggerProvider.LogMessages, log =>
+            log.Category == typeof(StreamableHttpServerTransport).FullName &&
+            log.LogLevel == LogLevel.Warning &&
+            log.Message.Contains(nameof(JsonRpcResponse)) &&
+            log.Message.Contains("no GET SSE stream"));
+    }
+
+    [Fact]
+    public async Task SendRequestAsync_LogsWarning_WhenGetRequestIsOpen()
+    {
+        // Even when the GET SSE stream is open and the request is delivered, server-to-client
+        // requests sent via the GET path are fragile (no per-request correlation, depend on a
+        // long-lived GET, race with startup/teardown). A warning is logged to direct callers at
+        // the RequestContext.RelatedTransport channel instead, without changing behavior.
+        McpServer? server = null;
+
+        Builder.Services.AddMcpServer()
+            .WithHttpTransport(options =>
+            {
+#pragma warning disable MCPEXP002 // RunSessionHandler is experimental
+                options.RunSessionHandler = (httpContext, mcpServer, cancellationToken) =>
+                {
+                    server = mcpServer;
+                    return mcpServer.RunAsync(cancellationToken);
+                };
+#pragma warning restore MCPEXP002
+            });
+
+        await StartAsync();
+
+        await CallInitializeAndValidateAsync();
+        Assert.NotNull(server);
+
+        using var getResponse = await HttpClient.GetAsync("", HttpCompletionOption.ResponseHeadersRead, TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.OK, getResponse.StatusCode);
+
+        // Send a request via the GET stream and assert it lands on the wire (proving behavior is unchanged).
+        // SendRequestAsync awaits a response that the test never produces, so use a CTS to cancel after
+        // confirming wire delivery.
+        var request = new JsonRpcRequest
+        {
+            Method = "roots/list",
+            Id = new RequestId(99),
+        };
+
+        using var requestCts = CancellationTokenSource.CreateLinkedTokenSource(TestContext.Current.CancellationToken);
+        var sendTask = server.SendRequestAsync(request, requestCts.Token);
+
+        await foreach (var sseEvent in ReadSseAsync(getResponse.Content))
+        {
+            var received = JsonSerializer.Deserialize(sseEvent, GetJsonTypeInfo<JsonRpcRequest>());
+            Assert.NotNull(received);
+            Assert.Equal("roots/list", received.Method);
+            break;
+        }
+
+        // Cancel the awaited response so SendRequestAsync completes — the wire delivery has already happened.
+        requestCts.Cancel();
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(() => sendTask);
+
+        Assert.Contains(MockLoggerProvider.LogMessages, log =>
+            log.Category == typeof(StreamableHttpServerTransport).FullName &&
+            log.LogLevel == LogLevel.Warning &&
+            log.Message.Contains("roots/list") &&
+            log.Message.Contains("RequestContext"));
     }
 
     [Fact]