mcp: fix reconnect semantics for hanging GET

A few problems with reconnection cropped up in the review of PR modelcontextprotocol#307.
We should allow for the hanging GET to fail with StatusMethodNotAllowed.
This simply means that the server does not support sending notifications
or requests over the GET, which is allowed in the spec.

Also, we should fix the initial delay of the hanging GET request: it
should start with 0 delay. Fix the math for this and subsequent
attempts. Incidentally, this makes the tests take 3s on my machine, down
from 9s.

Also address some comments from modelcontextprotocol#307.

Author: findleyr

internal/jsonrpc2/conn.go

@@ -742,8 +742,8 @@ func (c *Connection) write(ctx context.Context, msg Message) error {
 	var err error
 	// Fail writes immediately if the connection is shutting down.
 	//
-	// TODO(rfindley): should we allow cancellation notifations through? It could
-	// be the case that writes can still succeed.
+	// TODO(rfindley): should we allow cancellation notifications through? It
+	// could be the case that writes can still succeed.
 	c.updateInFlight(func(s *inFlightState) {
 		err = s.shuttingDown(ErrServerClosing)
 	})

mcp/streamable.go

@@ -49,6 +49,9 @@ type StreamableHTTPOptions struct {
 	// GetSessionID provides the next session ID to use for an incoming request.
 	// If nil, a default randomly generated ID will be used.
 	//
+	// Session IDs should be globally unique across the scope of the server,
+	// which may span multiple processes in the case of distributed servers.
+	//
 	// As a special case, if GetSessionID returns the empty string, the
 	// Mcp-Session-Id header will not be set.
 	GetSessionID func() string
@@ -58,7 +61,9 @@ type StreamableHTTPOptions struct {
 	// A stateless server does not validate the Mcp-Session-Id header, and uses a
 	// temporary session with default initialization parameters. Any
 	// server->client request is rejected immediately as there's no way for the
-	// client to respond.
+	// client to respond. Server->Client notifications may reach the client if
+	// they are made in the context of an incoming request, as described in the
+	// documentation for [StreamableServerTransport].
 	Stateless bool
 
 	// TODO: support session retention (?)
@@ -136,9 +141,10 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 		transport, _ = h.transports[sessionID]
 		h.mu.Unlock()
 		if transport == nil && !h.opts.Stateless {
-			// In stateless mode we allow a missing transport.
+			// Unless we're in 'stateless' mode, which doesn't perform any Session-ID
+			// validation, we require that the session ID matches a known session.
 			//
-			// A synthetic transport will be created below for the transient session.
+			// In stateless mode, a temporary transport is be created below.
 			http.Error(w, "session not found", http.StatusNotFound)
 			return
 		}
@@ -201,7 +207,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 				// stateless servers.
 				body, err := io.ReadAll(req.Body)
 				if err != nil {
-					http.Error(w, "failed to read body", http.StatusBadRequest)
+					http.Error(w, "failed to read body", http.StatusInternalServerError)
 					return
 				}
 				req.Body.Close()
@@ -272,9 +278,22 @@ type StreamableServerTransportOptions struct {
 // A StreamableServerTransport implements the server side of the MCP streamable
 // transport.
 //
-// Each StreamableServerTransport may be connected (via [Server.Connect]) at
+// Each StreamableServerTransport must be connected (via [Server.Connect]) at
 // most once, since [StreamableServerTransport.ServeHTTP] serves messages to
 // the connected session.
+//
+// Reads from the streamable server connection receive messages from http POST
+// requests from the client. Writes to the streamable server connection are
+// sent either to the hanging POST response, or to the hanging GET, according
+// to the following rules:
+//   - JSON-RPC responses to incoming requests are always routed to the
+//     appropriate HTTP response.
+//   - Requests or notifications made with a context.Context value derived from
+//     an incoming request handler, are routed to the HTTP response
+//     corresponding to that request, unless it has already terminated, in
+//     which case they are routed to the hanging GET.
+//   - Requests or notifications made with a detached context.Context value are
+//     routed to the hanging GET.
 type StreamableServerTransport struct {
 	// SessionID is the ID of this session.
 	//
@@ -285,7 +304,7 @@ type StreamableServerTransport struct {
 	// generator to produce one, as with [crypto/rand.Text].)
 	SessionID string
 
-	// Stateless controls whether the eventstore is 'Stateless'. Servers sessions
+	// Stateless controls whether the eventstore is 'Stateless'. Server sessions
 	// connected to a stateless transport are disallowed from making outgoing
 	// requests.
 	//
@@ -1225,9 +1244,18 @@ func (c *streamableClientConn) handleSSE(initialResp *http.Response, persistent
 			c.fail(err)
 			return
 		}
-
-		// Reconnection was successful. Continue the loop with the new response.
 		resp = newResp
+		if resp.StatusCode == http.StatusMethodNotAllowed && persistent {
+			// The server doesn't support the hanging GET.
+			resp.Body.Close()
+			return
+		}
+		if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+			resp.Body.Close()
+			c.fail(fmt.Errorf("failed to reconnect: %v", http.StatusText(resp.StatusCode)))
+			return
+		}
+		// Reconnection was successful. Continue the loop with the new response.
 	}
 }
 
@@ -1295,13 +1323,6 @@ func (c *streamableClientConn) reconnect(lastEventID string) (*http.Response, er
 				finalErr = err // Store the error and try again.
 				continue
 			}
-
-			if !isResumable(resp) {
-				// The server indicated we should not continue.
-				resp.Body.Close()
-				return nil, fmt.Errorf("reconnection failed with unresumable status: %s", resp.Status)
-			}
-
 			return resp, nil
 		}
 	}
@@ -1312,16 +1333,6 @@ func (c *streamableClientConn) reconnect(lastEventID string) (*http.Response, er
 	return nil, fmt.Errorf("connection failed after %d attempts", c.maxRetries)
 }
 
-// isResumable checks if an HTTP response indicates a valid SSE stream that can be processed.
-func isResumable(resp *http.Response) bool {
-	// Per the spec, a 405 response means the server doesn't support SSE streams at this endpoint.
-	if resp.StatusCode == http.StatusMethodNotAllowed {
-		return false
-	}
-
-	return strings.Contains(resp.Header.Get("Content-Type"), "text/event-stream")
-}
-
 // Close implements the [Connection] interface.
 func (c *streamableClientConn) Close() error {
 	c.closeOnce.Do(func() {
@@ -1361,8 +1372,11 @@ func (c *streamableClientConn) establishSSE(lastEventID string) (*http.Response,
 
 // calculateReconnectDelay calculates a delay using exponential backoff with full jitter.
 func calculateReconnectDelay(attempt int) time.Duration {
+	if attempt == 0 {
+		return 0
+	}
 	// Calculate the exponential backoff using the grow factor.
-	backoffDuration := time.Duration(float64(reconnectInitialDelay) * math.Pow(reconnectGrowFactor, float64(attempt)))
+	backoffDuration := time.Duration(float64(reconnectInitialDelay) * math.Pow(reconnectGrowFactor, float64(attempt-1)))
 	// Cap the backoffDuration at maxDelay.
 	backoffDuration = min(backoffDuration, reconnectMaxDelay)