Fix support for sending server notifications with request-specific McpServer instances used outside their original scope.

src/ModelContextProtocol.Core/Server/SseResponseStreamTransport.cs

@@ -66,6 +66,7 @@ public async ValueTask DisposeAsync()
     public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
         Throw.IfNull(message);
+        // If the underlying writer has been disposed, just drop the message.
         await _sseWriter.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
     }
 

src/ModelContextProtocol.Core/Server/SseWriter.cs

@@ -47,22 +47,22 @@ public Task WriteAllAsync(Stream sseResponseStream, CancellationToken cancellati
         return _writeTask;
     }
 
-    public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    public async Task<bool> SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
         Throw.IfNull(message);
 
         using var _ = await _disposeLock.LockAsync(cancellationToken).ConfigureAwait(false);
 
         if (_disposed)
         {
-            // Don't throw an ODE, because this is disposed internally when the transport disconnects due to an abort
-            // or sending all the responses for the a give given Streamable HTTP POST request, so the user might not be at fault.
-            // There's precedence for no-oping here similar to writing to the response body of an aborted request in ASP.NET Core.
-            return;
+            // Don't throw ObjectDisposedException here; just return false to indicate the message wasn't sent.
+            // The calling transport can determine what to do in this case (drop the message, or fall back to another transport).
+            return false;
         }
 
         // Emit redundant "event: message" lines for better compatibility with other SDKs.
         await _messages.Writer.WriteAsync(new SseItem<JsonRpcMessage?>(message, SseParser.EventTypeDefault), cancellationToken).ConfigureAwait(false);
+        return true;
     }
 
     public async ValueTask DisposeAsync()

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -72,7 +72,13 @@ public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken can
             throw new InvalidOperationException("Server to client requests are not supported in stateless mode.");
         }
 
-        await _sseWriter.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+        bool isAccepted = await _sseWriter.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+        if (!isAccepted)
+        {
+            // The underlying writer didn't accept the message because the underlying request has completed.
+            // Rather than drop the message, fall back to sending it via the parent transport.
+            await parentTransport.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+        }
     }
 
     public async ValueTask DisposeAsync()

src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs

@@ -131,6 +131,7 @@ public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken can
             throw new InvalidOperationException("Unsolicited server to client messages are not supported in stateless mode.");
         }
 
+        // If the underlying writer has been disposed, just drop the message.
         await _sseWriter.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
     }
 

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs

@@ -521,6 +521,41 @@ public async Task IdleSessionsPastMaxIdleSessionCount_ArePruned_LongestIdleFirst
         Assert.StartsWith("MaxIdleSessionCount of 2 exceeded. Closing idle session", idleLimitLogMessage.Message);
     }
 
+    [Fact]
+    public async Task McpServer_UsedOutOfScope_CanSendNotifications()
+    {
+        McpServer? capturedServer = null;
+        Builder.Services.AddMcpServer()
+            .WithHttpTransport()
+            .WithListResourcesHandler((_, _) => ValueTask.FromResult(new ListResourcesResult()))
+            .WithSubscribeToResourcesHandler((context, token) =>
+            {
+                capturedServer = context.Server;
+                return ValueTask.FromResult(new EmptyResult());
+            });
+
+        await StartAsync();
+
+        string sessionId = await CallInitializeAndValidateAsync();
+        SetSessionId(sessionId);
+
+        // Call the subscribe method to capture the McpServer instance.
+        using var response = await HttpClient.PostAsync("", JsonContent(Request("resources/subscribe")), TestContext.Current.CancellationToken);
+        var rpcResponse = await AssertSingleSseResponseAsync(response);
+        AssertType<EmptyResult>(rpcResponse.Result);
+        Assert.NotNull(capturedServer);
+
+        // Check the captured McpServer instance can send a notification.
+        await capturedServer.SendNotificationAsync(NotificationMethods.ResourceUpdatedNotification, TestContext.Current.CancellationToken);
+        using var getResponse = await HttpClient.GetAsync("", HttpCompletionOption.ResponseHeadersRead, TestContext.Current.CancellationToken);
+        JsonRpcMessage? firstSseMessage = await ReadSseAsync(getResponse.Content)
+            .Select(data => JsonSerializer.Deserialize<JsonRpcMessage>(data, McpJsonUtilities.DefaultOptions))
+            .FirstOrDefaultAsync(TestContext.Current.CancellationToken);
+
+        var notification = Assert.IsType<JsonRpcNotification>(firstSseMessage);
+        Assert.Equal(NotificationMethods.ResourceUpdatedNotification, notification.Method);
+    }
+
     private static StringContent JsonContent(string json) => new StringContent(json, Encoding.UTF8, "application/json");
     private static JsonTypeInfo<T> GetJsonTypeInfo<T>() => (JsonTypeInfo<T>)McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(T));