Streamable HTTP resumability + redelivery + SSE polling via server-side disconnect

src/ModelContextProtocol.AspNetCore/HttpServerTransportOptions.cs

@@ -43,6 +43,34 @@ public class HttpServerTransportOptions
     /// </remarks>
     public bool Stateless { get; set; }
 
+    /// <summary>
+    /// Gets or sets the event store for resumability support.
+    /// When set, events are stored and can be replayed when clients reconnect with a Last-Event-ID header.
+    /// </summary>
+    /// <remarks>
+    /// When configured, the server will:
+    /// <list type="bullet">
+    /// <item><description>Generate unique event IDs for each SSE message</description></item>
+    /// <item><description>Store events for later replay</description></item>
+    /// <item><description>Replay missed events when a client reconnects with a Last-Event-ID header</description></item>
+    /// <item><description>Send priming events to establish resumability before any actual messages</description></item>
+    /// </list>
+    /// </remarks>
+    public ISseEventStore? EventStore { get; set; }
+
+    /// <summary>
+    /// Gets or sets the retry interval to suggest to clients in SSE retry field.
+    /// </summary>
+    /// <value>
+    /// The retry interval. The default is 5 seconds.
+    /// </value>
+    /// <remarks>
+    /// When <see cref="EventStore"/> is set, the server will include a retry field in priming events.
+    /// This value suggests to clients how long to wait before attempting to reconnect after a connection is lost.
+    /// Clients may use this value to implement polling behavior during long-running operations.
+    /// </remarks>
+    public TimeSpan RetryInterval { get; set; } = TimeSpan.FromSeconds(5);
+
     /// <summary>
     /// Gets or sets a value that indicates whether the server uses a single execution context for the entire session.
     /// </summary>

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -23,6 +23,7 @@ internal sealed class StreamableHttpHandler(
     ILoggerFactory loggerFactory)
 {
     private const string McpSessionIdHeaderName = "Mcp-Session-Id";
+    private const string LastEventIdHeaderName = "Last-Event-ID";
 
     private static readonly JsonTypeInfo<JsonRpcMessage> s_messageTypeInfo = GetRequiredJsonTypeInfo<JsonRpcMessage>();
     private static readonly JsonTypeInfo<JsonRpcError> s_errorTypeInfo = GetRequiredJsonTypeInfo<JsonRpcError>();
@@ -88,10 +89,15 @@ await WriteJsonRpcErrorAsync(context,
             return;
         }
 
-        if (!session.TryStartGetRequest())
+        // Check for Last-Event-ID header for resumability
+        var lastEventId = context.Request.Headers[LastEventIdHeaderName].ToString();
+        var isResumption = !string.IsNullOrEmpty(lastEventId);
+
+        // Only check TryStartGetRequest for new connections, not resumptions
+        if (!isResumption && !session.TryStartGetRequest())
         {
             await WriteJsonRpcErrorAsync(context,
-                "Bad Request: This server does not support multiple GET requests. Start a new session to get a new GET SSE response.",
+                "Bad Request: This server does not support multiple GET requests. Use Last-Event-ID header to resume or start a new session.",
                 StatusCodes.Status400BadRequest);
             return;
         }
@@ -111,7 +117,7 @@ await WriteJsonRpcErrorAsync(context,
             // 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);
+            await session.Transport.HandleGetRequestAsync(context.Response.Body, isResumption ? lastEventId : null, cancellationToken);
         }
         catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
         {
@@ -194,7 +200,10 @@ private async ValueTask<StreamableHttpSession> StartNewSessionAsync(HttpContext
             {
                 SessionId = sessionId,
                 FlowExecutionContextFromRequests = !HttpServerTransportOptions.PerSessionExecutionContext,
+                EventStore = HttpServerTransportOptions.EventStore,
+                RetryInterval = HttpServerTransportOptions.RetryInterval,
             };
+
             context.Response.Headers[McpSessionIdHeaderName] = sessionId;
         }
         else

src/ModelContextProtocol.Core/Client/HttpClientTransportOptions.cs

@@ -106,4 +106,17 @@ public required Uri Endpoint
     /// Gets sor sets the authorization provider to use for authentication.
     /// </summary>
     public ClientOAuthOptions? OAuth { get; set; }
+
+    /// <summary>
+    /// Gets or sets the maximum number of reconnection attempts when an SSE stream is disconnected.
+    /// </summary>
+    /// <value>
+    /// The maximum number of reconnection attempts. The default is 2.
+    /// </value>
+    /// <remarks>
+    /// When an SSE stream is disconnected (e.g., due to a network issue), the client will attempt to
+    /// reconnect using the Last-Event-ID header to resume from where it left off. This property controls
+    /// how many reconnection attempts are made before giving up.
+    /// </remarks>
+    public int MaxReconnectionAttempts { get; set; } = 2;
 }

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -16,6 +16,8 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
     private static readonly MediaTypeWithQualityHeaderValue s_applicationJsonMediaType = new("application/json");
     private static readonly MediaTypeWithQualityHeaderValue s_textEventStreamMediaType = new("text/event-stream");
 
