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

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Stephen Halter 2026-06-08 14:44:10 -07:00 • committed by GitHub
commit ed07fdb6f4
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1 changed files with 255 additions and 4 deletions
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74
src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs
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@ -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.
// If no GET request has been made, drop the message.
return;
switch (message)
{
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);
}