+    private static readonly TimeSpan s_defaultReconnectionDelay = TimeSpan.FromSeconds(1);
+
     private readonly McpHttpClient _httpClient;
     private readonly HttpClientTransportOptions _options;
     private readonly CancellationTokenSource _connectionCts;
@@ -106,7 +108,17 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
         else if (response.Content.Headers.ContentType?.MediaType == "text/event-stream")
         {
             using var responseBodyStream = await response.Content.ReadAsStreamAsync(cancellationToken);
-            rpcResponseOrError = await ProcessSseResponseAsync(responseBodyStream, rpcRequest, cancellationToken).ConfigureAwait(false);
+            var sseState = await ProcessSseResponseAsync(responseBodyStream, rpcRequest, cancellationToken).ConfigureAwait(false);
+            rpcResponseOrError = sseState.Response;
+
+            // Resumability: If POST SSE stream ended without a response but we have a Last-Event-ID (from priming),
+            // attempt to resume by sending a GET request with Last-Event-ID header. The server will replay
+            // events from the event store, allowing us to receive the pending response.
+            if (rpcResponseOrError is null && rpcRequest is not null && sseState.LastEventId is not null)
+            {
+                var resumeResult = await SendGetSseRequestWithRetriesAsync(rpcRequest, sseState, cancellationToken).ConfigureAwait(false);
+                rpcResponseOrError = resumeResult.Response;
+            }
         }
 
         if (rpcRequest is null)
@@ -188,54 +200,135 @@ public override async ValueTask DisposeAsync()
 
     private async Task ReceiveUnsolicitedMessagesAsync()
     {
-        // Send a GET request to handle any unsolicited messages not sent over a POST response.
-        using var request = new HttpRequestMessage(HttpMethod.Get, _options.Endpoint);
-        request.Headers.Accept.Add(s_textEventStreamMediaType);
-        CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
+        var state = new SseStreamState();
 
-        // 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;
-        try
-        {
-            response = await _httpClient.SendAsync(request, message: null, _connectionCts.Token).ConfigureAwait(false);
-        }
-        catch (HttpRequestException)
+        // Continuously receive unsolicited messages until cancelled
+        while (!_connectionCts.Token.IsCancellationRequested)
         {
-            return;
+            var result = await SendGetSseRequestWithRetriesAsync(
+                relatedRpcRequest: null,
+                state,
+                _connectionCts.Token).ConfigureAwait(false);
+
+            // Update state for next reconnection attempt
+            state.UpdateFrom(result);
+
+            // If we exhausted retries without receiving any events, stop trying
+            if (result.LastEventId is null)
+            {
+                return;
+            }
         }
+    }
+
+    /// <summary>
+    /// Sends a GET request for SSE with retry logic and resumability support.
+    /// </summary>
+    private async Task<SseStreamState> SendGetSseRequestWithRetriesAsync(
+        JsonRpcRequest? relatedRpcRequest,
+        SseStreamState state,
+        CancellationToken cancellationToken)
+    {
+        int attempt = 0;
+
+        // Delay before first attempt if we're reconnecting (have a Last-Event-ID)
+        bool shouldDelay = state.LastEventId is not null;
 
-        using (response)
+        while (attempt < _options.MaxReconnectionAttempts)
         {
-            if (!response.IsSuccessStatusCode)
+            cancellationToken.ThrowIfCancellationRequested();
+
+            if (shouldDelay)
             {
-                return;
+                var delay = state.RetryInterval ?? s_defaultReconnectionDelay;
+                await Task.Delay(delay, cancellationToken).ConfigureAwait(false);
             }
+            shouldDelay = true;
+
+            using var request = new HttpRequestMessage(HttpMethod.Get, _options.Endpoint);
+            request.Headers.Accept.Add(s_textEventStreamMediaType);
+            CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion, state.LastEventId);
+
+            HttpResponseMessage response;
+            try
+            {
+                response = await _httpClient.SendAsync(request, message: null, cancellationToken).ConfigureAwait(false);
+            }
+            catch (HttpRequestException)
+            {
+                attempt++;
+                continue;
+            }
+
+            using (response)
+            {
+                if (!response.IsSuccessStatusCode)
+                {
+                    attempt++;
+                    continue;
+                }
+
+                using var responseStream = await response.Content.ReadAsStreamAsync(cancellationToken).ConfigureAwait(false);
+                var result = await ProcessSseResponseAsync(responseStream, relatedRpcRequest, cancellationToken).ConfigureAwait(false);
+
+                state.UpdateFrom(result);
+
+                if (result.Response is not null)
+                {
+                    return state;
+                }
 
-            using var responseStream = await response.Content.ReadAsStreamAsync(_connectionCts.Token).ConfigureAwait(false);
-            await ProcessSseResponseAsync(responseStream, relatedRpcRequest: null, _connectionCts.Token).ConfigureAwait(false);
+                // Stream closed without the response
+                if (state.LastEventId is null)
+                {
+                    // No event ID means server may not support resumability - don't retry indefinitely
+                    attempt++;
+                }
+                else
+                {
+                    // We have an event ID, so reconnection should work - reset attempts
+                    attempt = 0;
+                }
+            }
         }
+
+        return state;
     }
 
-    private async Task<JsonRpcMessageWithId?> ProcessSseResponseAsync(Stream responseStream, JsonRpcRequest? relatedRpcRequest, CancellationToken cancellationToken)
+    private async Task<SseStreamState> ProcessSseResponseAsync(
+        Stream responseStream,
+        JsonRpcRequest? relatedRpcRequest,
+        CancellationToken cancellationToken)
     {
+        var state = new SseStreamState();
+
         await foreach (SseItem<string> sseEvent in SseParser.Create(responseStream).EnumerateAsync(cancellationToken).ConfigureAwait(false))
         {
-            if (sseEvent.EventType != "message")
+            // Track event ID and retry interval for resumability
+            if (!string.IsNullOrEmpty(sseEvent.EventId))
+            {
+                state.LastEventId = sseEvent.EventId;
+            }
+            if (sseEvent.ReconnectionInterval.HasValue)
+            {
+                state.RetryInterval = sseEvent.ReconnectionInterval.Value;
+            }
+
+            // Skip events with empty data (priming events, keep-alives)
+            if (string.IsNullOrEmpty(sseEvent.Data) || sseEvent.EventType != "message")
             {
                 continue;
             }
 
             var rpcResponseOrError = await ProcessMessageAsync(sseEvent.Data, relatedRpcRequest, cancellationToken).ConfigureAwait(false);
-
-            // 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 (rpcResponseOrError is not null)
             {
-                return rpcResponseOrError;
+                state.Response = rpcResponseOrError;
+                return state;
             }
         }
 
-        return null;
+        return state;
     }
 
     private async Task<JsonRpcMessageWithId?> ProcessMessageAsync(string data, JsonRpcRequest? relatedRpcRequest, CancellationToken cancellationToken)
@@ -292,7 +385,8 @@ internal static void CopyAdditionalHeaders(
         HttpRequestHeaders headers,
         IDictionary<string, string>? additionalHeaders,
         string? sessionId,
-        string? protocolVersion)
+        string? protocolVersion,
+        string? lastEventId = null)
     {
         if (sessionId is not null)
         {
@@ -304,6 +398,11 @@ internal static void CopyAdditionalHeaders(
             headers.Add("MCP-Protocol-Version", protocolVersion);
         }
 
+        if (lastEventId is not null)
+        {
+            headers.Add("Last-Event-ID", lastEventId);
+        }
+
         if (additionalHeaders is null)
         {
             return;
@@ -317,4 +416,21 @@ internal static void CopyAdditionalHeaders(
             }
         }
     }
+
+    /// <summary>
+    /// Tracks state across SSE stream connections.
+    /// </summary>
+    private struct SseStreamState
+    {
+        public JsonRpcMessageWithId? Response;
+        public string? LastEventId;
+        public TimeSpan? RetryInterval;
+
+        public void UpdateFrom(SseStreamState other)
+        {
+            Response ??= other.Response;
+            LastEventId ??= other.LastEventId;
+            RetryInterval ??= other.RetryInterval;
+        }
+    }
 }

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -29,16 +29,32 @@ internal sealed partial class McpSessionHandler : IAsyncDisposable
         "mcp.server.operation.duration", "Measures the duration of inbound message processing.", longBuckets: false);
 
     /// <summary>The latest version of the protocol supported by this implementation.</summary>
-    internal const string LatestProtocolVersion = "2025-06-18";
+    internal const string LatestProtocolVersion = "2025-11-25";
 
     /// <summary>All protocol versions supported by this implementation.</summary>
     internal static readonly string[] SupportedProtocolVersions =
     [
         "2024-11-05",
         "2025-03-26",
+        "2025-06-18",
         LatestProtocolVersion,
     ];
 
+    /// <summary>
+    /// Checks if the given protocol version supports priming events.
+    /// </summary>
+    /// <param name="protocolVersion">The protocol version to check.</param>
+    /// <returns>True if the protocol version supports resumability.</returns>
+    /// <remarks>
+    /// Priming events are only supported in protocol version &gt;= 2025-11-25.
+    /// Older clients may crash when receiving SSE events with empty data.
+    /// </remarks>
+    internal static bool SupportsPrimingEvent(string? protocolVersion)
+    {
+        const string MinResumabilityProtocolVersion = "2025-11-25";
+        return string.Compare(protocolVersion, MinResumabilityProtocolVersion, StringComparison.Ordinal) >= 0;
+    }
+
     private readonly bool _isServer;
     private readonly string _transportKind;
     private readonly ITransport _transport;

src/ModelContextProtocol.Core/Protocol/JsonRpcMessageContext.cs

@@ -1,6 +1,5 @@
 using ModelContextProtocol.Server;
 using System.Security.Claims;
-using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;